BTPWE 302 Thermodynamics

Transcription

KARNATAKA STATE OPEN UNIVERSITY
COURSE NAME:
B.TECH IN POWER ENGINEERING
YEAR/SEMESTER:
3RD SEMESTER
PAPER NAME:
THERMODYNAMICS
PAPER CODE:
BTPWE 302
NATIONAL COLLABORATIVE PARTNER
INDEX
Structure
BLOCK!
Unit
Unit
Unit
Unit
1: Combustion of Fuel - Part I
2: Combustion of Fuel- Part II
3: Entropy - Part I
4: Entropy - Part II
BLOCK2
Unit
Unit
Unit
Unit
1: Air Compressor
2: Combustion of Fuel- Part II
3: IC Engine - Part I
4: IC Engine - Part II
1.1. Objective
1.2. Introduction
1.3. Mass and Mole:
1.4. Combustion B~
1.5. Classification 01
1.6. Solid Fuels
1.7. Liquid Fuels
1.8. Gaseous Fuels
1.9. Theoretical Air;
1.10. Let us Sum Up
1.11. Check Your Pre
1.12. Reference
1.1
Objective
After studying th
BLOCK3
Unit
Unit
Unit
Unit
1: Steam Process PART-I
2: Steam Process PART-II
3: Vapour Power Cycle
4: Air Standard Cycle
1.2
-
MassandMo
-
Combustion I
-
Classification
-
Solid Fuels
-
Liquid Fuels
-
Gaseous Fuel
-
Theoretical A
Introduction
Burning fuels such a
the emission of mal
slightly different coc
Combustion or burl
oxidant accompaniec
2
UNIT 1
COl\1BUSTION OF FUEL - PART I
Structure
1.1. Objective
1.2. Introduction
1.3. Mass and Mole Fractions
1.4. Combustion Basics
1.5. Classification of Fuels
1.6. Solid Fuels
1.7. Liquid Fuels
1.8. Gaseous Fuels
1.9. Theoretical Air and Excess Air
1.10. Let us Sum Up
1.11. Check Your Progressive Exercise
1.12. Reference
1.1
Objective
After studying this unit we are able to understand
- Mass and Mole Fractions
- Combustion Basics
- Classification of Fuels
- Solid Fuels
- Liquid Fuels
- Gaseous Fuels
- Theoretical Air and Excess Air
1.2
Introduction
Burning fuels such as coal, oil, gas and petrol to produce energy and to power vehicles causes
the emission of many different chemical species into the atmosphere. Each fuel delivers a
slightly different cocktail of pollutants into the air.
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an
oxidant accompanied by the production of heat and conversion of chemical species. The release
3
of heat can result in the production of light in the form of either glowing or a flame. Fuels of
interest often include organic compounds (especially hydrocarbons) in the gas, liquid or solid
phase.
Fuel may be chemical or nuclear. Here we shall consider briefly chemical fuels only. A chemical
fuel is a substance which releases heat energy on combustion. The principal combustible
elements of each fuel are carbon and hydrogen. Though sulphur is a combustible element too but
its presence in the fuel is considered to be undesirable.
1.3
Mass and Mole Fractions
Mass percentage
Multiplying mass j
obsolete term weigl
Mole fraction
In chemistry, the m<
amount of all consti
Mass Fraction:
U,i
Xi
is the fraction of one substance with mass m, to the mass of the total
mixture mtor. defined as:
The mass fraction
Wj
=--
ntof
The sum of all the D
tu,
Wi=--
11ltot
The sum of all the mass fractions is equal to I:
N
N
i=l
i=l
2.: mi = mtot; I:Wi = 1
It is one way of expressing the composition of a mixture in a dimensionless size (mole fraction is
another).
For elemental analysis, mass fraction (or "mass percent composition") can also refer to the
fraction of the mass of one element to the total mass of a compound. It can be calculated for any
compound using its empirical formula or its chemical formula
Mass concentration
The mole fraction is
is defined as the 111
molecules Mot. It is
(mass fraction is anr
X (chi) instead of a I
Mole percentage
Multiplying mole f
percent (abbreviatec
1.4
The mass fraction of a component in a solution is the ratio of the mass concentration of that
component pj (partial density of that component) to the density of solution p:
Pi
P
Wi=-
4
Cembustle»
Combustion is a cb
rapidly with certain
organic molecules.
atoms. Methane (n
alcohol are all exar
atoms in these mole
Oxygen atoms have
fact oxygen's poteni
apart the hydrocarb
a flame. Fuels of
as, liquid or solid
; only. A chemical
cipal combustible
Mass percentage
Multiplying mass fraction by 100 gives the mass percentage, sometimes referred to by the
obsolete term weight-weight percentage or weight percent (abbreviated as w/w% or wt %).
le element too but
Mole fraction
In chemistry, the mole fraction x, is defined as the amount of a constituent n, divided by the total
amount of all constituents in a mixture nlOt
Xi
mass of the total
ni
=-Tl.tot
The sum of all the mole fractions is equal to I:
N
Lni
N
= ntot;
i=l
e (mole fraction is
also refer to the
calculated for any
The mole fraction is also called the amount fraction. It is identical to the number fraction, which
is defined as the number of molecules of a constituent N, divided by the total number of all
molecules Mot. It is one way of expressing the composition of a mixture in a dimensionless size
(mass fraction is another). The mole fraction is sometimes denoted by the lower case Greek letter
X (chi) instead of a Roman x. For mixtures of gases, the letter y is recommended.
Mole percentage
Multiplying mole fraction by 100 gives the mole percentage, also referred as amount/amount
percent (abbreviated as n/n %).
1.4
centration of that
LX;i = 1
i=l
Combustion Basics
Combustion is a chemical process that occurs when oxygen atoms excited by heat energy bond
rapidly with certain fuel elements. The most common fuel sources are hydrocarbons and related
organic molecules. These are molecules that are made up primarily from hydrogen and carbon
atoms. Methane (natural gas), gasoline, kerosene, propane, butane, methyl alcohol and ethyl
alcohol are all examples of hydrocarbons and related compounds. The hydrogen and carbon
atoms in these molecules are the actual fuel in the combustion process.
Oxygen atoms have a very strong tendency to bond with both hydrogen and carbon atoms. In
fact oxygen's potential to bond with both hydrogen and carbon is so strong that it literally rips
apart the hydrocarbon molecules and forms new compounds. When two oxygen atom~ hont{
5
with a carbon atom, the result is CO2, carbon dioxide. When one oxygen molecule bonds with
two hydrogen molecules, the result is H20, water. Water and carbon dioxide are byproducts of
the combustion of any hydrocarbon fuel. Because the process of forming these molecules
produces heat (an exothermic reaction), these gasses incandesce, which means they glow or give
off light. These glowing gasses are what make up the flame.
You might see from this discussion that for any specific fuel molecule there is some amount of
oxygen that is "exactly right" in order to enable complete combustion. This is called the
Stoichiometric ratio. This ratio varies depending on the number of hydrogen and carbon atoms
in the fuel molecule. For example propane, C3Hg, has three carbon atoms and eight hydrogen
atoms. As each carbon atom needs two oxygen atoms, and every two hydrogen atoms need one
oxygen atom, one molecule of propane needs five oxygen molecules (oxygen occurs in nature as
a diatomic molecule O2, so five molecules provide ten atoms). This is usually written as a
reaction formula:
Stoichiometri
proportions 0
oxygen is SUI
is relatively s
If too much
results in a j
supplied witl
monoxide an
1.5
Class
Fuels can be
I. Theyoccu
If there is not enough oxygen available for complete combustion, some unburned fuel will
escape and its energy will be lost. Usually this takes the form of carbon monoxide being formed
in place of carbon dioxide.
2. They are i
summary for
Typeoffuel
It might seem that the best solution to the problem of incomplete combustion would be to insure
there is extra oxygen, more than is needed for me Stoichiometric ratio of the fuel. All this extra
oxygen would pretty much insure that there was always some available when it was needed.
Unfortunately, this causes another problem which is called "excess air". Since air is mostly
nitrogen, which doesn't contribute to the combustion reaction, adding even a little bit too much
oxygen adds a lot of extra nitrogen. This extra air takes heat away from the flame, causing it to
be coole and less stable.
Solid
Liquid
Combustion is the rapid combination of oxygen with a fuel, such as natural gas, resulting in the
release of heat. Most fuels contain carbon and hydrogen, and the oxygen usually comes from air.
Combustion generally consists of the following overall reactions:
Carbon + Oxygen
.____>
@
@
Carbon Dioxide +
Hydrogen + Oxygen
WalerVapor+
6
Gaseous
nolecule bonds with
Ie are byproducts of
ng these molecules
IS
they glow or give
: is some amount of
This is called the
n and carbon atoms
and eight hydrogen
gen atoms need one
I occurs in nature as
isually written as a
Stoichiometric or perfect combustion is obtained by mixing and burning exactly the correct
proportions of fuel and oxygen so that no oxygen remains at the end of the reaction. If too much
oxygen is supplied, the mixture is lean and the reaction is oxidizing. This results in a flame that
is relatively shorter.
If too much fuel is supplied, the mixture is rich and the reaction is reducing. This typically
results in a flame that is relatively longer and sometimes smoky. Most industrial burners are
supplied with some excess air to mitigate the formation of unburned hydrocarbons, carbon
monoxide and particulate matter.
1.5
Classification
of Fuels
Fuels can be classified according to:
1. They occur in nature called primary
unburned fuel will
oxide being formed
would be to insure
fuel. All this extra
len it was needed.
Since air is mostly
. little bit too much
flame, causing it to
fuels or are prepared called secondary fuels;
2. They are in solid, liquid or gaseous state. The detailed classification of fuels can be given in a
summary form as follows:
Prepared (Secondary)
Typeoffuel
Natural (Primary)
Solid
Wood
Coke
Peat
Charcoal
Lignite coal
Briquettes
Petroleum
Gasoline
Kerosene
Fuel oil
Alcohol
Benzol
Shale oil
Liquid
~as,resulting in the
lly comes from air.
Gaseous
Petroleum gas
Producer gas
Coal gas
Coke-oven gas
Blast furnace gas
Carburetted gas
Sewer gas
Natural gas
~'::~
~.
7
1.6
stability. In areas
bituminous coal.
Solid Fuels
Solid fuel refers to various types of solid material that are used as fuel to produce energy and
provide heating, oil sually released through combustion.
Solid fuels include wood (see wood fuel), charcoal, peat, coal, Hexamine fuel tablets, and pellets
made from wood (see wood pellets), corn, wheat, rye and other grains. Solid-fuel rocket
technology also uses solid fuel (see solid propellants).
Solid fuels have been used by humanity for many years to create fire. Coal was the fuel source
which enabled the industrial revolution, from firing furnaces, to running steam engines. Wood
was also extensively used to run steam locomotives. Both peat and coal are still used in
electricity generation today.
The use of some solid fuels (eg. coal) is restricted or prohibited in some urban areas, due to
unsafe levels of toxic emissions. The use of other solid fuels such as wood is increasing as
heating technology and the availability of good quality fuel improves. In some areas, smokeless
coal is often the only solid fuel used. In Ireland, peat briquettes are used as smokeless fuel. They
are also used to start a coal fire.
Lignite coal
The softest of the four types of coal. It is a brownish black in color, very crumbly and primarily
used for the generation of electricity. Because of its color, it is often referred to as "brown coal."
Lignite is the result of milljons of tons of plants and trees that decayed in a swampy atmosphere
about 50-70 million years ago.
..
The heating content of lignite is approximately 4,000-8,000 Btu's per pound. The carbon content
of lignite is 25%-35% and it has a very high water content - about 35 percent.lt has been
estimated that nearly half of the world's total proven coal reserves are made up of lignite and sub
bituminous coal, but lignite has not been exploited to any great extent, because it is inferior to
higher-rank coals (e.g., bituminous coal) in calorific value, ease of handling, and storage
I
...
8
See article of Otto
into two types. 11
lignite or perfect
appearance of ordii
great modification.
a weak solutior
The worlds biggest
the US.
Lignite is mined u
covers an area of sc
lignite-mining regi-
stability. In areas where other fuels are scarce, the production of brown coal far exceeds that of
bituminous coal.
ce energy and
:ts, and pellets
id-fuel rocket
ae fuel source
ngines. Wood
still used in
areas, due to
increasing as
as, smokeless
ess fuel. They
Lignite with a fossil embedded
md primarily
'brown coal."
y atmosphere
irbon content
t.It has been
gnite and sub
is inferior to
and storage
See article of Otto C. Kopp on Lignite in the Encyclopedia Britannica. Lignite can be separated
into two types. The first is xyloid lignite or fossil wood and the second form is the compact
lignite or perfect lignite. Although xyloid lignite may sometimes have the tenacity and the
appearance of ordinary wood it can be seen that the combustible woody tissue has experienced a
great modification. It is reducible to a fine powder by trituration and if submitted to the action of
a weak solution of potash it yields a considerable quantity of ulmic acid.
The worlds biggest Lignite or Brown Coal Producers are Germany, the Russian Federation and
the US.
Lignite is mined in so-called open pit mines. The Rhenish (Rhineland) lignite-mining region
covers an area of some 2,500km2 to the west of Cologne and is one of the world's most important
lignite-mining regions. Where Mechanised lignite mining began in the 1890s, the first bucket9
wheel excavator was commissioned in 1933 and, by 1940, output was over 60Mtly from 23
surface mines. began in the 1890s, the first bucket-wheel excavator was commissioned in 1933
and, by 1940, output was over 60Mtly from 23 surface mines. mechanised lignite mining began
in the 1890s, the first bucket-wheel excavator was commissioned in 1933 and, by 1940, output
was over 60Mtly from 23 surface mines.
sub-bitumin.
Btu's per poi
Peat
Peat is an ac
couple of mi
Bituminous
The Garzweiler Strip Mine in Germany and also visit: Rhineland Lignite Mining, Germany.
2) Sub-bituminous
coal
Bituminous c
bituminous c
pound - grea
electrical ger
making steel.
Anthracite (
Anthracite c
carbon conter
Sub-bitumous coal
Under greater pressure and heat, brown coal continued to lose moisture. The carbon content of
coal increased as water was lost. The properties of sub-bituminous coal range from those of
"lignite" (with a lower carbon content) to those of ''bituminous coal" (with a higher carbon
content). Sub-bituminous coal is primarily used for steam-electric power generation.
,
This is a medium soft coal that contains much less moisture than lignite and is not nearly as
crumbly. Like lignite, its primary use is in the generation of electricity. The carbon content of
10
find anthracit
that is roughl
much as seasr
burns cleanly
ground for 101
Coke. It con
hydrogen, nit
certain kinds •
is mainly usee
Briquettes.
pressure.
T
fer 60Mtly from 23
mmissioned in 1933
ignite mining began
nd, by 1940, output
sub-bituminous coal runs from 35%-45% and its heat value generally ranges from 8,000-13,000
Btu's per pound.
Peat
Peat is an accumulation of partially decayed vegetation matter. It is not yet coal, that will take a
couple of million years.
Bituminous Coal:
ing, Germany.
Bituminous coal contains even less moisture than the sub-bituminous type. The carbon content of
bituminous coal is generally from 45%-85%. Its heat value ranges from 10,500-15,000 Btu's per
pound - greater than either lignite or the sub-bituminous types. In addition to being used for
electrical generation, it is also used in making coke or coking coal, an essential ingredient in
making steel.
Anthracite Coal:
Anthracite coal - discovered in 1769 - is the hardest of the four types. It averages 85%-95%
carbon content and has the highest heating value of the four types of coal. It is not uncommon to
find anthracite that produces well in excess of 15,000 Btu's per pound. To put that in perspective,
that is roughly one and one-half times as much heat as the same volume of oil and four times as
much as seasoned hard-maple firewood. Anthracite makes excellent home heating fuel because it
burns cleanly, does not produce volatile gases and does not deteriorate. It can be stored on the
ground for long periods of time without creating environmental problems.
~ carbon content of
mge from those of
th a higher carbon
ration.
Coke. It consists of carbon, mineral matter with about 2% sulphur and small quantities of
hydrogen, nitrogen and phosphorus. It is solid residue left after the destructive distillation of
certain kinds of coals. It is smokeless and clear fuel and can be produced by several processes. It
is mainly used in blastfurnace to produce heat and at the same time to reduce the iron ore.
;
ad is not nearly as
: carbon content of
Briquettes. These are prepared from fine coal or coke by compressing the material under high
pressure.
11
1.7
Liquid Fuels
Liquid fuels are those combustible or energy-generating molecules that can be harnessed to
create mechanical energy, usually producing kinetic energy; they also must take the shape of
their container. Most liquid fuels, in widespread use, are or derived from fossil fuels; however,
there are several types, such as hydrogen fuel (for automotive uses), which are also categorized
as a liquid fuel. It is the fumes of Liquid fuels that are flammable instead of the fluid.
The chief source of liquid fuels is petroleum which is obtained from wells under the earth's
crust. These fuels have proved more advantageous in comparison to sold fuels in the following
respects.
Advantages:
1. Require less space for storage.
2. Higher calorific value.
3. Easy control of consumption.
4. Staff economy.
5. Absence of danger from spontaneous combustion.
6. Easy handling and transportation.
7. Cleanliness.
8. No ash problem.
9. Non-deterioration of the oil in storage.
PETROLEUM
Most liquid fuels used currently are produced from petroleum. The most notable of these is
gasoline. Scientists generally accept that petroleum formed from the fossilized remains of dead
plants and animals by exposure to heat and pressure in the Earth's crust.
Gasoline
Gasoline is the most widely used liquid fuel. Gasoline, as it is known in United States and
Canada, or petrol in India, Britain, Australia, New Zealand, South Africa and many Englishspeaking countries, is made of hydrocarbon molecules forming aliphatic compounds, or chains of
carbons with hydrogen atoms attached. However, many aromatic compounds (carbon chains
forming rings) such as benzene are found naturally in gasoline and cause the health risks
associated with prolonged exposure to the fuel.
12
harnessed to
the shape of
els; however,
J categorized
l.
!r the earth's
:he following
Production of gasoline is achieved by distillation of crude oil. The desirable liquid is separated
from the crude oil in refineries. Crude oil is extracted from the ground in several processes; the
most commonly seen may be beam pumps. To create gasoline, petroleum must first be removed
from crude oil.
Gasoline itself is actually not burned, but the fumes it creates ignite, causing the remaining liquid
to evaporate. Gasoline is extremely volatile and easily combusts, making any leakage extremely
dangerous. Gasoline for sale in most countries carries an octane rating. Octane is a measure of
the resistance of gasoline to combusting prematurely, known as knocking. The higher the octane
rating, the harder it is to bum the fuel, which allows for a higher compression ratio. Engines with
a higher compression ratio produce more power (such as in race car engines). However, such
engines actually require a higher octane fuel.
Diesel
Conventional diesel is similar to gasoline in that it is a mixture of aliphatic hydrocarbons
extracted from petroleum. Diesel may cost more or less than gasoline, but generally costs less to
produce because the extraction processes used are simpler. Many countries (particularly in
Europe, as well as Canada) also have lower tax rates on diesel fuels.
After distillation, the diesel fraction is normally processed to reduce the amount of sulfur in the
fuel. Sulphur causes corrosion in vehicles, ~cid rain and higher emissions of soot from the tail
pipe (exhaust pipe). In Europe, lower sulfur levels than in the United States are legally required.·
However, recent US legislation will reduce the maximum sulphur content of diesel from 3,000
ppm to 500 ppm by 2007, and 15 ppm by 2010. Similar changes are also underway in Canada,
Australia, New Zealand· and several Asian countries.
A diesel engine is a type of internal combustion engine which ignites fuel by injecting it into a
combustion chamber previously compressed with air (which in tum raises the temperature) as
opposed to using an outside source, such as a spark plug.
e of these is
iains of dead
Kerosene
Kerosene once used in kerosene lamps as an alternative to whale oil, is today mainly used in fuel
for jet engines. One form of the fuel known as RP-l is burned with liquid oxygen as rocket fuel.
These fuel grade kerosenes meet specifications. for smoke points and freeze points.
d States and
any English, or chains of
arbon chains
health risks
t
In the mid-20th century, kerosene or "TVO" (Tractor Vaporising Oil) was used as a cheap fuel
for tractors. The engine would start on gasoline, then switch over to kerosene once the engine
warmed up. A "heat valve" on the manifold would route the exhaust gases around the intake
pipe, heating the kerosene to the point where it can be ignited by an electric spark.
13
Kerosene is sometimes used as an additive in diesel fuel to prevent gelling or waxing in cold
temperatures.
NON-PETROLEUM
There is increasin,
This fuel blend ca
a modern Flexible
FOSSIL FUELS
When petroleum is not easily available, chemical processes such as the Fischer- Tropsch process
can be used to produce liquid fuels from coal and/or natural gas.
Ethanol for use iJ
synthesized from
fermentation of gr
Biodiesel
Butanol
to
Biodiesel is similar
diesel, but has differences akin to those between petrol and ethanol. For
instance, biodiesel has a higher cetane rating (45-60 compared to 45-50 for crude-oil-derived
diesel) and it acts as a cleaning agent to get rid of dirt and deposits. It has been argued that it only
becomes economically feasible above oil prices of $80 (£40 or €60 as of late February, 2007) per
barrel. This does however depend on locality, economic situation, government stance on
biodiesel and a host of other factors- and it has been proven to be viable at much lower costs in
some countries. Also, it gives about 10% less energy than ordinary diesel. NOTE: As with
alcohols and petrol engines, taking advantage of biodiesel's high cetane rating potentially
overcomes the energy deficit compared to ordinary number 2 diesel.
Alcohols
Generally, the term alcohol refers to ethanol, the first organic chemical produced by humans, but
any alcohol can be burned as a fuel. Ethanol and methanol are the most common, being
sufficiently inexpensive to be useful.
Butanol is an alco
without engine rn
bacterium Clostrii
was first delineate
making cordite, a :
The advantages 0
about 10% lower
100% more so tha
or 35 "C), toxicitj
that the fermentati
can only tolerate t
butanol from oil p
usage defeats the I
($1250-$l320 pel
than ethanol (appr
Methanol
1.8
Methanol is the lightest and simplest alcohol, produced from the natural gas component methane.
Its application is limited due to its toxicity (similar to gasoline). Small amounts are used in some
gasolines to increase the octane rating. Methanol-based fuels are used in some race cars and
model airplanes.
Methanol is also called methyl alcohol or wood alcohol, the latter because it was formerly
produced from the distillation of wood. It is also known by the name methyl hydrate.
Ethanol
Ethanol, also known as grain alcohol or ethyl alcohol, is most commonly used in alcoholic
beverages. However, it may also be used as a fuel, most often in combination with gasoline. For
the most part, it is used in a 9: 1 ratio of gasoline to ethanol to reduce the negative-environmental
effects of gasoline.
14
Gaseous F
Types of Gaseous
Natural Gas
Naturally occurrir
constituents vary
hydrocarbons whi
sulphide.
Terms used to de
dry or lean - high I
wet - high concent
There is increasing interest in the use of a blend of 85% fuel ethanol blended with 15% gasoline.
This fuel blend called E85, has a higher fuel octane than most premium gasolines. When used in
a modem Flexible fuel vehicle, it delivers more performance to the gasoline it replaces.
xing in cold
Ethanol for use in gasoline and industrial purposes may be called a fossil fuel because it is
synthesized from the petroleum product ethylene, which is cheaper than production from
fermentation of grains or sugarcane.
osch process
Butanol
Butanol is an alcohol which can b~ used as a fuel in most gasoline internal combustion engines
without engine modification. It is typically a product of the fermentation of biomass by the
bacterium Clostridium acetobutylicum (also known as the Weizmann organism). This process
was first delineated by Chaim Weizmann in 1916 for the production of acetone from starch for
making cordite, a smokeless gunpowder.
ethanol. For
»oil-deri ved
:l that it only
y, 2007) per
t stance on
wer costs in
'E: As with
; potentially
The advantages of butanol are its high octane rating (over 100) and high energy content, only
about 10% lower than gasoline, and subsequently about 50% more energy-dense than ethanol,
100% more so than methanol. Butanol's only major disadvantages are its high flashpoint (95 OF
or 35 "C), toxicity (note that toxicity levels exist but are not precisely confirmed), and the fact
that the fermentation process for renewable butanol emits a foul odour. The Weizmann organism
can only tolerate butanol levels up to 2% or so, compared to 14% for ethanol and yeast. Making
butanol from oil produces no such odour, but the limited supply and environmental impact of oil
usage defeats the purpose of alterytativefuels. The cost of butanol is about $0.57-$0.58 per pound
($1250-$1320 per metric ton or $4 approx. per US gallon). Butanol is much more expensive
than ethanol (approx. $1.50 per gallon) and methanol.
humans, but
unon, being
1.8
mt methane.
sed in some
ce cars and
Gaseous Fuels
Types of Gaseous Fuels:
Natural Gas
as formerly
Naturally occurring gas found in oil fields and coal fields (Fire damp). The quantities of the
constituents vary but the principle component is methane. Other components include higher
hydrocarbons which can be separated out as a condensate. Some gases also contain hydrogen
sulphide.
in alcoholic
Terms used to describe gases:
asoline. For
dry or lean - high methane content (less condensate)
wet - high concentration of higher hydrocarbons (C5 - CI0)
vironmental
I
15
sour - High concentration of H2S
sweet - low cone. of H2S
residue gas - gas remaining after the condensing process
casing head gas - gas extracted from an oil well by extraction at the surface.
Total world production of nat gas in 1986 was 100 trillion m3. It is used as feed stock as well as
fuel. It is preferred due to its high CV. Gas from coal mines is of equal quality to oil fields
however it is much more difficult to extract. In 1961 220 mill m3 of coal nat gas were extracted
in the UK. The North Sea gas has smashed the industry.
Natural gases can be liquefied for distribution by tanker. Liquefied natural gas (LNG) contains
mostly methane, LPG (Liquefied petroleum gas) mostly butane and propane.
Synthetic Gases
Carburetted .
unattractive to
gas methods. t
ratio of the tw
old and has 12
supplies of nat
Coal and C(
temperature cs
as fuel. If colo
coke to form b
of conversion i
Advantages
These are gases which are chemically made by some process.
Increased interest presently in power generation due to the gasification properties of waste and
biomass.
Main methods of synthesis:
Producer 'gas: The gas is produced by blowing air and sometimes steam through an
incandescent fuel bed (the process is self heating). The reaction with air is exothermic but
insufficient air is added hence CO is produced. Steam addition results in the formation of
hydrogen by the water gas reaction. This is endothermic and hence balances out the exothermic
air reaction.
:
1. Better contn
2. Much less e
3. Economy in
4. Easy mainte
5. Cleanliness.
6. No problem
7. The distribu
as such handlii
8. Gaseous ft
preheated in re
1.9
Theor(
Producer gas is low CV and is hence is only usually used on site
Blue Gas or Water Gas - This is produced in a similar manner to above but allows the
production of a higher CV fuel by intermittently blasting the incandescent bed with air and steam
such that the overall heat balance is maintained. The products of the air blast contain the nitrogen
which reduces CV. These are discharged to atmosphere. The products of the steam blast are kept
since they have a higher CV. CV is virtually doubled in this way. Often used as a synthesis gas
in the chemical industry.
Oil Gas This is the gas formed by the thermal cracking of crude oil. If oil is sprayed onto heated
checker work (refractor.) it cracks to form lower gaseous hydrocarbons. These depend entirely
on the feed stock but calorific values can increase to as much as 25MJ/m3 but can be as low as
half of this.
16
The minimum
the carbon, h
"theoretical a
contain no OX)
In practice, it i
air supplied is
means that air
methane with :
written as:
Carburetted Water Gas - Water gas has still to Iowa CV for most purposes and this makes it
unattractive to distribute. Carburetted water gas is the result of combining the water gas and oil
gas methods. Oil is sprayed into the hot water gas chamber to result in a good quality gas. The
eed stock as well as
quality to oil fields
gas were extracted
~as (LNG) contains
ratio of the two determines the quality. This was the method used to produce the "Town gas" of
old and has largely been superseded by natural gas in countries with an abundant supply. AS
supplies of natural gas diminish, however, it will become more important again.
Coal and Coke Oven Gas - As mentioned previously, gases are liberated in the high
temperature carbonisation (coking) of coal. These are cleaned, de tarred and scrubbed and used
as fuel. If coke is not required (coal gas), steam injection at the end of the cycle reacts with the
coke to form blue water gas. This reduces the CV of the gas produced but the thermal efficiency
of conversion rises.
Advantages
erties of waste and
:
1. Better control of combustion.
2. Much less excess air is needed for complete combustion.
3. Economy in fuel and more efficiency of furnace operation.
4. Easy maintenance of oxidizing or reducing atmosphere.
5. Cleanliness.
steam through an
is exothermic but
1 the formation of
out the exothermic
6. No problem of storage if the supply is available from public supply line.
7. The distribution of gaseous fuels even over a wide area is easy through the pipe lines and
as such handling of the fuel is altogether eliminated.
8. Gaseous fuels give economy of heat and produce higher temperatures (as they can be
preheated in regenerative furnances and thus heat from hot flue gases can be recovered).
1.9
we but allows the
with air and steam
:ontain the nitrogen
team blast are kept
Ias a synthesis gas
prayed onto heated
:se depend entirely
rt can be as low as
Theoretical
Air and Excess Air
The minimum amount of air that supplies sufficient oxygen for the complete combustion of all
the carbon, hydrogen, and any other elements in the fuel that may oxidise is called the
"theoretical air". When complete combustion is achieved with theoretical air, the products
contain no oxygen.
In practice, it is found that complete combustion is not likely to be achieved unless the amount
air supplied is somewhat greater than the theoretical amount. Thus 150 per cent theoretical
means that air actually supplied is 1.5 times the theoretical air. The complete combustion
methane with minimum amount of theoretical air and 150 per cent theoretical air respectively
written as:
17
of
air
of
is
CH, + 2<1.5}O:J + 2
G~)
(I.Sj N!l---+
co, + 2Hp
+ 02 + 3 (~)
Table 1.1. Symh
Nz
(with 150 per ~
theoretic:ll :lir)
.... 9
The amount of air actually supplied may also be expressed in terms of per cent excess air.
The excess air is the amount of air supplied over and above the theoretical air. Thus 150 per cent
theoretical air is equivalent to 50 per cent excess air.
Note. For complete combustion of fuel we need air. As per theoretical basis there is a minimum
amount of air which is required by the fuel to bum completely, but always, air in excess is used
because whole of air supplied for combustion purposes does not come in contact with the fuel
completely and as such portion of fuel may be left unburnt. But if a large quantity of excess air is
used it exercises a cooling effect on combustion process which however can be avoided by
preheating the air. The weight of excess air supplied can be determined from the weight of
oxygen which is left unused. The amount of excess air supplied varies with the type of fuel and
the firing conditions. It may approach a value of 100% but modem practice is to use 25% to 50%
excess air.
Carbo.. monazid.
Car'bon
diaid.
Sulphur~
MArSh C:U (MeUII
Ethylene
Ethane
1.10
Let us Su
In this unit we ha'
Basic Chemistry
-
Mass and
Combustir
-
Classificai
Solid Fuel
-
Liquid Fur
-
Gaseous F
-
Theoretica
1.11
Check Yo
Before considering combustion problems it is necessary to understand the construction and use
of chemical formulae. This involves elementary concepts which are discussed below briefly.
Atoms. It is not possible to divide the chemical elements indefinitely, and the smallest particle
which can take part in a chemical change is called an 'atom'. If an atom is split as in nuclear
reaction, the divided atom does not retain the original chemical properties.
Molecules. It is rare to find elements to exist naturally as single atom. Some elements have
atoms which exist in pairs, each pair forming a molecule (e.g. oxygen), and the atoms of each
molecule are held together by stronger inter-atomic forces. The isolation of a molecule of
oxygen would be tedious, but possible; the isolation of an atom of oxygen would be a different
prospect. The molecules of some substances are formed by the mating up of atoms of different
elements. For example, water has a molecule which consists of two atoms of hydrogen and one
atom of oxygen.
The atoms of different elements have different masses and these values are important when a
quantitative analysis is required. The actual masses are infinitesimally small, and the ratios of the
masses of atoms are used. These ratios are indicated by atomic weight quoted on a scale which
defines the atomic weight of oxygen as 16.
The symbols and molecular weights of some important elements, compounds and gases are given
in Table 1.1.
18
1. Definema
2. What is co
3. Classify tb
4. What is th.
5. Explain th.
1.12
~
Reference
Shuttle-Mi
MGBX. SI
Table 1.1. Symbols and Molecular weights
::t.ir)
Atcm
Nokcuk
.... 9
ElI1fU7Ih f ComJNIu7Ub I GIl:JU
Symbol
re is a minimum
'n excess is used
tct with the fuel
f of excess air is
be avoided by
'1l the weight of
!
type of fuel and
use 25% to 50%
Symbol
wright
[cess air.
hus 150 per cent
Mol«:Ul4r
Mokculor
wright
Hydrogen
Oxygen
Nitropn
~
2
H
03
32
0
N,
N
Carbon
C
28
12
1
16
1.
C
12
Sulphur
W~
5
82
S
S2
liP
-
Carbon monazYi-
CO
18
28
Carbon di..i.d.
co,
44
Sulphurclia:id.
MlttSh CII5 (Methane)
Et;hyJane
Ethane
SOa
M
16
1.10
~
CJI.
9\
2S
lI)
-
-
-
-
Let us Sum Up
In this unit we have studied
truction and use
low briefly.
imallest particle
dit as in nuclear
~ elements have
e atoms of each
, a molecule of
ld be a different
oms of different
«lrogen and one
iportant when a
the ratios of the
In a scale which
-
Mass and Mole Fractions
-
Combustion Basics
-
Classification of Fuels
Solid Fuels
-
Liquid Fuels
-
Gaseous Fuels
-
Theoretical Air and Excess Air
1.11
Check Your Progressive Exercise
1. Define mass and mole fractions and the relationship between them.
2. What is combustion?
3. Classify the different types of fuels and explain their detailed nature
4. What is theoretical air and how is it different from Excess Air?
5. Explain the difference between Gaseous and Solid Fuel.
1.12
)io>
Reference
Shuttle-Mir HistorylSciencelMicrogx.avity/Candle
Flame in Microgravity
MGBX. Spaceflight.nasa.gov (1999-07-16). Retrieved on 2010-09-28.
gases are given
19
(CFM) -
;.. A. A. Putnam and W. C. Dennis (1953) "Organ-pipe oscillations in a flame-filled tube,"
>-
2.1
Objecth
Fourth Symposium (International) on Combustion, The Combustion Institute, pp. 566--
------.:
.574.
After studying tl
E. C. Fernandes and M. V. Heitor, "Unsteady flames and the Rayleigh criterion" in F.
Culick, M. V. Heitor, and J. H. Whitelaw, ed.s, Unsteady Combustion (Dordrecht, the
Netherlands: Kluwer Academic Publishers, 1996), p. 4
2.2
-
Combust
-
Combust
-
Stoichioi
-
Air-Fuel
-
Weight c
-
Weight c
-
Analysis
. Introdu(
Combustion or
oxidant accomps
of -heat can resu
interest often in'
phase.
UNIT 2
COMBUSTION OF FUEL - PART II
In a complete (
oxygen or fluor
oxidizing elemer
Structure
•
2.1.0bjective
2.2.Introduction
2.3.Combustion of hydrogen
2.4.Combustion of carbon
2.5.Stoichiometric Air Fuel (AlF) Ratio
2.6.Air-Fuel Ratio from Analysis of Products
2.7.Weight of Carbon in Fuel Gases
2.8.Weight of Fuel Gases per KG of Fuel Burnt
2.9.Analysis of Exhaust and Flue Gas
2.10. Practical analysis of combustion products
2.11. Let us Sum Up
2.13. Reference
A simple exampl
used reaction in I
The result is watt
Complete combu
come to equilibr
monoxide and pi
is 78% nitrogen,
2.3
20
Combust
flame-filled tube,"
institute, pp. 566-
Objective
2.1
------~
----------After studying this unit we are able to understand
gh criterion" in F.
in
(Dordrecht, the
-
Combustion of hydrogen
-
Combustion of carbon
Stoichiometric Air Fuel (A/F) Ratio
-
Air-Fuel Ratio from Analysis of Products
-
Weight of Carbon in Flue Gases
-
Weight of Flue Gases per KG of Fuel Burnt
-
Analysis of Exhaust and Flue Gas
2.2
Introduction
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an
oxidant accompanied by the production of heat and conversion of chemical species. The release
of-heat can result in the production of light in the form of either glowing or a flame. Fuels of
interest often include organic compounds (especially hydrocarbons) in the gas, liquid or solid
phase.
In a complete combustion reaction, a compound reacts with an oxidizing element, such as
oxygen or fluorine, and the products are compounds of each element in the fuel with the
oxidizing element. For example:
CH4 + 2 02 -- C02 + 2 H20 + energy
CH2S + 6 F2 -- CF4 + 2 HF + SF6
•
A simple example can be seen in the combustion of hydrogen and oxygen, which is a commonly
used reaction in rocket engines:
2H2 + 02 -,2 H20(g)
+ heat
The result is water vapor.
Complete combustion is almost impossible to achieve. In reality, as actual combustion reactions
come to equilibrium, a wide variety of major and minor species will be present such as carbon
monoxide and pure carbon (soot or ash). Additionally, any combustion in atmospheric air, which
is 78% nitrogen, will also create several forms of nitrogen oxides.
2.3
Combustion of Hydrogen
21
Hydrogen gas (dihydrogen or molecular hydrogen) is highly flammable and will burn in air at a
very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion
for hydrogen is -286 kl/mol:
2 H2(g) + 02(g) -
2 H20(I) + 572 kl (286 kllmol)
Hydrogen gas forms explosive mixtures with air if it is 4-74% concentrated and with chlorine if
it is 5-95% concentrated. The mixtures spontaneously explode by spark, heat or sunlight. The
hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500°C
(932 OF). Pure hydrogen-oxygen flames emit ultraviolet light and are nearly invisible to the
naked eye, as illustrated by the faint plume of the Space Shuttle main engine compared to the
highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a burning
hydrogen leak may require a flame detector; such leaks can be very dangerous. The destruction
of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is
debated, but the visible flames were the result of combustible materials in the ship's skin.
Because hydrogen is buoyant in air, hydrogen flames tend to ascend rapidly and cause less
damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived the fire, and
many deaths were instead the result of falls or burning diesel fuel.
H2reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room
temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen
chloride and hydrogen fluoride, which are also potentially dangerous acid.
The above equation of combustion of hydrogen tell us that:
(I) Hydrogen reacts with water to form steam or water.
Carbon, by fai
14,500 British
ranges from II
hydrogen gene
not all of this
water vapor. "
relationship oc
partially oxide
contributed by
sulfur is only I
percent by wei
primarily to v~
The carbon dil
coal as POUll(
combustion w
Because the al
dioxide is 44
combines wit}
coal with a c:
about 204.3 J
combustion 0
.shorttons) of
To calculate t
the ratio of illl
(ii) Two molecules of hydrogen react with one molecule of oxygen to give two molecules of
steam or water,
Carbon Dioxi
The H20 may be liquid or a vapour depending on whether the product has been cooled
sufficiently to cause condensation.
where
2.4
QC01
Combustion of Carbon
The amount of heat emitted during coal combustion depends largely on the amounts of carbon,
hydrogen, and oxygen present in the coal and, to a lesser extent, on the sulfur content Hence, the
ratio of carbon to heat content depends on these heat-producing components of coal, and these
components vary by coal rank.
cf= specific c
hf
= specific
(
C", = specific
CC02
22
= specifi
= specij
vill bum in air at a
aJpy of combustion
md with chlorine if
at or sunlight. The
I in air, is 500°C
Iy invisible to the
ie compared to' the
:tion of a burning
JS. The destruction
stion; the cause is
in the ship's skin.
ily and cause less
vived the fire, and
i violently at room
halides, hydrogen
Carbon, by far the major component of coal, is the principal source of heat, generating about
14,500 British thermal units (Btu) per pound. The typical carbon content for coal (dry basis)
ranges from more than 60 percent for lignite to more than 80 percent for anthracite. Although
hydrogen generates about 62,000 Btu per pound, it accounts for only 5 percent or less of coal and
not all of this is available for heat because part of the hydrogen combines with oxygen to form
water vapor. The higher the oxygen content of coal, the lower its heating value. This inverse
relationship occurs because oxygen in the coal is bound to the carbon and has, therefore, already
partially oxidized the carbon, decreasing its ability to generate heat. The amount of heat
contributed by the combustion of sulfur in coal is relatively small, because the heating value of
sulfur is only about 4,000 Btu per pound, and the sulfur content of coal generally averages 1 to 2
percent by weight. Consequently, variations in the ratios of carbon to heat content of coal are due
primarily to variations in the hydrogen content.
The carbon dioxide emission factors in this article are expressed in terms of the energy content of
coal as pounds of carbon dioxide per million Btu. Carbon dioxide (C02) forms during coal
combustion when one atom of carbon (C) unites with two atoms of oxygen (0) from the air.
Because the atomic weight of carbon is 12 and that of oxygen is 16, the atomic weight of carbon
dioxide is 44. Based on that ratio, and assuming complete combustion, 1 pound of carbon
combines with 2.667 pounds of oxygen to produce 3.667 pounds of carbon dioxide. For example,
coal with a carbon content of 78 percent and a heating value of 14,000 Btu per pound emits
about 204.3 pounds of carbon dioxide per million Btu when completely burned. Complete
combustion of 1 short ton (2,000 pounds) of this coal will generate about 5,720 pounds (2.86
.short tons) of carbon dioxide.
To calculate the C02 emission from a fuel the carbon content of the fuel must be multiplied with
the ratio of molecular weight CO2 (44) to the molecular weight Carbon 12 -> 44/ 1j = 3.7
two molecules of
Carbon Dioxide emission can be calculated as
has been cooled
where
qC02 = specific C02 emission (C02lkWh)
cf = specific carbon content in the fuel (kgdkgfueJ)
mounts of carbon,
:ontent. Hence, the
of coal, and these
hf
= specific
Cm
= specific
energy content (kWhlkgfueV
mass Carbon (kg/mol Carbon)
CC02 = specific mass Carbon Dioxide (kg/mol COl)
23
When making carbon monoxide a source of carbon dioxide gas is needed. This could be from a
C02 cylinder or even dry ice (solid CO2). If neither is available carbon dioxide could be
generated by the neutralisation reactions between an acid and a carbonate or an acid and a
hydrogen carbonate.
2HCI (aq)
HCI (aq)
-_ Note. The recipr
+ CaC03 (s) ~CaCI2 (aq) + H20 (I) + CO2 (g)
Stoichiometric o
completely.
+ NaHC03 (s) ~ NaCI (aq) + H20 (I) + CO2 (g)
When carbon dioxide gas is passed over heated charcoal it forms carbon monoxide.
C02(g)
+ C (s) 42CO
The working va)
(g)
There will also be unreacted carbon dioxide, which needs to be removed. Carbon dioxide is
removed by reacting it with an aqueous solution of sodium hydroxide.
A complete com
(H20) and all the
With unburned «
uncompleted and
The combustion
[C + H (fuel)] +
2.5
Stoichiometric
Where
Air Fuels (AIF)Ratio
Stoichiometric (or chemically correct) mixture of air and fuel is one that contains just sufficient
oxygenfor complete combustion of thefuel.
C= Carbon
H=Hydrogen
A weak mixture is one which has an excess of air.
0= Oxygen
A rich mixture is one which has a deficiency of air.
N=Nitrogen
The perc -ntage of excess air is given as under:
(where A and F denote air andfuel respectively)
.
'
To determine th
stoichiometric ail
chemical mixing
excess left over.
The ratios are expressed as follows :
For gaseousfuels
Process heating e
high temperature
to 20% more than
By volume
For solid and liquid fuels
By mass
For boiler plant the mixture is usually greater than 20% weak ; for gas turbines it can be as much
as 300% weak.' Petrol engines have to meet various conditions of load and speed, and operate
over a wide range of mixture strength. The following definition is used :
24
If an insufficient j
monoxide exhaus
combustion effici.
could be from a
ioxide could be
r an acid and a
:Mixture strength
=
Stoichlmnetric AIF ratio
A -,-_,
..............
NFratio
The working value range between 80% (weak) and 120% (rich).
- Note. The reciprocal of the air fuel ratio is called thefuel-air (FIA) ratio.
Stoichiometric or Theoretical Combustion is the ideal combustion process where fuel is burned
completely.
Ie.
A complete combustion is a process burning all the carbon (C) to (C02)' all the hydrogen (H) to
(H20) and all the sulphur (S) to (S02).
With unburned components in the exhaust gas, such as C, H2, CO, the combustion process is
dioxide is
uncompleted and not stoichiometric.
The combustion process can be expressed as:
[C + H (fuel)] + [02 + N2 (Air)] -> (Combustion Process) -> [CO] + H20 + /12(Heat)]
Where
as just sufficient
C = Carbon
H=Hydrogen
0= Oxygen
•
N= Nitrogen
To determine the excess air or excess fuel for a combustion system we starts with the
stoichiometric air-fuel ratio. The stoichiometric ratio is the perfect ideal fuel ratio where the
chemical mixing proportion is correct. When all fuel and air burned is consumed without any
excess left over.
Process heating equipment are rarely run that way. "On-ratio" combustion used in boilers and
high temperature process furnaces usually incorporates a modest amount of excess air - about 10
to 20% more than what is needed to burn the fuel completely.
: can be as much
eed, and operate
If an insufficient amount of air is supplied to the burner, unburned fuel, soot, smoke, and carbon
monoxide exhausts from the boiler - resulting in heat transfer surface fouling, pollution, lower
combustion efficiency, flame instability and a potential for explosion.
25
..
To avoid inefficient and unsafe conditions boilers normally operate at an excess air level. This
excess air level also provides protection from insufficient oxygen conditions caused by variations
in fuel composition and "operating slops" in the fuel-air control system. Typical values of excess
air are indicated for various fuels in the table below.
•
•
The conversic
1. Multiply th
2. Add all th
if air content is higher than the stoichiometric ratio - the mixture is said to be fuel-lean
if air content is less than the stoichiometric ratio - the mixture is fuel-rich
percentage.
•
2.6
Howt
Howt
1. Divide the'
When analysis of combustion products is known air-fuel ratio can be calculated by the following
methods:
2. Add up th
percentage.
1. Fuel composition known
2.7
(I) Carbon balance method (il) Hydrogen balance method
The weight 0:
amounts of C(
Weigh
(iii) Carbon-hydrogen balance method.
In eqn. [4 (bJ
burnt. Hence ]
2. Fuel composition unknown
(I) Carbon-hydrogen balance method.
In eqn. [6 (a)].
3/7 kg of carlx
1. Fuel composition known
Therefore, wei
(I) Carbon balance method. When the fuel composition is known, the carbon balance method is
quite accurate if combustion takes place with excess air and when free (solid) carbon is not
present in the products. It may be noted that the Orsat analysis will not determine the quantity of
solid carbon in the products.
3
where C02 an
flue or exhaust
(ii) Hydrogen balance method. This method is used when solid carbon is suspected to be present.
•
(iiI) Carbon-hydrogen balance method. This method may be employed when there is some
uncertainty about the nitrogenpercentage reported by the Orsat analysis.
2.8
Weigh
The compositi
element in the
2. Fuel composition unknown
When the fuel composition
employed.
3
+= (.11CO:! 7
is not known the carbon-hydrogen
balance method has t~ be
For example, I
elemental fom
hydrogen, and
26
I
•
cess air level. This
aused by variations
.al values of excess
How to Convert Volumetric Analysis to Weight Analysis?
The conversion of volumetric analysis to weight analysis involves the following steps:
1. Multiply the volume of each constituent by its molecular weight.
2. Add all these weights and then divide each weight by the total of all and express it as
percentage.
Ito be fuel-lean
ich
•
How to Convert Weight Analysis to Volumetric Analysis?
1. Divide the weight of each constituent by its molecular weight.
2. Add up these volumes and divide each volume by the total of all and express it as a
ed by the following
percentage.
2.7
Weight of Carbon in Fuel Gases
The weight of carbon contained in one kg of flue or exhaust gas can be calculated from the
amounts of C02 and CO contained in it.
In eqn. [4 (b)], it was shown that 1 kg of carbon produces 1113 kg of C02 when completely
burnt. Hence 1 kg of C02 will contain 3/11 kg of carbon.
In eqn. [6 (a)], it can be seen that 1 kg of carbon produces 7/3 kg of CO, hence 1 kg CO contains
3/7 kg of carbon.
Therefore, weight of carbon per kg of fuel
I balance
method is
3
3 \
- (-~-+-coJ
'JUd) carbon is not
-
nine the quantity of
11"'"""'2
7
where C02 and CO are the quantities of carbon dioxide and carbon monoxide present in 1 kg of
ected to be present.
flue or exhaust gas.
then there is some
1.8
Weight of Fuel Gases per Kg of Fuel Burnt
The composition of a compound is often expressed in terms of the weight percent of each
element in the compound.
method has to be
For example, ethanol has the formula C2~O. One mole of ethanol has a mass of 46.07 g. The
elemental formula indicates that one mole of ethanol contains two moles of carbon, six moles of
hydrogen, and one mole of oxygen. Thus the composition of the compound by mass is
I
27
%C=
2 moles C (12.01 g/mole C)
______
100 %= 52.14 %
2.10
46.07 g ethanol
The most com
described belo
Practf
Construction.
Similarly the weight percents of hydrogen and oxygen in ethanol are
(i) A burette
6 moles H (l.008 g/mole H)
%H=
100 % = 13.13 %
(ii) A gas cleai
46.07 g ethanol
(iii) Four absoi
I mole 0 (16.00 g/mole 0)
%0 =
100 %
= 34.73
%
46.07 g ethanol
Notice that the sum of the weight percents of all the elements in a compound must equal 100 %.
The pipettes aJ
contain differe
(02). Each pi]
amount of sur:
under analysis
temperature an
Pipette 1: Cor
52.14 % + 13.13 % + 34.73 % = 100.00 %
Pipette 2 : Cor
Due to supply of air, the weight of fuel gas or exhaust gas is always more than
The actual weight of dry fuel gases can be obtained by comparing the weight
in the fuel gases with the weight of carbon in the fuel, since there is no loss of
combustion process. As the analysis of the exhaust gases is volumetric, so
that of fuel burnt.
of carbon present
carbon during the
this must first be
reduced to weight analysis.
Also, total weight of carbon in one kg offuel gas is
+-coJ
{3
3'
-CO2 7.
- .,11
. . The wcight of (lIM! gas I kg of fuel. burnt
-I.
Weight of carbon inone kg of fuel
.. Weight of carbon in one kg of llu~ gas
2.9
Analysisof Exhaust and Fuel Gas
The combustion products are mainly gaseous. When a sample is taken for analysis it is usually
cooled down to a temperature which is below the saturation temperature of the steam present,
The steam content is therefore not included in the analysis, which is then quoted as the analysis
of the dry products. Since the products are gaseous, it is usual to quote the analysis by volume.
An analysis which includes the steam in the exhaust is called a wet analysis.
28
2.10
Practical Analysis of Combustion
Products
The most common means of analysis of the combustion products is the Orsat apparatus which is
described below:
Construction.
An Orsat's apparatus consists of the following:
(i) A burette
(ii) A gas cleaner
(iii) Four absorption pipettes 1,2,3,4.
The pipettes are interconnected by means of a manifold fitted with cocks Sl, Sl, S3 and S4 and
contain different chemicals to absorb carbon dioxide (C02), carbonmonoxide (CO) and oxygen
(02). Each pipette is also fitted with a number of small glass tubes which provide a greater
amount of surface. These tubes are wetted by the absorbing agents and are exposed to the gas
under analysis. The measuring burrette is surrounded by a water jacket to prevent, changes in
temperature and density of the gas. The pipettes 1,2,3,4 contain the following chemicals:
st equal 100 %.
Pipette 1: Contains 'KOIf' (caustic soda) to absorb C02 (carbon dioxide)
Pipette 2: Contains an alkaline solution of 'pyrogallic acid' to absorb 02 (oxygen)
hat of fuel burnt.
If carbon present
arbon during the
his must first be
lysis it is usually
ie steam present.
ed as the analysis
alysis by volume.
I
29
The amount of J
the sum of CO2•
•
~
.z:
(5
.~
,.
0
~
ii.
;;;J
~
;;;J
2i.
:::J
()
0
:::J
0
(J
0
4
3c
Burette
~
E
Q
f
~
.Q
I
0
~
Orsat apparatus
to remove the SI
becomes saturat
products. This i
and the partial p
of the remaininj
total volume at (
Water
2.11
Let us S
In this unit we h,
.Il
2
1
~
-
Combust
Combust
Stoichior
Air-Fuel
-
Weight
-
Weighto
-
Analysis
2.12
CheekY,
..,
IU
4P
.2
II-
le\iel Un.gbottl.
Fit. H.1. Or&at's
IIppIlfttul.
0
Pipette 3, 4 : Contain an acid solution of 'cuprous chloride' to absorb CO (carbonmonoxide)
Furthermore the apparatus has a levelling bottle and a three way cock to connect the apparatus
either to gases or to atmosphere.
Procedure. 100 cm3 of gas whose analysis is to be made is drawn into the bottle by lowering the
levelling bottle. The stop cock S4 is then opened and the whole flue gas is forced to pipette 1.
The gas remains in this pipette for sometime and most of the carbondioxide is absorbed.
The levelling bottle is then lowered to allow the chemical to come to its original level. The
volume of gas thus absorbed is read on the scale of the measuring bottle. The flue gas is then
forced through the pipette 1 for a number of times to ensure that the whole of the C02 is
absorbed.
1. Explain tl
2. Elucidate
3. Explain tl
4. What is E
2.13
}l-
Referene
A. A. Put
Fourth S)
Further,
the remaining flue gas is then forced to the pipette 2 which contains pyrogallic acid to
-,
absorb whole of 02. The reading on the measuring burette will be the sum of volume of CO2 and
02.
The oxygen content can tnen be found out by subtraction. Finally, as before, the sample of gas is
forced through the pipettes 3 and 4 to absorb carbonmonoxide completely.
30
}l-
574.
E. C. Fen
Culick, .M
Netherlan
The amount of nitrogen in the sample can be determined by subtracting from total volume of gas
the sum of C02, CO and O2 contents.
Burette
Water
Orsat apparatus gives an analysis of the dry products of combustion. Steps may have been taken
to remove the steam from the sample by condensing, but as the sample is collected over water it
becomes saturated with water. The resulting analysis is nevertheless a true analysis of the dry
products. This is because the volume readings are taken at a constant temperature and pressure,
and the partial pressure of the vapour is constant. This means that the sum of the partial pressures
of the remaining constituents is constant. The vapour then occupies the same proportion of the
total volume at each measurement. Hence the vapour does not affect the result of the analysis.
2.11
Let us Sum UP
-
Combustion of hydrogen
Combustion of carbon
Stoichiometric Air Fuel (NF) Ratio
unonoxide)
-
Weight of Carbon in Flue Gases
-
Weight of Flue Gases per KG of Fuel Burnt
-
Analysis of Exhaust and Flue Gas
2.12
Cheek Your Progressive Exercise
ct the apparatus
by lowering the
:ed to pipette 1.
1. Explain the process of hydrogen combustion.
2. Elucidate the weight of carbon in fuel gases.
3. Explain the detailing stoichiometric air fuel (AlF) Ratio.
4. What is Heldane apparatus?
orbed,
~nal level. The
flue gas is then
of the C02 is
2.13
zrogallic acid to
ume of CO2 and
~
Reference
~ A. A. Putnam and W. C. Dennis (1953) "Organ-pipe oscillations in a flame-filled tube,"
Fourth Symposium (International) on Combustion, The Combustion Institute, pp. 566574.
E. C. Fernandes and M. V. Heitor, "Unsteady flames and the Rayleigh criterion" in F.
Culick, M. V. Heitor, and J. H. Whitelaw, ed.s, Unsteady Combustion (Dordrecht, the
Netherlands: Kluwer Academic Publishers, 1996),
sample of gas is
31
UNIT3
3.2
lnt
ENTROPY - PART I
Entropy i:
useful WOI
machines.
maximum
the system,
Structure
3.1. Objective
3.2. Introduction
3.3. Entropy as a Property
3.4. Limitation of First Law of Thermodynamics
3.5. Second Law of Thermodynamics
3.6. Clausius Statement
3.7. Kelvin-Planck Statement
3.8. Entropy and Irreversibility
3.9. Temperature-Entropy Diagram
3.10. Characteristics of Entropy
3.11. Entropy Changes in an Ideal Gas
3.12. Entropy Changes in System & Surrounding
3.13. Entropy Changes during Phase Transition
3.14. Let us Sum Up
3.16. Reference
3.1
In classica
second lav
increases 0
as a chem
determines
to regions,
the initial s
is the basis
entropy is
state of as
of this defii
I
Thermodyr
joules per k
Objective
3.3
Ent
Proof:
In order to
2-C-l as sh
Statements of Second Law of Thermodynamics
Clausius Statement
-
Kelvin-Planck Statement
Entropy and Irreversibility
Temperature-Entropy Diagram
Characteristics of Entropy
Entropy Changes in an Ideal Gas
Heating a Gas at Constant Volume
Heating a Gas at Constant Pressure
32
3
3.2
Introduction
Entropy is a thermodynamic property that can be used to determine the energy available for
useful work in a thermodynamic process, such as in energy conversion devices, engines, or
machines. Such devices can only be driven by convertible energy, and have a theoretical
maximum efficiency when converting energy to work. During this work, entropy accumulates in
the system, which then dissipates in the form of waste heat.
In classical thermodynamics, the concept of entropy is defined phenomenologically by the
second law of thermodynamics, which states that the entropy of an isolated system always
increases or remains constant. Thus, entropy is also a measure of the tendency of a process, such
as a chemical reaction, to be entropically favored, or to proceed in a particular direction. It
determines that thermal energy always flows spontaneously from regions of higher temperature
to regions of lower temperature, in the form of heat. These processes reduce the state of order of
the initial systems, and therefore entropy is an expression of disorder or randomness. This picture
is the basis of the modern microscopic interpretation of entropy in statistical mechanics, where
entropy is defined as the amount of additional information needed to specify the exact physical
state of a system, given its thermodynamic specification. The second law is then a consequence
of this definition and the fundamental postulate of statistical mechanics.
Thermodynamic entropy has the dimension of energy divided by temperature, and a unit of
joules per kelvin (J/K) in the International System of Units.
3.3
Entropy as a Property
Proof:
In order to prove that entropy is a property, we will suppose two cycles i.e. l-A-2-B-l and l-A2-C-l as shown in
T
3
•..Ji
33
For a reversible cycle l-A-2-B-l:
OQ I T + J2-8-1
T =0
3.6
For a reversible cycle l-A-2-C-l:
"It is impc
agency, to I
fl-A-2
fl-A-2
I2-C-I
so /
so / T + f2-C-I so / T = 0
OQ / T = J2-8-1 so / T
In other we
Hence, J oQ / T are a definite quantity independent of the path followed for the change and
depend only upon the initial and the final states of the system. Hence e-itropy is a property.
3.4
Cia
The Clausi
operates in
temperatun
Limitations of First Law of Thermodynamics
It has been observed that energy can flow from a system in the form of heat' or work. The first
law of thermodynamics sets no limit to the amount of the total energy of a system which can be
caused to flow out as work. A limit is imposed, however, as a result of the principle enunciated
in the second law of thermodynamics which states that heat will flow naturally from one energy
reservoir to another at a lower temperature, but not in opposite direction without 'assistance.This
is very important because a heat engine operates between two energy reservoirs at different
temperatures. Further the first law of thermodynamics establishes equivalence between the
quantity of heat used and the mechanical work but does not specify the conditions under which
conversion of heat into work is possible, neither the direction in which heat transfer Can take
place. This gap has been bridged by the second law of thermodynamics,
3.5
Second Law of Thermodynamics
Statements of Second Law of Thermodynamics
The second law of thermodynamics states that processes occur in a certain direction, not in just
any direction. Physical processes in nature can proceed toward equilibrium spontaneously:
•
•
•
Water flows down a waterfall.
Gases expand from a high pressure to a low pressure.
Heat flows from a high temperature to a low temperature.
The second law of thermodynamics has been enunciated meticulously by Clausius, Kelvin and
Planck in slightly different words although both statements are basically identical. Each
statement is based on an irreversible process. The first considers transformation of heat between
two thermal reservoirs while the second considers the transformation of heat into work.
34
Heat pump
Or energy 1
flow from a
Thus, the C
COP
A violation
violation ol
statement b
The output
temperature
reservoir.
3.6
Clausius Statement
"It is impossible for a self-acting machine working in a cyclic process unaided by any external
agency, to convey heat from a body at a lower temperature to a body at a higher temperature".
In other words, heat of, itself, cannot flow from a colder to a hotter body.
hange and
my.
The Clausius statement of the second law states that it is impossible to construct a device that
operates in a cycle and produces no effect other than the transfer of heat from a lowertemperature body to a higher-tel' perature body.
:. The first
iich can be
enunciated
me energy
tance. This
1 different
:tween the
ider which
:r can take
Heat pump that violates the Clausius statement of the second law
not in just
sly:
Or energy from the surroundings in the form of work or heat has to be expended to force heat to
flow from a low-temperature medium to a high-temperature medium.
Thus, the COP of a refrigerator or heat pump must be less than infinity.
COP <00
(elvin and
ical. Each
at between
A violation of either the Kelvin-Planck or Clausius statements of the second law implies a
violation of the other. Assume that the heat engine shown below is violating the Kelvin-Planck
statement by absorbing heat from a single reservoir and producing an equal amount of work W.
The output of the engine drives a heat pump that transfers an amount of heat QL from the lowtemperature thermal reservoir and an amount of heat QH + QL to the high-temperature thermal
reservoir. The combination of the heat engine and refrigerator in the left figure acts like a heat
35
pump that transfers heat QL from the low-temperature reservoir without any external energy
input. This is a violation of the Clausius statement of the second law.
';-':"ff:ifigh~1~d*~-';~~~~~,
Heat engin
<~~·~7itf~~'·}>;~~t;:·:-./.-
atTH
atTN
We know
now find tl
p
l
3.7
(
at T.c..
Gc_;,<
(b) '"In.. equival .... t r"jrigerator
(u) A re.t.rig&>rlltorwhi.,h i" po..........
dC
by a lOO'H. efficiellt heat engille
,
Kelvin-Planek Statement
Fig.:3
It is impossible for any device that operates on a cycle to receive heat from a single reservoir and
produce a net amount of work.
The Kelvin-Planck statement of the second law of thermodynamics states that no heat engine can
produce a net amount of work while exchanging heat with a single reservoir only. In other
words, the maximum possible efficiency is less than 100 percent.
n; < 100%
Consider a
L-2 and ret
Fig. above.
Since entro
•
tdS:= t)(l
(Subscript I
Now fora r
r
]{L)
(dS)R=
Substituting
!IEJU
ENGINE
Again, sine
applying CII
36
mal energy
Heat engine that violates the Kelvin-Planck statement of the second law
3.8
We know that cbange in entropy in a reversible process is equal to
(i)"
(Eqn. 1). Let us
now find the change in entropy in an irreversible process.
p
2
,,
,,
,,
I
Q,.
,1M
,,
,.""",.",
or
v
L-
Fig.:3
Consider a closed system undergoing a change from state 1 to state 2 by a reversible process 1L-2 and returns from state 2 to the initial state 1 by an irreversible process 2-M-l as shown in
Fig. above on the thermodynamic coordinates, pressure and volume.
:servoir and
: engine can
v, In other
Since entropy is a thermodynamic property, we can write
•
tdS '"
k)(dS).R +
tAl/ciS), =0
(Eqn.2)
(Subscript I represent the irreversible process).
Now for a reversible process, from (Eqn. 1), we have
(Eqn.3)
Substituting the value of
lu
(dS)B
in (Eqn. 2), we get
(Eqn.4)
Again, since in (Eqn. 2) the processes l-L-2 and 2-M-l together form an irreveesible cycle,
applying Clausius equality to this expression, we get
37
II
f OQ 12 (OQ)'
T
-=
-
TIL)
R
+
I' -OQ)
2(N),
T
1
process, as
that proces
<0
(Eqn.5)
If entropy'
called tern)
fluid receiv
Now subtracting (Eqn. 4) from (Eqn. 5), we get
II
II (0(/)
T
(dB) >
21M) .
121M)
temperatun
width of th.
1
(Eqn.6)
total heat n
Which for infinitesimal changes in states can be written as
will be con
T (TerJ1).)
(Eqn.7)
(Eqn. 7) states that the change in entropy in an irreversible process is greater than
Combining (Eqn. 6) and (Eqn. 7); we can write the equation in the general form as
oQIT.
dS> SQ
- T
(Eqn. 8)
Where equality sign stands for the reversible
irreversible process.
A"""".
process and inequality sign stands for the
I
•
I
!
I
It may be noted here that the effect of irreversibility is always to increase the entropy of the
system.
S"
Temperatur
Let us now consider an isolated system. We know that in an isolated system, matter, work or heat
cannot cross the boundary of the system. Hence according to first law of thermodynamics, the
internal energy of the system will remain constant.
Fig.:4
From above
Since for an isolated system,
(dS)i»l1lled
oQ = 0, from (Eqn. 8), we get
Entropycl:
>n
(Eqn.9)
(Eqn. 8) states that the entropy of an isolated system either increases or remains constant.
"Entropy m
when substa
This is a corollary of the second law. It explains the principle of increase in entropy.
Note. '8' sta
3.9
Temperature-Entropy
Diagram
3.10 eha
A temperature
entropy diagram, or T -s diagram, is used in thermodynamics to visualize
changes to temperature and specific entropy during a thermodynamic process or cycle. It is a
useful and common tool, particularly because it helps to visualize the heat transfer during a
38
The importa
process, as the area under the T-s curve of a process is the heat transferred to the system during
that process
If entropy is plotted-horizontally and absolute temperature vertically the diagram so obtained is
called temperature-entropy (T-s) diagram. Such a diagram is shown in Fig. below. If working
fluid receives a small amount of heat dQ in an elementary portion ab of an operation AB when
temperature is T, and if dQ is represented by the shaded area of which T is the mean ordinate, the
width of the figure must be dQrr. This is called 'increment of entropy' and is denoted by dS. The
total heat received by the operation
be given by the area under the curve AB and (SB - SA)
will
will be corresponding increase of entropy.
T(T~.)
B
than oQrr.
nds for the
A
.
I
I
i
tropy of the
1------'s....
" '------t~~t--dS--S.-'---...S(Entropy)
TemperatureEntropy Diagram
vork or heat
namics, the
Fig.:4
From above we conclude that:
En
mt.
...\.trupy ~,
tIS
Heatchange(Q)
Ab50lute temperature (,1')
"Entropy may also be defined as the thermal property of a substance which remains constant
when substance is expanded or compressed adiabatically in a cylinder".
Note. 's' stands for specific entropy whereas'S' means total entropy (i.e., S = ms).
3.10
o visualize
rcle. It is a
:r during a
The important characteristics of entropy are summed up below
(i) Entropy is an extensive property. Its value depends upon the amount of the substance present
in the system.
(ii) Entropy of a system is a state function. It depends upon the state variables (T, p, V, n).
(iii) The change in entropy in going from one state to another is independent of the path.
(ii) When 1
AS =nC,. ln-
(a) Thus, fc
(iv) The change in entropy for a cyclic process is always zero.
(b) For isol
(v) The total entropy change of an isolated system is equal to the entropy change of system and
entropy change of the surroundings. The sum is called entropy change of universe.
(c) For isoc
.1.Suniverse= .1.Ssys+ .1.Ssarr
(a) In a reversible process, and, therefore
3.12
(b) In an irreversible process, .1.Suniverse > O. This means that there is increase in entropy of
universe is spontaneous changes.
(vi) Entropy is a measure of unavailable energy for useful work.
Unavailable energy
= Entropy
x Temperature
(vii) Entropy, S is related to thermodynamic probability (W) by the relation,
S = k log, W and S = 2.303 k 10glOW
Ent
Heat increa
hence their
therefore, I
process, th,
surroundin]
In general,
disorder of
will be SPOI
3.13
Ent
Where, k is Boltzmann's constant
The change:
Such chang
(liquid to Vl
3.11
In going from initial to ..final state, the entropy change, .1.S for an ideal gas is given by the
following relations,
(i) When T and V are two variables,
AS "'nC In
•
•
7i +nR In ~.
Ii·
. ~.. Assuming C, is constant
40
When a so)
heat). Let II
Similarly, j
respectivelj
:ance present
(ii) When T and p are two variables,
AS"'nC
ath.
lnT2_nRlnh
"1i
V, n).
PI . Assuming Cp, is constant
(a) Thus, for an isothermal process (T constant),
AS=nC
(b) For isobaric process (p constant),
r
ln72
T.1
, system and
(c) For isochoric process (V constant);
3.12
entropy of
Entropy Changes in System & Surrounding
Heat increases the thermal motion of the atoms or molecules and increases their disorder and
hence their entropy. In case of an exothermic process, the heat escapes into the surroundings and
therefore, entropy of the surroundings increases on the other hand in case of endothermic
process, the heat enters the system from the surroundings and therefore. The entropy of the
surroundings decreases.
In general, there will be an overall increase of the total entropy (or disorder) whenever the
disorder of the surroundings is greater than the decrease in disorder of the system. The process
will be spontaneous only when the total entropy increases.
3.13
Entropy Changes during Phase Transition
The change of matter from one state (solid, liquid or gas) to another is called phase transition.
Such changes occur at definite temperature such as melting point (solid to liquid). boiling point
(liquid to vapours) etc, and are accompanied by absorption or evolution of heat.
ven by the
When a solid changes into a liquid at its fusion temperature, there is absorption of heat (latent
heat). Let Hfbe the molar heat of fusion. The entropy change will be
•
~Sf=~lff
Similarly, if the latent heat of vaporisation and sublimation are denoted by
respectively, the entropy of vaporisation and sublimation are given by
~\IlIP
and
~sub
•
41
. AS..
_ 48..
r.
and
>-
AS... _ I1HM
1;
"Entre
Amab
2009,
Since Hr, Hvapand Hsubare all positive, these processes are accompanied by increase of entropy
and the reverse processes are accompanied by decrease in entropy.
3.14
Let us Sum Up
-
3.1S
En6-opy as a Property
Limitations of First Law of Thermodynamics and Introduction to Second Law
Statements of Second Law of Th"Tlllodynamics
Clausius Statement
Kelvin-Planck Statement
Temperature-Entropy Diagram
Heating a Gas at Constant Volume
Heating a Gas at Constant Pressure
I. What is Entropy?
2. Explain in detail about Entropy Statements
3. What are the Characteristics of Entropy?
4. Explain Entropy Changes in an Ideal Gas.
3.16
»»»»»-
Reference
De Rosnay, Joel (1979). The Macroscope- a New World View (written by an M.IT.trained biochemist). Harper & Row, Publishers.
Baierlein, Ralph (2003). Thermal Physics. Cambridge University Press.
Schroeder, Daniel V. (2000). Introduction to Thermal Physics. New York: Addison
Wesley Longman.
Chang, Raymond (1998). Chemistry, 6th Ed.. New York: McGraw Hill.
Daintith, John (2005). Oxford Dictionary of Physics. Oxford University Press.
42
~ "Entropy production theorems and some consequences," Physical Review E; Saba,
Arnab; Lahiri, Sourabh; Jayannavar, A. M; The American Physical Society: 14 July
2009, pp. 1-10
e of entropy
an MIT.-
•. Addison
43
UNIT 4
ENTROPY - PART II
it is poss
constant,
Structure
4.3
4.1.0bjective
4.2.1ntroduction
4.3.Isothermal Process
4.4.Adiabatic Process (Reversible)
4.5.Polytrophic Process.
4.6.Approximation for Heat Absorbed
4.7.Entropy Changes for an Open System
4.8.The Third Law of Thermodynamics
4.9.Let us Sum Up
4.11. Reference
A thermo
Is
supply of
ISOTHE
Consider
fitted wit]
on the pi:
temperatu
system an
Accordin,
4.1
Objective
-
4.1
DQ=DU
Sincetem
Isothermal Process
Adiabatic Process (Reversible)
Polytropic Process
Approximation for Heat Absorbed
Entropy Changes for an Open System
The Third Law of Thermodynamics
i.e, DU=
As thewc
DQ=O+
DQ=-D'
Introduction
ISOTHK
An isothermal process is a change of a system, in which the temperature remains constant: llT=
O. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath),
and the change occurs slowly enough to allow the system to continually adjust to the temperature
of the reservoir through heat exchange. In contrast, an adiabatic process is where a system
exchanges no heat with its surroundings (Q = 0). In other words, in an isothermal process, the
value llT= 0 but Q #; 0, while in an adiabatic process, AT#; 0 but Q = O.
In a phase diagram, an isothermal process is indicated by following a vertical line (or plane, in a
3D phase diagram) along a constant temperature. Therefore, if the pressure and volume change,
44
in anothe
decreased
amount 0
maintaine
it is possible for a substance to change its state of matter even while its temperature remains
constant, if you're careful about how you apply or remove heat from the system.
4.3
Isothermal Process
A thermodynamic process in which the temperature of the system remains constant during the
supply of heat is called an ISOTHERMAL PROCESS.
iSOTHERMAL
COMPRESSION
Consider a cylinder of non-conducting walls and good heat conducting base. The cylinder is
fitted with a frictionless piston. An ideal gas is enclosed in the cylinder. In the first stage pressure
on the piston is increased and the cylinder is placed on a cold body. Due to compression , the
temperature of the system increases but at the same time DQ amount of heat is removed from the
system and the temperature of the system is maintained.
According to the first law of thermodynamics:
DQ=DU+DW
Since temperature is constant, therefore, there is no change in internal energy of the system.
i.e. DU = 0
As the work is done on the system, therefore, DW is negative,
DQ=O+(-DW)
DQ=-DW
ISOTHERMAL
istant: Il.T =
(heat bath),
temperature
'e a system
srccess, the
EXPANSION
in another situation the cylinder is placed over a hot body and the pressure on the system is
decreased. Due to expansion, the temperature of the system is decreased but at the same time DQ
amount of heat is absorbed from the hot body and the temperature of the system is again
maintained.
According to the first law of thermodynamics:
. plane, in a
me change,
45
DQ=DU+DW
Since temperature is constant, therefore, there is no change in internal energy of the system. i.e. ,
DU=O
As the work is done by the system, therefore, DW is positive,
(i) The COl
speed is q
and the vo
(ii) The e:
adiabatic,
surroundio
DQ=O+(DW)
DQ=+DW
p
Special cas
impcrtant
isentropic.
kind like n,
I
Therefore )
Mathemati.
4.5
Pol:
A process i
related as P
4.4
The path Ii
process. In
In an adiabatic process, there is no heat transfer from the system to the surroundings or from the
surroundings to the system i.e. dQ =0. Heat flow can be prevented either by surrounding the
system with thermally insulating material or by carrying out the process very quickly (RPM of
the machine has to be very high) so that there is not enough time for appreciable heat flow.
surrounding
when greate
line will be
For every adiabatic process, first law of thermodynamics gives
This proces
expansion 0
Sq = du + ow = 0,
du= -Sw
When a system expands adiabatically. dw is positive (the system does work on its surroundings),
sodu is negative and the internal energy decreases. When a system is compressed adiabatically,
dw is negative (work is done on the system by its surroundings) and du increases. In majority
(but not all) systems, an increase of internal energy is accompanied by a rise in temperature.
So the work
Examples of adiabatic processes:
46
system. i.e. ,
(i) The compression stroke in an intemal-combustion engine is nearly an adiabatic process as the
speed is quite high. The temperature rises as the air-fuel mixture in the cylinder is compressed
and the volume is decreased.
(ii) The expansion of the burned fuel air mixture during the power stroke is also nearly an
adiabatic expansion with a drop in temperature as the work is done by the system on the
surroundings and the volume is increased.
Special case of adiabatic process: A reversible adiabatic process is called Isentropic Process. It is
important to Dote that every isentropic process is adiabatic but every adiabatic process is not
isentropic. Only reversible adiabatic is isentropic. Reversible means there are no losses of any
kind like no heat loss, no friction loss, no leakage loss etc.
Therefore reversible process is only theoretical in nature since every process has some losses.
Mathematically reversible adiabatic=isentropic process is PVg =C.
4.5
PolytropicProcess
A process in which the pressure and volume of the gas in actual expansion and compression
related as PV"= C is called polytropic process.
or from the
rounding the
dy (RPM of
:flow.
r
The path line of the polytropic process falls between the path lines of isothermal and adiabatic
process. In the processes when the greater part of the work is done on the system from
surrounding then the path line will be closer to the path line of isothermal process. Similarly
when greater part of the work is done on the surrounding and it comes from the gas then the path
line will be closer to the adiabatic process.
This process is used for superheated vapors not for vapors the example of that process
expansion of combustion gases in the cylinder.
p,
I
I
I
I
I
Pt --~---------
rroundings ),
diabatically,
In majority
erature,
is
I
2
I
I
v
So the work done in the expansion of piston in the cylinder is giv~ as following
47
is
..I
Q=W -L\U
, P.dV
W = -,
(A)
Since we know that
PV"=c
Since
where L\T is
P= C. V-o
Now putting
So now putting it in the expression (A)
mR
f.
C. V-Il. dV
Q=
r
Q=
:l
W==l
W == cls.
=n:tl
mR((T
V-D.dV
1
mR((T
Q=
1
Now putting the value of C in the above expression
11
Q=m[ [(T]
Since we kn.
1
Dividing this
Since we know that
Now the above expression becomes
1
Cp
0 V -0 + I]
W = ---n + 1 . [P 2V2.0 V2-0 + 1- P 1V 1·
1
Where C,,='
1
W
=
R
-n + 1.[ P2V2 - PlVl]
According to ideal gas equation PV
y-l= CII
= mRT
so the above equation becomes
C;= y- 1
1
W
=
-n
R
+ 1.[ mRT2 - mRT1]
Now putting
mR
W= ::n+i.[T2-Tl]
Q=m((T]l
So according first law of thermodynamics
W=Q+L\U
Q=mR[((T:
48
Q=W -t:\u
(B)
t:\u
Since
= mCv t:\T
where t:\T is temperature difference.
Now putting the value of ~U and W .
mR
Q = -n + 1.[T2 - T 1] - mCy ~ T
Q=
mR(Th - T1)
1-n
-mCy[(Tl.
- T1)
mR((Tl1 - T2)
Q=
-mCy[{Tl, - 1'1)
11-1,
R
Q=m[ [(T], - T~)[n-l -Cy]
Since we know that
(C)
Cp - C,
=R
Dividing this expression by C, we have
ep
Where Cv = 1,putting it in the above equation
R
1-1=
Cy=
ell
R
y- 1
Now putting the value of in the above equation
1
Q=mR[I(T],
- T;z) [ii='i -]
49
y -n
Heat absorb,
mRICf) .. Q=
T2) (y -
<"-1)
.
0)
(y-1)
Where
mR
W= =-n+i.[ Tl - Tz]
In other won
absolute tern;
So the above expression becomes
.Summary
(y -n)
oj
Q=W. (r- 1)
,- "",
- 4.6.
S.lID.
Approximation
for Heat Absorbed
The curve LM shown in the Fig.4 below is obtained by heating 1 kg of gas from initial state L to
final state M. Let temperature during heating increases from T 1 to T 2. Then heat absorbed by the
gas will be given by the area (shown shaded) under curve LM.
3.
T
Tz
M
---------------
5.
6.
4.7
s
FlgA
& the curve on T-s diagram which represents the heating of the gas, usually has very slight
curvature, it can be assumed a straight line for a small temperature range. Then,
so
Entro
In an open sy:
the mass eros
mass transpor:
at the inlet an
during a small
Heat absorbed
= Area
under the curve LM
In other words, heat absorbed approximately equals the product of change of entropy and mean
absolute temperature .
.Summary of Formulae
S.Mo.
1.
C1wy:e r( er.trqJy (per klJ
Proms
Gmmdcue
(i)
{ii)
',1«&!l +R log, ~
Ii
'uk!r
1'2
+Colog.~ (mterms cipmdII)
II}
PI
aitial state L to
bsorbed by the
(iii)
2-
amt:u:t wlume
',k!r
(mtams of T md II)
II}
T2 -R~12.
(intmnsof:randp)
1'1
PJ.
( q, 1'2
•
Ii
3.
CaIut:mt pn=ure
'Iog,~
, Ii
4-
T:ttlrrr:W
R~
5.
Adia1Iati:
lao
6.
PaIytropc
'0 ln -1)
112
II}
4.7
(71-1'
q,.!l
Ii
In an open system, as compared with closed system, there is additional change of entropydue to
las very slight
the mass crossing the boundaries of the system. The net change of entropy of a system dueto
mass transport is equal to the difference between the product of the mass and its specificentropy
at the inlet and at the outlet of the system. Therefore, the total change of entropy ofthesystem
during a small interval is given by
51
Where
where, To::::Temperature of the surroundings,
Sj ::::
Specific entropy at the inlet,
And
~== Specific entropy at the outlet,
dm,
= Mass entering
For adiabatic s
the system, and
dm, ::::Mass leaving the system.
(Subscripts i and 0 refer to inlet and outlet conditions)
If the process i
The above equation in general form can be written as
J
as» ~:
4.8
+
Ls.dm
(Eqn.1)
TheTl
The third law
In (Eqn, 1) entropy flow into the system is considered positive and entropy out-flow is
considered negative. The equality sign is applicable to reversible process in which the heat
interactions and mass transport to and from the system is accomplished reversibly. The
inequality sign is applicable to irreversible processes.
/
The en'
For other mate
perfect crystal
The Third La'
If (Eqn. 1) is divided by dt, then it becomes a rate equation and is written as
•
•
(Eqn.2)
In a steady-state, steady flow process, the rate of change of entropy of the system (dS/dt)
becomes zero.
The Third Lav
lim
1 dQ + ~
O~ ----
To cit
the ent
tempe)
the te
approl
ST-+O::::
0(
din
B.-
dt
(Eqn.3)
Where
S = entropy (Ii
1.
To Q +l:B.
ria
so
T ::::absolute te
(Eqn.4)
52
dQ
tit
Q=
Where
.
dm
dt
m=-
And
For adiabatic steady flow process, Q = 0
(Eqn.7)
(Eqn.6)
If the process is Adiabatic then
4.8
The third law of thermodynamics
py out-flow is
which the heat
:-eversibly. The
The entropy of a perfect crystal approaches zero as temperature approaches absolute zero.
For other materials, the residual entropy is not necessarily zero, although it is always zero for a
perfect crystal in which there is only one possible ground state.
•
•
system (dS/dt)
is a statistical law of nature regarding entropy:
states that
the entropy of any pure substance in thermodynamic equilibrium approaches zero as the
temperature approaches zero (Kelvin), or conversely
the temperature
(Kelvin) of any pure substance in thermodynamic equilibrium
approaches zero when the entropy approaches zero
The Third Law of Thermodynamics can mathematically be expressed as
lim ST-+O = 0 (1)
Where
S = entropy (JIK)
T
= absolute
temperature (K)
S3
At a temperature of absolute zero there is no thermal energy or heat. At a temperature of zero
Kelvin the atoms in a pure crystalline substance are aligned perfectly and do not move. There is
no entropy of mixing since the substance is pure.
The temperature of absolute zero is the reference point for determination entropy. The absolute
entropy of a substance can be calculated from measured thermodynamic properties by integrating
the differential equations of state from absolute zero. For a gas this requires integrating through
solid, liquid and gaseous phases.
Various sources
thermodynamics:
show the following
three potential
formulations
P2 = 6 bar
P3 = 2 bar
p
2
of the third law of
1. It is impossible to reduce any system to absolute zero in a finite series of operations.
2. The entropy of a perfect crystal of an element in its most stable form tends to zero as the
temperature approaches absolute zero.
3. As temperature approaches absolute
4.9
Zt'TO,
the entropy of a system approaches a constant
Let us Sum Up
- Isothermal Process
-
Polytropic Process
-
Approximation for Heat Absorbed
Considering
II-
4.10
~ = (!!z..)' -;
CheckYour Progressive Exercise
7i
Problem 1: 5 m3 of air at 2 bar, 27°C is compressed up to 6 bar pressure followingpv'r' =
constant. It is subsequently expanded adiabatically to 2 bars. Considering the two processesto be
reversible, determine the net work. Also plot the processes on T-S diagram.
,PI
Considering
Solution:
~=(.ftJ"
Ts
Given:
P3
PI =2 bar
54
erature of zero
move. There is
T 1 = 27
+ 273
= 300 K
P2 = 6 bar
P3 = 2 bar
The absolute
by integrating
;rating through
T.
T(K)
p
2
2
third law of
-erations.
: to zero as the
1
1
es a constant
~
~3
__.v
T-s Diagram
p-v Diagram
Moss of air,
m
=
.s
PlVI = 2x 105 x5 = 1161...._.
RTl.
287 x 300
.
"'6
Considering Polytropic Compression process 1 - 2 we have
,._l
~=(ft)--;Tl
PI.
lowingpv'r' =
orocessesto be
13-1
or
..!-L=(!')li"
300
2,
or Ta = 386.5 K
Considering Polytropic Compression process 2 - 3 we get
(.:
To
386.5
T =~=--=282.3K
3
1369
13G9
ss
Pa =Pt)
Now, work done during polytropic compression 1-2,
T
- mR{7j_ - 12) _ 1181 x 0.287(300 - 388.5) _
kJ
11- 1
13_ 1
- - 960.'1
Tz
WI_II -
y -1
282.8)
,-,
"
: pVt-
Tt
Net work done = W 1-2 + W 2-3 = - 960.7
...
,
= 868 kJ
14-1
Hence net work done on the air
~~~
,,
,,,
,
and, work done during adiabatic expansion 2 - 3,
w~;;: mR<Tz - 13) = 1161 x 0..287(386.5 -
2
••
1
--r-- ---_
•
)1
+ 868 == - 92.7 kJ
14
= 92.7 kJ. (Ans)
The process plotted on T-s diagram is shown in Fig. above
Problem l:(a) Show that approximate change of entropy during a polytropic processequals the
quantity of heat transferred divided by the mean absolute temperature. (b) One kg of air at 290 K
is compressed in a cylinder according to the polytropic Iawpv'r' == constant. If the compression
ratio is 16, calculate the entropy change of air during thecompression process stating whether it
is an increase or decrease. What would be the percentage error if the entropy change is calculated
by dividing thequantity of heat exchanged by the mean absolute temperature during the process?
(Take j' = 1.4 and c.= 0.718 kJ/kg K.)
Entro
or,
Given:
m= 1 kg
Solutil)n:
T1=29OK
(a) In Fig. below curve 1-2 represents thepolytropic process (PY'
point
= c)
from state point 1 to state
2. The area under the process curve 1-2 on T-S diagramrepresents the heat transferred during the
process. The slopeof the curve 1-2 is usually small and can be considered to bea straight line
(shotted dotted).
pVI.3
= constan
r= 16
'Y == 1.4
Cv = 0.718
kJlle
Far a polytruJ
S6
T
2
1
•
I
14
)1
141'4--- dS--~~
s
Heat transferred = Area of trapezium 1-2-3-4
:::;:
Base x mean ordinate
essequals the
f air at 290 K
:::;:
Entropy change x mean absolutetemperature during the
compression
ng whether it
is calculated
:the process?
Process
or,
Entropy change
Heat transferred
:::;:---------Mean absolute temperature
Given:
m=lkg
r,=290K
.int 1 to state
pvl.3= constant
r= 16
ed during the
straight line
t= 1.4
c. = 0.718 kJ/kg K
T2:::;: 290 x (16)1.3-1:::;:666.2 K
57
Now,
82 - 81
=
t(72)
n-y\
n _ 1) l~ .. Tl
C,'
~13 - 14 )
= 0.718 ( -. 13-1
... per kg
log..
f --J
666.2\
= - 0.199 kJ/kg K
\. 290
(ADs.)
The -ve sign indicates decrease in entropy.
Structure
Heat transferred during the process is given by,
n
n
Q = 1- x W = 1- x R(1j -1;) =
1-1
1-1
n-l
c.,( 1- n)<Tl -
7;> •..per kg
.Y-l
=_!!_)
= 0.718 ( 14 -
13)(290 _ 800.2) = _ 90.04 kJlkg
13-1
( ._.c" y-l
Mean absolute t-......
~""'\A&",h...... T__ = 1i +
2 12 = 290+666.2
2
.
Approximate change of entropy = --
Q
T__
% age error =
0.199 -
= -90.04
478.1
russ x 100 = 5..53'1A
0.199
=-
= 418.1 K
0.188 kJllrg K
(ADs-)
The approximate value of entropy change is lower, because in the relation Q = Tav x dSactual
value of heat transferred is substituted instead of approximate value (i.e., Areaunder the straight
line) which is higher.
1.1.
1.2.
1.3.
1.4.
1.5.
1.6.
1.7.
1.8.
1.9.
1.10.
1.11.
1.12.
1.13.
1.14.
Air com
Volume
Method:
Let us S
Check ...
Referent
1.1
Objectr
Objecti-
Introdui
Uses of
Types 0
Recipro
Rotary ~
Centrifu
Capacit-
After studying t
4.11
);>-
);>-
Reference
Guggenheim, E.A. (1985). Thermodynamics. An Advanced Treatmentfor Chemists and
Physicists, seventh edition, North Holland, Amsterdam,
Kittel, C. Kroemer, H. (1980). Thermal Physics, second edition, W.H. Freeman, San
Francisco,
);>);>);>-
);>-
Adkins, C.J. (1968). Equilibrium Thermodynamics, McGraw-Hill, London,
Kondepudi D. (2008). Introduction to Modern Thermodynamics, Wiley, Chichester,
Lebon, G., Jou, D., Casas-Vazquez, J. (2008). Understanding Non-equilibrium
Thermodynamics.Foundations, Applications, Frontiers, Springer, Berlin.
Chris Vuille; Serway, Raymond A.; Faughn, Jerry S. (2009). College physics. Belmont,
CA: Brooks/Cole, Cengage Learning. p. 355.
58
-
Uses of COD
-
Types of Ai:
-
Automated ,
-
Types of Co
-
Reciprocatir
-
Rotary Sere'
Centrifugal:
Capacity Co
-
Air compres
-
Volumetric I
-
MethodsAd
1.2
Introdul
BLOCK-2
K.
UNITl
(ADs.)
AIR COMPRESSOR
Structure
Tav X dSactual
der the straight
1.1.
1.2.
1.3.
1.4.
1.5.
1.6.
1.7.
1.8.
1.9.
1.10.
1.11.
1.12.
1.13.
1.14.
Objective
Introduction
Uses of Compressed air for industries
Types of Compressors
Reciprocating Air Compressors
Rotary Screw Compressors
Centrifugal Compressors
....
Capacity Control
Air compressor Terminology
Volumetric Efficiency
Methods Adopted for Increasing Isothermal Efficiency
Let us Sum Up
Reference
1.1
Objective
.
. Chemists and
Freeman, San
iichester,
on-equilibrium
-
Types of Air Compressors
Automated Assembly Stations
Capacity Control
'sics. Belmont,
1.2
Introduction
59
An air compressor is a device that converts power (usually from an electric or diesel or gasoline
engine) into kinetic energy by pressurizing and compressing air, which is then released in quick
bursts. There are numerous methods of air compression, divided into either positivedisplacement or negative-displacement types
•
•
•
Compressed air, commonly called Industry's Fourth Utility, is air that is condensed and
contained at a pressure that is greater than the atmosphere. The process takes a given mass of air,
which occupies a given volume of space, and reduces it into a smaller space. In that space,
greater air mass produces greater pressure. The pressure comes from this air trying to return to its
original volume. It is used in many different manufacturing operations. A typical compressed air
system operating at 100 psig (7 bar) will compress the air down to 118of its original volume.
1.3
Uses of Compressed
1.S
drive m
lubricat
compre
package
Rec:ipr4
In this type of
engine: they s
cylinder feature
gas refrigerant
air for Industries
The air compressors seen by the public are used in 5 main applications:
•
To supply a high-pressure clean air to fill gas cylinders
•
To supply a moderate-pressure
diver
clean air to supply air to a submerged surface supplied
• To supply a large amount of moderate-pressure air to power pneumatic tools
_./ For filling tires
•
To produce large volumes of moderate-pressure air for macroscopic industrial processes
(such as oxidation for petroleum coking or cement plant bag house purge systems).
Most air compressors are either reciprocating piston type or rotary vane or rotary screw.
Centrifugal compressors are common in very large applications. There are two main types of air
compressor's pumps: Oillubed and oil-less. The oil-less system has more technical development,
but they are more expensive, louder and last for less time than the oiled lube pumps. However,
the air delivered has better quality.
1.4
Reciprocating a
the pressure of
of air which is
reciprocating ai
and displacing (
Types of Compresson
\The three basic types of air compressors are
• reciprocating
• rotary screw
• rotary centrifugal
These types are further specified by:
•
•
Reciprocating (
outside of the
compressor). In
drive motor are
maintenance. II
compressors an
compressors arc
of the cylinders
refrigerators an,
Reciprocating a
non-lubricated (
1.6
the number of compression stages
cooling method (air, water, oil)
60
Rotary
•
•
sel or gasoline
eased in quick
ither positive-
•
ondensed and
en mass of air,
In that space,
to return to its
compressed air
11 volume.
1.S
drive method (motor, engine, steam, other)
lubrication (oil, Oil-Free where Oil Free means no lubricating oil contacts the
compressed air)
packaged or custom-built
In this type of compressor, the pistons are designed in similar fashion to those used in a car
engine: they slide inside a cylinder, drawing in and compressing the gas refrigerant. Each
cylinder features a suction valve for the gas refrigerant and a delivery valve through which the
gas refrigerant is sent to the condenser after having been compressed,
rface supplied
s
trial processes
'Stems).
rotary screw.
in types of air
development,
aps, However,
Reciprocating compressors are defined as open-type when one end of the crankshaft protrudes
outside of the crankcase (the casing that contains the pistons and the mechanisms inside the
compressor). In semi-hermetic compressors, on the other hand, both the compressor itself and the
drive motor are housed inside the casing, which is designed so as to be opened for inspection and
maintenance. In this case, the drive shaft and the crankshaft are one single piece. Semi-hermetic
compressors are made so as to prevent air or dust from entering the mechanisms. Reciprocating
compressors are defined as hermetic when the casing is welded closed and sealed, and the heads
.of the cylinders cannot be accessed for inspection or maintenance. These are used in household
refrigerators and freezers and in medium-capacity air-conditioning units.
Reciprocating air compressors are positive displacement machines, meaning that they increase
the pressure of the air by reducing its volume. This means they are taking in successive volumes.
of air which is confined within a closed space and elevating this air to a higher pressure.·The
reciprocating air compressor accomplishes this by a piston within a cylinder as the compressing
and displacing element.
Reciprocating air compressors are available either as air-cooled or water-cooled in Iubricated and
non-lubricated configurations and provide a wide range of pressure and capacity selections.
1.6
61
The category of rotary compressors includes all compressors that perform the compression
function using mechanisms that involve impellers, involute scrolls or screws. Scroll compressors
(or orbiting scroll compressors) are based on a mechanism patented in 1905. This features two
involute scrolls: One stationary and one orbiting (but not rotating) around the first. Thanks to this
motion, the gas contained between the two elements reaches a very high pressure and discharged
through a hole in the centre.
Stationary or rotary vane compressors work due to the effect of the vanes located inside the
cylinders. The vanes may rotate on a cam in the centre of the cylinder (rotary), or be fixed to the
walls of the cylinder (stationary). In both cases, the vanes are responsible for the movement of
the gas, contributing fundamentally to the suction and compression phases.
Screw compressors are based on a mechanism made up of two threaded rotors (screws) that are
coupled together. The gas is compressed due to the progressive overlapping of the lobes, causing
a reduction in the volume occupied by the gas. There are also single-screw compressors that are
coupled together. The gas is compressed due to the progressive overlapping of the lobes, causing
a reduction in the volume occupied by the gas. There are also single-screw compressors that
operate by the rotation of just one cylindrical screw with a helical thread, onto which two
identical rotors are coupled.
1.7
Centrifugal
Compressors
Compressors
are"made up of a rotor located inside a special chamber. The rotor is rotated at high
speed, imparting high kinetic energy to the gas, which is forced through the narrow outlet
opening, thus increasing its pressure.
This type of compressor is used for high and very high cooling capacities.
The centrifugal air compressor is a dynamic compressor which depends on transfer of energy
from a rotating impeller to the air.
Centrifugal compressors produce high-pressure discharge by corierting angular momentum
imparted by the rotating impeller (dynamic displacement). In order to do this efficiently,
centrifugal compressors rotate at higher speeds than tl. other types of compressors. These types
62
of compressor:
continuous.
1.8
Capaci
Before review:
also called the
from fully loa,
band, although
There is a hi:
unregulated u:
possible unles
type, capacity
types of comp
First is auton
and stopping (
upper pressure
at full load or
tolerate only a
the applicatioi
system must r
dependent on
Unloading wi
while the air
demand, usua
modulating ty
Step controls
five-step). Th
open or fully
minimum pr€
control is ava
control systen
The basic pet
band up to th
then stays at
the unit imme
compression
II compressors
s features two
Thanks to this
md discharged
ted inside the
)e fixed to the
movement of
rews) that are
lobes, causing
essors that are
obes, causing
ipressors that
:0
which two
otated at high
tarrow outlet
fer of energy
r momentum
s efficiently,
. These types
of compressors are also designed for higher capacity because flow through the compressor is
continuous.
1.8
Capacity Control
Before reviewing the types of capacity controls, we should define the operating pressure band,
also called the dead band or proportional band. This is the pressure range the control can span
from fully loaded flow to fully unloaded (no flow). Compressors larger than 50 hp use a l O-psi
band, although others can be and are used.
There is a high cost to higher system pressure (1/2% per psi) and increased flow through
unregulated users (1% per psi). Most well planned systems try to hold as narrow a band as
possible unless there is a specific requirement for a larger band. Regardless of air compressor
type, capacity controls fall into several basic categories. Some will only be available on certain
types of compressors.
First is automatic start/stop. On any compressor, this control refers to the automatic starting
and stopping of the electric motor or driver. Usually, a pressure switch shuts off the motor at the
upper pressure limit and restarts the motor at a lower system pressure. Although operating either
at full load or off is the most efficient way to run an air compressor, most AC electric motors
tolerate only a limited number of starts over a given time for reasons of heat build up. This limits
the application of automatic start/stop, particularly for motors larger than 10 to 25 hp. A large
system must run above minimum system pressure to hold minimum pressure and performance is
dependent on adequate effective storage.
Unloading with continuous-run controls implies the driver or electric motor continues to run
while the air compressor is unloaded in some manner. The objective is to match supply to
demand, usually on the basis of system pressure. Continuous run controls either can be step or
modulating type.
Step controls are also called on line/offline; cut inlcut out; load/no load; two-step (or three-step,
five-step). The most common is two-step control, which keeps the compressor inlet either fully
open or fully shut. Over the complete operational band, the unit is at full load from the preset
minimum pressure point (load point) to the preset maximum pressure (no load point). Tnis
control is available on every type of air compressor as either a primary unit or part of a dualcontrol system.
The basic performance is fully loaded or full flow at points throughout the operational pressure
band up to the final preset maximum pressure when the air flow shuts off completely. The unit
then stays at no flow and full idles until the system pressure falls to the preset minimum when
the unit immediately goes to full flow capacity.
63
In this mode of control, the compressor runs at its two most efficient modes - full load and full
idle - which represents the lowest possible input power cost. Full idle at lowest input power
occurs almost immediately, except in the case of lubricated or lubricant-cooled rotary screw and
centrifugal compressors.
With lubricant-cooled rotary screws, full idle and lowest input power does not occur until the oil
sump pressure is bled down. This can often represent a time delay from 20 seconds to as much as
two minutes. Centrifugal compressors often have some time delay built into the controls before
they go to full idle. Double acting reciprocating compressors can also be equipped with threeand five-step unloading.
It is usually easy to tell if the unit is loaded or unloaded. Comparing the duty cycle gives an
accurate reflection of actual flow as a percent of rated capacity.
Correct piping and adequate storage is necessary to allow sufficient idle time over the
operational pressure band to generate significant energy savings. This is particularly true with
lubricant-cooled rotary screws, which must cover the bleed down time before any significant
power cost savings can occur.
This type of cc
multiple cylinde
normally operat
use, or the serio
In double actin!
place on both
revolution of th.
Also it has to he
(3) Single Stag,
In single stage
pressure is carri
(4) Multi-Stage
When the comp
than one cylind.
(5) Ratio of Co
1.9
Air Compressor
Terminology
It is the ratio of
(6) Free Air Dc
Definitions of the following terms are necessary in the study of the operation and theory of
reciprocating compressors.
(1) Single Acting Compressor
The free air de
pressure and tel
(7) Displaceme
This type of Coleman air compressor can have either one or more air cylinders. If there is more
than one cylinder, then they are both the same size. The main thing to remember about a single
stage compressor is that it should not be used on tools that require more than 100 PSI to operate
unless that tool is only used intermittently. These are good for the home owner who uses his
compressor occasionally, or just around the home.
For tools that require greater pressure, or if you use multiple tools back to back, then you need a
two stage compressor.
The swept volu
It is given by n.
of the cylinder 1
(8) Actual Cap
The actual free
of the compres
per stroke is ah
(9) Volumetric
(2) Double Acting Compressor
The ratio of ac
compressor is ~
64
ull load and full
est input power
otary screw and
This type of compressor has at least two cylinders with one being smaller than the other. If
multiple cylinders, the siz.e is the main way to tell if it is a single or two stage compressor. These
normally operate with a pressure around 175 PSI, so are good for heavier duty or professional
use, or the serious DIY guy.
:cur until the oil
is to as much as
controls before
lped with three-
In double acting reciprocating compressor, the suction, compression and delivery of the air take
place on both sides of the piston. Such compressor would have two delivery strokes per
revolution of the crankshaft.
. Also it has to have a piston rod, cross head and connecting rod arrangement.
· cycle gives an
(3) Single Stage Compressor
In single stage compressor, the compression of the air from the initial pressure to the final
time over the
ularly true with
any significant
pressure is carried out in one cylinder only.
(4) Multi-Stage
Compressor
When the compression of air from the initial pressure to the final pressure is carried out in more
than one cylinder, then the compressor is known as multi-stage compressor.
(5) Ratio of Compression
or Pressure Ratio
It is the ratio of absolute discharge pressure to absolute inlet pressure.
(6) Free Air Delivered (F. A.D.)
· and theory of
The free air delivered is the actual volume delivered at the stated pressure, reduced to intake
pressure and temperature, and expressed in cubic metre per minute.
(7) Displacement
there is more
· about a single
I PSI to operate
r who uses his
[f
of the Compressor
The swept volume of the piston in the first cylinder is known as displacement of the compressor.
It is given by nRlL for single acting and 2nR2L for double acting compressor. where R = Radius
of the cylinder bore and L = Stroke of the piston.
(8) Actual Capacity of the Compressor
hen you need a
The actual free air delivered by the cylinder per cycle or per minute is known as actual capacity
of the compressor. It is always given in cubic metre of free air per minute. The actual capacity
per stroke is always less than the displacement of the stroke.
(9) Volumetric Efficiency
The ratio of actual free air delivered by the compressor per stroke to the displacement of the
compressor is known as volumetric efficienc3 of the compressor.
65
•
Single phase galvanic motor - this type of motor is 115 volt or 230 volts and has a maximum
horsepower of 10. This motor can be found with either a appropriate duty capacitor starter or a
magnetic starter. I have no idea what the disagreement is between these two starters, but the
appropriate duty is typically found on a smaller, portable Coleman air compressor, or on
industrial single stage compressors with a tank capacity no larger than 60 gallons. The 230 volt
single phase compressors are heavy duty models, have the magnetic starter and are two-stage
compressors.
Three phase galvanic motors - these are larger 230 or 460 volt motors with up to 25
horsepower, so obviously a real heavy duty Coleman air compressor. These compressors are
more expensive, but also more efficient. They always come with a magnetic starter because it
acts as a voltage booster when the compressor is started, and it also protects the motor during
voltage fluctuations.
where
1.10
The volumetric efficiency of a compressor is the ratio of actual free air delivered to the
displacement of the compressor. It is obvious from. Fig. CAl-7 that the actual volume delivered
by compressor (1', - l'J having clearance volume l'C is less than the swept volume of the
compressor (1', - l'J). The volumetric efficiency or clearance factor is given by
". - '-4l)y .. ~.-.
Ya
-l')
where suffix 1n
11.
••.
The clearance volume of the compressor is always given in terms of percentage stroke volume
and say it is denoted by.K..
1.11
Methods
The following p
above one) for h
temperature Ti
isothermal comp
(1) Spray Inject
The practice of
stroke with the (
disadvantages:
(a) It requires spe
66
a maximum
tor starter or a
arters, but the
nessor, or on
. The 230 volt
are two-stage
18
...
s:
vith up to 25
mpressors are
rter because it
: motor during
I
I
=1-21(~)·-11=1-_!_[(!l)iI_l]
"J L PI
100 p,
K
where
.... 6
= -II. ')( 100.
""
(b) Volumetric efficieqcy of a c:ompI.'I:SSOr re(em:Id to ambieDt conditi(IDS is Biven by
livered to the
ume delivered
rolume of the
11 = Volume of air sucbd metnd to ambient oondition'
•
Swept ,oIubIC
(v, - v.() n:fcmd to ambient condi~
PIT. 10', - V..
:c
=---v,
POTt
V"
where suffix 1 represents the selection and suffix '0' represents the ambient conditions.
:. "l. (at lPlbi~"~
stroke volume
1.11
eoDdilia1) ==
.
't
1;
0
PoTI
I
-
[1 - _!_ (
(!.!)" - 1 } ].
100.'1
The following practical methods are used to achieve nearly isothermal compression (n is little
above one) for high speed compressors. The sole object of all the methods is to reduce the final
temperature Ti during compression so that actual work approaches more closely to the
isothermal compression.
(1) Spray Injection
The practice of injecting water into the compressor cylinder"towards the end of compression
stroke with the object of cooling the air was used some years ago. This system has following
disadvantages:
(a) It requires special gear for injection.
67
(b) The injected water interferes with the cylinder lubrication and attacks cylinder walls and
valves.
(c) The water mixed with air should be separated before using the air.
(2) Water jacketing
It is commonly and successfully used nowadays for all types of reciprocating air-compressors.
The water is circulated around the cylinder through the water jacket which helps to cool the air
during compression.
(3) Inter-cooling
Water-jacketing is not much effective when the speed of the compressor is high and pressure
ratio required is also high with single stage compression. Inter-cooling is used in addition to the
water jacketing by dividing the compression process into two or more stages. The air compressed
in first stage is cooled in a heat-exchanger known\as inter-cooler to its original temperature
before it is taken to the second stage
<-
(4) External Rns
Effective cooling can be achieved for small capacity air compressor with the use of fins on the
external surface of the compressor.
Multi-Stage Compression
It is obvious from the equation 6 that the volumetric efficiency of a reciprocating compare;,
It
(h)
a function of clearance ratio K, pressure ratio '. PI and index of expansion. The volumetric
efficiency of a compressor with the fixed clearance decreases with an increase in pressure ratio.
A stage may be reached, when the volumetric efficiency becomes zero as can be seen from Fig.
CAI-8.
It is obvious from the figure that the volume of gas taken into the compressor decreases in the
delivery pressure for the same clearance volume and fixed intake pressure. At some delivery the
compression line intersects the line of clearance volume (as at point - 2") and re-expansion
follows the same path as compression and there is no delivery of air. The attempt made to deliver
the air at a high pressure of p2" would result in compression and re-expansion of the same air
again and again without any delivery of the high pressure air. Therefore, the maximum pressure
ratio attainable with a single stage reciprocating compressor is limited by the clearance volume.
There are practical limits for the reduction of the clearance volume, and therefore to get high
pressure air it is necessary to use multistage compressions
68
The p-v diagram
In multi-stage ,
arrangement of
CAI-9.
In the two-stage
(L.P.) cylinder. ,
cylinder at cone
cooler is shown
possible to cool
supply of coolin.
H.P. cylinder an
linder
walls and
air-compressors.
)s to cool the air
.,
Fig.CAl-8
igh and pressure
n addition to the
e air compressed
inal temperature '"
se of fins on the
: compare;,
-11'
Fig.CAl-9
. The volumetric
in pressure ratio.
Ie seen from Fig.
The p-v diagram for Two-Stage compression is shown in Fig. CAl-9
In multi-stage compression, cooling of the air after it leaves each stage is possible. The
arrangement of two-stage compressor with inter-cooler and its p-v diagram are shown in Fig.
decreases in the
orne delivery the
ind re-expansion
,tmade to deliver
1 of the same air
aximum pressure
learance volume.
efore to get high
CAl-9.
In the two-stage air compressor with inter-cooler, the air is first taken into the low pressure
(L.P.) cylinder. After compress6n to some desired intermediate pressure p2, the air from the L.P.
cylinder at condition 2 is passed through inter-cooler. The condition of air leaving the intercooler is shown by the point 3 where its temperature is reduced from T2 to T3 (T3 c T2). It is
possible to cool the air in the inter-cooler up to initial temperature TI by properly regulating the
supply of cooling water in, the inter-cooler. Finally the air is compressed from condition 3, in the
H.P. cylinder and is discharged to the receiver.
69
'WATEROIIT
(d) The high r
is designed to
1.12
Let us
In this unit we
~~_qy~
7I11N'~""N
,,,,.
~1'AN7'
.
-
Uses of Cc
Types of I
-
Automatec
-
Types ofC
Reciprocal
-
Rotary SCI
-
Centrifuga
Capacity (
Aircompr
The work saved by introducing the inter-cooler is shown in Fig. CAI-9 by the area 2-3-4-6-2.
Sometimes coolers known as after-coolers are used to cool the air before passing to the receiver.
The cooling in after coolers helps to reduce the size of the receiver and not the work done.
Volumetri
Methods j
1.13
Check
1. What
Both the cylinders (L.P. and H.P.) are mounted on the same shaft and are driven by a primemover.
2. Explai
3. Explai
The advantages of multi-stage compressor with inter-cooling between stages are listed below:
(1) The work done per kg of air can be reduced by introducing an inter-cooler between the two
stages compared with single stage compressor for the same delivery pressure.
I
I
1.14
Refer.
};l>
Dixon
Pergar
(2) Better mechanical balance can be achieved with multi-stage compressors.
};l>
(3) The pressure and hence temperature range in each stage is reduced.
};l>
Aungi'
and AI
Bloch,
(a) The loss of air due to leakage is less.
};l>
(b) Higher volumetric efficiency can be achieved as the pressure of each stage is less than the
overall pressure ratio as the volumetric efficiency is also a function of the pressure ratio.
(c) Effective lubrication is possible due 'to lower temperature range.
};l>
};l>
};l>
70
Maint;
Recipr
Lubric
Screw
Techn
flowd
Richai
Pressu
(d) The high pressure cylinder is designed to withstand high pressure. whereas the L.P. cylinder
is designed to withstand low pressure. This reduces the cost of the compressor.
1.12
Let us Sun Up
a 2-3-4-6-2.
to the receiver.
Irkdone.
'en by a prime-
istedbelow:
etween the two
.s less than the
ratio.
Types of Air Compressors
Automated Assembly Stations
Capacity Control
1.13
1. What are the uses of compressed air?
2. Explain different types of air compressors.
3. Explain the methods adopted for increasing isothermal efficiency.
1.14
Reference
,.. Dixon S.L. (1978). Fluid Mechanics, Thermodynamics of Turbomachinery (Third ed.).
Pergamon Press.
,.. Aungier, Ronald H. (2000). Centrifugal Compressors A Strategyfor Aerodynamic design
and Analysis. ASME Press.
,.. Bloch, H.P. and Hoefner, J.J. (1996). Reciprocating Compressors, Operation and
Maintenance. Gulf Professional Publishing.
,.. Reciprocating Compressor Basics Adam Davis, Noria Corporation, Machinery
Lubrication, July 2005
,.. Screw Compressor Describes how screw compressors work and include photographs.
,.. Technical Centre Discusses oil-flooded screw compressors including a complete system
flow diagram
,.. Richardson, Jr., Hubert: "Scroll Compressor With Orbiting Scroll Member Biased By Oil
Pressure," U.S. Patent.
71
UNIT 2
rc ENGINE
- PART I
Structure
2. 1.Objective
2.2.Introduction
2.3.Classification of I.C. Engines
2.4.Fuel Supply in S.I. Engines
2.5.Fuel Supply to Diesel Engines
2.6.Battery Ignition System
2.7.Let us Sum Up
2.8.Check Your Progressive Exercise
2.9.Reference
rocket eng
previously
The intern.
such as ste
consisting ,
hot water, I
A large nu
different st
gasoline, a
application:
dominate a:
2.3
CIa
The interna
1. Cycle of
2.1
(a) Two-str
one rotatior
Objective
-
Classification ofl.C. Engines
Fuel Supply in S.1. Engines
-
Fuel Supply to Diesel Engines
Fuel Ignition
(b) Four-str
one during
1
2. Cycle of
(a) Otto-eye
The operatic
2.2
Introduction
3. The fuel
Internal combustion engine is an engine in which the combustion of a fuel (normally a fossil
fuel) occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion
engine, the expansion of the high-temperature and -pressure gases produced by combustion
applies direct force to some component of the engine, such as pistons, turbine blades, or a nozzle.
This force moves the component over a distance, generating useful mechanical energy.
The term internal combustion engine usually refers to an engine in which combustion is
intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with
variants, such as the six-stroke piston engine and the Wankel rotary engine. A second class of
internal combustion engines use continuous combustion: gas turbines, jet engines and most
72
(a) Petrol et
4. The met}
(a) Spark igi
5. The metl
(a) Air-cook
rocket engines, each of which are internal combustion engines on the same principle as
previously described.
The internal combustion engine (or ICE) is quite different from external combustion engines,
such as steam or Stirling engines, in which the energy is delivered to a working fluid not
consisting of, mixed with, or contaminated by combustion products. Working fluids can be air,
hot water, pressurized water or even liquid sodium, heated in some kind of boiler.
A large number of different designs for ICEs have been developed and built, with a variety of
different strengths and weaknesses. Powered by an energy-dense fuel (which is very frequently
gasoline, a liquid derived from fossil fuels). While there have been and still are many stationary
applications, the real strength of internal combustion engines is in mobile applications and they
dominate as a power supply for cars, aircraft, and boats.
2.3
Classification of I.C. Engines
The internal combustion engines are classified according to :
1. Cycle of operation. They are divided into the following group.
(a) Two-stroke engines. In two-stroke engines, there is one power stroke in every two strokes or
one rotation of the crankshaft.
(b) Four-stroke engines. In four-stroke engines, there is one power srrolte in every four strokes or
one during two rotations of the crankshaft,
2. Cycle of operation. They are divided into the following groups:
(a) Otto-cycle. (b) Diesel cycle. (c) Dual cycle.
The operations of these cycles are discussed in the previous chapter
3. The fuel used. On this basis they are classified as :
mallya
fossil
11 combustion
y combustion
5, or a nozzle.
.gy.
ombustion is
s, along with
cond class of
ies and most
(a) Petrol engines. (b) Diesel engines or heavy oil engines. (c) Gas engines.
4. The method of ignition. On this basis, they are divided into the two following classes.
(a) Spark ignition engines. (S.I. engines) and (b) Compression ignition engines. (C.1.engines).
S. The method of cooling. On this basis they are classified into two groups.
(a) Air-cooled engines. (b) Water-cooled engines.
73
6. The method of governing
Carburetor
(a) Quantity governing. (b) Quality governing. (c) Hit and Miss-governing.
A carbureto
7. The use of engines. The following is the classification on this basis:
Principles
(a) Stationary engines. (b) Automobile engines or engines for road vehicles.
(c) Marine engines. (6) Aero-engines. (e) Locomotive engines.
8. The arrangement
(a) Inline engine.
crankshaft.
of the cylinders. They can be classified as given below:
All the cylinders are arranged in a line and the power is taken from a single
The carburet,
and the highc
the flow of I
being pulled
amount of fur
When carbur
needed to pre
injection kno-
This arrangement is used in automobiles.
(b) V-type. It is a combination-of two inline engines set at an 'angle. The angle of V may vary
from 30" to 75".
Operation
The length of the crankshaft of V-type engine is half of the crankshaft used for inline engine.
This type is also used in automobiles.
• Fixed.
(c) Opposed piston engine. The pistons reciprocate in a common cylinder having common
combustion chamber at the centre. Opposed piston type is used in small air craft's and in some
diesel installations.
•
(d) Radial engines. All the cylinders are set along the radius of a circle. The connecting rods
point towards the centre of the circle. The connecting rods of all the pistons work on a single
crank pin which rotates around the centre of the circle. The radial engine occupies little floor
space and simplifies the balancing problems. This type was popular in aircrafts.
(e) Rotary engine. The engine consists of three-sided convex-type piston rotating in a cylinder.
This type of engine is known as 'Wankel' engine. It is of high speed-type, light in weight and
works on spark ignition system.
2.4
Fuel Supply in S.I. Engines
The fuels such as petrol, benzol and alcohol used in S.I. engines vaporise easily at atmospheric
conditions, therefore, the engine suction is sufficient to vaporise these fuels and no preheating is
required. The oil fuels such as light oils and paraffin oils used in diesel engines do not vaporise
easily and therefore the engine suction is not sufficient to vaporize therefore fuel injection
arrangement is used in oil engines.
Under all engi
•
•
•
Measu
Delive
(adjust
Mix tb
A carburetor
temperatures,
•
•
•
•
•
•
Basics
74
archit€
Varial
simult
vacuui
I
Cold st
Hotsta
Idling (
Accelei
Highs}:
Cruisin
Carburetor
A carburetor
is a device that blends air and fuel for an internal combustion engine.
Principles
11
from a single
The carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure,
and the higher its dynamic pressure. The throttle (accelerator) linkage does not directly control
the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air
being pulled into the engine. The speed of this flow, and therefore its pressure, determines the
amount of fuel drawn into the airstream.
When carburetors d."~used in aircraft with piston engines, special designs and features are
needed to prevent fuel starvation during inverted flight. Later engines used an early form of fuel
injection known as a pressure carburetor.
ofV may vary
Operation
r inline engine.
•
aving common
t's and in some
•
connecting rods
ork on a single
(pies little floor
19 in a cylinder.
Fixed-venturi, in which the varying air velocity in the venturi alters the fuel flow; this
architecture is employed in most carburetors found on cars.
Variable-venturi,
in which the fuel jet opening is varied by the slide (which
simultaneously alters air flow). In "constant depression" carburetors, this is done by a
vacuum operated piston connected to a tapered needle which slides inside the fuel jet.
Under all engine operating conditions, the carburetor must:
•
•
•
Measure the airflow of the engine
Deliver the correct amount of fuel to keep the fuel/air mixture in the proper range
(adjusting for factors such as temperature)
Mix the two finely and evenly
t in weight and
A carburetor must provide the proper fuel/air mixture across a wide range of ambient
temperatures, atmospheric pressures, engine speeds and loads, and centrifugal forces:
, at atmospheric
no preheating is
do not vaporise
e fuel injection
•
•
•
•
•
•
Basics
Cold start
Hot start
Idling or slow-running
Acceleration
High speed / high power at full throttle
Cruising at part throttle (light load)
.... '"
75
A carburetor basically consists of an open pipe through which the air passes into the inlet
manifold of the engine. The pipe is in the form of a venturi: it narrows in section and then widens
again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a
butterfly valve called the throttle valve -- a rotating disc that can be turned end-on to the airflow,
so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the
flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity
of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The
throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely
by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or
equipment.
Basic
Carburetor
(Cross Section)
As the throttl
additional fue
by the throttle;
the reduced \
metering fuel
Main open-tl
As the thrott
restriction on
the venturi sh
velocity incre
low pressure
venturi. Som
primary venn
As the thrott
insufficient 1<
Power valve
For open tlu
detonation, a
valve", whicl
the spring 0]
operation of
turned "off'.
stroke's tends
Accelerator
Fuel is introduced into the air stream through small holes at the narrowest part of the venturi and
at other places where pressure will be lowered when not running on full throttle. Fuel flow is
adjusted by means of precisely-calibrated orifices, referred to asjets, in the fuel path.
Off-idle circuit
76
Liquid gasol
the throttle i
the fuel flov
makes the er
throttle. This
by the throttl
This extra sl
pumps are a(
moving part
accelerator ]
renewed.
ito the inlet
then widens
venturi is a
, the airflow,
ly blocks the
the quantity
l speed. The
ints or rarely
:r vehicles or
'etor
n)
As the throttle is opened up slightly from the fully-closed position, the throttle plate uncovers
additional fuel delivery holes behind the throttle plate where there is a low pressure area created
by the throttle plate blocking air flow; these allow more fuel to flow as well as compensating for
the reduced vacuum that occurs when the throttle is opened, thus smoothing the transition to
metering fuel flow through the regular open throttle circuit.
Main open-throttle circuit
As the throttle is progressively opened, the manifold vacuum is lessened since there is less
restriction on the airflow, reducing the flow through the idle and off-idle circuits. This is where
the venturi shape of the carburetor throat comes into play, due to Bernoulli's principle (i.e., as the
velocity increases, pressure falls). The venturi raises the air velocity, and this high speed and thus
low pressure sucks fuel into the airstream through a nozzle or nozzles located in the center of the
venturi. Sometimes one or more additional booster venturis are placed coaxially within the
primary venturi to increase the effect.
As the throttle is closed, the airflow through the venturi drops until the lowered pressure is
insufficient to maintain this fuel flow, and the idle circuit takes over again, as described above.
Power valve
For open throttle operation a richer mixture will produce more power, prevent pre-ignition
detonation, and keep the engine cooler. This is usually addressed with a spring-loaded "power
valve", which is held shut by engine vacuum. As the throttle opens up, the vacuum decreases and
the spring opens the valve to let more fuel into the main circuit. On two-stroke engines, the
operation of the power valve is the reverse of normal - it is normally :'on" and at a set rpm it is
turned "off'. It is activated at high rpm to extend the engine's rev range, capitalizing on a twostroke's tendency to rev higher momentarily when the mixture is lean.
Accelerator pump
he venturi and
:. Fuel flow is
h.
Liquid gasoline, being denser than air, is slower than air to react to a force applied to it. When
the throttle is rapidly opened, airflow through the carburetor increases immediately, faster than
the fuel flow rate can increase. This transient oversupply of air causes a lean mixture, which
makes the engine misfire (or "stumble"}-an effect opposite what was demanded by opening the
throttle. This is remedied by the use of a small piston or diaphragm pump which, when actuated
by the throttle linkage, forces a small amount of gasoline through a jet into the carburetor throat.
This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator
pumps are adjustable for volume and/or duration by some means. Eventually the seals around the
moving parts of the pump wear such that pump output is reduced; this reduction of the
accelerator pump shot causes stumbling under acceleration until the seals on the pump are
renewed.
77
2.5
Fuel injection Systems and its Requirements
Fuel injection is a system for mixing fuel with air in an internal combustion engine. It has
become the primary fuel delivery system used in automotive petrol engines, having almost
completely replaced carburetors in the late 1980s.
A fuel injection system is designed ard calibrated specifically for the type(s) of fuel it will
handle. Most fuel injection systems are for gasoline or diesel applications. With the advent of
electronic fuel injection (EFI), the diesel and gasoline hardware has become similar. EFl's
programmable firmware has permitted common hardware to be used with different fuels.
Carburetors were the predominant method used to meter fuel on gasoline engines before the
widespread use of fuel injection. A variety of injection systems have existed since the earliest
usage of the internal combustion engine.
The primary difference between carburetors and fuel injection is that fuel injection atomizes the
fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies
on low pressure created by intake air rushing through it to add the fuel to the airstream.
Central to an E
engine operatinj
calculate the ap
operation by nu
of injected fuel
and workload, a
The electronic 1
electricity is ap
pulse width, is J
closely-controlli
fuel injection s~
system).
Since the nature
4-stroke engine
amounts. In a se
event. Every in.
calculation, and
requirements.
Detailed function
Typical EFI components
•
•
•
Injectors
Fuel Pump
Fuel Pressure Regulator
•
•
ECM - Engine Control Module; includes a digital computer and circuitry to communicate
with sensors and control outputs.
Wiring Harness
•
Various Sensors (Some of the sensors required are listed here.)
Functional
•
Crank/Cam Position: Hall effect sensor
•
•
Airflow: MAF sensor, sometimes this is inferred with a MAP sensor
Exhaust Gas Oxygen: Oxygen sensor, EGO sensor, UEGO sensor
description
78
Sprav lip,,-
It is necessary to
proportional to t
position. The am
be determined us
Central to an EFl system is a computer called the Engine Control Unit (ECU), which monitors
engine operating parameters via various sensors. The ECU interprets these parameters in order to
calculate the appropriate amount of fuel to be injected, among other tasks, and controls engine
operation by manipulating fuel and/or air flow as well as other variables. The optimum amount
of injected fuel depends on conditions such as engine and ambient temperatures, engine speed
and workload, and exhaust gas composition.
gme. It has
ving almost
fuel it will
.e advent of
nilar. EFI's
els.
; before the
the earliest
tomizes the
aretor relies
The electronic fuel injector is normally closed, and opens to inject pressurized fuel as long as
electricity is applied to the injector's solenoid coil. The duration of this operation, called the
pulse width, is proportional to the amount of fuel desired. The electric pulse may be applied in
.closely-controlled sequence with the valve events on each individual cylinder (in a sequential
fuel injection system), or in groups of less than the total number of injectors (in a batch fire
system).
Since the nature of fuel injection dispenses fuel in discrete amounts, and since the nature of the
4-stroke engine has discrete induction (air-intake) events, the ECU calculates fuel in discrete
amounts. In a sequential system, the injected fuel mass is tailored for each individual induction
event. Every induction event, of every cylinder, of the entire engine, is a separate fuel mass
calculation, and each injector receives a unique pulse width based on that cylinder's fuel
requirements.
1.
Electrical Attachment
Solenoid
Off
unmunicate
Filter
Sprav
InjeCtOr Casing
It is necessary to know the mass of air the engine "breathes" during each induction event. This is
proportional to the intake manifold's air pressure/temperature, which is proportional to throttle
position. The amount of air inducted in each intake event is known as "air-charge", and this can
be determined using several methods.
79
The three elemental ingredients for combustion are fuel, air and ignition. However, complete
combustion can only occur if the air and fuel is present in the exact stoichiometric ratio, which
allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air, with
no undesirable polluting leftovers. Oxygen sensors monitor the amount of oxygen in the exhaust,
and the ECU uses this information to adjust the air-to-fuel ratio in real-time.
Injection syst
(1) Air inject]
(2) Air-less
0
Air Injection
To achieve stoichiometry. the air mass flow into the engine is measured and multiplied by the
stoichiometric air/fuel ratio 14.64: I (by weight) for gasoline. The required fuel mass that must be
injected into the engine is then translated to the required pulse width for the fuel injector. The
stoichiometric ratio changes as a function of the fuel; diesel, gasoline, ethanol, methanol,
propane, methane (natural gas), or hydrogen.
Deviations from stoichiometry are required during non-standard operating conditions such as
heavy load, or cold operation, in which case. the mixture ratio can range from 10:1 to 18:1 (for
gasoline). In early fuel injection systems this was accomplished with a thermotime switch.
Pulse width is inversely related to pressure difference across the injector inlet and outlet. For
example. if the fuel line pressure increases (injector inlet), or the manifold pressure decreases
(injector outlet), a smaller pulse width will admit the same fuel. Fuel injectors are available in
various sizes and spray characteristics as well. Compensation for these and many other factors
are programmed into the ECU's software.
The air inject
the exhaust r
combustion c
of the fuel mi
Air injection
Emissions La
system desigr
Pump Type:
as the smog I
stream throu
components 0
known as exh
The fuel injection system must satisfy the following fundamental requirements:
1. Inject the quantity of the fuel demanded by the load on the engine and maintain this metered .
quantity:
(a) constant supply of fuel from cycle to cycle operation.
(b) Constant supply of fuel from cylinder to cylinder.
2. Inject the fuel at correct time in the cycle throughout the speed range of the engine.
3. Inject the fuel at desired rate to control the combustion and resulting rate of pressure-rise.
4. Automize the fuel to the required degree.
Pulse Type:
5. Distribute the fuel throughout the combustion chamber for better mixing.
6. Injection should begin and end sharply.
•
Injection systems are m ifactured with great accuracy, especially the parts that actually meter
and inject the fuel. Sud; .iosely fitting parts require special attention during their manufacture
and, as a result, injection ~stems are costly items.
80
r
only relies on
and passages .
The air is then
duct or hose s
valve, and ther
ever, complete
ic ratio, which
Injection system'lri~y be divided into two general types, as follows:
(1) Air injection
in the air, with
in the exhaust,
(2) Air-less or solid or mechanical injection.
Air Injection System
iltiplied by the
that must be
I injector. The
101, methanol,
5S
litions such as
:1 to 18:1 (for
switch.
md outlet. For
sure decreases
re available in
yother factors
The air injection system (AIS) is designed to introduce clean air to the
the exhaust manifold or exhaust headers. Exhaust gases are at their
combustion chambers. Introducing oxygen to the exhaust at this point
of the fuel mixture as it travels down the exhaust system and ultimately
engine exhaust as it exits
hottest as they leave the
allows continued burning
out the tailpipe.
Air injection systems consist of mainly two different designs. Your vehicle's Underhood
Emissions Label can provide you with information regarding the requirements of this emission
system design and equipped components.
Pump Type: The first system known as the Pump Type includes an air pump, commonly known
as the smog pump, which is responsible for supplying fresh pressurized oxygen to the exhaust
stream through header or exhaust manifold, and/or before the Catalytic Converter. Tbe
components of this system are the air pump, the diverter valve, the air distribution manifold; both
known as exhaust manifold and exhaust headers, and the air check valve.
n this metered .
re.
sure-rise.
actually meter
r manufacture
Pulse Type: The second type of system known as the Pulse Air Sys m is much sunnkr . '.1
only relies on the vacuum created in the exhaust stream as it travels down thr exhaust d<l ,; L :.;
and passages. As the engine cycles, this vacuum draws fresh oxygen into the Air Injection ~l'l .
The air is then used to prolong thorough exhaust burning. This system should consist 0! a metal
duct or hose approximately I" in diameter around the air cleaner leading to a metal :!H .. hi ck
valve, and then the exhaust manifold.
81
The arrangemen
(a) Common R~
The Common R.
which leads to b
in engine noise c
In the Common.
under a consister
Operation:
Pump Type - The spinning vanes of the air pump force air into the diverter valve. During
acceleration air is forced through the diverter valve, the check valve, the air injection manifold,
. and into the exhaust stream. During deceleration the diverter valve blocks air flow, preventing a
backfire that could damage the exhaust system. When needed, the diverter valve will release
excess pressure to the air cleaner.
Pulse Type - As exhaust gases travel down the exhaust passages, the vacuum created draw fresh
oxygen into the air injection system. Fresh air then travels through the diverter valve, check
valve and to the exhaust manifold.
Air-less or Solid Injection
In this system, the fuel is supplied at a very high pressure (150 bar) from the fuel pump to the
fuel injector and then it is injected to the combustion chamber with the help of an injector. The
main parts of this system are fuel pump and fuel injector. The fuel pump is operated by a cam
which is mounted on cam shaft. The power required to operate the camb taken from the engine
crankshaft. Depending upon the location of the fuel pumps and injectors, and upon the method
used to meter the fuel, solid injection may further be classified as follows:
(a) Common Rail System. (A single pump for compressing the fuel, plus a metering element for
each cylinder).
(b) Distributor system. (A single pump for metering and compressing the fuel, plus a driving
mechanism for distributing the fuel to the various cylinders).
(c) Individual pump and nozzle system. (A separate metering and compressing pump and
nozzle for each cylinder of the engine).
82
A high-pressure I
PSI. The pressure
quantity of fuel b
pipes to the fuel iJ
The injectors use
Control, or EDC l
the fuel rail and th
Diesel fuel injectc
fuel injectors use(
position and spee
injector. Hydraulj
The arrangement and working of each system are given below.
(a) Common Rail System.
The Common Rail Diesel Injection System delivers a more controlled quantity of atomized fuel,
which leads to better fuel economy; a reduction in exhaust emissions; and a significant decrease
in engine noise during operation.
In the Common Rail system, an accumulator, or rail, is used to create a common reservoir of fuel
under a consistent controlled pressure that is separate from the fuel injection points.
Common raJ
valve. During
tion manifold,
" preventing a
-e
will release
ted draw fresh
~valve, check
!l pump to the
1 injector. The
ated by a cam
om the engine
on the method
ng element for
A high-pressure pump increases the fuel pressure in the accumulator up to 1,600 bar or 23,200
PSI. The pressure is set by the engine control unit and is independent of the engine speed and
quantity of fuel being injected into any of the cylinders. The fuel is then transferred through rigid
pipes to the fuel injectors, which inject the correct amount of fuel into the combustion chambers.
The injectors used in Common Rail systems are triggered externally by an Electronic Diesel
Control, or EDC unit, which controls all the engine injection parameters including the pressure in
the fuel rail and the timing and duration of injection.
plus a driving
ing pump and
Diesel fuel injectors used in Common Rail injection systems operate differently to conventional
fuel injectors used in the jerk pump system, where the plungers are controlled by the camshaft
position and speed. Some common rail injectors are controlled by a magnetic.solenoid on the
injector. Hydraulic force from the pressure in the system is used to open and close the injector,
83
but the available pressure is controlled by the solenoid triggered by the Electronic Diesel Control
unit.
Some injectors use Piezo crystal wafers to actuate the injectors. These crystals expand rapidly
when connected to an electric field. In a Piezo inline injector, the actuator is built into the
injector body very close to the jet needle and uses no mechanical parts to switch injector needles.
The electronic diesel control unit precisely meters the amount of fuel injected, and improves
atomization of the fuel by controlling the injector pulsations. This results in quieter, more fuel
efficient engines; cleaner operation; and more power output.
Advantages:
1. It fulfills the requirements of either, (a) the constant load with variable speed or (b) constant
speed with variable load.
2. Only one pump is sufficient for multi-cylinder engines.
3. Variation in pump supply pressure will affect all the cylinders uniformly.
4. The arrangement of the system is very simple and maintenance cost is less.
Disadvantages:
The distributor
the distributor
injection and oi
1. Very accurate design and workmanship are required.
2. There is tendency to develop leaks in the injection valve.
(b) Distributor
System.
This system, like common rail system, employs a single high pressure pump as shown in Fig. 5.
The high pressure pump in this system is used for metering and compressing the fuel and then
the fuel is delivered to the common rotating distributor. The fuel is supplied to each cylinder by
the distributor In every cycle, the injection strokes of the pump are equal to the number cf
cylinders.
The quantity of fuel supplied and timing of fuel supply is done by single piunger (main pump)
therefore equal amount of fuel is supplied to each cylinder and at the same point in the cycle.
The function of the distributor is merely to select the cylinder to receive the fuel.
84
Fuel Injection
An Injection I
typically, a gas
gears, chains 0]
cam engines ( (
timing is such 1
compression st
directly from tl:
Construction
Earlier diesel p
line, rather lik
injection volun
aligns with a
simultaneously
pumps still fin
plant, static en!
Diesel Control
expand rapidly
, built into the
ljector needles.
, and improves
eter, more fuel
5TIJRAtX
or (b) constant
TANI(
Fig. 5
The distributor block selects a particular cylinder according to the cam coming in contact with
the distributor as shown in figure. The appropriate valve opens just before the beginning of
injection and oil is supplied to the required cylinder.
Fuel Injection Pump
hown in Fig. 5.
e fuel and then
ach cylinder by
the number cf
!r (main pump)
nt in the cycle.
An Injection Pump is the device that pumps fuel into the cylinders of a diesel engine or less
typically, a gasoline engine. Traditionally, the pump is driven indirectly from the crankshaft by
gears, chains or a toothed belt (often the timing belt) that also drives the camshaft on overheadcam engines ( OHC ). It rotates at half crankshaft speed in a conventional four-stroke engine. Its
timing is such that the fuel is injected only very slightly before top dead centre of that cylinder's
compression stroke. It is also common for the pump belt on gasoline engines to be driven
directly from the camshaft. In some systems injection pressures can be as high as 200Mpa.
Construction
Earlier diesel pumps used an in-line layout with a series of cam-operated injection cylinders in a
line, rather like a miniature inline engine. The pistons have a constant stroke volume, and
injection volume (ie, throttling) is controlled by rotating the cylinders against a cut-off port that
aligns with a helical slot in the cylinder. When all the cylinders are rotated at once, they
simultaneously vary their injection volume to produce more or less power from the engine. Inline
pumps still find favour on large multi-cylinder engines such as those on trucks, construction
plant, static engines and agricultural vehicles.
8S
There are two
a superior sp
generally flo,
injectors, as j
superior, lead
about the spra
Inline diesel injection pump
For use on cars and light trucks, the rotary pump or distributor pump was developed. It uses a
single injection cylinder driven from an axial cam plate, which injects into the individual fuel
lines via a rotary distribution valve. Later ir I amations such as the Bosch VE pump vary the
injection timing with crank speed to allow '~eater power at high crank speeds, and smoother,
more economical running at slower revs. Some VE variants have a pressure-based system that
allows the injection volume to increase over normal to allow a turbocharger or supercharger
equipped engine to develop more power under boost conditions.
Inline diesel metering pump
All injection pumps incorporate a governor to cut fuel supply if the crank speed endangers the
engine - the heavy moving parts of diesel engines do not tolerate overspeeding well, and
catastrophic damage can occur if they are over-revved.
Fuel Injector
So when do y.
injector duty (
time), you she
according to Sl
working as a
course upgradi
works best on
Air Flow sense
the available h
(the other tyt:
recalibrating it
Honda people
should think se
ECU systems (
Or, you can re
size.
The fuel from 1
following funct
86
There are two common types of fuel injectors, pintle and disc (Lucas style). Pintle injectors have
a superior spray pattern to disc actuated injectors, but disc injectors are less expensive and
generally flow large amounts of fuel easily. If possible, always choose high flow pintle style
injectors, as fuel atomization at anything other than full throttle (high velocity port flow) is
superior, leading to better drivability and economy. Below is a picture illustrating what I mean
about the spray patterns.
loped. It uses a
individual fuel
pump vary the
and smoother,
sed system that
rr supercharger
Iendangers the
ding well, and
So when do you upgrade to larger fuel injectors? The general rule of thumb is that anytime the
injector duty cycle goes beyond 80% (meaning the injector is open .and firing over 80% of the
time), you should upgrade. Fuel injector performance can become unstable beyond this point
according to some experts, plus it doesn't pay to have a proper sequential fuel injection system
working as a simple always open spray nozzle (which is neither efficient nor powerful). Of
course upgrading their size is only one way to add more fuel, and although a very good idea, it
works best on car's that can be easily retuned to work with them. In the case of MAF cars (Mass
Air Flow sensor), system recalibration is generally just a matter of changing the sensor (many of
the available larger sensors offer multiple calibrations). For speed density fuel injection systems
(the other type of airflow sensing), its generally necessary to have an ECU capable of
recalibrating it's fuel tables for different injector flow rates. This is a problem mainly for the
Honda people out there (all of which come with speed density systems), in which case you
should think seriously about giving Zdyne or Hondata a call. Both offer very nice programmable
ECU systems capableof keeping a big time forced induction motor running reliably and strong.
Or, you can read on and see a few other ways to increase fuel flow without changing injector
size.
The fuel from the fuel pump is supplied to the fuel injector. The fuel injector has to perform the
following functions:
87
(a) To atomize the fuel to the required degrees.
(6) 10 distribute the fuel in such a way that there is complete mixing of fuel
find
a:; ..
(c) It must prevent the injection on the walls of the cylinder and piston top surface.
(t.}
It must stan and stop the fuel injection instantaneously.
A '~'pil'JI spring leaded Bosch fuel injector is shown in Fig.S. The spring loaded valve is lifted
by the high pressure oil supplied by the pump and the hel is injected into the combustion
chamber of the engine cylinder. As the pressure of oil falls, the valve is automatically closed by
the spring force. The amount of fuel injected is regulated by the duration of the open period of
the valve,
Battery Ignltk
The presurc of the controlling spring can be adjusted by means of the adjusting screw as shown
iil r.gure A, ',y leakage of oil port, the valve and spindle is taken out through the leak-off
connection as shown In figure.
The ignition sy
The figure shoi
surges of curre
electric spark.
Adjusting
screw
Controlling
spring
Construction
It consists of a 1
Battery: The b.
Ignition coil (I
It has two wind
up the voltage i
Fig.8
The primary w
consists of mor
battery voltage
the spark plug.
Condenser: It i
sparking at com
a more rapid co
2.6
Contact break,
Battery Ignition System
88
-
------_-
valve is lifted
combustion
:ally closed by
open period of
ie
:.::::'8"
Battery Ignition System (Coil Ignition System)
erew as shown
1 the
leak-off
The ignition system is based on the principle of mutual electromagnetic induction.
The figure shows a battery ignition for a four cylinder engine. The ignition system supplies high
surges of current gas high as 30000 volts) to the spark plug. This surge in current produces the
electric spark.
Construction
It consists of a battery, ignition coil, consider, contact breaker, distributor and spark plug.
Battery: The battery provides a voltage of 6 to 12volts.
Ignition coil (Induction coil)
It has two windings namely, primary winding and secondary winding. It is a transformer to step
up the voltage in the ignition system.
The primary winding consists of thick wire with less number of turns. The secondary winding
consists of more number of turns of thin wire. The purpose of the ignition coil is to set up the
battery voltage (6 or 12 volts) to 20000 to 30000 volts required to produce spark for ignition in
the spark plug.
Condenser: It is connected across the contact breaker. It serves two purposes 1) prevents excess
sparking at contact breaker points 2) It induces a high voltage in the secondary circuit by causing
a more rapid contact break of the primary circuit
Contact breaker:
It makes and breaks the contact in primary ignition circuit.
89
Distributor: It distributes the high voltage to the respective spark plugs at regular intervals in
the sequence of firing order of the engine.
--2.8
1.
2.
3.
4.
Spark plugs: The spark plug is fitted on the combustion chamber of the engine. It produces
spark to ignite the fuel-air mixture. Each cylinder has an individual spark plug ..
Chec
Class
ExplG
Expla
Expl2
Working
The rotor of the distributor and contact breaker cam are driven by the engine, there are two
circuits in this system.
(a)Primary circuit contact breaker.
It consists of battery, primary coil of the ignition coil, condenser and
(b) Secondary circuit - It consists of secondary coil, distributor and spark plug.
The ignition switch is switched on and the engine crankshaft is turned (cranking is done by a
starting motor), the cranking opens and closes the contact .breaker points through a cam.
When the contact breaker points are closed
1. The current flows from the battery to the contact breaker points through the switch and
primary winding and then returns to earth through the battery.
2. The current builds up a magnetic field in the primary winding of the ignition coil.
3. When the primary current is at the maximum, the contact breaker points are opened by the
cam.
When the contact breaker points are opened
1. The magnetic field set up in the primary winding is suddenly collapsed.
2. A high voltage (20,000 volts) is generated in the secondary winding of the ignition coil.
3. This high voltage is directed to the rotor of the distributor.
4. The rotor directs the high voltage to the individual spark plugs in the sequence of the firing
order of the engine.
5. This high voltage tries to cross or jump the spark plug gap (0.45 t)
2.7
Let us Sum Up
-
Classification of I.C. Engines
-
Fuel Supply in S.I. Engines
-
Fuel Ignition
90
2.9
Refel
~
~
"Intel
"Colt
2010·
~ Laser
Acce:
~ "Imp]
intervals in
1. Classify the IC Engines.
2. Explain fuel supply in S.l. Engines
3. Explain fuel supply in Diesel Engines
4. Explain Fuel Ignition
It produces
rere are two
2.8
Reference
2.9
)l>
ndenser and
)l>
)l>
s done by a
am.
)l>
"Internal combustion engine". Answers.com. 2009-05-09. Retrieved 2010-08-28.
"Columbia encyclopedia: Internal combustion engine". Inventors.about.com. Retrieved
2010-08-28.
Laser sparks revolution in internal combustion engines Physorg.com, April 20, 2011.
Accessed April 2011
"Improving IC Engine Efficiency". Courses.washington.edu. Retrieved 2010-08-28.
switch and
ened by the
coil.
of the firing
91
UNIT 3
Ie ENGINE - PART II
Structure
3.1.0bjective
3.2.Introduction
3.3.Governing of I.C. Engines
3.4.Cooling ofH.C. Engines
3.5.Lubricating Systems
3.6.Lubrication of Different Engine Parts
3.7.I.C. Engine and its Components
3.8.Working of 4-Stroke Cycle Diesel Engine
3.9. Two-Stroke Engines
3.10. Let us Sum Up
3.12. Reference
3.1
A large num
different stre
gasoline, a Ii,
applications,
dominate as 1
Internal com
portable mac
provide high
fossil fuel (n
(automobiles
Where very l
form of gas t
generators.
Objective
- Governing of I.C. Engines
- Cooling ofH.C. Engines
- Lubrication of I.C. Engines
- Lubricating Systems
- Lubrication of Different Engine Parts
- I.C. Engine and its Components
- Working of 4-Stroke Cycle Diesel Engine
- Two-Stroke Engines
3.2
The internal
such as .stea
consisting of
hot water, pn
Introduction
The term internal combustion engine usually refers to an engine in which combustion is
intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with
variants, such as the six-stroke piston engine and the Wankel rotary engine. A second class of
internal combustion engines use continuous combustion: gas turbines, jet engines and most
rocket engines, each of which are internal combustion engines on the same principle as
previously described
92
3.3
Geve:
As a matter
particular Sl
time to time.
change its Sf
changing the
engine is incr
Now, in or
conditions
of providi
(according
MElHODSOI
Through th
following are
The internal combustion engine (or ICE) is quite different from external combustion engines,
such as steam or Stirling engines, in which the energy is delivered to a working fluid not
consisting of, mixed with, or contaminated by combustion products. Working fluids can be air,
hot water, pressurized water or even liquid sodium, heated in some kind of boiler.
A large number of different designs for ICEs have been developed and built, with a variety of
different strengths and weaknesses. Powered by an energy-dense fuel (which is very frequently
gasoline, a liquid derived from fossil fuels). While there have been and still are many stationary
applications, the real strength of internal combustion engines is in mobile applications and they
dominate
as a power
supply for cars, aircraft, and boats.
Internal combustion engines are most commonly used for mobile propulsion in vehicle .. and
portable machinery. In mobile equipment, internal combustion is advantageous since it can
provide high power-to-weight ratios together with excellent fuel energy density. Generally using
fossil fuel (mainly petroleum), these engines have appeared in transport in almost all vehicles
(automobiles, trucks, motorcycles, boats, and in a wide variety of aircraft and locomotives).
Where very high power-to-weight ratios are required, internal combustion engines appear in the
form of gas turbines. These applications include jet aircraft, helicopters, large ships and electric
generators.
3.3
Governing of I.C. Engines
As a matter of fact, all the I.C. engines like other engines, are always designed torun at a
particular speed. But in actual practice, load on the engine keeps on fluctuating from
time to time. A little consideration will show, that change of load, on an I.C. engine, is sure to
change its speed. It has been observed that if load on an I.C. engine is decreased without
changing the quantity of fuel, the engine will run at a higher speed. Similarly, ifload on the
engine is increased without changing the quantity of fuel, the engine will run at a lower speed.
Now, in order to have a high efficiency
of an I.C. engine, at different
load
conditions,
its speed must be kept constant
as far as possible.
The process
of providing
any
arrangement,
which
will
keep
the
speed
constant
(according
to the changing load conditions) is known as governing ofI.C. engines.
nbustion is
along with
md class of
:s and most
principle as
MEIHODS OF GOVERNING iC.ENGINES
Through
there are many methods
for the governing
following are important from the subject point of view:
93
of 1. C. engines,
yet the
L Hit and miss governing:
This method of governing is widely used for IC engines of smaller
capacity
or gas
engines
This method
is most suitable
for engines,
which
are frequently
subjected to reduced loads and ,LS a result of this, the engines tend to run at higher
speeds. In this system of governing, whenever the engine starts running at higher speed (due to
decreased load), some explosion are omitted or missed. This is done with help of
centrifugal governor in which the inlet valve of fuel is closed and the explosions are omi tted
till the engine speed reaches its normal value. The only disadvantage
of this method
is that there is uneven turning moment due to missing of explosions. As a result of
this, it requires a heavy flywheel.
2. Qualitative governing:
In this system of governing, a control valve is fitted in the fuel del i v e r y pip e, w h i c h
controls
the
quantity
of fuel
to be mixed
in the
charge.
The
movement of control valve is regulated by the centrifugal
governor through rack
and pinion arrangement.
It may be noted that in this system, the amount of air used
in each cycle remains the same. But with the change in the quantity of fuel (with quantity of
air remaining
constant),
the quality
of charge (i.e. air-fuel
ratio of mixture)
changes.
Whenever the engine starts running at higher speed (due to decreased load), the
quantity of fuel is reduced till the engine speed reaches its normal value. Similarly, whenever the
engine
starts running
at lower speed (due to increased
load), the quantity
of fuel is increased. In automobile engines, the rack and pinion arrangement is connected
with the accelerator.
3. Quantitative governing
In this system of governing,
the quality of charge (i.e. air-fuel ratio of the mixture) is
kept constant. But quantity of mixture supplied to the engine cylinder
is varied by means
of a throttle
valve which is regulated
by the centrifugal
governor through
rack and pinion arrangement.
Whenever the engine starts running at higher speed
(cue to decreased
load), the quantity of charge is reduced till the engine speed
reaches
its normal
value.
Similarly,
whenever
the engine
starts running
at
lower speed (due to increased load), the quantity of charge is increased. This method is used
for governing large engines.
3.4
Cooling (
Internal combust
energy in the fuel
the surroundings
from the engine.
2500° C. The en;
this heat. Such hi
Necessity for En
1)
Engine
2)
Damag,
3)
Lubrica
4)
Thermal stn
and cracking of c(
5)
Pre - i
overheat . ': of spr
6)
Reduce:
7)
Overher
To avoid the abo'
engine at the des
efficiency reduces
Requirements
of
(i) It should remot
cooling reduces th
4. Combination system of governing:
In this system of governing, the above mentioned two methods of governing
(i.e. qualitative and quantitative)
are combined together, so that quality
as quantity
of the
charge
is varied
according
to the
conditions. This system is complicated, and has not proved to be successful
94
as well
changing
(ii) A good coolin
starting, the coolin
The components it
3.4
, or gas
requently
at higher
:d (due to
th help of
e omitted
is method
L result of
l
, which
~e. The
nigh rack
f air used
quantity of
mixture)
l load), the
neneverthe
quantity
: connected
mixture) is
)y means
rr through
.her speed
.ine speed
unning at
hod is used
Cooling of H.C. Engines
Internal combustion engines at best can transform about 25 to 35 percentage of the chemical
energy in the fuel in to mechanical energy. About 35 percentage of the heat generated is lost in to
the surroundings of combustion space, remainder being dissipated through exhaust' and radiation
from the engine. The temperature of the burning gases in the engine cylinder is about 2000 to
25000 C. The engine components like cylinder head, cylinder wall piston and the valve absorb
this heat. Such high temperatures are objectionable for various reasons state below.
Necessity for Engine Cooling
I)
Engine valves warp (twist) due to over heating.
2)
Damage to the materials of cylinder body and piston.
3)
Lubricating oil decomposes to form gummy and carbon particles.
4) Thermal stresses are set up in the engine parts and causes distortion (twist or change shape)
and cracking of components.
5)
Pre - ignition occurs (i.e. ignition occurs before it is required to igniter due to the
overheat ' :of spark plug.
6)
Reduces the strength of the materials used for piston and piston rings.
7)
Overheating also reduces the efficiency of the engine.
To avoid the above difficulties, some form of cooling is provided to keep the temperature of
engine at the desired level. It should be noted that if the engine becomes every cool the
efficiency reduces, because starting the engine from cold requires more fuel.
Requirements of a good Cooling System
(i) It should remove only about 30% of the heat generated in the combustion chamber. Too much
cooling reduces the thermal efficiency of the engine.
as well
(ii) A good cooling system should remove heat at a faster rate when the engine is hot. During
starting, the cooling should be very slow.
.h a n g in g
The components in the cylinder must be reasonably hot (250°C).
95
Over-cooling of the engine results in insuffici~lit v~~ori~ati~n of fuel, loss of pow_er, high fuel
consumption, higher emissions, starting troubles, excessive formation of sludge, lower thermal
efficiency and greater wear and tear of parts.
Methods of cooling
The engine des
of cooling are
engine and sco,
Advantages of
There are two methods of cooling an l.C. engine. They are:
1. Air cooling or Direct cooling.
2. Water cooling or Indirect cooling
Ct)()Ung fins
b~tween walls
1. The eng
2. Lighter
etc. It al
3. There is
4. This is,
5. Less spe
6. Warmin
Disadvantages
1. Air cool
2. Engine I
is coolec
Coo
3. Not suit
4. Fans use
5. Such en!
Water cooling (.
Air cooling system
In air cooling, air is circulated around the cylinder block and cylinder head, fins are provided
outside the cylinder and on the cylinder head. Fins increase the surface area exposed to the
atmosphere and the heat radiation from the surface also increases. More air passes over the fins
and comes contact with the cylinder, thus the engine heat is removed efficiently.
The use of fins increases the heat transfer surface by 5 to 10 times its original value (i.e. without
the use of fins). The high velocity of air required for cooling is obtained by the forward motion
of the engine (vehicle) itself. In stationary engine, air circulating fan is provided.
Application
Water cooling (11
96
er, high fuel
.wer thermal
The engine design is much simpler and lighter in weight than water cooling engine. These types
of cooling are used for small industrial engines and small capacity engines such as motor cycle
engine and scooter engine.
Advantages of air cooling
1. The engine design is much simpler.
2. Lighter in weight than water cooled engines since there is no water jacket, radiator, pump
etc. It also minimizes the maintenance and operating cost.
3. There is no danger from freezing water in cold climates.
4. This is very much useful in water scarcity areas and desert.
5. Less space is required.
6. Warming up the engine is faster than water cooled engine.
Disadvantages of air cooling
1. Air cooling is not as effective as water cooling and efficiency of the engine is reduced.
2. Engine parts are not uniformly cooled. The front portion of the engine which faces the air
is cooled more than the rear portion. This results in slight distortion.
3. Not suitable for multi-cylinder engines. This requires a separate fan for circulation.
4. Fans used for a stationary engine consumes 5% of engine power.
5. Such engines are suitable only for low horsepower engines.
Water cooling (Indirect cooling)
: are provided
xposed to the
) over the fins
ie (i.e.
without
orward motion
Water cooling (Indirect cooling)
97
In this system, water is circulated around the cylinder and cylinder heat to carry away the heat.
The water passes through a passage called "water jacket" There are two methods of water
cooling;
gets circulated a
transfer of heat fr
Since water is cir
go below 75°C,
barrel. A tempers
the temperature 1
thermostat valve «
(i) Natural circulation of water.
(ii) Forced circulation of water.
Cooling by Natural Circulation of Water
The cooling by natural circulation of water is also known thermo-syphon cooling. The principle
that water becomes less dense ' heating is the basis of this method of cooling. The radiator is
connection the water jacket at the top and bottom ends. As the water gets heat moves up and
travels through the radiator. There it gets cooled by the radiator fins and travels downwards.
The word radiator is not the correct word. In a radiator the transfer of heat from coolant to the air
is by conduction and forced convention and not radiation.
A drain tap is pro'
Application
This system is use
buses, lorries, cars
Advantages of w:
A drain tap is provided for removing water periodically.
1.
2.
3.
4.
5.
Forced Circulation of Water
Cooling is
Uniform c(
Water cool
Chances of
Engine tern
Disadvantages of
1. More weigl
2. Requires m
of water is.
3. In cold we,
heat the rad
4. Watercircu
5. Water caus
hence great.
Cooling by Forced Circulation of Water
This system has a centrifugal water pump. The water pump gets the power from the rotating
engine crankshaft.
The water pump draws cold water from the radiator. This cold water is forced into the water
jackets of the cylinder. The rate of circulation of water is increased. Thus the engine parts are
cooled efficiently. After circulating, the hot water enters the radiator top. The hot water in the
radiator flows from top to bottom. The heat from the water is cooled by the radiator fins and it
98
3.5
Lubricatin:
Requirement of lu
opposite friction f
rubbing the surface
ry the heat.
ls of water
gets circulated again by the water pump. A fan also be provided near the radiator for rapid
transfer of heat from radiator to the outside air.
Since water is circulated by a pump it may become very cold. The water temperature should not
go below 75°C, since this will cause corrosion and acid formation which attack the cylinder
barrel. A temperature controller thermostat valve is provided to control the cooling of water. If
the temperature below 75°C, water is bypassed and when temperature reaches 75°C or above
thermostat valve opens, hot water flows through the radiator and get cooled.
A drain tap is provided at the bottom of the radiator for the removal of water periodically.
he principle
e radiator is
rves up and
wards.
Application
This system is used in light and heavy duty vehicles. It is general in automobile engines such as
buses, lorries, cars and trucks.
tnt to the air
Advantages of water cooling
1.
2.
3.
4.
S.
Cooling is more efficient, thus engine efficiency is more.
Uniform cooling is obtained.
Water cooled engines can be installed anywhere.
Chances of engine overheating are greatly reduced.
Engine temperature can be controlled.
Disadvantages of water-cooling
I
1. More weight, since it uses radiator, pump, fan etc.
2. Requires more maintenance. The engine may have to be stopped even if a small leakage
of water is detected in the radiator.
3. In cold weather, freezing of water causes trouble. An electric heater may be required to
heat the radiator.
4. Water circulating pump consumes more power.
S. Water causes scale formation in the water circulating jacket and corrosion of materials,
hence greater maintenance is required.
the rotating
nto the water
gine parts are
; water in the
.or fins and it
3.5
Lubricating Systems
Requirement of lubrication: When we rub two surfaces the force required has to overcome the
opposite friction force loss between the surfaces. Thus some part of the energy applied in
rubbing the surfaces is lost in the form of friction. To overcome the friction we apply certain
99
fluids between the two surfaces which are known as lubricants. Which minimizes the force
required to rub the surfaces and some part of the energy lost in friction can be saved.
In an automobile, chemical energy of fuel is converted into mechanical energy by the
engine. In engine there are many rubbing surfaces. The friction loss occurs between these
surfaces which lead to loss in mechanical energy. To minimize the friction loss between the
rubbing surfaces we use some lubricating systems. Hence the main purpose of the lubricating
system is to save the mechanical energy. These lubricating systems are designed to provide the
proper lubrication to the rubbing surfaces of the engines.
Functions of lubricating oil: A good lubricating oil should perform the following function.
•
It reduces the friction between the moving parts.
•
•
It cools the piston so it also acts as a cooling medium.
It also prevents the leakage of gas between the piston and cylinder because it makes a
film of lubricant between them.
It also reduces the noise between the rubbing surfaces.
•
Methods of lubrication: Following methods are used for the lubrication of an engine.
•
•
•
Mist lubrication or petrol lubrication system
Wet sump lubrication system
Dry sump lubrication system
The lubrication system is subdivided mainly into three groups.
1. Charge Lubrication System. This is the simplest method of lubrication and does not require
oil-filter and oil pump. In this system, the lubricating oil is pre-mixed with the petrol therefore
the fuel carries the lubricating oil in the cylinder which helps for lubricating the piston and
cylinder. Most of the oil burns with the fuel due to high temperature and burnt oil is carried wi:h
the exhaust gases. The lubricating oil cannot be recovered in this system.
This type of lubrication is generally used for two stroke spark ignition engines of scooter and
motor cycle. The quantity oflubricating oil mixed with the petrol is 3 to 6% of petrol.
The advantages of this system are listed below
I. It does not require separate lubricating system so it is most economical.
2. There is no risk of failure of lubrication system.
3. The lubricating oil supplied is regulated at various loads and speeds by the increased fuel flow.
100
The carbon dep
recover of the oi
Wet Sump Lub
These system er
the different par
the purpose. Tl
system is furtheI
Dry Sump Lubi
In this system, 1
cylinder block. 1
of a pump. The ,
oil cooler. This :
system lies betw.
3.6
Lubricat
Hydrostatic bea
.Hydrostatic film
(parallel) surface
direct contact.
micrometers thic
relative motion
Figure I is a set
systems generall
where relative I
coefficient of fric
I
:s the force
The carbon deposits due to the burning of the oil on the spark plug and on other parts and nonrecover of the oil used are the main disadvantages of this system.
Wet Sump Lubrication System
.ergy by the
tween these
between the
e lubricating
• provide the
These system employees a large capacity oil sump at the base of crank case and oil is passed to
the different parts with the help of pressure pump. The oil returns back to the sump after serving
the purpose. The oil under-pressure is circulated generally through the different parts. This
s-ystemis further subdivided into splash lubrication and pressure lubrication.
Dry Sump Lubrication System
motion.
se it makes a
ne.
In this system, the oil from the sump is carried to a separate storage tank outside the engine
cylinder block. The oil from the sump is pumped through filter into the storage tank with the help
of a pump. The oil from the storage tank is further pumped by the pump to the cylinder through
oil cooler. This is generally used for high capacity engines. The pressure of the oil used in this
system lies between 3 to 8 bar.
3.6
Lubrication of Different Engine Parts
Hydrostatic bearings
oes not require
•etrol therefore
the piston and
is carried wi:h
,Hydrostatic films are created when a high-pressure lubricant is injected between opposing
(parallel) surfaces (pad and runner), thereby separating them and preventing their coming into
direct contact. Hydrostatic bearings require external pressurization. The film is 5-50
micrometers thick, depending on application. Though hydrostatic lubrication does not rely on
relative motion of the surfaces, relative motion is permitted and can even be discontinuous.
Figure I is a schematic of a hydrostatic bearing pad. To handle asymmetric loads, hydrostatic
systems generally employ several evenly spaced pads. Hydrostatic bearings find application
where relative positioning is of extreme importance. They are also applied where a low
coefficient of friction at vanishing relative velocity is required.
of scooter and
:rol.
·easedfuel flow.
101
TYPical
<.&
A typical h)
1. The rotati
2. The supp<
3. Lubricant
0111_
Working p,
Hydrostatic bearings are bearing systems that, because they are extremely precise and almost
wear-free, are used especially in high-precision applications in measuring, testing and machine
tool engineering.
The principle behind them is that an external pressure supply is used to continuously force fluid
lubricant through inlet channels into chambers between the bearing surfaces. That means these
bearing surfaces are always separated by a thin lubricant film which prevents any friction
between the bearing surfaces. This allows highly precise position control in the sub-micrometre
range.
Unlike hydrodynamic plain bearings, they avoid the slip resistance caused by mixed friction
during start-up and rundown, which causes increased bearing wear.
Hydrodynamic
1. Before the
weight as sh.
I
I
j
bearings
Hydrodynamic bearings are self-acting. To create and maintain a load-carrying hydrodynamic
film, it is necessary only that the bearing surfaces move relative to one another and ample
lubricant is available. The surfaces must be inclined to form a clearance space in the shape of a
wedge, which converges in the direction of relative motion. The lubricant film is then created as
the lubricant is dragged into the clearance by the relative motion. This viscous action results in a
pressure build-up within the film (Fig. 2). The fact that hydrodynamic bearings are selfgenerating and do not rely on auxiliary equipment makes these bearings very reliable.
Hydrodynamic journal bearings and thrust bearings are designed to support radial and axial
loads, respectively, on a rotating shaft.
Hydrodynamic Lubrication is formed basically due to the dynamic motion or action of the
moving parts:-when sufficient quantity of lubricant is present between two surfaces in which at
least one surface tends to move, the relative velocity of the moving surfaces tends to pump the
lubricant between the two surfaces separating the two surfaces by a dynamic film of the
lubricant.
102
2. As the sha
squeezed and
slight amount
Typical Construction Features of a Hydrodynamic Bearing:
A typical hydrodynamic bearing consists of the following:
1. The rotating member - Shaft
2. The supporting bearing - Outer Sleeve
3. Lubricant
Working Principle of the Hydrodynamic Bearing:
and almost
nd machine
force fluid
means these
any friction
-micrometre
1. Before the shaft starts to rotate, both the shaft and the outer sleeve are in contact due to the self
weight as shown below in the figure.
I
xed friction
zdrodynamic
r and ample
Ie shape ofa
en created as
n results in a
.gs are se1fery reliable .
.al and axial
l
I
2. As the shaft tends to rotate the lubricant in between the shaft and the sleeve tends to get
squeezed and ultimately forms a boundary lubrication between the shaft and the sleeve but still a
slight amount of metal to metal contact remains as shown in the figure.
iction of the
s in which at
to pump the
film of the
103
ApplicatiQ.!
Though all (
harsh and w,
, The crank
Hydrodynan
. All hand Of-
ROiling cent
3. As the shaft tends to rotate rapidly, the fluid lubricant is pumped by the shaft. The lubricant
surface near to the shaft has a velocity which differs from the surface velocity of the oil surface
near the sleeve. This causes a pumping action in the lubricant. The pumped lubricant ensures that
the shaft and the sleeve remain out of contact. (bis phenomenon of the lubricant pumping itself
to keeps two mating surfaces out of contact is iermed as "Hydrodynamic Lubrication" and hence
the name "Hydrodynamic Bearing". See Figure below.
Journal and
conform in
counterforma
the contact ar
gigapascals (
lubricant vise.
Lubrication (
ring or chain t:
Prlndple of Hydrodynamic Beam,
This principle ensures that the shaft remains out of contact when required i.e. during running.
But a problem exists in this, the oil in between the sleeve and shaft tends to flow towards you or
away from you when you look at. Hence a copious-continuous flow of the lubrication has to be
ensured for the bearing to work perfectly compensating the lubricant loss.
104
Fig.5
The oil ring rest
away as shown
as shown in the
bath to the bear
Applications
of Hydrodynamic
Bearings:
.
Though all of us may not have seen the hydrodynamic bearing, they tend to work in some of the
harsh and well know environments to us.
. The crankshaft and camshaft bearings in an automobile engine is a very good example for
Hydrodynamic bearing
. All hand operated lube oil pumps do posses hydrodynamic bearings.
Rolling contact bearings
lubricant
il surface
sures that
ring itself
md hence
Journal and thrust bearings are conformal bearings; that is, the opposing bearing surfaces
conform in shape. Ball and roller bearings, also known as rolling contact bearings, are
counterformal. Counterformal bearings always operate in the hydrodynamic mode, but because
the contact area in these bearings is small the pressure attains high values, in the range of 1-3
gigapascals (10,000-30,000 atm). In consequence, the surfaces deform elastically and the
lubricant viscosity increases by several orders of magnitude.
Lubrication of Main Bearings: The main bearings are lubricated satisfactorily with the help of
ring or chain type feeder. The arrangement of the system is shown in Fig. 5.
oil riltf
19 running.
irds you or
n has to be
Fig.S
The oil ring rests on the main shaft where a small portion of the main bearing shell has been cutaway as shown in the figure. The lower end of the oil ring is allowed to submerge in the oil bath
as shown in the figure. The oil ring rotates with the main shaft and carries the oil from the oil
bath to the bearing and it is distributed to the bearing through the oil groove. The surplus oil
lOS
flows to the ends of the bearing and drops back into the oil bath. Chains or more rings are
provided instead of single ring to carry more oil.
This type of lubrication is more useful for medium speed engines because at high speeds, the oil
will be thrown off due to centrifugal force and at low speeds, the amount of oil carried is not
sufficient.
3.7
I.C. Engine and its Components
For a four-stroke engine, key parts of the engine include the crankshaft (purple), connecting rod
(orange), one or more camshafts (red and blue), and valves. For a two-stroke engine, there may
simply be an exhaust outlet and fuel inlet instead of a valve system. In both types of engines
there are one or more cylinders (grey and green), and for each cylinder there is a spark plug
(darker-grey, gasoline engines only), a piston (yellow), and a crankpin (purple). A single sweep
of the cylinder by the piston in an upward or downward motion is known as a stroke. The
downward stroke that occurs directly after the air-fuel mix passes from the carburetor or fuel
injector to the cylinder (where it is ignited) is also known as a power stroke.
All four-:
air into tl
uncOvere,
Piston en,
In piston
air and '01
and the or
opened b)
Contro/VG
Continuou
close to ac
flow to COl
Exhaust
s:
Exhaust rna
Internal cor
gas from th
chemical an
frequently ti
have system
as heat-sensi
A Wankel engine has a triangular rotor that orbits in an epitrochoidal (figure 8 shape) chamber
around an eccentric shaft. The four phases of operation (intake, compression, power, and
exhaust) take place in what is effectively a moving, variable-volume chamber.
Valves
106
For jet prop
velocity noz
the engine it:
~ rings are
eds, the oil
Tied is not
necting rod
,there may
of engines
spark plug
ngle sweep
stroke. The
etor or fuel
All four-stroke internal combustion engines employ valves to control the admittance of fuel and
air into the combustion chamber, Two-stroke engines use ports in the cylinder bore, covered and
uncovered by the piston, though there have been variations such as exhaust valves.
Piston engine valves
In piston engines, the valves are grouped into 'inlet valves' which admit the entrance of fuel and
air and 'outlet valves' which allow the exhaust gases to escape. Each valve opens once per cycle
and the ones that are subject to extreme accelerations are held closed by springs that are typically
opened by rods running on a camshaft rotating with the engines' crankshaft.
Control valves
Continuous combustion engines-as well as piston engines-usually have valves that open and
close to admit the fuel and/or air at the startup and shutdown. Some valves feather to adjust the
flow to control power or engine speed as well.
Exhaust systems
Exhaust manifold with ceramic plasma-sprayed system
Internal combustion engines have to effectively manage the exhaust of the cooled combustion
gas from the engine. The exhaust system frequently contains devices to control pollution, both
chemical and noise pollution. In addition, for cyclic combustion engines the exhaust system is
frequently tuned to improve emptying of the combustion chamber. The majority of exhausts also
have systems to prevent heat from reaching places which would encounter damage from it such
as heat-sensitive components, often referred to as Exhaust Heat Management.
>e) chamber
power, and
For jet propulsion internal combustion engines, the 'exhaust system' takes the form of a high
velocity nozzle, which generates thrust for the engine and forms a colimated jet of gas that gives
the engine its name.
107
Cooling systems
Combustion generates a great deal of heat, and some of these transfers to the walls of the engine.
Failure will occur if the body of the engine is allowed to reach too high a temperature; either the
engine will physically fail., or any lubricants used will degrade to the point that they no longer
protect the engine. The1ubricants must be clean as dirty lubricants may lead to over formation of
sludge in the engines.
Cooling systems usually employ air (air cooled) or liquid (usually water) cooling while some
very hot engines using radioactive cooling (especially some Rocket engines). Some high altitude
rocket engines use ablative cooling where the walls gradually erode in a controlled fashion.
Rockets in particular can use regenerative cooling which uses the fuel to cool the solid parts of
the engine.
A cranks]
Most.reci
motion ol
Flywheel
valve by covering and uncovering ports in the cylinder wall.
The flywi
rotational
over from
reciprocat
most auto
flywheel a
at idle. Th
out of bal.
be balance
torque can
Propelling nozzle
Starter sy:
Piston
A piston is a component of reciprocating engines. It is located in a cylinder and is made gas-tight
by piston rings. Its purpose is to transfer force from expanding gas in the cylinder to the
crankshaft via a piston rod and/or connecting rod. In two-stroke engines the piston also acts as a
For jet engine forms of internal combustion engines, a propelling nozzle is present. This takes
the high temperature, high pressure exhaust and expands and cools it. The exhaust leaves the
nozzle going at much higher speed and provides thrust, as well as constricting the flow from the
engine and raising the pressure in the rest of the engine, giving greater thrust for the exhaust
mass that exits.
Crankshaft
All interna
piston engi
systems. r,
to one of th
the ground
cords. Mot(
electric-stat
compressed
assistance f
starting ref€
rolling dow:
Heat shield
lOB
e engine.
either the
10 longer
nation of
rile some
h. altitude
fashion.
1 parts of
gas-tight
er to the
, acts as a
A crankshaft for a 4-cylinder engine
Most-reciprocating internal combustion engines end up turning a shaft. This means that the linear
motion of a piston must be converted into rotation. This is typically achieved by a crankshaft.
Flywheels
The flywheel is a disk or wheel attached to the crank, forming an inertial mass that stores
rotational energy. In engines with only a single cylinder the flywheel is essential to carry energy
over from the power stroke into a subsequent compression stroke. Flywheels are present in most
reciprocating engines to smooth out the power delivery over each rotation of the crank and in
most automotive engines also mount a gear ring for a starter. The rotational inertia of the
flywheel also allows a much slower minimum unloaded speed and also improves the smoothness
at idle. The flywheel may also perform a part of the balancing of the system and so by itself be
out of balance, although most engines will use a neutral balance for the flywheel, enabling it to
be balanced in a separate operation. The flywheel is also used as a mounting for the clutch or a
torque converter in most automotive applications.
Starter systems
'his takes
eaves the
. from the
e exhaust
All internal combustion engines require some form of system to get them into operation. Most
piston engines use a starter motor powered by the same battery as runs the rest of the electric
systems. Large jet engines and gas turbines are started with a compressed air motor that is geared
to one of the engine's drive shafts. Compressed air can be supplied from another engine, a unit on
the ground or by the aircraft's APU. Small internal combustion engines are often started by pull
cords. Motorcycles of all sizes were traditionally kick-started, though all but the smallest are now
electric-start. Large stationary and marine engines may be started by the timed injection of
compressed air into the cylinders - or occasionally with cartridges. Jump starting refers to
assistance from another battery (typically when the fitted battery is discharged), while bump
starting refers to an alternative method of starting by the application of some external force, e.g.
rolling down a hill.
Heat shielding systems
109
Control s~
Most engi
parameters
to stabilise
ignition. s,
Internal COl
function to
below. It is
Flexible ceramic heat shield commonly used on high-performance automobiles
These systems often work in combination with engine cooling and exhaust systems. Heat
shielding is necessary to prevent engine heat from damaging heat-sensitive components. The
majority of older cars use simple steel heat shielding to reduce thermal radiation and convection.
It is now most common for modem cars are to use aluminium heat shielding which has a lower
density, can be easily formed and does not corrode in the same way as steel. Higher performance
vehicles are beginning to use ceramic heat shielding as this can withstand far higher temperatures
as well as further reductions in heat transfer.
Lubrication
01) CylindE
Function- I
piston and e:
02) Cylindei
Function-It
burnt gases a
of diesel engi
03) Piston
systems
Internal combustions engines require lubrication in operation that moving parts slide smoothly
over each other. Insufficient lubrication subjects the parts of the engine to metal-to-metal
contact, friction, heat build-up, rapid wear often culminating in parts becoming friction welded
together e.g. pistons in their cylinders. Big end bearings seizing up will sometimes lead to a
connecting rod breaking and poking out through the crankcase.
1. Several different types of lubrication systems are used. Simple two-stroke engines are
lubricated by oil mixed into the fuel or injected into the induction stream as a spray. Early
slow-speed stationary and marine engines were lubricated by gravity from small
chambers similar to those used on steam engines at the time - with an engine tender
refilling these as needed. As engines were adapted for automotive and aircraft use, the
need for a high power-to-weight ratio led to increased speeds, higher temperatures, and
greater pressure on bearings which in turn required pressure-lubrication for crank
bearings and co necting-rod journals. This was provided either by a direct lubrication
from a pump, or indirectly by a jet of oil directed 'at pickup cups on the connecting rod
.ends which had the advantage. of providing higher pressures as the engine speed
increased.
110
Function-Dll
and compress
power from tJ
out of the cyli
04) Piston RiJ
Function-It pr
the piston. Oil
ring prevents tJ
OS)Connectin:
Function-It ch
way connecting
06) Gudgeon P
Control systems
Most engines require one or more systems to start and shutdown the engine and to control
parameters such as the power, speed, torque, pollution, combustion temperature, efficiency and
to stabilise the engine from modes of operation that may induce self-damage such as preignition. Such systems may be referred to as engine control units.
Internal combustion engines are made from various parts. Each part has its own location and
function for proper working of engine. Some important parts and its ..function is as described
below. It is most essential to know right information from engineering person.
01) Cylinder Block
:ms. Heat
ients. The
onvection.
is a lower
rformance
nperatures
Function- In the bore of cylinder the fresh charge of air-fuel mixture is ignited, compressed by
piston and expanded to give power to piston.
02) Cylinder Head
Function-It carries inlet and exhaust valve. Fresh charge is admitted through inlet valve and
burnt gases are exhausted from exhaust valve. In case of petrol engine, a spark plug and in case
of diesel engine, a injector is also mounted on cylinder head.
03) Piston
: smoothly
il-to-metal
on welded
: lead to a
Function-During suction stroke, it sucks the fresh charge of air-fuel mixture through inlet valve
and compresses during the compression stroke inside the cylinder. This way piston receives
power from the expanding gases after ignition in cylinder. Also forces the burnt exhaust gases
out of the cylinder through exhaust valve.
04) Piston Rings
nginesare
oray, Early
rom small
~ne tender
rft use, the
atures, and
for crank
lubrication
iecting rod
gine speed
Function-It prevents the compressed charge of fuel-air mixture from leaking to the other side of
the piston. Oil rings, is used for removing lubricating oil from the cylinder after lubrication. This
ring prevents the excess oil to mix with charge.
OS) Connecting Rod
Function-It changes the reciprocating motion of piston into rotary motion at crankshaft. This
way connecting rod transmits the power produced at piston to crankshaft.
06) Gudgeon Pin
111
Function- Connects the piston with small end of connecting rod.
07) Crank Pin
Function- hand over the power and motion to the crank shaft which come from piston through
connecting rod.
08) Crank Shaft
Principle
Function-Receives oscillating motion from connecting rod and gives a rotary motion to the main
shaft. It also drives the camshaft which actuate the valves of the engine.
The princip
that there w
is used for'
trucks use a
09) Cam Shaft
Function-It takes driving force from crankshaft through gear train or chain and operates the inlet
valve as well as exhaust valve with the help of cam followers, push rod and rocker arms.
0:
In every Ot
adiabatic ex
engine is as
10) Inlet Valve & Exhaust Valve
Function-Inlet valve allow the fresh charge of air-fuel mixture to enter the cylinder bore. Exhaust
valve permits the burnt gases to escape from the cylinder bore at proper timing.
11) Governor
Function-It controls the speed of engine at a different load by regulating fuel supply in diesel
engine. In petrol engine, supplying the mixture of air-petrol and controlling the speed at various
load condition.
12) Carburettor
Function-It converts petrol in fine spray and mixes with air in proper ratio as per requirement of
the engine.
13) Fuel Pump
Function-This
device supply the petrol to the carburettor sucking from the fuel tank.
Working of
14) Spa! k Plug
Function-This
device
ed in petrol engine only and ignite the charge of fuel for combustion.
15) Fuel Injector
112
A stroke is
Function-This
device is used in diesel engine only and delivers fuel in fine spray under pressure.
iston through
Principle of a Four Stroke Petrol Engine
the main
The principle used in a four stroke petrol engine is commonly known as Otto Cycle. It states
that there would be one power stroke for every four strokes. Such engines use a spark plug which
is used for the ignition of the combustible fuel used in the engine. Most of the cars, bikes and
trucks use a 4 stroke engines.
rates the inlet
In every Otto cycle there is an adiabatic compression, addition of heat at constant volume, an
adiabatic expansion and the release of heat at constant volume. The P-V diagram for a 4 stroke
engine is as follows:
m to
irms.
: bore.Exhaust
II'
P
IPply in diesel
leed at various
requirement of
' ..
.... 1ataJa
-EDaut -co.,._rie ...
-,
J\
'v
ik,
Working of a Four Stroke Petrol Engine
r combustion.
A stroke is the movement of the piston from the top, to the bottom of the cylinder.
113
As the name suggest the Four Stroke Petrol Engine uses a cycle of four strokes and petrol as
the fuel. Each cycle includes 2 rotations of the crankshaft and four strokes, namely:
I. An Intake Stroke
2. A Compression Stroke
3. A Combustion Stroke also called Power Stroke
4. An Exhaust Stroh
The steps involved are as follows:
1. Intake Stroke: As the name suggests in this stroke the intake of fuel takes place. When the
engine starts, the piston descends to the cylinder's bottom from the top. Thus the pressure inside
the cylinder reduces. Now the intake valve opens and the fuel and air mixture enters the cylinder.
The valve then closes.
3. Combustion
to the maximu
combustion lea
piston is drivel
crankshaft rotan
2. Compression Stroke:
This stroke is known as compression stroke because the compression of the fuel mixture takes
place at this stage. When the intake valve closes (exhaust valve is already closed), the piston
forced back to the top of the cylinder and the fuel mixture gets compressed. The compression is
around 1I8thof the original volume. An engine is considered more efficient if its compression
.
ratio is higher.
4. Exhaust Strok
114
ad petrol as
Compression Stroke
Cylinder
e. When the
essure inside
the cylinder.
3. CombustionIPower Stroke: Now in case of petrol engine when the fuel mixture compresses
to the maximum value the spark plug produces spark which ignites the fuel mixture. The
combustion leads to the production of high pressure gases. Due to this tremendous force the
piston is driven back to the bottom of the cylinder. As the piston moves downwards, the
crankshaft rotates which rotates the wheels of the vehicle.
Power Stroke
ixture takes
.the piston
mpression is
:ompression
Piston Rod ----
4. Exhaust Stroke:
115
As the wheel moves to the bottom the exhaust valve opens up and due to the momentum gained
by the wheel the piston is pushed back to the top of the cylinder. The gases due to combustion
are hence expelled out of the cylinder into the atmosphere through the exhaust valve.
The induction
cylinder. As th
combustion an
engines use tur
Exhaust Stroke
Outlet Burnt GotSes
The exhaust valve closes after the exhaust stroke and again the intake valve opens and the four
strokes are repeated.
3.8
Working of 4-Stroke Cycle Diesel Engine
A diesel engine (also known as a compression-ignition engine) is an internal combustion
engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected
into the combustion chamber. This is in contrast t~ spark ..ignition engines such as a petrol eng.ne
(gasoline engine) or gas engine (using a gaseous fuel as opposed to gasoline), which uses a spark
plug to ignite an air-fuel mixture. The engine was developed by Rudolf Diesel in 1893.
The diesel engine has the highest then s
-1iclency of any regular internal or external
combustion engine due to its very higl, compression ratio. Low-speed Diesel engines (as used in
ships and other applications where overall engine weight is relatively unimportant) often have a
thermal efficiency which exceeds 50 percent
The Diesel cycle is the cycle used
heat is transferred to the working
injection and burning of the fuel in
consists of induction, compression,
in the Diesel (compression-ignition) engine. In this cycle the
fluid at constant pressure. The process corresponds to the
the actual engine. The cycle in an internal combustion engine
power and exhaust strokes.
Induction Stroke
116
Compression S
The compressic
cylinder bore b~
ientum gained
to combustion
e.
lS
The induction stroke in a Diesel engine is used to draw in a new volume of charge air into the
cylinder. As the power generated in an engine is dependent on the quantity of fuel burnt during
combustion and that in tum is determined by the volume of air (oxygen) present, most diesel
engines use turbochargers to force air into the cylinder during the induction stroke.
and the four
al combustion
rich is injected
a petrol eng.ne
ch uses a spark
893.
Compression Stroke
The compression stroke begins as the inlet valve closes and the.piston is driven upwards in the
cylinder bore by the momentum of the crankshaft and flywheel.
al or external
nes (as used in
t) often have a
1 this
cycle the
esponds to the
ibustion engine
117
The purpose of the compression stroke in a Diesel engine is to raise the temperature of the
charge air to the point where fuel injected into the cylinder spontaneously ignites. In this cycle,
the separation of fuel from the charge air eliminates problems with auto-ignition and therefore
allows Diesel engines to operate at much higher compression ratios than those currently in
production with the Otto Cycle.
Compression
Ignition
Compression ignition takes place when the fuel from the high pressure
spontaneously ignites in the cylinder.
fuel injector
In the theoreti
(ignition lag) i
to ensure that
ignition advan
Power Stroke
The power stn
the rapidly bu
pressure whie
converted int(
momentum to
the work of co
alternator, fuel
118
rature of the
In this cycle,
md therefore
currently in
In the theoretical cycle, fuel is injected at IDe, but as there is a finite time for the fuel to ignite
(ignition lag) in practical engines, fuel is injected untothe cylinder before the piston reaches TDC
to ensure that maximum power can be achieved, This is synonymous with automatic spark
ignition advance used in Otto cycle engines.
fuel injector
Power Stroke
The power stroke begins as the injected fuel spontaneously ignites with the air in the cylinder. As
the rapidly burning mixture attempts to expan4.. within the cylinder walls, it generates a high.
pressure which forces the piston down the cyl'inlderbore. The linear motion of the piston is
converted into rotary motion through the crankshaft. The rotational energy is imparted as
momentum to the flywheel which not only provides power for the end use, but also overcomes
the work of compression and mechanical losses incurred in the cycle (valve opening and closing,
alternator, fuel injector pump, water pump, etc.).
I
I
j
119
Exhaust Stroke
Exhaust and 1
The exhaust stroke is as critical to the smooth and efficient operation of the engine as that of
induction. As the name suggests, it's the stroke during which the gases formed during
combustion are ejected from the cylinder. This needs to be as complete a process as possible, as
any n-naining gases displace an equivalent volume of the new charge air and leads to a reduction
in the maximum possible power.
120
Exhaust and iJ
practical nece
constraints im:
manifold, it is
Centre (TDC)
combustion ga
full cycle whe
occurs and the
the speed at wI
Exhaust and Inlet Valve Overlap
as that of
Exhaust and inlet valve overlap is the transition between the exhaust and inlet strokes and is a
practical necessity for the efficient running of any internal combustion engine. Given the
constraints imposed by the operation of mechanical valves and the inertia of the air in the inlet
manifold, it is necessary to begin opening the inlet valve before the piston reaches Top Dead
Centre (TDC) on the exhaust stroke. Likewise, in order to effectively remove all of the
combustion gases, the exhaust valve remains open until after TDC. Thus, there is a point in each
full cycle when both exhaust and inlet valves are open. The number of degrees over which this
occurs and the proportional split across TDC is very much dependent on the engine design and
the speed at which it operates.
led during
sosslble, as
a reduction
I
I
j
121
cylinder. T
by the pistc
2nd stroke
ejection pi
compressec
upward thn
ignited by 1
time the fn
point.
It will be ee
completing
C'tUNDER-
3.'
Two-Stroke Engines
A two-stroke engine is an internal combustion engine that completes the process cycle in one
revolution of the crankshaft (an up stroke and a down stroke of the piston, compared to twice that
number for a four-stroke engine). This is accomplished by using the end of the combustion
stroke and the beginning of the compression stroke to perform simultaneously the intake and
exhaust (or scavenging) functions. In this way, two-stroke engines often provide high specific
power, at least in a narrow range of rotational speeds. The functions of some or all of the valves
required by a four-stroke engine are usually served in a two-stroke engine by ports that are
opened and closed by the motion of the piston(s), greatly reducing the number of moving parts.
Gasoline (spark ignition) versions are particularly useful in lightweight (portable) applications,
such as chainsaws, and the concept is also used in diesel compression ignition engines in large
and weight insensitive applications, such as ships and locomotives.
Working of Two Stroke-petrol
Engine
Mode of operation of the two-stroke engine
1st stroke: The piston is at the bottom of the cylinder. A pipe at the left side is opened and lets
the fuel mix~,
,which is already compressed a bit, flow from the lower to the upper
122
part of
the
Fig.17
(a) The posi
produced by
gases increa
piston uncov
expanded bi
uncovers (op
directly com
charge in: the
this position,
the transfer t:
of fresh char
loss of fresh
BDC positioi
known as "S
During this SI
(8) Power is
gases.
cylinder. The fresh gases expulse now the exhaust through an ejection pipe, which is not closed
by the piston at this moment.
2nd stroke: After being hurried upward, the piston now covers the pipe on the left side and the
ejection pipe. Because there is no way out any more, the upper, fresh gas mixture gets
compressed now. At the same time in the part below fresh gas is taken in by the piston driving
upward through the open suction pipe. At the upper dead-center, the compressed fuel mixture is
ignited by the sparking plug, the piston is pressed downward while he compresses at the same
time the fresh gas below. The process begins again as soon as the piston arrives at its lowest
point.
It will be easier to describe the cycle beginning at the point when the piston has reached to TOC
completing the compression stroke.
(c)
Fig.17
(a) The position of the piston at the end of compression is shown in Fig. 17 (a). The spark is
produced by the spark plug as the piston reaches the TDC. The pressure and temperature of the
gases increase and the gases push the piston downward producing the power stroke. When the
piston uncovers (opens) the exhaust port as shown in Fig. 17 (b) during the downward stroke, the
expanded burnt gases leave the cylinder through the exhaust port. A little later, the piston
uncovers (opens) the transfer ports also as shown in Fig. 17 (c). In this position, the crank-case is
directly connected to the cylinder through port. During the downward stroke of the piston, the
charge in the erank-case is compressed by the underside of the piston to a pressure of 1.4 bar. At
this position, as shown in Fig. 17 (c), the compressed charge (fuel + air) is transferred through
the transfer port to the upper part of the cylinder. The exhaust gases are swept out with the help
of fresh charge. The piston crown shape helps in this sweeping action as well as it prevents the
loss of fresh charge carried with the exhaust gases. This is continued until the piston reaches
BDC position. This action of sweeping out the exhaust gases with the help of fresh charge is
known as "Scavenging" The scavenging helps to remove the burnt gases from the eylindc.
During this stroke of the piston (downward stroke) the following processes are completed.
(8) Power is developed by the downward movement of the piston caused by the high pressure
gases.
cycle in one
to twice that
combustion
e intake and
ugh specific
)f the valves
orts that are
loving parts.
applications,
ines in large
ned and lets
t" part
of the
I
..l
123
(b) The exhaust gases are removed completely from the cylinder by scavenging.
(c) The charge is compressed in the crank-case with the help of underside of the piston.
As the piston moves upward, it covers the transfer ports stopping the flow of fresh-charge into
the cylinder. A little later, the piston covers the exhaust ports and actual compression of the
charge begins. This position of the piston is shown in Fig. 17. The upward motion of the piston
during this stroke lowers the pressure in the crank case below atmosphere, therefore, a fresh
charge is induced in the crank case through the inlet ports as they are uncovered by the piston.
From:
-- The com
extent,
ai
-- Each tirr.
through the
3.10
The compression of the charge is continued until the piston reaches to its original position (TDC)
and the cycle is completed as shown in Fig. 17 (a).
In this unit
-
GO\
-
Coc
Lub
Lub
-
Lub
-
I.C.
-
WOl
-
Twc
3.11
Cht
In this stroke of the piston, the following processes are completed.
1. Partly scavenging takes place as the piston moves from BDC to the position shown in Fig.17
(c).
2. The fresh charge is sucked in the crankcase through the carburetor.
3. Compression of the charge is completed as the piston moves from the position shown in Fig.
17 (c) to TDC as shown in Fig, 17 (a).
The cycle of the engine is completed within two strokes of the piston.
Advantages of 2 Stroke Engines:
-Two-stroke
-Two-stroke
revolution).
-Two-stroke
engines do not have valves, simplifying their construction.
engines fire once every revolution (four-stroke engines fire once every other
This gives two-stroke engines a 1-·'.gmficantpower boost
engines are ligh ..cr, and cost less to manufacture.
- Two-stroke engines have the potential for about twice the power in the same size because there
are twice as many power strokes per revolution.
Disadvantages
of 2 Stroke Engines:
- Two-stroke engines don't live as long as four-stroke
lubrication system means that the parts of a two-stroke
engines require a mix of oil in with the gas to lubricate
cylinder
- Two-stroke oil can be expensive. Mixing ratio is about
about a gallon of oil every 1,000 miles.
engines. The lack of a dedicated
engine wear-out faster. Two-stroke
the crankshaft, connecting rod and
walls.
4 ounces per gallon of gas: burning
- Two-stroke engines do not use fuel ·~fficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution.
124
Let
1. Exp
2. Hov
3. Exp
4. Wh;
5. Wh;
3.12
Rd
~ "Inti
~ "Co
2011
~ Lase
Ace
~ "1m]
i.
charge into
sion of the
f the piston
Ire, a fresh
:piston.
From:
-- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some
extent, and a badly worn two-stroke engine can emit more oily smoke.
-- Each time a new tT'!X of air/fuel is loaded into the combustion chamber, part of it leaks out
through the exhaust port.
3.10
Let us Sum Up
tion (TDC)
n in Fig.17
rwn in Fig.
- Governing of I.C. Engines
- Cooling ofH.C. Engines
- Lubrication of I.C. Engines
- Lubricating Systems
- Lubrication of Different Engine Parts
- I.C. Engine and its Components
- Working of 4-Stroke Cycle Diesel Engine
- Two-Stroke Engines
3.11
very other
cause there
1.
2.
3.
4.
5.
3.12
Explain the process of governing of I.C. Engines
How is lubrication important to IC Engines?
Explain lubrication of different engine parts.
What is I.C. Engine and explain the different components in detail.
What is working procedure of 4-Stroke Cycle Diesel Engine?
Reference
"Internal combustion engine". Answers.com. 2009-05-09. Retrieved 2010-08-28.
"Columbia encyclopedia: Internal combustion engine". Inventors.about.com. Retrieved
2010-08-28.
~ Laser sparks revolution in internal combustion engines Physorg.com, April 20, 2011.
Accessed April 2011
~ "Improving IC Engine Efficiency". Courses.washington.edu. Retrieved 2010-08-28.
~
~
dedicated
.wo-stroke
fg rod and
walls.
s: burning
125
Hydraulic Dynamometer.
ferromagnetic (
counterwci ght.
eddy currents u
The B.P. of an engine coupled to the dynamometer is - given by
the electromagr
:I
In the hydraulic dynamometer, as the arm length (R) is fixed, the factor (2nR/(60 x 1000)1 is
constant and its value is generally given on the name plate of the dynamometer by the
manufacturer and is known as brake or dynamometer constant. Then the B.P. measured by the
dynamometer is given by
B.P.:It
w;
where W
4.4
Morse Test is a
is noted. Then
other cylinders
original value.
Thus for eacl
... (23.5)
= Weight
measured on the dynamometer, N
K = Dynamometer constant
(
60 x 1000 )
2'JtR
Let,
BP = Brake Po
BPI = Brake P
BP2 = Brake F
BP3 = Brake F
IP = Indicated
IPI = Indicate,
and N = R.P.M. of the engine.
The arm length 'R' is selected in such a way that K is a whole number.
These dynamometers are directly coupled with the engine shaft,
Electric Dynamometer.
A device that is designed to measure the torque of electric motors. Electric dynamometers are
used in bench tests of motors to determine mechanical or electromechanical characteristics of the
motors. Such a dynamometer is an electric machine that operates as a generator and is
mechanically coupled to a motor to be tested. DC generators are most often used as electric
dynamometers.
!
I
j
1
(
The torque developed by an electric motor is given by the equation
Uxl
TmolQf
= 9.6'l}xn
MeasUl
f
t-
newton-meters
I
IP2 = Indicate
IP3 = Indicate
FPI, FP2, FP:
When,
All cylinders i
IP = (IPI + IP
BP = (IPI + IJ
First Cylinder
BPI = (IP2 +
Where, ( FP 1speed.
Subtracting E
Indicated Pov
IPI = (BP - E
Similarly,
Indicated Poi
IP:: -= (BP - .E
Indicated POl
where U is the voltage across the generator terminals in volts, lis the current in the field winding
in amperes, n is the rotation rate in rpm, and 11is the efficiency of the generator. The torque is
varied by adjusting the load resistance and the current in the generator's field winding.
Electric dynamometers are used to test high-power traction machines. The torque of low-power
electric i:notors is sometimes measured by means of a very simple dynamometer that consists of a
130
_At1
ferromagnetic disk mounted on the shaft of an electric motor and a DC electromagnet with a
counterweight. As the disk rotates, a braking torque is produced as a result of the interaction of
ven by
eddy currents in the disk and the magnetic field of the electromagnet. The angle through which
}pe electromagnet and counterweight swing is proportional to the torque to be measured.
x 1000)1 is
eter by the
ured by the
4.4
Measurement
of I.P. of Multi-Cylinder
Engine (Morse Test)
Morse Test is applicable to multi-cylinder engines. The engine is run at desired speed and output
is noted. Then one of the cylinders is cut out by short circuiting spark plug. Under this condition
other cylinders "motor" this cut cylinder. The output is measured by keeping speed constant to
original value. The difference in output is measure of the indicated power of cut-out cylinder.
Thus for each cylinder indicated power is obtained to find out total indicated power.
Let,
BP = Brake Power when all cylinders are in working condition.
BPI = Brake Power when first cylinder cut-off.
BP2 ~ Brake Power when second cylinder cut-off.
BP3 = Brake Power when third cylinder cut-off.
imeters are
sties of the
tor and is
as electric
IP = Indicated Power of Engine
IP 1 = Indicated Power of first cylinder
IP2 = Indicated Power of second cylinder
IP3 = Indicated Power of third cylinder
FPI, FP2, FP3 = Friction power of each cylinder
t
I(
(
I
d winding
! torque is
ow-power
nsists of a
When,
All cylinders in working condition,
IP = (IPI + IP2 + IP3)
BP = (IPI + IP2 + IP3) - (FP1+ FP2
+ FP3 )
,
(i)
(ii)
First Cylinder Cut-off,
BPI = (IP~ + IP3) - (FPI+ FP2 + FP3)
(iii)
Where, ( FPI + FP2 + FP3 ) in above both eqs.{ii)&{iii) remains almost constant at constant
speed.
Subtracting Eq.{iii) from Eq.{ii), We get,
Indicated Power of first cylinder,
IPI = (BP - BPI)
{iv)
Similarly,
Indicated Power of second cylinder
IPi = (BP - BP2)
{v)
Indicated Power of third cylinder
131
IP3 = (BP - BP3)
0
Putting the values of IPI, IP2, IP3-, in eq.(i),we get,
(vi)
4.6
IP = (BP - BPI) + (BP - BP2) + (BP - BP3)
Frictional Power,
(vii)
. FP = ( IP - BP )
Mechanical Efficiency,
(viii)
?m = (BP / IP)
(ix)
Heat Bal
A heat balance
system .
Necessary infor
balance.
The heat balanc
4.5
Measurement
of Fuel Consumption
The heat supplie
The arrangement for measuring the fuel supplied to the engine is shown in Fig. 3 (a).
Qs=m/XC.V.
Where mr is the
of the fuel.
BURETTE
-
The various wa
fUEl TANK r-
....._-_
-... _-..-_
-----.,--... ----_
---
""'---------__._.- ... _-----_-----------._
-----... -----------_---..---------------------------------------------------_.'----------------... ----_.--_.
:~-:-:-:-:-:-::-:~
(a) Heat equiva
~:~::~~:§~*:~~-..LX.~;t
~----------- VALVE'
TO ENGINE:
(a)
TO ENGINE
(b)
Two glass vessels of 100 C.C. and 200 C.C. capacity are connected in between the engine and
main fuel tank through two, three-way cocks. When one is supplying the fuel to the engine, the
other is being filled. The time for the consumption of 100 or 200 C.C. fuel is measured with the
help of stop watch.
Another simple arrangement for measuring the fuel consumption rate is shown in Fig. 3 (b).
A small glass tube is attached to the main fuel tank as shown in figure. When fuel rate is to be
measured, the valve is closed so that fuel is consumed from the burette. The time for a known
value of fuel consumption can be measured and fuel consumption rate can be calculated.
•.
..
Xt:c
X
= Cpwx mw, (T,
Where mw is t
jacket and
(T wo-Twi) is the
and Cpwis the ~
Fig. 3: Fuel consumption measurement
Fuel consumption kg/hr!!:
(b) Heat carried
Sp. gravity of fuel
1000 x r
(c) Heat carrie
= mg Cpg (TgeWhere mg is tb
the methods al
T ge = Tempera
Tin = Ambient
Cpg
= Sp. heat
(d) A pan ofb
of the power (
132
4.6
(vii)
Heat Balance Sheet
A heat balance sheet is an (lccount of heat supplied and heat utilized in various ways in the
system.
viii)
Necessary information concerning the performance of the engine is obtained from the heat
balance.
(ix)
The heat balance is generally done on second basis or minute basis or hour basis.
The heat supplied to the engine is only in the form of fuel-heat and that is given by
1).
Where mr is the mass of fuel supplied per minute or per sec and C.V. is the lower calorific value
of the fuel.
The various ways in which heat is used up in the system is given by
(a) Heat equivalent ofB.P.
= kW = kJ/sec. = 60 kJ/min.
(b) Heat carried away by cooling water
= Cpwx mw,(Two-T,",.;)kJ/min.
Where mw is the mass of cooling water in kg/min. or kg/sec., circulated through the cooling
jacket and
(Two-Twi) is the rise in temperature of the water passing through the cooling jacket of the engine,
and Cpwis the specific heat of water ih kJ/kg-K.
he engine and
he engine, the
sured with the
(c) Heat carried away by exhaust gases
= mg Cpg(Tge- Ta) (kJ/min.) or (kJ/sec)
Where mg is the mass of exhaust gases m kg/min. or kg/sec., and it is calculated by using one or
the methods already explained.
ig. 3 (b).
I rate is to be
: for a known
ated.
Tge= Temperature of burnt gases comIng out of the engine.
Ttl> = Ambient Temperature.
Cpg= Sp. heat of exhaust gases in (kJ/kg-K).
(d) A pan of heat is lost by convection and radiation as well as due to the leakage of gases. Part
of the power developed inside the engine is also used, ':0 run the accessories as lubricating pump,
133
i .
-_ Entre
BLOCK-3
UNIT 1- PART I
-
Enth,
-
Deter
STEAM PROCESS
1.2
Intro
Structure
Process and
1.1.
Introduction
Whenever or
1.2.
Objectives
path of the s
1.3.
Heating-Cooling and Expansion of Vapours
system in a ~
1.4.
Internal Latent Heat
and finally
1.5.
Internal Energy of Steam
conclusion
1.6.
Entropy of Water
(water) that
1.7.
Entropy of Evaporation
reactor) unde
1.8.
Entropy of Wet Steam
1.9.
Entropy of Superheated Steam
1.3
1.10. Enthalpy-Entropy (h-s) Chart or Motlier Diagram
1.11.
Determination of Dryness Fraction of Steam
1.12.
Summary
1.13.
Keywords
1.14.
Exercise
J
0
Heat
The basic en,
The various
1.1
Objective
j
1. Constant
on p-v, T-s.
steam is in ,
-
pressure incr
-
-
Entropy of Water
-
Since the
136
rna
-
-- Enthalpy-Entropy (h-s) Chart or Mollier Diagram
1.2
Introduction
Process and cvcles
Whenever one or more of the properties of a system change, a Change in State has occurred. The
path of the succession of states through which the system passes is called the process. When a
system in a given initial state goes through a number of different changes of state or processes
and finally returns to its initial state, the system has undergone a Cycle. Therefore, at the
conclusion of a cycle, all the properties have the same value they had at the beginning. Steam
(water) that circulates through a steam power plant (like the conventional side of a nuclear
reactor) undergoes a cycle.
1.3
Heating-Cooling
and Expansion of Vapours
The basic energy equations for non-flow and flow processes are also valid for vapours.
:. When M<E = 0 and ~PE
dQ
=0
= du + p.dv
for non-flow process.
= dh -
for flow process.
dQ
v.dp
The various processes using vapour are discussed below:
1. Constant
Volume heating
or Cooling:
The constant volume heating process is represented
on p-v, T-s and h-s diagram as shown in Fig. 1 (a), (b), (c) respectively. It is assumed that the
steam is in wet condition before heating at pressure Ph becomes superheated after heating and
pressure increases from PI to Pl·
Since the mass of steam, m, remains constant during the heating process,
137
m == --
v
Xll!Cl
V
= ---
USllP2
) where V is the total constant volume of steam
p
L-
~
____+
v
(a)
T
where vg2 can
Applying the f
i.e.,
(I.)
In case the cor
Also
In the cooling
Vg2 and Ts2 can be found from the steam tables corresponding to pressure P2 and then Tsup2 can
Example 1
be calculated by using the above equation. When the final condition is known, the change in all
A rigid cylindi
other properties can be found easily.
until the pressi
If after cooling the condition of steam remains wet, then the mass fraction is obtained as follows:
(i) The state of
138
team
(c)
Fig.1: Constant volume process
where vg2can be found from the steam tables corresponding to pressure P2·
Applying the first law of thermodynamics, we have
=1t~-1~1
i.e.)
In case the condition of steam remains wet after heating, then
In the cooling process, the same equations are used except that the suffixes 1, 2 are interchanged.
m
Tsup2
can
Example 1
lange in all
A rigid cylinder of volume 0.028 m3 contains steam at 80 bar and 350°C. The cylinder is cooled
until the pressure is 50 bar. Calculate:
as follows:
(i) The state of steam after cooling;
139
(ii) The amount of heat rejected by the steam.
Solution: Volume of rigid cylinder
= 0.028
m3
Pressure of steam before cooling, PI = 80 bar
(ii) Heat reje
Temperature of steam before cooling = 350°C
Pressure of steam after cooling,
!
P2
Internal ene
= 50 bar
Steam at 80 bar and 350°C is in a superheated state, and the specific volume from tables is
0.02995m3Ikg. Hence the mass of steam in the cylinder is given by
0.:
m
= 0.935
kg
At constant
Internal energy at state 1, (80 bar, 350°C)
= 0.935(22,
U1 ~
hl -
PLVI
= 2987.3 -
80 x 105 x U02995
10'
or
U1
= 2747.7 kJlkg.
i.e., Heat re
2. Constat
heating pre
Generation
Applying f
Fig.: 2
Fig. 2 shows the process drawn on T -S diagram, the shaded area representing the heat rejected by
the system.
(i) State of steam after cooling:
At state 2,
P2
= 50 bar
and
V2
= 0.02995
m3/kg, therefore, steam is wet, and dryness fraction is
given by,
140
x? =~
-
= 0_02995 ::.:0_76_
L'~2
0.0394
= (1 -
X2) UI2
(ii) Heat rejected by the steam:
Internal energy at state 2 (50 bar),
.m tables is
U2
= (1 At constant volume, Q
= 0.935(2249.48
i.e., Heat rejected
=
0.76) x 1149
U2 - U 1= m(u2 -
- 2747.7)
= 465.5
=-
+ 0.76
+ x2ug2
x 2597
= 2249.48
kJ/kg
UI)
465.5 kJ
kJ.
2. Constant pressure Heating or Cooling: Fig. 3 (a), (b) and (c) shows the constant pressure
heating process on p-v, T-s and h-s diagrams respectively.
Generation of steam in the boilers is an example of constant pressure heating.
Applying first law of thermodynamics, we have
Q
J:
= .w + i' . dlJ
= (u2 - u1) + p(u2 -
VI) ::
(u!!' + J102) -
(UI +
PUl)
= hz -
'1
II, =11.
T,
t rejected by
V,
---'"-- ------------- ------,.y
v,
.s fraction is
141
hI
The heat adde'
fa)
during the pro,
the work done.
Example: 0.0
occupied is 0.1
Calculate:
(i) Heat suppln
s
(ii) Work done
(1))
h
p,
Solution: Mass
= P2
Pressure of stes
----'+--T2
Volume occupi
Initially the ste
Finally the stea
s
(c)
Fig. 3 Constant pressure process
If the initial condition of steam is wet and final condition is superheated, then
Q...=~~u.P:!)
= (h2here hand
-
(UI
+P . Xl
V'l)
hI)
h2 are the actual enthalpies of steam per kg before and after heating.
142
Hence the steal
superheat tables
= 3071.8kJ/'.l(g.
The heat added during the constant pressure process is equal to the change
1D
enthalpy of steam
during the process. When the steam is wet before heating and becomes superheated after heating
the work done,
W
Example:
= P (Vsup2 - XIVgI ) ...
(4.74)
0.08 kg of dry steam is heated at a constant pressure of 2 bar until the volume
occupied is 0.10528 m3•
Calculate:
(i) Heat supplied;
(ii) Work done.
Solution: Mass of steam, m = 0.08 kg
Pressure of steam, p = 2 bar
Volume occupied after heating = 0.10528 m3
Initiaily the steam is dry saturated at 2 bar, hence
hI
= hg
(at 2 bar) = 2706.3 kJikg
Finally the steam is at 2 bar and the specific volume is given by
I~
= 0..10.528
0.08
= 1.316 mG/kg
Hence the steam is superheated finally (since the value of vg at 2 bar
=
0.885 m3/kg). From
superheat tables at 2 bar and 1.316 m3/kg the temperature of steam is 300°C, and the enthalpy, h2
= 3071.8kJ/'.l{g.
.g.
I
j
143
3. Constant 1
p(Nlm2)
temperature
0
region, the co
and as well ,
constant temp'
When the wet
heat transfer (c
and work don€
Fig.:4
(i) Heat supplied:
[Vg2= VgIas pr
Heat supplied,
Q = H2 - HI = m(h2 - hI)
= 0.08(3071.8
- 2706.3)
This process
isothermal pro
work done dur
= 29.24
kJ. (Ans.)
(ii) Work done:
The..process is shown on a p-V diagram in Fig.4. The work done is given by the shaded area
i.e., W
= P(V2-
VI) Nmlkg
Here VI = Vgat 2 bar = 0.885 m3/kg and V2= 1.316 m3/kg
:. W
=2 x
105 (1.316 - 0.885)
=2 x
105 x 0.431 Nmlkg
Now work done by the total mass of steam (0.08 kg) present
= 0.08
x 2 x 105 x 0.431 x 10-3 kJ
= 6.896 kJ. (Ans.)
144
3. Constant Temperature
or Isothermal
Expansion:
Fig.5 (a), (b) and (c) shows the constant
temperature or isothermal expansion on p-v, T -s and h-s diagrams respectively.
In the wet
region, the constant temperature process is also a constant pressure process during evaporation
and as well as condensation.
When the steam becomes saturated it behaves like a gas and
constant temperature process in superheated region becomes hyperbolic (pv
= constant).
When the wet steam is heated at constant temperature till it becomes dry and saturated, then the
heat transfer (Q) is given by :
and work done,
= pVgJ(1 -
xl)
[vg2= VgJas pressure remains constant during this process]
=
This process is limited to wet region only. Hyperbolic process (pv
constant) is also an
isothermal process in the superheat region as the steam behaves like a gas in this region. The
work done during the hyperbolic expansion in a non-flow system is given by
led area
V,
V2
(0)
I
I
I
j
I
1
145
V
T
Sinm
..
Example: S
isothermally
found to be'
•
enthalpy;
(b)
(iii) The wor]
h
Solution: Init
Final pressur
Heat suppliee
The process
Therefore, th
(c)
Fig.: 5 Constant temperature or isothermal expansion
W=J2pdv=J2
1
1
c dV=Clog.(IJ2)
v
1'1
= P1V1 10& (~)
\\ ~ .e VI and V2 are the specific volumes of steam before and after expansion.
Applying first law of energy equation,
146
j
-Q
= /l:u. +
r
P . du
(J
~\
:: (h3 - P.J'2) - (hI Pll'l::
P1VI) + PI'}l
l~
I)
P2V2
Example: Stearn at 7 bar and dryness fraction 0.95 expands in a cylinder behind a piston
isothermally and reversibly to a pressure of 1.5 bar. The heat supplied during the process is
found to be 420 kJlkg. Calculate per kg : (i) The change of internal energy; (ii) The change of
enthalpy;
(iii) The work done.
Solution: Initial pressure of steam, PI
= 7 bar = 7 x
105N/m2
Final pressure of steam, P2 = 1.5 bar = 1.5 xl 05 N/m2
Heat supplied during the process, Q = 420 kl/kg,
The process is shown in Fig.6. The saturation temperature corresponding to 7 bar is 165°C.
Therefore, the steam is superheated at the state 2.
.'
1
147
Enthalpy at st
At 7 bar. hr::
...hi == 697.1-1
Interpolating
J
...Changeofe
Fig.: 6
(iii) Work don
From non-floe
(i) Change of internal energy:
The internal energy at state 1 is found by using the relation:
UI
= (l-x) ur+ XUg
= (1 :. UI
0.95) 696
= 2479.15
+ (0.95
:. W=Q-(U2
i.e., Work don
x 2573)
~.
kJ/kg
Interpolating from superheat tables at 1.5 bar and 165°C. we have
4. Revenible
process on P-'
16
Us = 2580 + 50 (2866 - 2580)
reversible adia
"'" 2eOO_8 k.JIkg
:. Gain in internal energy.
U
\
- UI
j
= 2602.8 -
2479.15
As 1Dr adiabal
= 123.65 kJ/kg.
(Ans.)
Change of enthalpy:
148
Enthalpy at state 1 (7 bar),
At 7 bar. hr= 697.1 kJ/kg and hra = 2064.9 kJ/kg
:. hI
= 697.1 + 0.95
x 2064.9
= 2658.75
kJ/kg
Interpolating from superheat tables at 1.5 bar and 165°C, we have
h2
= 2772..6 +
15
50 (2872.9 - 2772.6)
= 2802.69 kJlkg
:. Change of enthalpy
= h2 - hI
= 2802.69
- 2658.75 = 143.94 kJ/kg. (Ans.)
(iii) Work done:
From non-flow energy equation,
:. W
= Q-
(U2 - UI)
= 420 -
i.e., Work done by the steam
Note. The ~
123.65;"; 296.35 kJ/kg
= 296.35
done is also ~
kJ/kg.
by tAe -
OIl
tba ~.
4.fIO
lJ: pdv) .
this eo. 0Dl.y _1IIi:lbat84
~.
4. Revenlble Adiabatic or IHDtropic Preeess, Fig. 7 (a), (b) and (c) shows the isentropic
process on p-v, T-s and h-s diagrams respectively. Let us consider that the
reversible adiabatic ..Now applying first law energy equation, we have
Q = 611 +
As _
.
1
=
r
p. au
= (lis -
adiabatia: pWi!l!lS. Q 0
W = ("I - u,)
149
"l) +
W
process is non-flow
In case the process is steady flow reversible adiabatic, then first law energy equation can be
written as
where VI and V2 are the specific volumes of steam before and after executing the process.
hI +O=h2+W
(Q=O)
p
Example: 1
v
(0)
insulated cylii
Calculate the
Solution: Mas
T
Initial pressun
Initial tempers
(b)
150
h
ion can be
55.
= 0)
(e)
Fig.7 : Reversible adiabatic or isentropic process
Example: 1 kg of steam at 120 bar and 400°C expands reversibly in a perfectly thermrIy
insulated cylinder behind a piston until the pressure is 38 bar and the steam is then dry saturated.
Calculate the work done by the steam.
Solution: Mass of steam, m = 1 kg
Initial pressure of steam, PI
= 120 bar = 120 x
105 N/m2
Initial temperature of steam, tl = 400°C.
!
I
I
I
I
I
J
151
.I.
p(Nlm)
Polytrophic
on p-v, T-s and
The work done
'\pplying the fi
Fig.:
Final pressure of steam, P2 =
8
38 bar
From superheat tables, at 120 bar and 400°C
hi = 3051.3 kJ/kg and
VI
= 0.02108 m3/kg
Now, using the equation:
u=h-pv
U1
Also,
UI
= 3061.3-
12O)(~;
= Ug at 38 bar
= 2602
OD21OS= 2'198_34. kJlkg
kJ/kg.
Since the cylinder is perfectly thermally insulated then no heat flows to or from the steam during
the expansion, the process therefore is adiabatic.
Work done by the steam, W =
= 2798.34
UI - U2
- 2602 = 196.34 kJ/kg. (Ans.)
The process is shown on p-v diagram in Fig. 8, the shaded area representing the work done.
152
',. Polytrophic
process: In this process, the steam follows the law pvn = constant. This process
on p-v, T-s and h-s diagrams is shown in Fig. 9 (a), (b) and (c).
The work done during this process is given by
\pplying the first law energy equation to non-flow process, we have
Q=Au+W
p
kJlkg
y
(0)
steam during
k done,
1
153
T
T2
In adiabatic p
isentropic. Su
index n in this
Tl
Q=O
L_
~~------~
82 SI
8
(b)
h
Adiabatic pro.
value of n for
steam When t
reduces to:
When the end
In adiabatic process
Q = 0 and if ~s f:. 0 then the process behaves like adiabatic process and not
isentropic. Such a process with steam will be a particular case of the law pvn = constant. The
index n in this case will be that particular index which will satisfy the condition:
Q=O
:.O=~u+W
(4.80)
i.e.,
Adiabatic process (not reversible) is also a polytrophic process with an index n. The appropriate
value of n for adiabatic compression of steam are n = 1.13 for wet steam n = 1.3 for superheated
steam When the initial condition and end condition are both in wet region then PlVl
n
= 1'2 V2n
reduces to:
As PI. xl;n and 1'2 are specified the value ofx2 can be calculated.
When the end condition is superheated, then
Solving for V2,then using
T sup2 can be calculated. Knowing T 51 and T sup all properties at the end condition can be
calculated.
1
155
1.4
The latent i~ternal
energy of a system is the internal energy a system requires to undergo a
phase change .. Its value is specific to the substance or mix of substances in question. The value
can also
vary with temperature
and pressure. Generally speaking the value is different for the
type of phase change being accomplished.
Examples can include Latent internal energy of
vaporization (liquid to vapor), Latent internal energy of crystallization (liquid to solid) Latent
internal energy of sublimation (solid to vapor). These values are usually expressed in units of
It is defined
energy per mass or per mole such as J/mol or BTU/lb. Often a negative sign will be used to
steam is sun
represent energy being withdrawn from the system, while a positive value represents energy
evaporation
being added to the system. However, reference sources do vary so check the source to be sure.
energy of su
The latent heat consists of true latent heat and the work of evaporation. This true latent heat is
In otherwor
called the internal latent heat and may also be found as follows:
Internal latent heat
= ~_ _
P~L
or
J
= 1 in SI units.
In case of we
I.S
Internal Energy of System
Potential energy and kinetic energy are macroscopic forms of energy. They can.be visualized in
terms of the position and the velocity of objects. In addition to these macroscopic forms of
energy, a substance possesses several microscopic forms of energy. Microscopic forms of energy
include those due to the rotation, vibration, translation, and interactions among the molecules of
a substance. None of these forms of energy can be measured or evaluated directly, but techniques
have been developed to evaluate the change in the total sum of all these microscopic forms of
energy. These microscopic forms of energy are collectively called internal energy, customarily
represented by the symbol U. In engineering applications, the unit of internal energy is the
British thermal unit (Btu), which is also the unit of heat
The specific internal energy (u) of a substance is its internal energy per unit mass. It equals the
total internal energy (U) divided by the total mass (m).
156
and if steam
1.6
Entrl
.-D
III
• =
undergo a
The value
ent for the
~--CIIliJ
t1
=
Dmul.-g(!b)
•
=
JIIJ5l
(lb'1D)
(liar)
energy of
,lid) Latent
in units of
It is defined as the actual energy stored in the steam. As per previous articles, the total heat of
be used to
steam is sum of sensible heat, internal latent heat and the external work of evaporation. Work of
ents energy
evaporation is not stored in the steam as it is utilised in doing external work. Hence the internal
be sure.
energy of steam could be found by subtracting work of evaporation from the total heat.
itent heat is
Inother words,
or
PJ
h
=
u
= 11 -
~ u, where u is internal
;;'
Incase of wet steam with dryness fraction 'x'
u
= 11 _
pJCV,
J
visualized in
sic forms of
nsofenergy
molecules of
1t techniques
pic forms of
, customarily
mergy
and if steam is superheated to a volume of vsup per kg.
is the
equals the
1.6
Entropy of Water
157
energy of' 1kg of'steam at pressure p
Consider I kg of water being heated from temperature T I to T2 at constant pressure. The change
in entropy will be given by,
1.8
-3_ =
u...
d'rQ
= cpw
-
dT
T
Enn
The total en
(srg).
Integrating both sides, we get
In otherwor
dT
cpa'T
T?
82 - $1
= epw l~ 7f
where
If 00C is taken as datum, then entropy of water per kg at any temperature T above this datum will
Sf=
Swet =
Entropy
be
~
•
=Ent
If steam is dJ
1.7
1.9
The change of entropy (ds) is given by,
..3 __
Let I kg of
d,Q
WI-T
or
82 _ 81
= ~,
entre
specific heat
where
Q is the bent absmbed.
pressure p
When water is evaporated to steam completely the heat absorbed is the latent heat and this heat
goes into water without showing any rise of temperature.
Then
Total entropj
Q=hfg
Ssup
and
However, in case of wet steam with dryness fraction x the evaporation will be partial and heat
~
absorbed will be xhfg per kg of steam. The change of entropy will be T.
158
1.10
Enth:
The change
1.8
The total entropy of wet steam is the sum of entropy of water (Sf) and entropy of evaporation
(Sfg).
In other words,
where
is datum will
= Total
Swet
entropy of wet steam,
Sf= Entropy of water, and
~
•
= Entropy
of evaporation.
If steam is dry and saturated: i.e., x
1.9
= 1, then
entropy of Super Heated Steam
Let 1 kg of dry saturated steam at T, (saturation temperature of steam) is heated to Tsup. If
specific heat at constant pressure is cJlS' then change of entropy during superheating at constant
pressure p
at and this heat
Total entropy of superheated steam above the freezing point of water.
ssup= Entropy of dry saturated steam
+ change of entropy during superheating
partial and heat
1.10
Enthalpy-Entropy (h-s) Chart or Mollier Diagram
159
Dr. Mollier, in 1904, conceived the idea of plotting total heat against entropy, arid his diagrarr
I. Tank or bu
more widely used than any other entropy diagram, since the work done on vapour cycles can
2. Throttling.
scaled from this diagram directly as a length; whereas on T-s diagram it is represented by
area.
3. Separating
A sketch of the h-s chart is shown in Fig. 10.
Tank or Bue
The dryness fi
~~=r--T5
--=f----T
-.+t-----T:
_........
---T,
2805
A known rna
Line
of coostanl
temperatwe
condensed. Tl
arrangement c
calorimeter cc
mixing with s
water before a
steam passed t
EntrqJy. s iWIkg K
Fig. 10: Enthalpy-entropy (h-s) chart
Lines of constant pressure are indicated by PI. P2etc., lines of constanttemperature by TI, T2
etc.
-
Any two independent properties which appear on the chart are sufficient to define the state
(e.g., PI and xi define state 1 and h can be read off the vertical axis).
-
In the superheat region, pressure and temperature can define the state (e.g., P3 and T4 define
the state 2, and h2 can be read off),
-
A line of constant entropy between two state points 2 and 3 defines the properties at all points
during an isentropic process between the two states.
1.11
The dryness fraction of steam can be measured by using the following calorimeters:
160
"
~
s diagrarr
I. Tank or bucket calorimeter
ycles can
2. Throttling calorimeter
.ented by
3. Separating and throttling calorimeter.
Tank or Bucket Calorimeter
The dryness fraction of steam can be found with the help of tank calorimeter as follows:
A known mass of steam is passed through a known mass of water and steam is completely
condensed. The heat lost by steam is equated to heat gained by the water. Fig. 11 shows the
arrangement of this calorimeter. The steam is passed through the sampling tube into the bucket
calorimeter containing a known mass of water. The weights of calorimeter with water before
mixing with steam and after mixing the steam are obtained by weighing. The temperature of
water before and after mixing the steam are measured by mercury thermometer. The pressure of
steam passed through the sampling tube is measured with the help of pressure gauge.
[Pressure
I
Q9.uQ~
Bucket
calorimeter
lefine the state
Mixing
bcx
ies at all points
rs:
Fig. I1 Tank or bucket calorimeter
161
Insulation
Let, ps = Gauge pressure of steam (bar),
The value
01
convection a
PI = Atmospheric pressure (bar),
The calculat,
is = Daturation temperature of steam known from steam table at pressure (ps + pa),
the dryness.
hfg = Latent heat of steam,
Example: S
x = Dryness fraction of steam,
condenSed. 1
20°C respect
cPw == Specific heat of water,
gets condensl
cpc = Specific heat of calorimeter,
tank as 1.5k~
me = Mass of calorimeter, kg,
Solution: Prel
mew= Mass of calorimeter and water, kg,
Mass of water
mw = (mew - me) = Mass of water in calorimeter, kg,
Initial temper.
mews= Mass of calorimeter, water and condensed steam, kg,
Amount of ste
m, = (mews - fficw) = Mass of steam condensed in calorimeter, kg,
Final temperat
lc:w = Temperature of water and calorimeter before mixing the steam, °C, and
Water equivah
lcws = Temperature of water and calorimeter after mixing the steam, °C.
Dryness {rae/i,
Neglecting the losses and assuming that the heat lost by steam is gained by water and
At 5 bar. Fron
calorimeter, we have
hr= 640.1 kJ/k
(mews - mew)
= (mew
.•
or
c..., (t, -
m.Wa,r
+
m.~
+ c,....
-
[xhfg
m,,:C_
+ Cpw (1. -lcws)]
.t.....- t_.) + me
Total mass of,
Cpc (1-.. -
t"",1
t_.i] - (tell.- tew) [mCIII - m)<c:"..,+ ,,\c:;J
(t. - t_)] It_ - t_,)(m.,.p..... + m"eFW)
=
Also, heat lost 1
The 1llcCpc is known as water equivalent of calorimeter.
The value of dryness fraction 'x' can be found by solving the above equation.
162
1
The value of dryness fraction found by this method involves some inaccuracy since losses due to
convection and radiation are not taken into account.
The calculated value of dryness fraction neglecting losses is always less than the actual value of
the dryness.
Example:
Steam at a pressure of 5 bar passes into a tank containing water where it gets
condensed. The mass and temperature in the tank before the admission of steam are 50 kg and
20°C respectively. Calculate the dryness fraction of steam as it enters the tank if 3 kg of steam
gets condensed and resulting temperature of the mixture becomes 40°C. Take water equivalent of
tank as 1.5 kg.
Solution: Pressure of steam, p = 5 bar
Mass of water in the tank = 50 kg
Initial temperature of water = 20°C
Amount of steam condensed, mil = 3 kg
Final temperature after condensation of steam = 40°C
y water and
I
I
Water equivalent of tank = 1.5 kg
Dryness fraction ors/eam. x:
At 5 bar. From steam tables,
hr= 640.1.kJ/kg; hfg= 2107.4 kJ/kg
Total mass of water, mw
= mass
of water in the tank
+ water
equivalent of tank
= 50 + 1.5 = 51.5 kg
Also, heat lost by steam
m, [(hr+
= heat gained
xhra) -1
or 3[(640.1
+x
by water
x 4.18 (40 - 0)]
= mw[1 x 4.18
x 2107.4) - 4.18 x 40]
163
= S1.S
(40 - 20)]
x 4.18 x 20
);>
or 3(472.9
+ 2107.4x) = 4305.4
);>
or 472.9 + 2107.4x = 1435.13
Boilers (
x = 1436.13 - 472.9 = 0.466.
2107.4
Hence dryness fraction of steam = 0.456. (Ans.)
1.11
Let us Sum Up
_
-
-
-
Entropy of Water
-
-
-
Enthalpy-Entropy (h-s) Chart or Mollier Diagram
-
1.13
1. Explain the following
a. Internal Energy of Steam
b. Entropy of Water
c. Entropy of Evaporation
d. Entropy of Wet Steam
2. Elucidate the Enthalpy-Entropy (h-s) chart with example.
3. What are the determinant of dryness fraction of steam
1.14
Reference
164
Frederic)
Technica
Frederic)
1
>>-
Frederick M. Steingress (2001). Low Pressure Boilers (4th Edition ed.). American
Technical Publishers.
Frederick M. Steingress, Harold J. Frost and Darryl R. Walker (2003). High Pressure
Boilers (3rd Edition ed.). American Technical Publishers.
165
UNIT 2 - PART II
-
Combus
STEAM PROCESS
-
Auxiliar
-
Boiler W
Structure
-
Boiler \\
1.15.
-
Boiler fif
-
Controlli
1.16.
1.17.
1.18.
1.19.
1.20.
1.21.
1.22.
Introduction
Objectives
Boiler Terminology
Location of Fire and Water Spaces
Type of Boilers
Arrangement of Steam and Water Spaces
Type of Super heaters
Control of Superheat
1.23. Boiler Components
1.24. Combustion Air
1.25. Auxiliary Boiler
1.26. Boiler Mounting and Accessories
1.27. Boiler Water Quality Control
1.28. Boiler fittings and accessories
1.29. Controlling draught
1.30. Summary
1.31. Keywords
1.32. Exercise
2.2
Intro
Boilers
The function
naval boilers
efficiency. TJ
and skill on
1
should have
boiler, and th.
review combi
used in propi
nature and do
particular shit:
Upon comple
2.1
Objective
understand bo
interpret gaug
-
Boiler Terminology
understand bo:
-
functions. Alsc
-
Type of Circulation
off.
-
-
Type of Super heaters
2.3
-
Before studyin
-
Boiler Components
know the boil,
Boiler'
section we hay
166
-
Combustion Air
-
Auxiliary Boiler
-
Boiler Mounting and Accessories
-
Boiler Water Quality Control
-
Boiler fittings and accessories
-
Controlling draught
2.2
Introduction
Boilers
The function of a boiler in the steam cycle is to convert water into steam. Reliability in operating
naval boilers and associated equipment is important for the power plant to operate at maximum
efficiency. The complex design of naval boilers requires a high degree of technical knowledge
and skill on the part of the fire room personnel responsible for boiler operations. All engineers
should have some knowledge of the principles of combustion, how combustion occurs in a
boiler, and the combustion requirements for operating a boiler more efficiently. You may want to
review combustion in chapter 2 and 3 of this textbook. This chapter describes boilers commonly
used in propulsion plants of naval steam-driven surface ships. This information is general in
nature and does not relate to anyone class of ship. For detailed information on the boilers in any
particular ship, consult the manufacturer's technical manuals furnished with the boilers.
Upon completion of this chapter, you will have the knowledge to be able to identify and
understand boiler terminology, the basic types of naval boilers and their operating principles,
interpret gauges and indicators that aid in monitoring operating parameters of naval boilers, and
understand boiler construction. You should be able to identify the major parts of a boiler and its
functions. Also, you will learn about safety precautions that must be observed during boiler lightoff.
1.3
Boiler Terminology
Before studying the types of boilers used in propulsion plants aboard Navy ships, you need to
know the boiler terms and definitions used most frequently by shipboard personnel. In this
section we have listed some of the terms used in this chapter and by fireroom personnel on the
167
job. It is not an all-inclusive list, but it will help form a basis for your understanding of the
• Check va
feedwater ente
or else as a to]
cold water.
information presented on boilers.
• Ashpan: A container beneath the furnace, catching ash and clinker that falls through the
firebars. This may be made of brickwork for a stationary boiler, or steel sheet for a
locomotive. Ashpans are often the location of the damper. They may also be shaped into
hoppers, for easy cleaning during disposal.
• Blow-down cock: a valve mounted low-down or. he !.J .iler, often around the foundation
. ring, which is used to periodically vent water from the b.i.Ier. This water contains the most
concentrated precursors for sludge build-up, so by venting it whilst still di: nlved, the buildup is reduced. When early marine boilers were fed with salt water, they .ould b'~ blowndown several times an hour.
• Cladding:
that of a stean
bands. Later :
hazardous, fibr
simplification c
Also termed "c
I
• Blower: the blower provides a forced draught on the fire, encouraging combustion. It
consists of a hollow ring mounted either in the base of the chimney or on top of the blastpipe.
Holes are drilled in the top of the blower ring, and when steam is fed into the ring, the steam
jets out of the holes and up the chimney, stimulating draught.
• Boilermaker: a craftsman skilled in the techniques required for construction and repair
of boilers. Historically known as a boilersmith.
• Boiler suit: heavy-duty one-piece protective clothing, worn when inspecting the inside of
a firebox for steam leaks, for which task it is necessary to crawl through the firehole door.
• Boiler ticket: the safety certificate issued for a steam (locomotive) boiler on passing a
formal inspection after a major rebuild, and generally covering a period of ten years.
Additional annual safety inspections must also be undertaken, which may result in the
locomotive being withdrawn from service if the boiler requires work. When the ticket
"expires" the locomotive cannot be used until the boiler has been overhauled or replaced, and
a new ticket obtained.
• Brick areh: A horizontal baffle of firebrick within the furnace, usually of a locomotive
boiler. This forces combustion gases from the front of the furnace to flow further, back over
the rest of the furnace, encouraging. efficient combustion. The invention of the brick arch,
along with the blastpipe and forced draught, was a major factor in allowing early locomotives
to begin to bum coal, rather than coke.
• Carryover: the damaging condition where water droplets are carried out of the' boiler
along with the dry steam. These can cause scouring in turbines or hydraulic lock in cylinders.
The risk is accentuated by dirty feedwater.
168
• Crinolines:
the similar hoo]
.,
C,own she
part of tile fireb
the crown sheet
stays.
• . Damper: A
part of the ashps
• Disposal: 11
the fire and blov
• Dome: a rais
which to collect
• Downcomer
water from the st
• Drowned tul
the operating boi
this reduces wear
• Exhaust inje
waste steam, sud
• Field-tube: A
circulation.
ding of the
through the
sheet for a
shaped into
! foundation
ins the most
d, the buildd b;~blown-
• Check valve: or clack valve, from the noise it makes. A non-return valve where the
feed water enters the boiler drum. They are usually mounted halfway along the boiler drum,
or else as a top feed, but away from the firebox, so as to avoid stressing it with the shock of
cold water.
• Cladding: The layer of insulation and outer wrapping around a boiler shell, particularly
that of a steam locomotive. In early practice this was usually wooden strips held by brass
bands. Later and modem practice is to use asbestos insulation matting (or other, less
hazardous, fibres) covered with rolled steel sheets. The outer shape of the cladding is often a
simplification of the underlying boiler shell.
Also termed "clothing" in LMS practice.
• Crinolines: The framework of hoops used to support cladding over a boiler. Named from'
the similar hoops under a crinoline skirt.
.mbustion. It
he blastpipe.
ig, the steam
., Crown sheet: The upper sheet of the inner firebox on a locomotive boiler. It is the hottest
part of tile firebox, and sometimes at risk of boiler explosion, should the water level drop and
the crown sheet be exposed and thus allowed to overheat. Supported from above by complex
stays.
m and repair
• Damper: An adjustable flap controlling the air admitted beneath the fire-bed. Usually
part of the ashpan .
. the inside of
iole door.
on passing a
of ten years.
result in the
en the ticket
replaced, and
.a locomotive
ier, back over
he brick arch,
y locomotives
• Disposal: The cleanup process at the end of the working day, usually involving dropping
the fire and blowing down the boiler.
• Dome: a raised location on the top of the main boiler drum, providing a high point from
which to collect dry steam, reducing the risk of priming.
• Downcomer: large external pipes in many water-tube boilers, carrying unheated cold
water from the steam drum down to the water drum as part of the circulation path.
• Drowned tube: Either a fire-tube or water-tube that is entirely below the water-level of
the operating boiler. As corrosion and scaling is most active in the region of the water-level,
this reduces wear and maintenance requirements.
• Exhaust injector: a feedwater injector that economizes on steam consumption by using
waste steam, such as engine exhaust.
t of the boiler
k in cylinders.
• Field-tube: A form of single-ended thimble water tube with an internal tube to encourage
circulation.
169
• Manhol
Manholes aJ
be turned at
one or two I
cutting of a
patch.
• Firebar: Replaceable cast-iron bars that form the base of the furnace and support the fire.
These wear out frequently, so as designed for easy replacement.
• Fire dropping: Emptying out the remains of the fire after a day's work. A timeconsuming and filthy task; labour-saving ways to improve this became important in the final
days of steam locomotives.
•
Fire-tube
• MUd: a:
in the lower
to localized ~
boiler: A boiler where the primary heating surface is tubes with hot gas
flowing inside and water outside.
• Flue: A large fire tube, either used as the main heating surface in a flued boiler, or used
as enlarged firetubes in a locomotive-style boiler where these contain the superheater
• Mud dn
primarily to 1
elements.
•
Foundation
• MUdhoI«
either as an iJ
ring: The base of the firebox, where the inner and outer shells are joined.
• Fusible plug: A safety device that indicates if the water level becomes dangerously low.
It melts when overheated, releasing a jet of steam into the firebox and alerting the crew.
•
ROcking
or tipped to e
•
Galloway
tubes:
tapered thermic syphon water-tubes
inserted in the furnace of a
Lancashire boiler.
•
• Gauge glass: part of the water level gauge, which normally consists of a vertical glass
tube connected top and bottom to the boiler backplate. The water level must be visible within
• Scale dis:
the water-lev
contaminants:
the glass at all times.
Safetyv8
• Grooving: erosion of a boiler's plates from the internal water space, particularly where
there is a step inside the shell. This was a problem for early boilers made from lapped plates
rather than butted plates, and gave rise to many boiler explosions. In later years it was a
• Scum val
down lighter I
problem for the non-circular water drums of Yarrow boilers.
•
•
Sludge, at
• Smokebo:
gases from the
Handhole: A small manhole, too small for access but useful for inspection and washing
out the boiler.
• Injector: a feedwater pump without moving parts that uses steam pressure and the
Bernoulli effect to force feedwater into the boiler, even against its pressure.
• Steam dn
dry steam rna
priming.
• Klinger gauge glass: A form of gauge glass where the water level is visible through a
flat glass window in a strong metal frame, rather than a cylindrical tube. These were popular
with some operators, and increasingly so for high pressure boilers.
• Steam &
water, with l
. .. I
170
interchangeabl
_ _L__
)Ort the fire.
rk, A timet in the final
vith hot gas
iiler, or used
superheater
rejoined.
;.
Manhole: an oval access door into the boiler shell, used for maintenance and cleaning,
Manholes are sealed with a removable door from the inside. As they are oval, this door may
be turned and lifted out through the hole. Doors are clamped in place from the outside with
one or two bridge clamps spanning the hole and tightened down with a nut on a stud. As the
cutting of a manhole weakens the boiler shell, the surrounding area is strengthened with a
patch.
• Mud: a sludge of boiler scale particles, precipitates and general impurities that builds up
in the lower parts of a boiler. Mud reduces water circulation and so a local buildup may lead
to localized overheating and possibly explosion.
• Mud drum: a water drum, particularly one mounted low on the boiler whose function is
primarily to trap mud from circulation.
• Mudhole: A small manhole, too small for access but useful for washing out the boiler,
either as an inlet for a hose or as a drain for removed mud.
gerously low.
e crew.
• Rocking grate: An advanced form of firebar, where sections of the grate may be rocked
or tipped to either break up clinker within the fire, or to drop the fire after a day's work.
furnace of a
•
Safety valve: an automatic valve used to release excess pressure within the boiler.
visible within
• Scale dissolved minerals from hard water that precipitate out in the steam space around
the water-level. Where this scale falls to the bottom of the boiler and mixes with other
contaminants, it is termed mud.
icularly where
• Scum valve: A blow-down valve mounted at the water-level of a boiler, used to blowdown lighter oily or foamy deposits within a boiler that float on the water-level.
vertical glass
I lapped
plates
years it was a
•
Sludge, another term for mud.
n and washing
• Smokebox: an enclosed space at the extremity of a fire-tube boiler, where the exhaust
gases from the tubes are combined together and pass to the flue or chimney.
essure and the
• Steam drum: a cylindrical vessel mounted at a high point of a water-tube boiler, where
dry steam may separate above the water level, so that it may be drawn off without risk of
priming.
sible through a
;e were popular
• Steam & water drum: a steam drum that contains a turbulent mixture of steam and
water, with a substantial part of this being water. The terms are used somewhat
interchangeably.
171
• Steam drier, a form of mild superheater that adds additional heat to wet- or saturated
steam, thus ensuring that all water in the steam has been evaporated, thus avoiding problems
with water droplets in the cylinders or turbine. Unlike the superheater, the steam drier does
not attempt to raise the temperature of the steam significantly beyond the boiling point.
• Suction valve: an automatic non-return valve, which opens when the boiler is at less than
atmospheric pressure. This avoids any risk of vacuum collapse, when a hot boiler is allowed
to cool down out of service.
•
Throatplate:
In the fire bI
Cochran, La
tubes and he
3. Externall
The boiler i:
Wilcox boik
the shell. Ex:
a plate forming the lower front of the outer firebox of a locomotive boiler,
4. Forced ch
below the barrel.
• Top-feed: in locomotive boilers, a feed water check valve placed on the top of the boiler
drum. This encourages rapid mixing of the cold feedwater with the hot steam, reducing the
risk of thermal shock to the heated parts of the boiler.
• Tubeplate: a plate across the barrel of a fire-tube boiler, containing many small holes to
receive the fire-tubes. A locomotive boiler has two tubeplates: one at the front of the inner
firebox and one at the front of the boiler, adjacent to the smokebox.
• W'ater-wall: a furnace or other wall within a boiler enclosure that is composed of
numerous closely-set water-tubes. These tubes may be either bare, or covered by a mineral
In forced ci
Examples: V
water in the 1
heat. Exampl
5. Higher Pr
The boilers 1
boilers. Exan
produce stean
Examples: C(
cement.
6. Stationary
• Washout plug: A small mudhole used for washing out the boiler. Plugs, as compared to
mudholes, are usually screwed into a taper thread, rather than held by clamps.
• Primarily, th
• Stationary b
plant process:
• Water-tube boiler: a boiler whose primary heating surface is composed of many small
tubes, filled with water. Tubes of 3 inch diameter and above are termed "large-tube" boilers.
Later water-tube designs used smaller "small-tubes" of2 inches or less.
Boiler Classification
1. Horizontal, Vertical or Inclined Boiler.
If the axis of the boiler is horizontal,the boiler is called horizontal, if the axis is vertical, it is
called verticalboiler and if the axis is inclined it is called as inclined boiler.The parts of
horizontal boiler is can be inspected and repaired easily but it occupies more space.The vertical
boiler occupies less floor area.
• Mobile boil€
use at sites.
7. Single Tub
The fire tube I
the fire tube i
tube boiler anc
2.4
Locad.
One of the ba:
water spaces. I
BOILERS and
through the tul
water flows tlu
2. Fire Tube and Water Tube
172
. saturated
; problems
drier does
In the fire boilers, the hot gases are inside the tubes and the water surrounds the tubes. Examples:
Cochran, Lancashire and Locomotive boilers. In the water tube boilers, the water is inside the
tubes and hot gases surround them. Examples: Babcock and Wilcox, Stirling, Yarrow boiler etc.
oint.
at less than
is allowed
3. Externally Fired and Internally Fired
The boiler is known as externally fired if the fire is outside the shell. Examples: Babcock and
Wilcox boiler, Stirling boiler etc. In case of internally fired boilers, the furnace is located inside
the shell. Examples: Cochran, Lancashire boiler etc.
itive boiler,
rf the boiler
·educing the
4. Forced circulation and Natural Circulation
In forced circulation type of boilers, the circulation of water is done by a forced pump.
Examples: Ve1ox, Lamomt,Benson Boiler etc. In natural circulation type of boilers, circulation of
water in the boiler takes place due to natural convention currents produced by the application of
heat. Examples: Lancashire, Babcock and Wilcox boiler etc.
nall holes to
of the inner
.omposed of
5. Higher Pressure and Low Pressure Boilers
The boilers which produce steam at pressures of 80 bar and above are called high pressure
boilers. Examples: Babcock and Wilcox, Ve1ox, Lamomt, Benson Boiler etc. The boilers which
produce steam at pressure below 80 bar are called low pressure boilers.
Examples: Cochran, Cornish, Lancashire and Locomotive boiler etc.
by a mineral
6. Stationary and Portable
compared to
• Primarily, the boilers are classified as either stationary or mobile.
• Stationary boilers are used for power plant steam, for central station utility power plants, for
plant process steam etc.
• Mobile boilers or portable boilers include locomotive type, and other small units for temporary
If many small
.tube" boilers.
use at sites .
7. Single Tube and Multi Tube Boiler
The fire tube boilers are classified as single tube and multi-tube boilers, depending upon whether
the fire tube is one or more than one. Examples: Cornish, simple vertical boiler are the single
tube boiler and rest of the boilers are multi-tube boiler.
s vertical, it is
r.The parts of
ce.The vertical
2.4
One of the basic classifications of boilers is according to the relative location of the fire and
water spaces. By this method of classification, boilers are divided into two classes, FIRE-TUBE
BOILERS and WATERTUBE BOILERS. In the fire-tube boilers, the gases of combustion flow
through the tubes and thereby heat the water that surrounds the tubes. In water-tube boilers, the
water flows through the tubes and is heated by the gases of combustion that fill the furnace and
173
heat the outside metal surfaces of the tubes. All propulsion boilers used in naval ships are of the
water-tube type. Auxiliary boilers may be either fire-tube or water-tube boilers.
l.S
Types of Boilers
resistance of
releases steru
bum out the
where steam
Steammayal
Based on the order of Evaporation, the boilers are categorized into:
4) Steam Gel
1) Cast Iron sectional Boilers
These are used for hot water services with a maximum operating pressure of 5 bar and a
inaximum output in the order of 1500 KW.Site assembly of the unit is necessary and will consist
of a bank of cast iron sections. Each section has internal waterways.
Thi!__coiltype
steam pressu
available, alt}:
with liquid an
The sections are assembled with screwed or taper nipples at top and bottom for water circulation
and sealing between the sections to contain the products of combustion. Tie rods compress the
sections together.
The coiled tul
and converter
demand. Feed
A standard section may be used to give a range of outputs dependent on the number of sections
used. After assembly of the sections, the mountings, insulation and combustion appliance are
fitted.
The remainin
Because there
suitable for sit
This system makes them suitable for locations where it is impractical to deliver a package unit,
eg.basements where inadequate access is available or rooftop plant rooms where sections may be
faken up using the elevator shafts. Models available use liquid, gaseous and solid fuel.
Also, as the- .
respond to flu
control of suit
2) Steel Boilers
5) Vertieal Sh
These are similar in rated outputs to the cast iron sectional boiler. Construction is of rolled steel
annular drums for the pressure vessel. They may be of either vertical or horizontal configuration,
depending upon the manufacturer. In their vertical pattern they may be supplied for steam rising.
This is a cyli
comprised a cl
The gases ros
tubes were fitt.
chimney. Late
running horizo
hemispherical f
3) Electrode Boilers
These are available for steam raising up to 3600kglh and manufacture is of two designs. The
smaller units are element boilers with evaporation less than 500kglh.In these, an immersed
electric element heats the water and a set of water-level probes positioned above the element
controls the water level being interconnected to the feed water pump and the element electrical
supply.
Larger units are electrode boilers. Normal working pressure would be lObar but higher pressures
are available. Construction is a vertical pattern pressure shell containing the electrodes. The
lengths of the electrodes control the maximum and minimum water level. The electrical
174
The present ve
generation or J
same shell ma:
there is insuffic
6) Waste Beat
rs are of the
resistance of the water allows a current to flow through the water, which in tum, boils and
releases stearn. Since water has to be present within the electrode system, lack of water cannot
burn out the boiler. The main advantage with these units is that they may be located at the point
where steam is required and, as no combustion fumes are produced, no chimney is required.
Steam may also be raised relatively quickly, as there is little thermal stressing to consider.
4) Steam Generators
This coil type. boilers works in the evaporative range up to 3600 kglh of steam. Because of the
; bar and a
will consist
stearn pressure being contained within the tubular coil, pressures of 35bar and above are
available, although the majority is supplied to operate at up to 10 bar. They are suitable for firing
with liquid and gaseous fuels, although the use of heavy fuel oil is unusual.
r circulation
ompress the
The coiled tube is contained within a pressurized combustion chamber and receives both radiant
and converted heat. A control system matches the burner-firing rate proportional to the steam
demand. Feed water is pumped through the coil and partially flashed to steam in a separator.
r of sections
ppliance are
The remaining water is recirculated to a feed water heat exchanger before being run to waste.
Because there is no stored water in this type of unit they are lighter in weight and therefore
suitable for sitting on mezzanine or upper floors adjacent to the plant requiring steam.
ackage unit,
:ions maybe
l.
Also, as the- water content is minimal, steam raising can be achieved very quickly and can
respond to fluctuating demand within the capacity of the generator. It must be noted that close
control of suitable water treatment is essential to protect the coil against any build-up of deposits.
5) Vertical Shell Boilers
f rolled steel
onfiguration,
team rising.
designs. The
ill immersed
the element
ent electrical
her pressures
ctrodes. The
he electrical
This is a cylindrical boiler where the shell axis is vertical to the firing floor. Originally it
comprised a chamber at the lower end of the shell, which contained the combustion appliance.
The gases rose vertically through a flue surrounded by water. Large diameter (lOOIllll1} c:rou
tubes were fitted across this flue to help extract heat from the gases which then proceeded-to the
chimney. Later versions had the vertical flue replaced by one or two banks of small-bcn', tubes
running horizontally before the gases discharged to the chimney. The steam was contained in a
hemispherical chamber forming the top of the shell.
The present vertical boiler is generally used for heat recovery from exhaust gases from power
generation or marine applications. The gases pass through small-bore vertical tube banks. The
same shell may also contain an independently fired section to produce steam at such times, as
there is insufficient or no exhaust gas available.
6) Waste Heat Boilers
175
These may be horizontal or vertical shell boilers or water tube boilers. They would be designed
to suit individual applications ranging through gases from furnaces, incinerators, gas turbines and
diesel exhausts. The prime requirement is that the waste gases must contain sufficient usable heat
to produce steam or hot water at the condition required.
Supplementary firing equipment may also be included if a standby heat load is to be met and the
waste-gas source is intermittent. Waste-heat boilers may be designed to us either radiant 01
convected heat sources.
In some cases, problems may arise due to the source of waste heat, and due consideration must
be taken of this, with examples being plastic content in waste being burned in incinerators, carryover from some type of furnaces causing strongly bonded deposits and carbon from heavy oil
fired engines.
Several app
used being t
combustor i:
In the com]
single-pass :
required wil
To its adva
availability (
bed during c
solid-fuel ch
TYPES OF
Some may be dealt with by maintaining gas-exit temperatures at a predetermined level to prevent
dew point being reached and others by soot blowing. Currently, there is a strong interest in small
combined heat and power (CHP) stations, and these will normally incorporate a waste-heat
boiler.
Auxiliary bo
WATER-TV
Fire-TubeB
7) Fluid-bed Boilers
The name derives from the fire bed produced by containing a mixture of silica sand and ash
through which air is blown to maintain the particles in suspension.
Fire-tube boi
boiler, the gr
number of au
cutaway view
The beds are in three categories
Water-Tube,
i) Shallow bed
ii) Deep bed and
iii) Recirculating bed
Shallow beds are mostly used and are about 150-250mm in depth in their slumped condition and
around twice that when fluidized. Heat is applied to this bed to raise its temperature to around
600C by auxiliary oil or gas burners. At this temperature coal and/or waste is fed into the bed,
which is controlled to operate at 800-900C.Water-c00ling surfaces are incorporated into this bed
connected to the water system of the boiler.
Water-tube, r
connected by
tubes, which
beneath the r
boilers, the ste
2.6
Arran
Natural circuli
The deep bed, as its name implies, is similar to the shallow bed but in this case may be up to 3m
deep in its fluidized state, making it suitable only for large boilers. Similarly, the recirculating
fluid bed is only applicable to large water tube boilers.
176
Boilers Or He
Drum-type bo
well). Header
serve the same
Number of FlI
be designed
turbines and
t usable heat
Several applications of the shallow-bed system are available for industrial boilers; the two most
used being the open-bottom shell boiler and the composite boiler. With the open-bottom shell the
combustor is sited below the shell and the gases then pass through two banks of horizontal tubes.
In the composite boiler the combustion space a water tube chamber directly connected to a
: met and the
er radiant 01
single-pass shell boiler forms housing the fluid bed. In order to fluidize the bed the fan power
required will be greater than that with other forms of firing equipment.
leration must
rators, carryom heavy oil
To its advantage, the fluid bed may utilize fuels with high ash contents, which affect the
availability of other systems. It is also possible to control the acid emissions by additions to the
bed during combustion. They are also less selective in fuels and can cope with a wide range of
solid-fuel characteristics.
TYPES OF BOILERS
vel to prevent
erest in small
a waste-heat
Auxiliary boilers on Navy ships may be divided into two groups: FIRE-TUBE BOILERS and
WATER-TUBE BOILERS.
Fire- Tube Boilers
sand and ash
Fire-tube boilers are generally similar to Scotch marine or locomotive boilers. In this type of
boiler, the gases of combustion pass through tubes that are surrounded by water. There are a
number of auxiliary boilers of the fire-tube type in use in diesel-driven ships. Figure illustrates a
cutaway view of the fire-tube boiler shown in figure.
Water-Tube, Natural-Circulation Boilers
condition and
ture to around
i into the bed,
d into this bed
Water-tube, natural-circulation boilers consist basically of a steam drum and a water drum
connected by a bank of generating tubes. The two drums are also connected by a row of water
tubes, which fOmISa water-cooled sidewall opposite the tube bank. The water-wall tubes pass
beneath the refractory furnace floor before they enter the water drum. In natural-circulation
boilers, the steam and water
2.6
Natural circulation water-tube boilers are classified as DRUM-TYPE
ly be up to 3m
recirculating
ie
Boilers Or Header type Boilers, depending on the arrangement of the steam and water spaces.
Drum-type boilers have one or more water drums (and .usually one or more water headers as
well). Header type boilers have no water drum; instead, the tubes enter many headers which
serve the same purpose as water drums.
Number of Furnaces
177
ooilers commonly used in the propulsion plants of naval ships may be classified as either
SINGLE-FURNACE BOILERS or DOUBLEFURNACE BOILERS. The D-type boiler is a
single-furnace boiler; the M-type boiler is a double-furnace (divided-furnace) boiler.
Burner Location
The stand a
cylindrical s
bypass for te
Naval boilers are also classified on the basis of where their burners are located. Most burners in
naval propulsion plants are located at the front of the boiler. These are called FRONT-FIRED
BOILERS. Other ships, such as the AO-I?? and LKA-113 class ships, have their burners on the
top of the boilers. These are called TOP-FIRED BOILERS.
Furnace Pressure
Super heater.
The super hI
super heater
temperature (
Another convenient boiler classification is based on the air pressure used in the furnace. Most
boilers in use in naval propulsion plants operate with a slight air pressure (seldom over 5 psig) in
the boiler furnace. This slight pressure is not enough to justify calling these boilers pressurized
furnace boilers. However, some boilers installed on naval ships are truly pressurized-furnace
boilers. They are called
Locating the
ex..:h:mgers it
gas flows frc
control is pos
These furnaces are maintained under a positive air pressure of about 65 psig (about 50 psig)
when operated at full power. The air pressure in these boiler furnaces is maintained by special air
compressors called superchargers.
2.7
Type of Super Heater
A super heater is a device used to convert saturated
steam or wet steam into dry steam used
for power generation or processes. There are three types of superheaters namely: radiant,
convection, and separately fired. A superheater can vary in size from a few tens of feet to several
hundred feet (a few metres or some hundred metres).
Process Gas
When a supe
temperature (
heater is loca
However, wb
Superheated
When a stean
super heater i!
_ steam temper.
equipment.
Plain vs. Finn
A radiant superheater is placed directly in the combustion chamber.
In cases of hig
A convection superheater is located in the path of the hot gases.
A separately fired superheater, as its
name implies,
is totally separated from the boiler.
A superheater is a device in a steam engine, when considering locomotives, that heats the steam
generated by the boiler again, increasing its thermal energy and decreasing the likelihood that it
will condense inside the engine. Super heaters increase the efficiency of the steam engine, and
were widely adopted. Steam which has been 'superheated is logically known as superheated
steam; non-superheated steam is called saturated steam or wet steam.
Types and Location of Super heaters
In acid plants, super heaters have been designed as stand alone units, integral with a waste heat
boiler and as units for installation inside our converters.
178
because the ad
acceptable limi
heat transfer. j
2.8
Contr(
A boiler that I
rate of steam ~
separate contro
Normally, the
I
uncontrolled su
Operating
Pre
:d as either
boiler is a
The stand alone units are simply rectangular tube bundles contained inside the traditional box or
cylindrical shells. The gas must be brought to and from the super .heaterby ducting. A gas side
bypass for temperaturecontrolis possible with these units.
burners in
NT-FIRED
:ners on the
Superheaters designed as part of a waste heat boiler are simplytube bundles l?cated in the gas flow.
The super heater section will be located near the gas inlet where the gas is hottest. However, the
super heater bundle will often be protected by a convection or screen bundle so the tube metal
temperaturedoes not exceed the design limits.
t
mace. Most
er 5 psig) in
pressurized
zed-furnace
out 50 psig)
'y special air
'steam used
ely: radiant,
eet to several
Locating the super heater inside the converterhas the same advantages as locating our gas/gas heat
exchangers inside the converter. The super heater bundle is located in a cylindricalarea where the
gas flows from the exit of the bed down through the tubes. A gas side bypass for temperature
control is possible but is difficultto implement.
Process Gas Temperature Control
When a super heater is located between catalyst beds in the contact section of an acid plant, the
temperature of the gas exit the super heater to the next bed must be controlled. When the super
heater is located outside the converter a gas bypass is used to regulate the gas exit temperature.
However, when the super heater is located inside the converter,a steam side bypass is used.
Superheated Steam Temperature Control
When a steam system is designed for superheat, the designer should ensure that the steam exit the
super heater is superheatedabout 5.6°C (10°C)higher than the desired superheat temperature. The
_ steam temperature is not controlled using bypasses on the super heaters but by desuper heating
equipment.
Plain vs. Finned Tubes
In cases of high gas temperatures (i.e. downstream of a regeneration furnace) plain tubes are used
because the added thickness of the fins increases metal temperatures and thermal stresses beyond
er.
eats the steam
:lihood that it
n engine, and
superheated
acceptablelimits. Where the temperaturesare more 'moderate',finned tubes can be used to enhance
heat transfer. This is typicalfor super heaters within tho.contact section of the acid plant.
2.8
A boiler that provides some means of controlling the degree of superheat independently of the
rate of steam generation is said to have CONTROLLED SUPERHEAT. A boiler in which such
separate control is not possible is said to have UNCONTROLLED SUPERHEAT.
Normally, the term superheat control boiler is used to identify a double-furnace boiler. The term
I
a waste heat
uncontrolled superheat boiler is used to identify a single-furnace boiler.
Operating Pressure
179
For some purposes, it is convenient to classify boilers according to operating pressure. Most
classification of this type are approximate rather than exact. Header-type boilers and some older
drum-type boilers are often called 400-PSI BOILERS even though their operating pressures
range from about 435 psi to 700 psi.
The term high-pressure boiler is at present used rather loosely to identify any boiler that operates
off the top 0
mixture entei
the water drc
steam drum j
drum. Its ac
water of boil
along the len:
at a substantially higher pressure than the socalled 600-PSI BOILERS. In general, we will
consider any boiler that operates at 751 psi or above as a high-pressure boiler. Many boilers in
SafeI'
valvi
naval ships operate at about 1200 psi. These boilers are referred to as 1200-PSI BOILERS.
Saturatec
stean
Stean
drun
Boilll'll
As you can see, classifying boilers by operating pressure is not very precise since actual
operating pressure may vary widely within anyone
group. Also, any classification based on
operating pressure may easily become obsolete. What is called a high-pressure boiler today may
wate
well be called a low-pressure boiler tomorrow.
Downcome:
tubE
2.9
Boiler Components
Fue
bumet
Boilers used onboard naval ships have essentially the same components: steam and water drums,
generating and circulating tubes, superheaters, economizers,
and accessories and fittings for
Fue
controlling steam pressure and temperature and other aspects of boiler control and operation.
Figure lshows a cutaway view of a D-type boiler. You should refer to this figure as a guide to
the arrangement of the boiler components. As we discuss the boiler and its components, imagine
that you are assembling a similar boiler. As you add each part to your boiler, follow the line
drawings introduced in the following paragraphs that describe the position of each component.
Steam Drum
A steam drum is a standard feature of a water-tube boiler. It is a reservoir of water/steam at the
top end of the water tubes. The drum stores the steam generated in the water tubes and acts as a
phase-separator for the steam/water mixture. The difference in densities between hot and cold
water helps in the accumulation of the "hotter"-water/and saturated-steam into the steam-drum.
Construction
Made from high Carbon Steel with high tensile strength and its working involves temperatures
around 390°C and pressures well above 350 psi(2.4MPa). The separated steam is drawn out from
the top section of the drum and distributed for process. Further heating of the saturated steam
will make superheated steam normally used to drive a steam turbine. Saturated steam is drawn
180
A steam drur
located at a 10
boiler and a I
construction i:
designed for h
2.10
Comb:
Air is forced iJ
fan that can b.
air into the ou
outer casing t<
air registers ar
:ure. Most
:ome older
. pressures
at operates
off the top of the drum and re-enters the furnace in through a superheater. The steam and water
mixture enters the steam drum through riser tubes; drum internals consisting of demister separate
the water droplets from the steam producing dry steam. The saturated water at the bottom of the
steam drum flows down through the downcomer pipe, normally unheated, to headers and water
drum. Its accessories include a safety valve, water-level indicator and level controller. Feedwater of boiler is also fed to the steam drum through a feed pipe extending inside the drum,
along the length of the steam drum.
LI, we will
{boilers in
~RS.
ince actual
n based on
.today may
Safety
valve
Saturated
steam
Steam
drum
Boiling
water
~
Saturated
steam outlet
Superheated
steam
Exhaust
gasses
Superheater
Oowncomer
tube
rater drums,
Fuel
burner
fittings for
Fuel
..,__
_.....
nts, imagine
low the line
mponent.
Water
Feedwater
drum
eration.
s a guide to
Water
tubes
A steam drum is used without or in the company of a mud-drum/feed water drum which is
located at a lower level. A boiler with both steam drum 'and mud/water drum is called a bi-drum
boiler and a boiler with only a steam drum is called a mono-drum boiler. The bi-drum boiler
construction is normally intended for low pressure-rating boiler while the mono-drum is mostly
designed for higher pressure-rating.
Isteam at the
2.10
and acts as a
hot and cold
Air is forced into the furnace by a forced draft blower. The forced draft blower is a large volume
earn-drum.
fan that can be powered by an electric motor or a steam turbine. The forced draft blower blows
Combustion Air
air into the outer casing of the boiler (Fig 12). The air then travels between the inner casing and
temperatures
awn out from
turated steam
eam is drawn
outer casing to the boiler front where it is forced into the furnace through the air registers. The
air registers are part of the fuel oil burner assembly that consists of four main parts: air ~rs,
181
a
diffuser, air foils, and the atomizer assembly. Figure 13 shows a side view of a fuel-oil burner
assembly.
A~!RREGISTERS: The air entering the furnace through the air registers mixes with a fine fueloil spray through the atomizer. Figure 13 shows the arrangement of an air register in a fuel oil
burner assembly. The air doors are used to open or close the register, as necessary. They are
usually kept either fully opened or fully closed. When the air doors are open, air rushes in and is
given a whirling motion by the diffuser plate. The diffuser plate causes the air to mix evenly with
the atomized oil in such a way that the flame will not blow away from the atomizer (atomizers
are discussed in the next paragraph). The air foils guide the major quantity of air so it mixes with
the larger particles of fuel oil spray beyond the diffuser.
ATOMIZERS: Atomizers (devices for produc ing a fine spray) break up the fuel oil into very fine
particles. In the following paragraphs we w.ll briefly discuss the three types of atomizers. These
three types are the return-flow atomizer., the steam-assist atomizer, and the vented plunger
atomizer.
Steam-Ass is
fuel oil to he
Return-Flow Atomizer: The return-flow atomizer provides a constant supply of fuel-oil pressure.
Navy are the
Any fuel oil not needed to meet steam demand is returned to the fuel-oil service tank. This is
pressure air I
accomplished by the return control valve installed in the piping between the boiler front and the
Vented-Plun
service tank. As the return control valve is closed, more fuel oil is forced through the sprayer
the only
plate into the furnace. The return-flow atomizer is shown in figure 14.
~tOIl
barrel and
at
cartridge tim
begins to spi
oil pressure
pressure. The
and forced i
recalls the pi:
torch is used
Fig. 14 - Return-flow atomizer
182
_ .....
,
-oil burner
a fine fueln a fuel oil
....nM..,~
i•
.."u_
.......
»» ......
~
)~.~.~
~
::::~
~
y. They are
es in and is
Fig.l5: TODD LVS atomizer
evenly with
. (atomizers
mixes with
Ito very fine
izers. These
Fig.16 : Y-Jet steam atomizer
Jed plunger
Steam-Assist Atomizer.- The steam-assist atomizer employs 150 psi of steam mixed with the
fuel oil to help atomize the fuel oil. The two most common steam-assist atomizers in use by the
oil pressure.
Navy are the TODD LVS fig 15 and the Y-Jet fig 16. All steam-assist atomizers must have low-
:ank. This is
pressure air hook up for use as a substitute when suitable auxiliary steam is not available.
ront and the
Vented-Plunger Atomizer: The vented plunger atomizer shown in figure 17 is unique in that it is
the sprayer
the only ~tomizer in use in the Navy that has moving parts. The fuel oil flows down the atomizer
I
barrel and around the atomizer cartridge. The pressure in the barrel forces the fuel oil into the
cartridge through the holes drilled in the cartridge. As the fuel is forced into the cartridge, it
begins to spin. This motion forces the fuel out through the orifice in a fine mist Increasing fueloil pressure in the atomizer barrel and cartridge will cause the piston to overcome the spring
pressure. The piston is then forced back, uncovering more holes and allowing fuel to be atomized
and forced into the furnace. As pressure decreases, the opposite occurs. The spring tension
recalls the piston, covering the holes and allowing less fuel oil to be atomized. In most boilers, a
torch is used to light fires. However, some boilers may have electric igniters
183
• Do not op
EDO, and al
• Always de
• Report to y
*00 not be a
Fig. 17: Vented-plunger atomizer
2.11
Auxi
We will describe the more common method lighting fires with a torch. Boiler light off is always
a two-person operation. One person is needed to handle the torch, the air register, and the
furnace, and the other to open the fuel-oil root valve. If fires do not light in 2 or 3 seconds, you
must secure the fuel oil and investigate the reason for the failure to light. The boiler furnace must
An auxiliary
heating, wate
and controls
steam-genera
be inspected and repurged before the next attempt to light.
The basic light-off procedure involves the following steps:
I. Ensure that all fuel-oil manifold and atcmizersafety
shut-ofIvalves
are shut.
2. Insert a clean atomizer with a lighting off sprayer plate into the No. 1 burner.
3. Adjust the combustion air and fuel-oil pressures for lighting the fires.
Waste-Heat
On some cla
Spruance clar
using the hot
4. Ignite the lighting-off torch,
5. Insert the lighted torch into the lighting off port and close the port cover; visually check to
on waste-heal
ensure that the torch remains lighted. However, you should never insert a torch into a furnace
until you are sure that no fuel is (In the furnace deck and that the boiler has been purged of all
combustible gases.
6. Open the No.1 burner fuel-oil atomizer! safety shut-offvalve(s).
2.12
Boilei
Boiler mount
for the safety
boiler mounti
7. Open the No.1 burner fuel-oil supply manifold valve one-half tum.
8. Observe the furnace through the No.1 burner observation port to ensure that the ignition is
1)
2)
successful.
9. Adjust the flame with the burner air register handle.
10. Open the No. 1 burner fuel-oil supply manifold to the fully open position.
11. Withdraw and extinguish the torch.
For specific lighting-off instructions, always refer to your ship's EOSS.
The following are a few simple suggestions to make your job easier and safer:
184
3)
4)
i)
ii)
5)
6)
7)
o
00 not operate any valves or start equipment until you have permission from the EOOW or
EOO, and always refer to the BOSS.
o
Always clean up any spills or debris.
o
Report to your supervisor any condition that you think may be abnormal.
*00not be afraid to ask questions!
2.11
Tis always
;T,
and the
conds, you
mace must
Auxiliary Boiler
An auxiliary boiler is a small boiler that supplies steam for distilling plants; space heating, oil
heating, water heating, galley, and laundry. These boilers have all the auxiliaries, accessories,
and controls to form a unit assembly. They are arranged to operate as complete self-contained,
steam-generating plants.
Waste-Heat Boilers
On some classes of ships, you may find wasteheat boilers. Waste-heat boilers are used by the
Spruance class and Ticonderoga class CGs. These boilers supply the steam for ship's services by
using the hot exhaust gases from the gas turbine generator sets (GTGSs). For further information
on waste-heat boilers, refer to GSM 3 & 2, NAVEOTRA 10548-2, chapter 6.
ly check to
) a furnace
2.12
Boiler Mounting and Accessories
irged of all
Boiler mountings are the machine components that are mounted over the body of the boiler itself
for the safety of the boiler and for complete control of the process of steam generation. Various
boiler mountings are as under:
: ignition is
1) Pressure gauge
2) Water Level Indicator
3) Fusible plug
4) Safety Valve
i) Lever Safety Valve
ii) Spring Loaded safety Valve
5) Steam stop valve
6) Feed check valve
7) Blow off cock
185
List of boiler Mounting and their functions
Name of
Component
valves, rno
attachment:
Function in boiler operation
i
I
Safety valves
To permit the steam in the boiler to escape to atmosphere when pressure in
the steam space exceeds a certain specified limit.
Water level
indicators
To ascertain constantly and exactly the level of water in the boiler shell.
Ipressure gauge
Steam stop valve
To shut off or regulate the flow of steam from the boiler to the steam pipe or
from the steam pipe to the engine
Safety valv
without thei
Function
ITo extinguish fire in the event of water level in the boiler shell falling below a
certain specified limit.
IBIOW-Offcock
ITo drain out the water from the boiler for internal cleaning, inspection or
other purposes.
~andmud
oles
ITo allow men to enter inside the boiler fur inspection and repair.
/
~preh
eater
Function in boiler operation
I
Ilwaste heat recovery device in which the air to on its way to the furnace is
heated utilizing the heat of exhaust gases
Foo=-
ITo recover some of the heat being carried over by exhaust gases (This heat is
used to raise the temperature of feedwater supplied to the boiler)
ISteam
superheater
liTOsuperhearthe steam generated by boiler
IFeedpump
ITo raise the pressure of water and force it into the boiler
IInjector
liTo feed water in vertical and locomotive boilers
While this Tutorial can offer advice on this subject, definitive information should always be
sought from the appropriate standard. In the UK, the standard relating to the specification of
186
a
The earliest
pressure of
were easily
safety valve
sudden acce
spring balan
dangerous p
1856 John F
railways.
List 6f boiler Accessories and their functions
Name of
Component
A safety v
pressure ve
It is part of
other parts
lowpressur
ITo record the pressure at which the steam is generated in the boiler.
i) To allow the feed water to pass into the boiler.
Feed check valve ii) To prevent the back flow of water from the boiler in the event of the failure
of the feed pump.
IFUSibleplug
Safety vah
I
I
I
valves, mountings and fittings in connection with steam boilers is BS 759.Several key boiler
attachments will now be explained, together with their associated legislation where appropriate.
~
Safety valve regulations
pressure in
-.
A safety valve is a valve mechanism for the automatic release of a substance from a boiler,
pressure vessel, or other system when the pressure or temperature exceeds preset limits.
It is part of a bigger set named pressure safety valves (PSV) or pressure relief valves (PRV). The
other parts of the set are named relief valves, safety relief valves, pilot-operated relief valves,
low pressure safety valves, vacuum pressure safety valves.
I
mpipe or
'the
I
Safety valves were first used on steam boilers during the industrial revolution. Early boilers
without them were prone to accidental explosion.
failurel
Function and design
mg below a
-..
The earliest and simplest safety valve on the steam digester in 1679 used a weight to hold the
pressure of the steam, (this design is still commonly used on pressure cookers); however, these
were easily tampered with or accidentally released. On the Stockton and Darlington Railway, the
safety valve tended to go off when the engine hit a bump in the track. A valve less sensitive to
sudden accelerations used a spring to contain the steam pressure, but these (based on Salter
spring balances) could still be screwed down to increase the pressure beyond design limits. This
dangerous practice was sometimes used to marginally increase performance of a steam engine. In
1856 John Ramsbottom invented a tamper-proof spring safety valve which became universal on
railways.
~
d always be
:cification of
J
187
In the petrol
generation,
associated \\
valve. The s
be noted, as
that PSV's hi
•
•
Relie
vesse
Safet
opens
•
•
Safet
and Ii
Pilot-
comn
•
Safety valves also evolved to protect equipment such as pressure vessels (fired or not) and heat
exchangers. The term safety valve should be limited to compressible fluid application (gas,
vapor, steam).
The two general types of protection encountered in industry are thermal protection and flow
protection.
conne
Low
gas. l
is sms
•
Vacul
on a g
presS\l
•
Low
I
pressu
pressu
For liquid-packed vessels, thermal relief valves are generally characterized by the relatively '
small size of the valve necessary to provide protection from excess pressure caused by thermal
expansion. In this case a small valve is adequate because most liquids are nearly incompressible,
and so a relatively small amount of fluid discharged through the relief valve will produce a
substantial reduction in pressure.
Feed water ci
The feed watt
boiler. A boil
equivalent to
Flow protection is characterized by safety
in thermal protection. They are generally
of gas or high volumes of liquid must be
the vessel or pipeline. This protection can
pressure protection system (HIPPS).
valves that are considerably larger than those mounted
sized for use in situations where significant quantities
quickly discharged in order to protect the integrity of
alternatively be achieved by installing a high integrity
Technical terms
,.,.
188
This prevents
i
In the petroleum refining, petrochemical, chemical manufacturing, natural gas processing, power
generation, food, drinks, cosmetics and pharmaceuticals industries, the term safety valve is
associated with the terms pressure relief valve (PRV), pressure safety valve (PSV) and relief
valve. The generic term is Pressure relief valve (PRV) or pressure safety valve (PSV) It should
be noted, as most people think PRY and PSV are the same thing, they are not. The difference is
that PSV's have a manual lever to open the valve in case of emergency.
•
•
•
•
•
lot) and heat
lication (gas,
•
ion andflow
•
Relief valve (RV): automatic system that is actuated by static pressure in a liquid-filled
vessel. It specifically opens proportionally with increasing pressure.
Safety valve (SV): automatic system that relieves the static pressure on a gas. It usually
opens completely, accompanied by a popping sound.
Safety relief valve (SRV): automatic system that relieves by static pressure on both gas
and liquid.
Pilot-operated safety rellef valve (POSRV): automatic system that relieves by remote
command from a pilot on which the static pressure (from equipment to protect) is
connected.
Low pressure safety valve (LPSV): automatic system that relieves static pressure on a
gas. Used when difference between vessel pressure and the ambient atmospheric pressure
is small.
Vacuum pressure safety valve (vpSV): automatic system that relieves static pressure
on a gas. Used when the pressure difference between the vessel pressure and the ambient
pressure is small, negative and near the atmospheric pressure.
Low and vacuum pressure safety valve (LVPSV): automatic system that relieves static
pressure on a gas. The pressure is small, negative or positive and near the atmospheric
pressure.
he relatively .
d by thermal
Feed water check valves
eompressible,
The feed water check valve is installed in the boiler feed water line between the feed pump and
ill produce a
boiler. A boiler feed stop va,lve is fitted at the boiler shell. The check valve includes a spring
equivalent to the head of water in the elevated feed tank when there is no pressure in the boiler.
lose mounted
ant quantities
e integrity of
high integrity
This prevents the boiler being flooded by the static head from the boiler feed tank.
189
The maintel
The measur
by a numbe
the next few
TDS centre
This control
also referree
system may
water is cor
released to b
Boiler check valve
the water in
Under normal steaming conditions the check valve operates in a conventional manner to stop
return flow from the boiler entering the feed line when the feed pump is not running. When the
Ona manual
feed pump is running, its pressure overcomes the spring to feed the boiler as normal.
A typical au!
Because a good seal is required, and the temperatures involved are relatively low (usually less
than 100°C) a check valve with a EPDM (Ethylene Propylene) soft seat is generally the best
option.
)0.
Location of feed check valve
2.13
Boiler Water Quality Control
190
The maintenance of water quality is essential to the safe and efficient operation of a steam boiler.
The measurement and control of the various parameters is a complex topic, which is also covered
by a number of regulations. It is therefore covered in detail later in this Block. The objective of
the next few Sections is simply to identify the fittings to be seen on a boiler.
TDS control
This controls the amount of Total Dissolved Solids (TDS) in the boiler water, and is sometimes
also referred to as 'continuous'blowdown'. The boiler connection is typically DNl5 or 20. The
system may be manual or automatic. Whatever system is used, the IDS in a sample of boiler
water is compared with a set point; if the TDS level is too high, a quantity of boiler water is
released to be replaced by feedwater with a much lower TDS level. This has the effect of diluting
the water in the boiler, and reducing the TDS level.
nner to stop
On a manually controlled TDS system, the boiler water would be sampled every shift.
g. When the
A typical automatic TDS control system is shown in Figure
(usually less
ally the best
j
191
Pressure C
All boilers
Typical automatic TDS control system
gauge cons
Bottom blowdown
Bourdon n
This ejects the sludge or sediment from the bottom of the boiler. The control is a large (usually
maximum
DN25 to DN50) key operated valve. This valve might normally be opened for a period of about 5
Pressure ga
seconds, once per shift.
siphon tube
Figure illustrates a key operated manual bottom blowdown valve whereas Figure 3.7.8 illustrates
temperature;
an automated bottom blowdown valve and its typical position in a blowdown system.
vessels, and
Key operated manual bottom blowdown valve
I
192
~3
j
j
SMllboler
Typical position for an automated bottom blowdown valve
Pressure Guage
All boilers must be fitted with at least one pressure indicator. The usual type is a simple pressure
gauge constructed to EN 12953.The dial should be at least 150 mm in diameter and of the
Bourdon tube type, it should be marked to indicate the normal working pressure and the
arge (usually
od of about 5
7.8 illustrates
maximum permissible working pressure / design pressure.
Pressure gauges are connected to the steam space of the boiler and usually have a ring type
siphon tube which fills with condensed steam and protects the dial mechanism from high
temperatures. Pressure gauges may be fitted to other pressure containers such as blowdown
vessels, and will usually have smaller dials as shown in Figure.
1.
193
A gauge glass
conditions. Gi
level at 50 mr
Typica I pressure gauge with ring siphon
protector arou
Gauge glasses
Gauge glasses and fittings
All steam boilers are fitted with at least one water level indicator, but those with a rating of 100
kW or more should be fitted with two indica~ors. The indicators are usually referred to as gauge
chemicals in 1:
of corrosion
0]
When testing
glasses complying with EN 12953.
the gauge glas
the following]
• Close the wa
• Close the drs
relatively quicJ
and remedial a
• Close the stet
• Close the dr
working level
be taken
as
I
S(
gauges at least
hands, as a safe
194
Gauge glass and fittings
A gauge glass shows the current level of water in the boiler, regardless of the boiler's operating
conditions. Gauge glasses should be installed so that their lowest reading will show the water
level at 50 mm above the point where overheating will occur. They should also be fitted with a
protector around them, but this should not hinder visibility of the water level.
Gauge glasses are prone to damage from a number of sources, such as corrosion from the
chemicals in boiler water, and erosion during blowdown, particularly at the steam end. Any sign
ing of 100
) as gauge
of corrosion or erosion indicates that a new glass is required.
When testing the gauge glass steam connection, the water cock should be closed. When testing
the gauge glass water connections, the steam cock pipe should be closed. To test a gauge glass,
the following procedure should be followed:
• Close the water cock and open the drain cock for approximately 5 seconds
• Close the drain cock and open the water cock. Water should return to its normal working level
relatively quickly. If this does not happen, then a blockage in the water cock could be the reason,
and remedial action should be taken as soon as possible.
• Close the steam cock and open the drain cock for approximately 5 seconds.
• Close the drain cock and open the steam cock. If the water does not return to its normal
working level relatively quickly, a blockage may exist in the steam cock. Remedial action should
be taken as soon as possible. The authorised attendant should systematically test the water
gauges at least once each day and should be provided with suitable protection for the face and
hands, as a safeguard against scalding in the event of glass breakage.
195
Gauge glass guards
• Automatic
The gauge glass guard should be kept clean. When the guard is being cleaned in place, or
blowdown
removed for cleaning, the gauge should be temporarily shut-off. Make sure there is a satisfactory
amount of h
water level before shutting off the gauge and take care not to touch or knock the gauge glass.
needed as the
After cleaning, and when the guard has been replaced, the gauge should be test-d and the cocks
set in the correct position.
• Hand holej
installation
Maintenance
0
0
• Steam drum
The gauge glass should be thoroughly overhauled at each annual survey. Lack of maintenance
• Low- water
can result in hardening of packing and seizure of cocks. If a cock handle becomes bent or
burner or shi
distorted special care is necessary to ensure that the cock is set full open. A damaged fitting
certain point.
should be renewed or repaired immediately. Gauge glasses often become discoloured due to
catastrophic f
water conditions; they also become thin and worn due to erosion. Glasses, therefore, should be
• SUiface ble
renewed at regular intervals.
condensible s
A stock of spare glasses and cone packing should always be available in the boiler house.
• Circulating
of its heat.
• Feedwater e
2.14
Boiler Fitting and Accessories
be fitted to th(
• Safety valve: It is used to relieve pressure and prevent possible explosion of a boiler.
.• Tqpfeed: A
• Water level indicators: They show the operator the level of fluid in the boiler, also known as a
intended to re
sight glass, water gauge or water column is provided.
the limescale 1
• Bottom blowdown valves: They provide a means for removing solid particulates that condense
• Desuperheat
and lie on the bottom of a boiler. As the name implies, this valve is usually located directly on
steam drum de
the bottom of the boiler, and is occasionally opened to use the pressure in the boiler to push these
not need, or m
particulates out.
• Chemical inj.
• Continuous blowdown valve: This allows a small quantity of water to escape continuously. Its
Steam aecesse
purpose is to prevent the water in the boiler becoming saturated with dissolved salts. Saturation
• Main steam s
would lead to foaming and cause water droplets to be carried over with the steam -
• Steam traps:
a condition
known as priming. Blowdown is also often used to monitor the chemistry of the boiler water.
• Main steam s
• Flash Tank: High pressure blowdown enters this vessel where the steam can 'flash' safely and
Combustion a
be used in a low-pressure
• Fuel oil syste
system or be vented to atmosphere while the ambient pressure
• Gas system:
blowdown flows to drain.
196
• Automatic BlowdowniContinuous Heat Recovery System: This system allows the boiler to
place, or
blowdown only when makeup water is flowing to the boiler, thereby transferring the maximum
.tisfactory
amount of heat possible from the blowdown to the makeup water. No flash tank is generally
rge glass.
needed as the blowdown discharged is close to the temperature of the makeup water.
the cocks
• Hand holes: They are steel plates installed in openings in "header" to allow for inspections &
installation of tubes and inspection of internal surfaces.
• Steam drums internals, A series of screen, scmbber & cans (cyclone separators).
iintenance
• Low- water cutoff: It is a mechanical means (usually a float switch). that is used to tum off the
!s bent or
burner or shut off fuel to the boiler to prevent it from mnning once the water goes below a
ged fitting
certain point. If a boiler is "dry-fired"
-ed due to
catastrophic failure.
should be
• Surface blowdown line: It provides a means for removing foam or other lightweight non-
(burned without water in it) it can cause rupture or
condensible substances that tend to float on top of the water inside the boiler.
• Circulatin~pump: It is designed to circulate water back to the boiler after it has expelled some
se.
of its heat.
• Feedwater check valve or clack valve: A non-return stop valve in the feedwater line. This may
be fitted to the side of the boiler, just below the water level, or to the top of the boiler.
_. Top feed.' A check valve (clack valve) in the feed water line, mounted on top of the boiler. It is
intended to reduce the nuisance of timescale. It does not prevent timescale formation but causes
known as a
the limescale to be precipitated in a powdery form which is easily washed out of the boiler.
• Desuperheater tubes or bundles,' A series of tubes or bundles of tubes in the water dmm or the
It condense
steam drum designed to cool superheated steam. Thus is to supply auxiliary equipment that does
directly on
not need, or may be damaged by, dry steam .
•push these
• Chemical injection line.' A connection to add chemicals for controlling feedwater
Steam accessories
nuously, Its
• Main steam stop valve:
. Saturation
• Steam traps:
a condition
• Main steam stop/Check valve: It is used on multiple boiler installations,
r water.
Combustion
, safely and
accessories
• Fuel oil system:
ent pressure
• Gas system:
I
.1.
197
• Coal system:
• Snot blower
• Balanced
Other essential items
draught, Thi
• Pressure gauges;
distance throi
l<
forced draugh
• Feed pumps
• Fusible plus
• Inspectors test pressure gauge attachment:
2.16
Let
• Name plate
In this chapn
• Registration plate
function. To 1
U!
a BT to shou
2.15
Controlling
retention of
Draught
Most boilers now depend on mechanical draught equipment rather than natural draught. This is
knowledge on
because natural draught is subject to outside air conditions and temperature of flue gases leaving
the furnace, as well as the chimney height. I,n these factors make proper draught hard to attain
2.17
Checlc
and therefore make mechanical draught equipment much more economical. There are three types
1. Explai
of mechanical draught:
2. What~
3. Explai
• Induced draught: This is obtained one of three ways, the first being the "stack effect" of a
The denser column of ambient air forces combustion air into and through the boiler. The second
4. Whati
S. Explai
method is through use of a steam jet. The steam jet oriented in the direction of flue gas flow
6. Drawt
heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler.
induces Due gasses into the stack and allows for a greater flue gas velocity increasing the overall
draught in the furnace, this method was common on steam driven locomotives which could not
have tall chimneys. The third method is by simply using an induced draught fan (10 fan) which
removes flue gases from the furnace and foroes the exhaust gas up the stack. Almost all induced
draught furnaces operate with a slightly negative pressure.
• Forced draught: Draught is obtained by forcing air into the furnace by means of a fan (FD fan)
and ductwork. Air is often passed through an air heater; which, as the name suggests, heats the
air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are
used to control the quantity of air admitted to the furnace, Forced draught furnaces usually have
a positive pressure.
198
2.18
Refere
~ Frederi
Techni
~ Frederi
Boilers
• Balanced draught: Balanced draught is obtained through use of both induced and forced
draught. This is more common with larger boilers where the flue ga~es have to travel a long
distance through many boiler passes. The induced draught fan works in conjunction with the
forced draught fan allowing the furnace pressure to be maintained slightly below atmospheric.
2.16
Let us Sum Up
In this chapter we have discussed boiler terminology, construction, types, co.nponents, and
function. To help you understand this information, go down to the fireroom on your ship and ask
a BT to show and explain to you the things you have just read about. This should help your
retention of the information you have studied and perhaps provide you with additional
knowledge on boilers.
It.
This is
~sleaving
2.17
I to attain
tree types
1. Explain Boiler Terminology.
2. What are the different types of circulations in boilers?
feet" of a
he boiler.
he second
gas flow
3. Explain the detailed arrangement of steam and water spaces.
4. What is Auxiliary boiler?
5. Explain Boiler mounting and different types accessories.
6. Draw the diagram of boiler fittings and accessories and label the same.
he overall
could not
an) which
llinduced
1(FD
fan)
2.18
>
>
Reference
Frederick M. Steingress (2001). Low Pressure Boilers (4th Edition ed.). American
Technical Publishers.
Frederick M. Steingress, Harold J. Frost and Darryl R. Walker (2003). High Pressure
Boilers ,(3rdEdition ed.). American Technical Publishers.
, heats the
mpers are
ually have
1~
UNIT 3
VAPOUR POWER CYCLE
Structure
3.1.
3.2.
3.3.
3.4.
3.5.
3.6.
3.7.
3.8.
Objective
Introduction
Rankine Cycle
Camot Vapor Power Cycle
Thermal Efficiency of Rankine Cycle
Let us Sum Up
Reference
3.1
Objective
- Carnot Vapor Power Cycle
Rankine Cycle
3.2
Introduction
In any thermodynamic process, the use of working fluid gas or vapour is an essential working
medium to convert heat into work. A cycle, which continuously converts heat into work is called
the power cycle. In a power cycle, the working fluid performs the various processes, which are
suction, compression, expanding, condensing, etc. All these processes are performed repeatedly
to generate the work or converting heat in to work. If the steam is alternatively vaporised and
condensed, then the working cycle is called vapour power cycle.
There are various types of working fluids available such as steam, sodium, potassium and
mercury. Some working fluids are used at high temperatures and some are at low temperatures.
The steam is the mostly used working fluid in the vapour power cycles. The steam has the
various desirable characteristics such as low cost, easy availability and high enthalpy of
vaporization.
In this unit we will be discussing about the vapour power cycles, which are mostly used for
steam power plants. The steam power plants are classified as coal plants, nuclear plants, natural
gas or geothermal plants, depending on the type of fuel used to supply the heat to generate the
steam.
200
3.3
Ran
The Rankir
closed loop,
throughout
plants. It is
cycle is the j
Description
Physicallayo
A Rankine c
in power gen
the combustii
The Rankine
efficient turb
difference is 1
Rankine cyc).
working fluid
pumping the
vaporization (
condensation
through the c.
very small fra
fluid as a gas j
The efficiencj
reaching sUP,e
3.3
Rankine Cycle
---------------------------
The Rankine cycle is a cycle that converts h~at into work. The heat is supplied externally to a
closed loop, which usually uses water. This cycle generates about 80% of all electric power used
throughout the world, including virtually all solar thermal, biomass, coal and nuclear power
plants. It is named after William John Macquorn Rankine, a Scottish polymath. The Rankine
cycle is the fundamental thermodynamic underpinningof the steam engine.
Description
Physical layout of the four main devices used in the Rankine cycle
l working
(is called
which are
epeatedly
irised and
sium and
peratures.
n has the
.thalpy of
{ used for
ts, natural
:nerate the
A Rankine cycle describes a model of steam-operated heat engine most commonly found
in power generation plants. Common heat sources for power plants using the Rankine cycle are
the combustion of coal, natural gas and oil, and nuclear fission.
.,
The Rankine cycle is sometimes referred to as a practical Camot cycle because, when an
efficient turbine is used, the TS diagram begins to resemble the Camot cycle. The main
difference is that heat addition (in the boiler) and rejection (in the condenser) are isobaric in the
Rankine cycle and isothermal in the theoretical Camot cycle. A pump is used to pressurize the
working fluid received from the condenser as a liquid instead of as a gas. All of the energy in
pumping the working fluid through the complete cycle is lost, as is all of the energy of
vaporization of the working fluid, in the boiler. This energy is lost to the cycle in that first, no
condensation takes place in the turbine; all of the vaporization energy is rejected from the cycle
through the condenser. But pumping the working fluid through the cycle as a liquid requires a
very small fraction of the energy needed to transport it as compared to compressing the working
fluid as a gas in a compressor (as in the Camot cycle).
The efficiency of a Rankine cycle is usually limited by the working flu~ Without the pressure
reaching sup,ercritical levels for the working fluid, the temperature range the cycle can operate
201
over is quite small: turbine entry temperatures are typically 565°C (the creep limit of stainless
steel) and condenser temperatures are around 30°C. This gives a theoretical Carnot efficiency of
about 63% compared with an actual efficiency of 42% for a modem coal-fired power station.
This low turbine entry temperature (compared with a gas turbine) is why the Rankine cycle is
often used as a bottoming cycle in combined-cycle gas turbine power stations.
Ts diagram
There are fou
in the diagran
•
The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The
water vapor with entrained droplets often seen billowing from power stations is generated by the
cooling systems (not from the closed-loop Rankine power cycle) and represents the waste energy
heat (pumping and vaporization) that could not be converted to useful work in the turbine. Note
that cooling towers operate using the latent heat 'of vaporization of the cooling fluid. The white
billowing clouds that form in cooling tower operation are the result of water droplets that are
entrained in the cooling tower airflow; they are not, as commonly thought, steam. While many
substances could be used in the Rankine cycle, water is usually the fluid of choice due to its
favorable properties, such as nontoxic and unreactive chemistry, abundance, and low cost, as
well as its thermodynamic properties.
One of the principal advantages the Rankine cycle holds over others is that during the
compression stage relatively little work is required to drive the pump, the working fluid being in
its liquid phase at this point. By condensing the fluid, the work required by the pump consumes
only 1% to 3% of the turbine power and contributes to a much higher efficiency for a real cycle.
The benefit of this is lost somewhat due to the lower heat addition temperature. Gas turbines, for
instance, have turbine entry temperatures approaching 1500°C. Nonetheless, the efficiencies of
actual large steam cycles and large modem gas turbines are fairly well matched.
0
•
•
•
Proce
liquid
Proce
pressu
requin
chart a
Proces
decrea.
Theou
steam 1
Proces
pressur
In an ideal Ra
would generat
would be repre
Carnot cycle.
region after the
Equations
The four processes in the Rankine cycle
In general, the '
400
TJtherm
350
E
300
2
250
i
200
Each of the nex
volume. ?therm d
to heat input. A
can be simplifie
i!
§
150
100
--_
50
0
0.0
1.0
2.0
3.0
S.O
4.0
entropy, s (kl/kgK)
202
6.0
7.0
8.0
m.
stainless
.iency of
. station.
cycle is
Ts diagram of a typical Rankine cycle operating between pressures of O.06bar and 50bar
There are four processes in the Rankine cycle. These states are identified by numbers (in brown)
in the diagram above.
•
tly. The
:d by the
e energy
ne. Note
he white
that are
ile many
ue to its
cost, as
ring the
being in
onsumes
:at cycle.
nnes, for
encies of
•
•
•
Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a
liquid at this stage the pump requires little input energy.
Process 2-3: The high pressure liquid enters a boiler where it is heated at constant
pressure by an external heat source to become a dry saturated vapor. The input energy
required can be easily calculated using mollier diagram or h-s chart orenthalpy-entropy
chart also known as steam tables.
Process 3-4: The dry saturated vapor expands through a turbine, generating power. This
decreases the temperature and pressure of the vapor, and some condensation may occur.
The output in this process can be easily calculated using the Enthalpy-entropy chart or the
steam tables.
Process 4-1: The wet vapor then enters a condenser where it is condensed at a constant
pressure to become a saturated liquid.
In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump and turbine
would generate no entropy and hence maximize the net work output Processes 1-2 and 3-4
would be represented by vertical lines on the T -S diagram and more closely resemble that of the
Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat
region after the expansion in the turbine, which reduces the energy removed by the condensers.
Equations
In general, the efficiency of a simple Rankine cycle can be defined as:
.
~
_H';_f:ur:..;:.:...::,:bI:..:.,:n:.=.,f!
c:
Each of £he next four equations is easily derived from the energy and mass balance for a control
volume. ?therm defines the thermodynamic efficiency of the cycle as the ratio of net power output
to heat input. As the work required by the pump is often around 1% of the turbine work output, it
can be simplified.
...
r7pM)
-
Qin
_- h3 -'''2
J.._
-.m
Qmd
-.-=h",-h1
m
203
fllpump
= ~ _ h1
m.
'{tilturbine
•
m
_
-
h
3 -
h
4
Variations
Real Rankine cycle (non-ideal)
r-~------------------
-.
T-s diagram for steam
Rankine cycle
Rankine cycle with superheat
The overall tI
In a real Rankine cycle, the compression by the pump and the expansion in the turbine are not
isentropic. In other words, these processes are non-reversible and entropy is increased during the
two processes. this somewhat increases the power required by the pump and decreases the
power generated by the turbine.
In particular the efficiency of the steam turbine will be limited by water droplet formation. As
the water condenses, water droplets hit the turbine blades at high speed causing pitting and
erosion, gradually decreasing the life of turbine blades and efficiency of the turbine. The easiest
way to overcome this problem is by superheating the steam. On the Ts diagramabove, state 3 is
above a two phase region of steam and water so after expansion the steam will be very wet. By
superheating, state 3 will move to the right of the diagram and hence produce a dryer steam after
expansion.
average heat i
the steam into
basic Rankine
two of these a
Rankine cycle
In this variati
pressure. Afte
reheated befor
prevents the Vl
blades, and im
higher tempers
Regenerative
204
Variations of the basic Rankine cycle
.---~~------------------~
T-$ diagram for steam
'rei
7011
600
500
400
300
200
100
Rankine cycle with reheat
The overall thermodynamic efficiency (of almost any cycle) can be increased by raising the
e are not
luring the
eases the
ation.As
tting and
b.eeasiest
state 3 is
ywet By
:eamafter
(
average heat input temperature
tin =
ds)
T
Qin.
-=-=--/23
of that cycle. Increasing the temperature of
the steam into the superheat region is a simple way of doing this. There are also variations of the
basic Rankine cycle which are designed to raise the thermal efficiency of the cycle in this way;
two of these are described below.
Rankine cycle with reheat
In this variation, two turbines work in series. The first accepts vapor from the boiler at high
pressure. After the vapor has passed through the first turbine, it re-enters the boiler and is
reheated before passing through a second, lower pressure turbine. Among other advantages, this
prevents the vapor from condensing during its expansion which can seriously damage the turbine
blades, and improves the efficiency of the cycle, as more of the heat flow into the cycle occurs at
higher temperature.
Regenerative Rankine cycle
205
T-s diagram for steam
absorptic
2. Isentrop
r rc]
700
this step
thennall)
working
temperan
3. Reversib
(isothern
surroundi
low tempe
4. Isentropi
in Figure
During th
temperatu
600
500
400
)00
200
tOO
Regenerative Rankine cycle
The regenerative Rankine cycle is so named because after emerging from the condenser
(possibly as a subcooled liquid) the working fluid is heated by steam tapped from the hot portion
.of the cycle. On the diagram shown, the fluid at 2 is mixed with the fluid at 4 (both at the same
pressure) to end up with the saturated liquid at 7. This is called "direct contact heating". The
Regenerative Rankine cycle (with minor variants) is commonly used in real power stations.
Another variation is where bleed steam from between turbine stages is sent to feedwater heaters
to preheat the water on its way from the condenser to the boiler. These heaters do not mix the
input steam and condensate, function as an ordinary tubular heat exchanger, and are named
"closed feedwater heaters".
The regenerative features here effectively raise the nominal cycle heat input temperature, by
reducing the addition of heat from the boiler/fuel source at the relatively low feedwater
temperatures that would exist without regenerative feedwater heating. This improves the
efficiency of the cycle, as more of the heat flow into the cycle occurs at higher temperature.
3.4
gas mak
Carnot Cycle
The Carnot cycle when acting as a heat engine consists of the following steps:
1. Reversible isothermal expansion of the gas at the "hot" temperature, TH (isothermal
heat addition). During this step (A to B on Figure 1, 1 to 2 in Figure 2) the expanding
206
imount of
r
L...--...l+--f1---s
n will be
.ervoir will
[cycle in h- 8 coordinates]
Figure 8.7: Carnot cycle with two-phase medium
A Carnot cycle that uses a two-pease fluid as the working medium is shown below in Figure 8.7.
Figure 8.7(a) gives the cycle in P - v coordinates, Figure 8.7(b) in T - 8 coordinates, and
Figure 8.7(c) in h- s coordinates. The boundary of the region in which there is liquid and vapor
both present (the vapor dome) is also indicated. Note that the form of the cycle is different in the
T - 8 and h : s representation; it is only for a perfect gas with constant specific heats that
cycles in the two coordinate representations have the same shapes.
The processes in the cycle are as follows:
1. Start at state awith saturated liquid (all of mass in liquid condition). C~
out a
reversible isothermal expansion to b( a - b) until all the liquid is vaporized. During
~;
this process a quantity of heat
temperature
qH
per unit mass is received from the heat source at
T2
Tl
2. Reversible adiabatic (i.e., isentropic) expansion ( b -;. C) lowers the temperature to
Generally state cwill be in the region where there is both liquid and vapor.
Tl
3. Isothermal compression ( c -;. d) at
gy extracted
usually the
qL
to state d. During this compression, heat
r.
per unit mass is rejected to the source at
4. Reversible adiabatic (i.e., isentropic) compression ( d condenses to liquid and the state returns to a.
a) in which the vapor
dee!
tlH
abe!
qL
. The
T - s diagram the heat received,
, is
and the heat rejected,
, is
net work is represented by abed, The thermal efficiency is given by
In the
coordinates]
209
In the h: II diagram, the isentropic processes are vertical lines as in the T - S diagram. The
isotherms in the former, however, are not horizontal as they are in the latter. To see their shape
we note that for these two-phase processes the isotherms are also lines of constant pressure
P=P(T)
(isobars), since
Similarly
. The combined first and second law is
dp
Tds=dh--.
p
dqrc,'
For a constant pressure reversible process,
pressure line in h: s ~rdinates
is thus,
( :~)
p
= Tds = dh
The slope of a constant
= T = constant; slope of constant pressure line for two-phase medium.
The heat received and rejected per unit mass isgiven in terms of the enthalpy at the different
states as
qH
=hb-ha
The pump
Therefore,
(In accord with our convention this is less than zero.)
The thermal efficiency is
Wnct
qH
TJ=--=--qH
+ qL
qH
(hE, - hG,) + (hd - he)
(hb - ha)
or, in terms of the work done during the isentropic compression and expansion processes, which
correspond to the shaft work done on the fluid and received by the fluid,
1J =
3.5
(hE, - he) - (hG - htJ.)
.
(hb - hll.)
Consider one kg of working fluid, and applying first law to flow system to various processes
with the assumption of neglecting changes in potential and kinetic energy, we can write,
&!-w=dh
For process 2-3, ~ w
= 0 (heat
addition process), we
can write,
Note that t
4-1' with
temperatur
than the co
Reasons fo
Power PIa
1) It is ver
(refer T-s
condense t
2) In the ra
difficulty. ]
constant tel
means that
to achieve i
3.6
Let
In this unit
For process 3-4; ~ q
= 0 (adiabatic
working flu
steam powe
improving t
process)
210
9"am. The
heir shape
t
pressure
1
constant
Similarly,
edium.
e different
The pump work (ow)pumpis negligible, because specific volume of water is very small.
Therefore,
Note that the rankine cycle has a lower efficiency compared to corresponding Carnot cycle 2'-34-1' with the same maximum and minimum temperatures. The reason is that the average
temperature at which heat is added in the rankine cycle lies between T2and T12 and is thus less
than the constant temperature TI2 at which heat is added to the Carnot cycle.
Reasons for Considering Rankine Cycle as an Ideal Cycle For Steam
:ses, which
; processes
.e,
Power Plants:
1) It is very difficult to build a pump that will handle a mixture of liquid and vapor at state l'
(refer T-s diagram) and deliver saturated liquid at state 2'. It is much easier to completely
condense the vapor and handle only liquid in the pump.
2) In-the rankine cycle, the vapor may be superheated at constant pressure from.J to 3" without
difficulty. In a Camot cycle using superheated steam, the superheating will have to be done at
constant temperature along path 3-5. During this process, the pressure has to be dropped. This
means that heat is transferred to the vapor as it undergoes expansion doing work. This is difficult
to achieve in practice.
3.6
Let us Sum Up
In this unit we have studied about vapour power cycles. We have also studied about various
working fluids used in the vapour power cycles and their effects. It explains that in most of the
steam power plants, Carnot vapour cycle is used as an Ideal cycle. We have learned about
improving the efficiency of vapor power cycles, by changing the thermodynamic variables. The
211
thermal efficiency of Rankine cycle's increased by Increasing the average temperature at which
heat is added to the cycle. Decreasing the average temperature at which heat is rejected to the
cycle. Finally, we conclude this unit explaining various advantages and disadvantage of vapour
power cycles used in steam power plants.
Structure
4.1. Obje
3.7
4.2. Intro
1. Briefly describe the working of Ideal reheat Rankine cycle. Also explain the advantages
of reheat Rankine cycle.
2. What are the various types of feed water heaters used in the regenerative Ranking cycle.
Explain its properties.
3. Explain the working ofCarnot cycle with the aid ofPV and T-S diagram.
4. Explain the differences in Carnot cycle and Rankine cycle used in stearn power plants.
4.3. Clas
4.4. eycl
4.5. eycl
4.6. COlI
4.7. Let I
4.8. ChC4
3.8
Reference
4.9. Refe
~ Wiser, Wendell H. (2000). Energy resources: occurrence, production, conversion, use.
Birkhauser. p. 190.
~ Canada, Scott; O. Cohen, R. Cable, D. Brosseau, and H. Price (2004-10-25). "Parabolic
Trough Organic Rankine Cycle Solar Power Plant". 2004 DOE Solar Energy
Technologies (Denver, Colorado: US Department of Energy NREL). Retrieved 2009-0317.
~ Batton, Bill (2000-06-18). "Organic Rankine Cycle Engines for Solar Power". Solar 2000
conference. Barber-Nichols, Inc.. Retrieved 2009-03-18.
4.1
OJ
After stud
- Otto (
- Diesel
- Dural
- Comp
- Diese
- Airar
- Cones
4.2
III
The air ~
medium.
called ide
In
made. TIl
engine p
212
UNIT 4
e at which
l to the
of vapour
Air Standard Cycle
Structure
4.1. Objective
4.2. Introduction
idvantages
4.3. Classification
4.4. Cycles
cing cycle.
4.5. Cycles in Engines
4.6. Comparison of Otto, Diesel and Dual Cycles
r plants.
4.7. Let us Sum Up
4.9. Reference
rsion, use.
4.1
"Parabolic
zr Energy
d 2009-03Solar2000
Objective
After studying this unit, you should be able to know
- Otto Cycle
- Diesel Cycle
- Dural Combustion Cycle
- Comparison of Otto
- Diesel Dual Cycle
- Air and Fuel-vapour mixtures
- Concept of air fuel Cycle
4.l
Introduction
The air standard cycle is a cycle followed by a heat engine which uses air as the working
medium. Since the air standard analysis is the simplest and most idealistic, such cycles are also
called ideal cycles and the engine running on such cycles are called ideal engines.
In order that the analysis is made as simple as possible, certain assumptions have to be
made. These assumptions result in an analysis that is far from correct for most actual combustion
engine processes, but the analysis is of considerable value for indicating the upper limit of
213
performance. The analysis is also a simple means for indicating the relative effects of principal
variables of the cycle and the relative size of the apparatus.
Step 4
Assumptions
The exhaust
1.
2.
3.
4.
5.
For these
The working medium is a perfect gas with constant specific heats and molecular weight
corresponding to values at room temperature.
No chemical reactions Occur during the cycle. The heat addition and heat rejection
processes are merely heat transfer processes.
The processes are reversible.
Losses by heat transfer from the apparatus to the atmosphere are assumed to be zero in
this analysis.
The working medium at the end of the process (cycle) is unchanged and is at the same
condition as at the beginning of the process (cycle).
..
In the selecting an idealized process (j te is always faced with the fact that the simpler the
assumptions, the easier the analysis, but the iJrther the result from reality. The air cycle has the
advantage of being based on a few simple assumptions and of lending itself to rapid and easy
mathematical handling without recourse to thermodynamic charts or tables or complicated
calculations. On the other hand, there is 'tlways the danger of losing sight of its limitations and of
trying to employ it beyond its real usefulness.
fOI
times. Durill
two to and f
of classifical
other variabl
T~bIe 1: rc
Sl
No.
.
L
<r.
2.
Pe
3.
Lij
Oi
4.3
Classification
The IC Engines are classified in several ways. The criteria of classifications are fuel, cycle of
4.
He
Oi
operation, method of ignition, number of strokes of piston to complete cycle.
The necessary operations have already been spelt out in last section. Here we enumerate again
.The fuel, and
starting from the piston position at C , which is called top dead centre.
I
which cycle'
Step 1
turn depend .
The suction, the charge is inducted and cylinder is completely filled, the piston reaching C
2
which is called bottom dead centre.
such that car
Step 2
spark plug w
The compression,
the charge is compressed to clearance volume, resulting in high pressure and
temperature. Ignition occurs here.
highest temp
ceramic tube
Step3
The expansion,
design stage.
A petrol engi
due to temperature and pressure gas expands, piston is pushed to bottom dead
centre under great pressure and torque is generated on crank. It is also called power stroke.
214
self ignition.
of the engine
principal
Step 4
The exhaust, the residual gases are pushed out of the cylinder by motion of the piston.
For these four steps to complete the piston may move between TDC and BDC two times or four
IT
weight
times. During one to and fro motion the crank rotates once (i.e. one rotation) but if piston has
two to and fr? motions the crank rotates two times (i.e. two rotations). We will take up this type
rejection
of classification at a later stage. Table 1 describes the engine classification based on fuels while
other variables are also mentioned.
e zero in
the same
T~ble 1: IC Engine Classification
SL
No.
L
npler the
e has the
and easy
nplicated
as and of
Fuel
Type
Gas
Based on Fuels
Specific
Idea)
Fuel
Cycle
Coal gas,
3.
4.
Spark or
Heated
tube
Hit and miss.,
Throttle
Spm
1hrottle
Spark or
Throttle
Petrol
Petrol
Benzol
Alcohol
Otto cycle
FOUl'or
Two
Light
Paraffin
Otto cycle
Four or
Two
ate again
Heavy
Oil
Diesel or
Paraffin
Governinl
Fouc
Producer
gas
2.
Ipinon
Otto cycle
Oil
cycle of
Piston
Strokes
Diesel 01
Dual cycle
Four or
Two
Hot
Tube
Se1f
.
Change of off
point of cut
The fuel, and ideal cycles have already been discussed in the last unit. Which fuel is to be used in
which cycle will depend upon the highest temperature achieved after compression, which will in
turn depend upon the compression ratio. The compression ratio of course is to be fixed during
ching C
design stage. If compression ratio is around 8 or 8.5 then the highest temperature may not be
2
such that can cause the ignition and a separate device to initiate ignition may be required. A
spark plug which strikes a spark at desired moment is very commonly used in IC Engine where
isure and
highest temperature may not cause self ignition. The other igniting devices are porcelain or
ceramic tube which may be heated from outside or may retain heat from previous combustion.
A petrol engine works on Otto cycle in which highest temperature is not high enough to cause
om dead
:e.
self ignition. Hence, a spark plug ignition is used. In this case, the charge sucked in the cylinder
of the engine contains a mixture of vapourized .petrol and air. In earlier engine, a carburetor was
215
used for vapourizing the petrol and allow the vapours to mix with air in its passage. In new
design, the liquid petrol is injected in air stream through inlet manifolds which are the air
passages attached to the body of the engine. The injection occurs at several points hence the
system is known as multipoint fuel injection (MPFI). The control of quantity of fuel becomes
much more convenient and accurate in MPFI than in carburettor system.
In a diesel engine which works on Diesel standard air cycle and bums diesel fuel the
compression ratio is higher than 14. The resulting temperature and pressure are much higher than
in petrol engine. The fuel is allowed to enter in the clearance space at high pressure and
o
.
0
.
temperature (675 C) is greater than the self ignition temperature (400 C) of fuel. The fuel burns
and expansion occurs causing a great force on the piston. Since compression produces required
temperature for ignition the engine is also called compression ignition (Cl) engine. For pushing
the fuel inside the cylinder at high pressure a device, called injector, is used and process is called
injection. The amount of fuel to be injected is controlled by the injector itself.
In gas engines, the gas and air through separate passages are entered through a single valve
controlled passage into the cylinder.
4.4 Cycles
A cycle has already been defmed as sequence of processes which end in the same final state of
the substance as the initial. The heat engines are devices which produce work by using heat from
a reservoir and rejecting heat to another constant temperature reservoir called heat sink. Perhaps
in earlier days some heat engines were developed which directly used the heat from sun, hitherto
all engines have been using heat produced from combustion of fuel. Apart from heat source the
engine has to have some working fluid that will absorb and reject heat and undergo such
processes as expansion are compression. For theoretical study of cycles for engines it is assumed
that some working fluid remains in the machine and undergoes different processes over and over
again. A number of standard cycles, consisting of well known processes have been developed.
We will study a few of them.
Carnot Cycle
A~
This cycle was proposed-by Sadi Carnot in 1824 and has the highest possible efficiency for any
cycle. Figures 1 and 2 show the P-V and T-s diagrams of the cycle.
216
compressi(
is added is
e. In new
re the air
hence the
l becomes
TH
. fuel the
igher than
:ssure and
T
fuel burns
s required
or pushing
;s is called
Tc
agle valve
Fig. I
ial state of
p
;heat from
k, Perhaps
10, hitherto
source the
T,
lergo such
is assumed
T,
er and over
developed.
V
Figl
lCY
for any
Assuming that the charge is introduced into the engine at point 1, it undergoes isentropic
compression from 1 to 2. The temperature of the charge rises from Tmin to Tmax. At point 2, heat
is added isothermally. This causes the air to expand, forcing the piston forward, thus doing work
217
on the piston. At point 3, the source of heat is removed and the air now expands isentropically to
point 4, reducing the temperature to Tmin in the process. At point 4, a cold body is applied to the
end of the cylinder and the piston reverses, thus compressing the air isothermally; heat is rejected
to the cold body. At point 1, the cold body is removed and the charge is compressed
isentropically till it reaches a temperature Tmax once again. Thus, the heat addition and rejection
processes are isothermal while the compression and expansion processes are isentropic.
Hen
From thermodynamics, per unit mass of charge
Heat supplied from point 1to 2= P2v2ln
Heat rejected from point 4 -to 1= P4 v4ln
Now
P2V2
= RTmax
v)
(5)
v2
2
v4
(6)
(7)
FroII
the maximu
minimum te
According t<
whichhappe
(8)
Since Work done, per unit mass of charge, W
= Rln(rXr_
-Tmin)
= heat
supplied - heat rejected
(9)
We have assumed that the compression and expansion ratios' are equal, that is
218
This
the heat engi
In other wor
should reject
combustion (
possible tern
Moreover, in
to obtain th€
modem heat
temperature <
It is iJ
engine, it W(J
oically to
ed to the
(10)
l rejected
npressed
rejection
Heat supplied Qs
= R Tmax In (r)
(10)
Hence, the thermal efficiency of the cycle is given by
n; =
Rln(rXTmax - Tmin)
Rln (r )
t.:
(11)
From Eq. 11 it is seen that the thermal efficiency of the Carnot cycle is only a function of
the maximum and minimum temperatures of the cycle. The efficiency will increase if the
minimum temperature (or the temperature at which the heat is rejected) is as low as possible.
According to this equation, the efficiency will be equal to 1 if the minimum temperature is zero,
which happens to be the absolute zero temperature in the thermodynamic scale.
This equation also indicates that for optimum (Carnot) efficiency, the cycle (and hence
the heat engine) must operate between the limits of the highest and lowest possible temperatures.
In other words, the engine should take in all the heat at as high a temperature as possible and
should reject the heat at as Iowa temperature as possible. For:the first condition to be achieved,
combustion (as applicable for a real engine using fuel to provide heat) should begin at the highest
possible temperature, for then the irreversibility of the chemical reaction would be reduced.
Moreover, in the cycle, the expansion should proceed to the lowest possible temperature in order
to obtain the maximum amount of work. These conditions are the aims of all designers of
modem heat engines. The conditions of heat rejection are governed, in practice, by the
temperature of the atmosphere.
It is impossible to construct an engine which will work on the Carnot cycle. In such an
engine, it would be necessary for the piston to move very slowly during the first part of the
219
forward stroke so that it can follow an isothermal process. During the remainder of the forward
stroke, the piston would need to move very quickly as it has to follow an isentropic process. This
variation in the speed of the piston cannot be achieved in practice. Also, a very long piston stroke
would produce only a small amount of work most of which would be absorbed by the friction of
the moving parts of the engine.
Since the efficiency of the cycle, as given by Eq. 11, is dependent only on the maximum
and minimum temperatures, it does not depend on the working medium. It is thus independent of
the properties of the working medium.
Th
constant,
these proc
Th
Piston Engine Air Standard Cycles
The cycles described here are air standard cycles applicable to piston engines. Engines bases on
these cycles have been built and many of the engines are still in use.
Otto Cycle
The Otto cycle, which was first proposed by a Frenchman, Beau de Rochas in 1862, was first
used on an engine built by a German, Nicholas A. Otto, in 1876. The cycle is also called a
constant volume or explosion cycle. This is the equivalent air cycle for reciprocating piston
engines using spark ignition. Figures 3 show the P-V and T -s diagrams respectively.
P
1
Adiabatic
3
v
Fig 3
At the start of the cycle, the cylinder contains a mass M of air at the pressure and volume
indicated at point 1. The piston is at its lowest position. It moves upward and the gas is
compressed isentropically to point 2. At this point, heat is added at constant volume which raises
the pressure to point 3. The high pressure charge now expands isentropically, pushing the piston
down on its expansion stroke to point 4 where the charge rejects heat at constant volume to the
initial state, point 1.
220
the heat re
and the the
forward
icess. This
ston stroke
friction of
ie
The isothermal heat addition and rejection of the Carnot cycle are replaced by the
constant volume processes which are, theoretically more plausible, although in practice, even
these processes are not practicable.
maximum
pendent of
The heat supplied, Qs, per unit mass of charge, is given by
es bases on
the heat rejected, Qr per unit mass of charge is given by
2, was first
so called a
uing piston
and the thermal efficiency is given by
(19)
and volume
I the gas is
which raises
19 the piston
alume to the
Now
7;
= (V2
T2
V;
),-1 = (V3V )'-1 = T41;
4
221
Hence, substituting in Eq. 19, we get, assuming that r is the compression ratio
V,N2
=l-(~r
1
=1--
-r:'
From the u
(20)
CRis incre
In a true thermodynamic cycle, the term expansion ratio and compression ratio
are synonymous. However, in a real engine, these tw; ratios need not be equal because of the
valve timing and therefore the term expansion ratio is preferred sometimes.
Equation 20 shows that the thermal efficiency of the theoretical Otto cycle increases with
increase in compression ratio and specific heat ratio but is independent of the heat added
(independent of load) and initial conditions of pressure, volume and temperature.
A plot of thermal efficiency versus compression ratio for an Otto cycle. It is seen that the
inc~se in efficiency is significant at lower compression ratios. This is also seen in Table 1
given below.
R
n
1
0
2
0.242
3
0.356
4
0.426
5
0.475
6
0.512
7
0.541
222
CR is incre
CRis incre
Meaneff~
It is seen tl:
However, 1
done; allde
point 3, bes
mean effec
piston dispi
constant pn
equal to the:
wbereQ2-3
8
0.565
9
0.585
10
0.602
16
0.67
20
0.698
50
0.791
From the table it is seen that if:
CR is increased from 2 to 4, efficiency increase is 76%
ission ratio
ause of the
:reases with
heat added
een that the
in Table 1
CR is increased from 4 to 8, efficiency increase is only 32.6%
CR is increased from 8 to 16, et,]iciency increase is only 18.6%
MeaD efYedive pressure:
It is seen that the-air standard efficiency of the Otto cycle depends only on the compression ratio.
However, the pressures and temperatures at the various points in the cycle arid the net work
done; all depend upon the initial
pressure and temperature and the heat input from point 2 to
point 3, besides the compression ratio.
A quantity of special interest in reciprocating engine analysis is the
mean effective pressure. Mathematically, it is the net work done on the piston, W, divided by the
piston displacement volume, VI - V2. This quantity has the units of pressure. Physically, it is that
constant pressure' which, if exerted on the piston for the whole outward stroke, would yield work
equal to the work of the cycle. It is given by
mep=
W
V; - V2
(21)
where
<b-3 is the heat
added from points 2 to 3.
223
Now
v.I - v,'- = V.I
(1_ V.
V2 )
I
·
n;
Smce-=(
(22)
m
Here r is the compression ratio, VIN2
From the equation of state:
(23)
Ito is the universal
The dimens
compression
pressure is (
ambient) ten
gas constant
We can sub
Otto cycle ir
Substituting for VI from Eq. 3 in Eq. 2 and then substituting for VI - V2 in Eq. 1 we get
In terms oft
(24A)
The quantity Q2.31M is the heat added between points 2 and 3 per unit mass of air (M is
the mass of air and m is the molecular weight of air); and is denoted by Q', thus
We can obta
mep
="
Q'Plm
RT.
0
I
1-!
(24B)
r
We can non-dimensionalize
following equation
Another pan
following ex
the mep by dividing it by PI so that we can obtain the
ChoiceofQ
We have sai:
224
I15
Since ~
m
= cy(y
-1), we can substitute it in Eq. 25 to get
mep
Q'
PI
Cy~
-=T/-
1
(26)
[1-;]rr-1]
The dimensionless quantity mep/pi is a function of the heat added, initial temperature,
compression ratio and the properties of air, namely, Cv and 1. We see that the mean effective
pressure is directly proportional to the heat added and inversely proportional to the initial (or
ambient) temperature.
We can substitute the value of 11 from Eq. 20 in Eq. 26 and obtain the value of mep/pi for the
~et
Otto cycle in terms of the compression ratio and heat added.
In terms of the pressure ratio, P3/P2 denoted by rp we could obtain the value of mep/pi as follows:
mep
-;:=
r~p
-IXrr-1
-1)
(r-lXr-l)
(27)
of air (M is
We can obtain a value of
r, in terms ofQ' as follows:
r
p
= C rr::
Q'
y
+1
(28)
I
Another parameter, which is.of importance, is the quantity mep/pj. This can be obtained from the
n obtain the
following expression:
_m_ep_= _m_ep
__ l __ 1__
P3
PI rr
Q'
+1
c T.rrI
I
y
ChoiceofQ'
We have said that
225
(29)
Q' == Q2-3
M
(30)
M is the mass of charge (air) per cycle, kg.
At an ambient
value ofQ'/Cy'
Now, in an actual engine
Under fuel ric]
Under fuel lea:
(31)
Example;
Calculate effii
working fluid.
Me is the mass of fuel supplied per cycle, kg
Solution
Use r == 1.4 foi
Qc is the heating value of the fuel, kJ/kg
M, is the mass of air taken in per cycle
F is the fuel air ratio == MtIMa
r
Substituting for Eq. (B) in Eq. (A) we get
7
8
9
(32)
10
M" 1'; -V2
Now -~_,__~
M
And
1';
V; - V2 = 1_ .!.
1';
So, substituting for
MaIM from
Diesel
r
(33)
Eq. (33) in Eq. (32) we get
For isooctane, FQc at stoichiometric conditions is equal to 2975 kJ/kg, thus
226
This cycle, pre
engine, is also
for the reciprc
shown in Figs
Q' = 2975(r - 1)/r
(35)
At an ambient temperature, T1 of 300K and c, for air is assumed to be 0.718 kJ/kgK., we get a
value of Qvc.T, = 13.8(r- 1)/r.
Under fuel rich conditions, <p = 1.2, Q'/ c.T, = 16.6{r-l)/r.
Under fuel lean conditions, cp = 0.8, Q'I CvT. = 11.1(r- l)1r
Example;
Calculate efficiencies of a Carnot cycle for compression ratios of 7, 8, 9 and 10 for air as
working fluid.
Solution
Use 'Y = 1.4 for air,
1
fl-1---.,(r)1-1
r
,,-1
11 (r),-l
1"1 = l_l/(r),-l
7
2.18
0.46
0.54
8
2.30
0.435
0.565
9
2.41
0.415
0.584
10
2_51
0.400
0_600
Diesel Cycle
This cycle, proposed by a German engineer, Dr. Rudolph Diesel to describe the processes of his
engine, is also called the constant pressure cycle, This is believed to be the equivalent air cycle
for the reciprocating slow speed compression ignition engine. The P-V and T-s diagrams are
shown in Figs 8 and 9 respectively.
227
Fron
same as that
1-2: Adiabatic expansion
2-3: Constant volume heat rejection
3-4 : Adiabatic COI'Jl)ression
4-1 : Constant pressure heat addition
3
v
Fig. 4: Diesel Cycle
The cycle has processes which are the same as that of the Otto cycle except that the heat
is added at constant pressure.
The heat supplied, Qs is given by
whereas the heat rejected, Qr is given by
Substi
1
=1--
(36)
r
228
16
From the T-s diagram, Fig. 9, the difference in enthalpy between points 2 and 3 is the
same as that between 4 and 1, thus
hat the heat
..
:. T4
T.
=(1;)'T2
and ~
T2
=(VV;2)'-1
=
1
rr:'
Substituting in eq. 36, we get
TJdo
= 1 __ yl (-r1)' -I
(~r-
1
(37)
T3 -1
T2
Now T3
T2
= VV3 = r = cut - off ratio
2
c
(38)
]6
229
When Eq. 38 is compared with Eq. 20, it is seen that the expressions are similar except
for the term in the parentheses for the Diesel cycle. It can be shown that this term is always
greater than unity.
Now rc
= -V3 = -V3IV'--"-= -r
V2
V4~
r.
. the compreSSIOnratio
..
. rano
.
w here r IS
an d rvi
r, IS th e expanSIon
Substi
Thus, the thermal efficiency of the Diesel cycle can be written as
1
1] =1-r-I
(39)
r
Since
brackets in Eq
Let
r, = r - !l. since r is greater than reoHere, !l. is a small quantity. We therefore have
a quantity gre
than that for a
!l. .
If -
1:
r
thermal efficie
We can expand the last term binomially so that
From 1
forward stroki
involved, intrc
injection gear.
In prac
the compressi
With a mixtw
developed by
definite limit i
Thus (
cycle engines
We can expand the last term binotnially so that
230
WecaJ
lar except
is always
Substituting in Eq. 39, we get
:io
(40)
/1 /12 /13
Since the coefficients of -'-'-3 ' etc are greater than unity, the quantity in the
r r' r
brackets in Eq. 40 will be greater than unity. Hence, for the Diesel cycle, we subtract
have
}-l
times
r
a 'quantity greater than unity from one, hence for the same r, the Otto cycle efficiency is greater
than that for a Diesel cycle.
If /1 is small, the square, cube, etc of this quantity becomes progressively smaller, so the
r
thermal efficiency of the Diesel cycle will tend towards that of the Otto cycle.
From the foregoing we can see the importance of cutting off the fuel supply early in the
forward stroke, a condition which, because of the short time available and the high pressures
involved, introduces practical difficulties with high speed engines and necessitates very rigid fuel
injection gear.
In practice, the diesel engine shows a better efficiency than the Otto cycle engine because
the compression of air alone in the former allows a greater compression ratio to be employed.
With a mixture of fuel and air, as in practical Otto cycle engines, the maximum temperature
developed by compression must not exceed the self ignition temperature of the mixture; hence a
definite limit is imposed on the maximum value of the compression ratio.
Thus Otto cycle engines have compression ratios in the range of 7 to 12 while diesel
cycle engines have compression ratios in the range of 16 to 22.
We can obtain a value of'r, for a Diesel cycle in terms ofQ' as follows:
231
Use r=
Q'
r =----+1
C
cp T.lr r-I
(41)
We can substitute the value of n from Eq. 38 in Eq. 26, reproduced below and obtain the value of
mep/p, for the Diesel cycle.
or
User=
or
In terms of the cut-off ratio, we can obtain another expression for mep/p, as follows:
Dual
-1)- r~: -1)
(r -IX, -1)
mep _ rr1(rc
-;;:- -
(42)
For the Diesel cycle, the expression for mep/p, is as follows:
mep _ mep( 1 )
----P3
PI r"
(43)
Modern high speed diesel engines do not follow the Diesel cycle. The-process of heat
addition is partly at constant volume and partly at constant pressure. This brings us to the dual
cycle.
Example
Calculate the efficiency of a diesel cycle for which compression ratio is 14and cut off ratio is 2.
What will be the efficiency if cut off ratio is increased to 3. Given y = 1.4.
Solution
23~
An importar
maximum PI
acting on the
the strength 1
seen that the
that for a giv
ratio increas
maximumpr
rises to abou
limit the mru
also indicate
withstand hig
Use r = 14 and p = 2 with Y = 1.4 in Eq. (6.3).
1
'1=1-
=
1
the value of
or
TJ
21.4 - 1
x--1.4 (14)°·4
2- 1
1 - _1_ x 1.64
4.02
1
= 0.59
... (i)
= 59%.
Use r = 14 and p = 3 with Y = 1.4 in Eq. (6.3)
'1 =
=
or
1
31.4-1
1- 1.4(14)0.43-1
1 __ 1_ x 3.655 = 0.545
4.02
2
... (ii)
TJ = 54.5 0/0
follows:
Dual Combustion
rocess of heat
us to the dual
Cycle
An important characteristic of real cycles is the ratio of the mean effective pressure to the
maximum pressure, since the mean effective pressure represents the useful (average) pressure
acting on the piston while the maximum pressure represents the pressure which chiefly affects
the strength required of the engine structure. In the constant-volume cycle, shown in Fig. 5, it is
seen that the quantity mep/p, falls off rapidly as the compression ratio increases, which means
that for a given mean effective pressure the maximum pressure rises rapidly as the compression
ratio increases. For example, for a mean effective pressure of 7 bar and Q'!c"T1 of 12, the
maximum pressure at a compression ratio of 5 is 28 bar whereas at a compression ratio of 10, it
rises to about 52 bar. Real cycles follow the same trend and it becomes a practical necessity to
limit the maximum pressure when high compression ratios are used, as in diesel engines. This
also indicates that diesel engines will have to be stronger (and hence heavier) because it has to
withstand higher peak pressures.
it off ratio is 2.
233
p
5
1
whereas the
3
v
and the ther
Fig 5
Constant pressure heat addition achieves rather low peak pressures unless the compression ratio
is quite high. In a real diesel engine, in order that combustion takes place at constant pressure,
fuel has to be injected very late in the compression stroke (practically at the top dead center). But
in order to increase the efficiency of the cycle, the fuel supply must be cut off early in the
expansion stroke, both to give sufficient til ne for the fuel to bum and thereby increase.
combustion efficiency and reduce after burning but also reduce emissions. Such situations can be
achieved if the engine was a slow speed type so that the piston would move sufficiently slowly
for combustion to take place despite the late injection of the fuel. For modem high speed
compression ignition engines it is not possible to achieve constant pressure combustion. Fuel is
injected somewhat earlier in the compression stroke and has to go through the various stages of
combustion. Thus it is seen that combustion is nearly at constant volume (like in a spark ignition
engine). But the peak pressure is limited because of strength considerations so the rest of the heat
addition is believed to take place at constant pressure in a cycle. This has led to the formulation
of the dual combustion cycle. In this cycle, for high compression ratios, the peak pressure is not
allowed to increase beyond a certain limit and to account for the total addition, the rest of the
heat is assumed to be added at constant pressure. Hence the name limitedpressure cycle.
In the cycle, compression and expansion processes are isentropic; heat addition is partly
at constant volume and partly at constant pressure while heat rejection is at constant volume as in
the case of the Otto and Diesel cycles.
The heat supplied, Qs per unit mass of charge is given by
234
From
whereas the heat rejected, Qr per unit mass of charge is given by
(44A)
iression ratio
ant pressure,
Icenter). But
early in the
eby increase
ations can be
iently slowly
I high speed
stion. Fuel is
JUS stages of
park ignition
st of the heat
: formulation
.essure is not
Ie rest of the
(44B)
=1-
(44e)
From thermodynamics
icle.
tion is partly
volume as in
(45) the explosion or pressure ratio and
(46) the cut-off ratio.
Now T4 = P4 = P4 ELP3 P2
'7;
PI P3' P3 P2 PI
235
Sine
usually speci
correlate rp w
Thus
T
T.
__1_
I
= rpcrr
Wei
follows:
Therefore, the thermal efficiency of the dual cycle is
Wei
as follows:
(46)
We can substitute the value of n from Eq. 46 in Eq. 26 and obtain the value of mep/pj for
the dual cycle.
Figun
In terms of the cut-off ratio and pressure ratio, we can obtain another expression for
mep/p, as follows:
mep
.-;;:=
yrp rr(rc
-1)+rr~p -1)- r~p r: -1)
(r-1Xr-l)
For the dual cycle, the expression for mep/p, is as follows:
236
limited press
ratio and the
pressure cycl
(47)
Figur
for the same
that the air Sl
of 8 for the Ii
Since the dual cycle is also called the limited pressure cycle, the peak pressure, P3, is
usually specified. Since the initial pressure, PI. is known, the ratio P3/PI is known. We can
correlate rp with this ratio as follows:
We can obtain an expression for rc in terms of Q' and rp and other known quantities as
follows:
r. =
![[{
r
Q' _
cvipT'r'
1
}_!_]
+ (r -I)J
r
(50)
We can also obtain an expression for rp in terms ofQ' and rc and other known quantities
as follows:
nep/p, for
ession for
Figure 13 shows a constant volume and a constant pressure cycle, compared with a
limited pressure cycle. In a series of air cycles with varying pressure ratio at a given compression
ratio and the same Q', the constant volume cycle has the highest efficiency and the constant
pressure cycle the lowest efficiency.
Figure 14 compares the efficiencies of the three cycles for the same value of
Q,(_r_)
r-I
for the same initial conditions and three values of P~Pl for the dual cycle. It is interesting to note
that the air standard efficiency is little affected by compression ratio above a compression ratio
of 8 for the limited pressure cycle.
237
The sucti.
between t1
It is seen that a considerable increase in this ratio is obtained for a limited pressure cycle
as compared to the constant volume or constant pressure cycles.
4.5
Cycles in Engines
Standard air cycles were discussed in last unit. Out of these the Camot cycle is practically not
used. It involves an isothermal process followed by an adiabatic process in one stroke of the
piston. Isothermal process is very slow whereas adiabatic is very fast and practically it is
impossible to vary the speed of the piston in a single stroke.
The Otto and Diesel cycles are practically followed in petrol and diesel engines respectively.
There are gas engines and paraffin engines which work on Otto cycle. Diesel engine also works
on dual combustion cycle. The paraffin engine can also work on Diesel cycle.
It may be understood that for same compression ratio Otto cycle is more efficient than Diesel
cycle. Even otherwise, when diesel cycle works with a higher compression ratio this cycle is not
as efficient as Otto cycle. It is because burning of fuel at constant pressure is less efficient than
burning of fuel at constant volume. For this reason modern diesel engines operate on dual
combustion cycle in which, part of combustion occurs at constant volume also.
In actual engine cylinders, the entry and exit of gases takes place through valves which are
opened and closed at right moments. Separate mechanisms for valves are provided in the engine.
The pressure losses occur at the valves and the ideal cycles lose their sharpness at points where
process changes. For example, sharp change at point 2 in Figure 5 will not be practically as sharp
as shown in this Figure. If we obtain actual p V diagram from an engine, this diagram must be the
operation cycle of the engine. For example if pV diagram
is obtained from a spark ignition,
.
(petrol) engine then this diagram should be an Otto cycle. Further the ideal cycle assumes sofDe
air or medium being heated and cooled cycle after cycle but in actual engine fresh charge is
taken in and spent gases are exhausted. Therefore, this effect will also appear on the diagram
obtained from the engine. The p V diagram stnsed from the engine is called its indicator diagram.
Figure 6 shows an indicator diagram of a petrol engine and an Otto cycle is superimposed upon it
(shown in broken lines).
238
4.6
Co
The impor
compressic
compare th
factors mu
compressio
rejection ar
set of opere
Case 1: Sar
The Otto c
shown in p
and heat in}
The suction and exhaust lines can be clearly seen in indicator diagram. The area enclosed
between these lines represent the loss of work
ssure cycle
=
=
=
=
EVO Exhaust Valve Opens
EVC = Exhaust Valve Closes
SVO Suction Valve Opens
SVC Suction Valve Closes
IS
Ignition
,,
,,
,,
,
t
'-
p
ctically not
,,
,
'- ,
rake of the
SVC
ically it is
SVO
EVC
V-+
espectively.
also works
than Diesel
cycle is not
ficient than
ate on dual
Fig. 6: An Indicator Diagram of a Petrol Engine
4.6
Comparison of Otto, Diesel and Dual CYcles
The important variable factors which are used as the basis for comparison of the cycles are
compression ratio, peak pressure, heat addition, heat rejection and the net work. In order to
compare the performance of the Otto, Diesel and Dual combustion cycles, some of the variable
factors must be fixed. In this section, a comparison of these three cycles is made for the same
; which are
.the engine.
oints where
compression ratio, same heat addition, constant maximum pressure and temperature, same heat
rejection and net work output. This analysis will show which cycle is more efficient for a given
set of operating conditions.
illy as sharp
mustbethe
Case I: Same Compression Ratio and Heat Addition:
ark ignition
;
sumes sofIle
sh charge is
The Otto cycle 1-2-3-4-1, the Diesel cycle 1-2-3'-4'-1 and the Dual cycle 1-2-2"-3"-4"-1 are
shown in p-V and T-9 diagram in Fig.7 (a) and (b) respectively for the same compression ratio
and heat input.
the diagram
tor diagram.
osed upon it
239
3
(area 5-1
Conseque
efficiencj
4"
One more
4
whereas )
1
Isentropic Process
case of (
expansior
4.7
Volume
Ca)
L,
In this un
Otto,Diel
Con.tant Pr••• ur.
3
4.8
C)
1. A gas
cleara
2. Expla
1
5
3. What
con.tant Volum.
Entropy
(b)
4. Explai
6 6"6'
Rt
~ Wl
Fig. 7: Same compression ratio and heat addition
From the T-s diagram, i~can be seen that Area 5-2-3-6 = Area 5-2-3'-6'
4.9
= Area 5-2-2"-3"-6" as
~
Yc
M,
Th
~ Gll
Pal
this area represents the heat input which is the same for all cycles. All the cycles start from the
~ Gll
same initial state point 1 and the air is compressed from state 1 to 2 as the compression ratio is
Pri
same. It is seen from the T-s diagram for the same heat input, the heat rejection in Otto cycle
240
(area 5-1-4-6)
is minimum
and heat rejection
In
Diesel cycle (5-1-4'-6') is maximum.
Consequently, Otto cycle has the highest work output and efficiency. Diesel cycle has the least
efficiency and Dual cycle having the efficiency between the two.
One more observation can be made i.e., Otto cycle allows the working medium to expand more
whereas Diesel cycle is least in this respect. The reason is heat is added before expansion in the
case of Otto cycle and the last portion of heat supplied to the fluid has a relatively short
expansion in case of the Diesel cycle.
4.7
Let us Sum Up
In this unit we have studied Otto Cycle, Diesel Cycle, Dual Combustion Cycle, Comparison of
Otto, Diesel Dual Cycle, Air and Fuel-vapour mixtures Concept of air fuel Cycle.
4.8
1. A gas engine working on Otto cycle has a cylinder dia 178 mm and stroke of 254 mm. The
6
3
clearance volume is 1.5 x 10 mm . Calculate air standard efficiency.
2. Explain the different types of Cycles.
3. What is the concept of air fuel cycle?
4. Explain the similarities and differences between Otto, Diesel and Dual Cycles
4.9
'-3"-6" as
from the
Reference
)- Wu, Chih. Thermodynamic Cycles: Computer-aided Design and Optimization. New
York: M. Dekker, 2004. Print.
)- Moran, Michael 1., and Howard N. Shapiro. Fundamentals of Engineering
Thermodynamics. 6th ed. Hoboken, N.J. : Chichester: Wiley; John Wiley, 2008. Print.
)- Gunston, Bill (1999). Development of Piston Aero Engines (2 ed.). Sparkford, UK:
Patrick Stephens Ltd. p. 21.
)- Gupta, H. N. Fundamentals of Internal Combustion. New Delhi: Prentice-Hall, 2006.
Print.
In ratio is
)tto cycle
241
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BTPWE 302 Thermodynamics

Transcription

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