AJAYI, OLASUNKANMI `TUNDE

MODIFICATION AND TESTING OF AN
EVAPORATIVE COOLING FACILITY FOR STORING
VEGETABLES
BY
AJAYI, OLASUNKANMI ‘TUNDE
MATRIC. NO : 20007/ 0582
A PROJECT REPORT SUBMITTED TO THE
DEPARTMENT OF AGRICULTURAL ENGINEERING,
COLLLEGE OF ENGINEERING
UNIVERSITY OF AGRICULTURE, ABEOKUTA,
OGUN STATE
IN PARTIAL FUFILMENT O F THE REQUIREMENTS FOR THE
AWARD OF BACHELOR OF ENGINEERING (B. ENG. Hons)
SUPERVISED BY: PROF. E. B. LUCAS
JUNE, 2011.
i
CERTIFICATION
The undersigned certify that this project report prepared by AJAYI, OLASUNKANMI
‘TUNDE (MATRIC. No. 2007/0582) Titled: Modification and Testing of an Evaporative
Cooling Facility for Storing Vegetables meets the requirements of the Department of
Agricultural Engineering for the Award of Bachelor of Engineering (B.ENG) Degree in
Agricultural Engineering.
--------------------------------------
--------------
Canon Prof. E.B. LUCAS
DATE
(Supervisor)
-------------------------------------
----------------
Prof. B. A. ADEWUMI
DATE
(Head of Department)
ii
ABSTRACT
A 1.05 cubic metre capacity storage facility which is to increase the shelf life of stored
vegetables as designed and fabricated was modified and tested. The equipment operates on
the principle of evaporative cooling and increasing the relative humidity of the interior of the
equipment. The door and the back wall were of particle board of 12mm thickness while the
two sides were made of jute bag through which water can flow from a trough at the top. The
facility designed was able to contain 1.8kg of tomatoes for 12 days and Celosia spp. for nine
(9) days without appreciable deterioration while the vegetables outside the facility which
were used as control were totally spoilt .The cooling efficiency was found to be 85.15% and
the average cooler temperature was 19.5°C while the average outside temperature was found
to be 29°C. The material cost of modification of the facility was 21,000 naira.
iii
DEDICATION
This project is dedicated to Almighty God; the most beneficent and merciful
iv
ACKNOWLEDGMENTS
Firstly my acknowledgment goes to my Heavenly Father who is the source of my life
and without whom I’m nothing.
I am also highly appreciative to the effort of my supervisor Canon Prof. E.B. Lucas for
his patience and support shown towards me during the course of this project.
I also want to use this medium to appreciate my lovely parents; Mr. Rufus Ayodele and
Mrs. Olubunmi Ajayi for their financial, moral and spiritual supports for me during my stay
at the University.
I also want to appreciate my Head Of Department, Prof. B.A. Adewumi including all my
lecturers like Engr. P.O Dada, Engr. O.J Adeosun, Engr. A. Sobowale, Dr. J.K Adewunmi,
Dr. A.F Adisa, Engr. I.A Ola, Engr. A.A Aderinlewo, Dr. T.M.A Olayanju, Dr. O.U Dairo
and all the members of staff of the department of Agricultural Engineering including the
technicians and technologists like Mr. Ọwonikoko, Mr. Adeniji, Mr. Razak, and the others I
want to say thanks for always been there at anytime.
My deep appreciation also goes to my siblings; Bayode, Seunfunmi, Bamidele and
Olamide for your invaluable supports; may God continue to bless you real good.
I am also highly indebted to all my course mates; victor, chris, Vivian toluwani, Isaac,
Toluene, Idris, Musty, moshood, bro glorious, prof. lanky and my HOC, Segun Odebiyi, I
pray that you all succeed in all your dealings.
I cannot forget all my friends which include Adebola Adewunmi, Biodun, Bunmi, Yinka,
Lanre, Beejay, Afeez, Toheeb, Alli, Sola, Kayode, Ronke Odebiyi, Tolu Odebiyi, and Salami
Oluwakemi among others. I appreciate you all for playing various roles in my life. I also
thank Abiola Alonge, you are a treasure.
All of you have their individual handwriting in aspects of my life and to the rest of my
friend I love you all.
v
TABLE OF CONTENTS
TITLE
CERTIFICATION
i
ABSTRACT
ii
ACKNOWLEDGEMENTS
iii
DEDICATION
iv
TABLE OF CONTENTS
v
LIST OF TABLES
viii
LIST OF FIGURES
ix
LIST OF PLATES
x
CHAPTER ONE: INTRODUCTION
1.1 COLD STORAGE
-----------------------------------------------------------------------------
1
1.2 PRINCIPLES OF EVAPORATION……………………………………………………………………………………….4
1.3 STATEMENT OF PROBLEM -----------------------------------------------------------------------------4
1.4 AIM ------------------------------------------------------------------------------------…………………………-4
1.5 JUSTIFICATION -------------------------------------------------------------------------------------------5
1.4 Basic principles of evaporation ------------------------------------------------------------------------5
CHAPTER TWO: LITERATURE REVIEW
2.1 HISTORY OF EVAPORATIVE COOLERS --------------------------------------------------------------- 7
2.2 ADVANCES IN EVAPORATIVE COOLING TECHNOLOGY ------------------------------------------ 8
2.3 FACTORS AFFECTING SHELF LIFE OF FRUITS AND VEGETABLES ------------------------------ 10
2.3.1 Ambient conditions ------------------------------------------------------------------------------ 10
2.3.1.1 Temperature ------------------------------------------------------------------------------------- 11
2.3.1.2 Relative humidity ------------------------------------------------------------------------------- 11
2.3.2 Variety and stage ripening -------------------------------------------------------------------- 14
vi
2.4 FACTORS ACCOUNTABLE FOR DETERIORATION IN FRUITS AND VEGETABLES ----------- 14
2.4.1 Physiological activity ------------------------------------------------------------------------------14
2.4.2 Pathological infection ---------------------------------------------------------------------------- 14
2.4.3 Mechanical injury ---------------------------------------------------------------------------------15
2.4.4 Evaporation of water -----------------------------------------------------------------------------15
2.5 POST-HARVEST CHANGES IN THE QUALITY OF FRUITS AND VEGETABLES ----------------- 16
2.5.1 Colour change --------------------------------------------------------------------------------------16
2.5.2 Loss of weight --------------------------------------------------------------------------------------16
2.5.3 Fruit firmness ---------------------------------------------------------------------------------------17
2.5.4 Change in total soluble solid --------------------------------------------------------------------17
2.6 PRINCIPLES OF EVAPORATIVE COOLING ----------------------------------------------------------- 18
2.6.1 Evaporative cooling and the Psychrometric chart -----------------------------------------18
2.6.2 Factors affecting evaporative cooling -------------------------------------------------------- 19
2.7 METHODS OF EVAPORATIVE COOLING ------------------------------------------------------------- 20
2.7.1 Direct evaporative cooling ----------------------------------------------------------------------- 21
2.7.2 Indirect evaporative cooling -------------------------------------------------------------------- 21
2.8 FORMS OF DIRECT EVAPORATIVE COOLING -------------------------------------------------------22
2.8.1 Passive-direct evaporative cooling ----------------------------------------------------------- 22
2.8.2 Non-passive direct evaporative cooling ------------------------------------------------------23
2.9 ENERGY CHANGES DURING EVAPORATIVE COOLING ------------------------------------------- 23
2.9.1 Vapour transmission through materials ------------------------------------------------------ 23
2.9.2 Heat and mass balance for pad-end -----------------------------------------------------------25
2.10 COOLING PAD MATERIAL ----------------------------------------------------------------------------- 29
CHAPTER THREE: MATERIALS AND METHODS
3.1 DESIGN OF EVAPORATIVE COOLING DEVICE
-----------------------------------------------32
3.1.1 Design principles ----------------------------------------------------------------------------------- 32
vii
3.2 MATERIALS OF CONSTRUCTION ----------------------------------------------------------------------34
3.3 FEATURES OF THE COOLER ----------------------------------------------------------------------------35
3.4 MODIFICATION REQUIREMENTS -------------------------------------------------------------------------37
3.5 EXPERIMENTAL METHOD AND PROCEDURE -----------------------------------------------------------43
CHAPTER FOUR: RESULTS AND DISCUSSIONS
4.1 RESULTS
------------ -------------------------------------------------44
4.1.1 No-Load Test of the Evaporative Cooler with Jute Bag --------------------------------------44
4.2 DISCUSSION…………………………………………………………………………………………………………………48
4.2.1 ASSESSMENT OF THE QUALITY OF STORED PRODUCTS ----------------------------------------48
CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS
5.1
CONCLUSIONS………………………………………………………………………………………………………..52
5.2 RECOMMENDATIONS…………………………………………………………………………………………………..53
REFERENCES ----------------------------------------------------------------------------------------------------54
APPENDICES ----------------------------------------------------------------------------------------------------58
viii
LIST OF TABLES
Table No.
Title
Page
2.1
Temp& relative humidity for various fruits and vegetables
13
3.1
Table of materials
39
4.1
Temperature Readings inside the Storage Chamber
44
4.2
Temperature Readings outside the Chamber
45
4.2.1
Cooling Efficiency of Cooler without Products
46
4.3
Percentage weight losses in tomatoes
48
4.3.1
Percentage weight losses in Celosia spp.
49
A1
Daily Temperature and Relative Humidity Reading
51
B1
Physiological Weight Measurement of Tomatoes
55
C2
Physiological Weight Measurement of Celosia spp.
56
ix
LIST OF FIGURES
Fig. No.
4.1
Title
Page
cooling efficiencies
46
4.3
Percentage Weight loss for Tomatoes
49
4.4
Percentage Weight loss for Celosia spp.
50
x
LIST OF PLATES
Plate No.
Title
Page
4.1
Front View of the Evaporative Cooler
47
4.2
Side View of Pad Section
47
4.3
The Overhead Tank with Delivery Pipe
47
4.4
Storage Chamber with Vegetables and in ambient
52
xi
CHAPTER ONE
INTRODUCTION
1.1 Cold storage
Deterioration of fruits and vegetables during storage depends partly on temperature. One
way to slow down temperature induced deterioration and thus increases the length of time fruits
and vegetables can be stored, is by lowering the temperature to an appropriate level. If the
temperature is too low the produce will be damaged and also that as soon as the produce leaves
the cold store, deterioration starts again and often at a faster rate. It is essential that fruits and
vegetables are not damaged during harvest and that they are kept clean. Damaged and bruised
produce have much shorter storage lives and very poor appearance after storage.
All fruits and vegetables have a 'critical temperature' below which undesirable and irreversible
reactions or 'chill damage' takes place. Carrots for example blacken and become soft, and the cell
structure of potatoes is destroyed. The storage temperature always has to be above this critical
temperature. One has to be careful that even though the thermostat is set at a temperature above
the critical temperature, the thermostatic oscillation in temperature does not result in storage
temperature falling below the critical temperature. Even 0.5°C below the critical temperature can
result in chill damage.
Sushmita et al., (2008) stated that keeping products at their lowest safe temperature (0°C
for temperate crops or 10-12°C for chilling sensitive crops) will increase storage life by lowering
respiration rate, decreasing sensitivity to ethylene gas and reducing water loss .He further said
that the most common method followed by the vegetable traders in local famers is to add
12
moisture to the air around the commodity as mists, sprays, or at last resort, by wetting the store
room floor.
Post-harvest respiration is a deterioration process. It results in the depletion of reserve
carbohydrate by oxidizing them to carbon di oxide, water and energy (Dzivama, 2000)
represented in the equation below:
C6H12 + 3O6 = 6CO2 + 6H2O + 67Kcal (energy)
Much of the post-harvest losses of fruits and vegetables in developing countries is due to the
lack of proper storage facilities. While refrigerated cold stores are the best method of preserving
fruits and vegetables they are expensive. Consequently, in the developing countries there is an
interest in simple, low-cost alternatives, many of which depend on evaporative cooling which is
simple.
The basic principle relies on cooling by evaporation. When water evaporates, it draws energy
from its surroundings which produces cooling effect. Evaporative cooling occurs when air, that
is not too humid, passes over a wet surface; the faster the rate of evaporation the greater the
cooling. The efficiency of an evaporative cooler depends on the humidity of the surrounding air.
Very dry air can absorb moisture. Therefore, greater cooling occurs. In the extreme case of air
that is totally saturated with water, no evaporation can take place and no cooling occurs.
Generally, an evaporative cooler is made of a porous material that is fed with water. Hot, dry air
is drawn over the material. The water evaporates into the air raising its humidity and at the same
time reducing the temperature of the air. For fresh market produce, any method of increasing the
13
relative humidity of the storage environment (or decreasing the vapor pressure deficit (VPD)
between the commodity and its environment) will slow the rate of water loss. The best method of
increasing relative humidity is to reduce temperature.
Evaporation not only lowers the air temperature surrounding the produce, it also
increases the moisture content of the air. This helps prevent the drying out of produce, and
therefore extends its shelf life.
In general, evaporative cooling can be best effective where:
•
temperatures are high; The consumption of vegetables is important because the nutrients
contained in them can be used in the treatment of CDR (Cardio Vascular diseases) and
cancer. In Nigeria research showed that at the national level, 24.8% of ch
1.2
•
humidity is low;
•
water can be spared for this use; and,
•
air movement is available (from wind or fans)
Basic Principles of Evaporation and Evaporative Cooling
Evaporation is the process of changing a liquid into a gas. In this case liquid water becomes
water vapour, and this gas becomes part of the mixture of gases that compose the air. The change
from the liquid state to a vapour requires the addition of energy, or heat. The energy that is added
to water to change it to a vapour comes from the environment, thus leaving the environment
cooler (Rusten, 1985) Not all substances need to gain or lose the same amount of energy to
change from one physical state to another. For example, it takes much more heat energy to cause
a given amount of water to vaporize than to cause the same amount of alcohol to do so. Water is
unique in that it requires a relatively large quantity of heat energy to change from a liquid to a
14
gas. It is this characteristic that enables evaporating water to lower substantially the temperature
of its environment.
On the other hand, the amount of water vapour that can be taken up and held by the air is not
constant; it depends on two factors. The first is the temperature (energy level) of the air, which
determines
the
potential
of
the
air
to
take
up
and
hold
water
vapour. The second factor is the availability of water. If little or no water is present, the air will
be unable to take up very much. (Beiler, 2009).
The measurement of the amount of water vapour present in the air is referred to as the air's
humidity. There are two ways of measuring the humidity of the air:
(1) Absolute humidity
(2) Relative humidity.
(1) Absolute humidity is the measurement of the actual quantity of water (measured in grams) in
a given volume of air (measured in cubic meters or litres).
(2) Relative humidity the more common measurement, is the measurement of the water vapour
in
the
air
as
a
percentage
of
the
maximum
quantity
of
water
vapour
that the air would be capable of holding at a specific temperature. Air that is fully saturated--that
is, contains as much water vapour as possible--has a relative humidity of 100 percent, while air
that has only half as much water vapour as it possibly could hold at a specific temperature has a
relative humidity of 50 percent.
15
The relative humidity varies with the temperature. As the air cools (i.e., loses energy), its ability
to hold water vapour decreases, which results in an increase in the relative humidity. This is
because the ability of the air to hold water vapour has been reduced by the drop in temperature,
but the absolute humidity (the actual amount of water vapour in the air) has remained unchanged.
If the air temperature continues to fall, the relative humidity will approach 100 percent, or
complete saturation. The point at which the air is fully saturated is referred to as the dew point.
At temperatures lower than the dew point, water vapour condenses out of the air onto cooler
surfaces. (Bachmann et al, 2000)
There are two types of cooling that may be achieved through this method
(1) Direct evaporating cooling
(2) Indirect evaporating cooling
1.3 Statement of problem
•
About 5 children suffered from subclinical vitamin A deficiency while 4.7% were
vitamin A deficient, making a total of 29.5% who suffered from clinically deficiency
(IITA, 2004).
The effect of lack of adequate storage facilities for vegetables after being harvested leads
to the reduction in the quantity of vegetable that get to the market which also has a direct
effect on the distribution and consumption of the needed quantity for healthy living.
1.4 Aim
The objective is to modify an existing evaporating cooling device, so as to improve its capability;
¾ To increase the shelf life of fruits and vegetables
16
¾ To reduce the cost of storing fruits and vegetable after harvesting
And consequently increasing the shelf lives of fruits and vegetables, using the modified
device.
1.5 Justification
Embarking on this project would muscle retarding the rate of spoilage of fruits and vegetables. In
turn, this will improve the shelf life and extend the life span of crops after post harvesting.
17
CHAPTER TWO
2.0
LITERATURE REVIEW
2.1 History of Evaporative Coolers
In the early Ancient Egyptian times, paintings depicting slaves fanning large, porous clay
jars filled with water which was an early form of evaporative cooling. The first man-made
evaporative coolers consisted of towers that caused water to flow through the base, and into a
building, in order to cool the building (dualheating.com).
In 1800 B.C the New England textiles factory began to use the evaporative cooling systems
to cool their mills (www.evaprocool.com). In the 1930’s the Beardmore tornado airship engine
used to reduce and completely remove the effect of using a radiator which reduces the effect of
lag.(coco.cooler.com)
Bamboo coolers were constructed with bricks with hessian cloth which were used to wrap
the bricks. Also, charcoal coolers were also produced together with the Almirah coolers.
Rusten, (1985) described some types of evaporative cooling that was been used in New
Delhi, India in which a wetted mat with fan was used to cool a local restaurant.
The concept of water-cooling a roof has a long history but it is estimated that less than 60
million square feet of roof have ever been water cooled Tiwari et al.(1992).
If only a small amount of water is placed on the roof, the evaporation is highly accelerated as
compared to what would be if the roof surface was flooded. Carrasco, (1987)
18
2.2 Advances in Evaporative Cooling Technology
Vakis, (1981) developed a cheap cool store in Kenya, with the help of local grass for
storage of vegetables. He kept the roof and walls wet by dripping water from the top of the roof.
Evaporative coolers, which rely on wind pressures to force air through wet pads, have also
been designed and constructed, especially in some developing countries like India, China and
Nigeria (FAO, 1986).
Construction of various evaporative systems was done by Rusten, (1985) using available
materials as absorbent (pads). Materials used include canvas, jute curtains and hourdis clay
blocks. Also a mechanical fan was introduced to some of the coolers constructed.
Rusten, (1985) did an extensive research in the construction of different evaporative
cooling systems using locally available absorbent materials such as canvas, jute curtains, etc.
Mechanical fans were used in some of the designs which drew air through a continuously wetted
pad. The continuous wetting of the pad was achieved by placing elevated water basins on the
fabric material, which absorbed the water gradually and eventually got saturated. He described
the functionality of a hourdis clay block coolers which was constructed by two researchers.
Roy and Khurdiya, (1986) constructed an evaporative cooled structure for storage of fruits
and vegetables with a double wall made of baked bricks and the top of the storage space covered
with khaskhas/gunny cloth in a bamboo framed structure.
Abdalla and Abdalla, (1995) worked on the development of a fan driven evaporative
cooler. The research was study the suitability of using palm leaves as a wetted medium. This
research was made possible due to the availability of palm leaves in Saudi Arabia. According to
the research it was claimed that palm leaves could be used as the wetted media which is locally
available to the masses.
19
Sanni, (1999) did a research on the development of evaporative cooling system on the
storage of vegetable crops .The major development was implemented by adding a regulated fan
speed, water flow rate and wetted-thickness .This was possible as a result of varying
temperature and relative humidity within the facility.
Dzivama, (2000) did a research on the performance evaluation of an active cooling system
using the principles of evaporative cooling for the storage of fruits and vegetables. He developed
mathematical models for the evaporative process at the pad-end and the storage chamber and a
stem variety of sponge was considered to be the best pad material from the local materials tested
as pad material.
Mordi and Olorunda, (2003) in their study on storage of tomatoes in Evaporative cooler
environment reported a drop of 8.2°c from ambient condition of 33°c while the relative humidity
increased by 36.6% over an ambient 60.4%. They further reported storage life of unpacked fresh
tomatoes in evaporative cooler environment as 11 days from the 4 days. Storage life under
ambient conditions while in combination with sealed but perforated polyethylene bags; it was 18
days and 13 days respectively.
Olosunde, (2006) also did a research on the performance evaluation of absorbent,
materials in evaporative cooling system for the storage of fruits and vegetables. Three materials
were selected to be used as pad materials: jute, Hessian and cotton waste. The design
implemented a centrifugal fan, high density polystyrene plastic, Plywood used as covering for
the walls and basement and the top and the main body frame was made of thick wood. The
performance criteria included the cooling efficiency, amount of heat load removed and the
quality assessments of stored products. The result showed that the jute material had the overall
20
advantage over the other materials. The cooling efficiency could be increased if two sides were
padded.
Sushmita et al., (2008) did a research on Comparative Study on Storage of Fruits and
Vegetables in an Evaporative Cooling Chamber and in Ambient. An evaporative cooling
chamber was constructed with the help of baked bricks and riverbed sand. It was recorded that
weight loss of fruits and vegetables kept inside the chamber was lower than those stored outside
the chamber. The fruits and vegetables were fresh up to 3 to 5 days more inside the chamber than
outside.
2.3 Factors affecting the shelf life of fruits and vegetables
There are various factors that affect the shelf life of fruits and vegetables which would lead
to their spoilage. The factors include:
i) Ambient Condition
ii) Variety and stage of ripening of fruits
2.3.1 Ambient Condition
The environmental condition has a great influence on the shelf life of fruits and vegetables
and the factors can be sub-divided into temperature and relative humidity.
2.3.1.1 Temperature
Temperature has a great influence on the shelf life on agricultural products.
FAO, (1998) found that all produce are subject to damage when exposed to extreme
temperatures which will lead to increase in their level of respiration. Also, it was further
disclosed that agricultural products vary in their temperature tolerance.
21
Gravani, (2008) observed that for every 18°F (-7.7°C) rise in temperature within the
moderate temperature range (50°F-100°F)/(10°C-37.8°C) where most food is handled, the rate of
chemical reactions is approximately doubled. As a result, excessive temperatures will increase
the rate of natural food enzyme reactions and the reactions of other food constituents.
2.3.1.2 Relative Humidity
This is the measurement of the amount of water vapour in the air as a percentage of the
maximum quantity that the air is capable of holding at a specific temperature. Mathematically it
can be represented by
× 100%
Relative Humidity = actual vapour density
Saturation vapour density
Relative humidity can also be mathematically represented by the equation below
Ø = EW/ EW*
Where EW = partial pressure of water vapour
EW*= saturated vapour pressure
EW* = (1.0007 +3.46 × 10-6 P) × (6.1121) ℮ (17.502T/240.97 + T)
Where T = the dry bulb temperature, °C
22
P = absolute pressure, millibar(mbar)
It has a great effect on the deterioration of fruits and vegetable because it has a direct
relationship with the moisture content in the atmosphere which determines whether the shelf life
will not be exceeded.
Bachmann and Earles, (2000) noted that the relative humidity of the storage unit directly
influences water loss in produce. Wilson et al., (1995) also found that water loss means salable
weight loss and reduced profit.
23
Table2.1; temperature and relative humidity for various fruits and vegetables
Fruits and
vegetables
Temperature(deg.
Celsius)
Relative
humidity
(%)
Apple
0-49
90-95
Maximum storage time
recommended
(ASHRAE hand book,
1982)
2-6m
Storage time in
cold stores for
vegetables in
tropical countries
Beetroot
0
95-99
-
Cabbage
0
95-99
5-6m
2m
Carrots
0
98-99
5-9m
2m
Cucumber
10-13
90-95
Cauliflower
0
95
2-4w
1w
Lettuce
1
95-99
Eggplant
8-10
90-95
Leeks
0
95
1-3m
1m
Oranges
0-4
85-90
3-4m
Pears
0
90-95
2-5m
Spinach
0
95
1-2w
Tomatoes
13-21
85-90
1w
2.3.2 Variety and Stage ripening
Post-harvest operation does not stop the fruits and vegetables from respiring which if not
controlled will lead to the over-ripening of the fruits which will lead to early deterioration.
Depending on the stage the fruits are harvested, which in practice varies from mature green
to fully ripened, the commodities have different storage conditions (Earles, 2004).
24
2.4 Factors Accountable for Deterioration in Fruits and Vegetables
2.4.1 Physiological Activity
Major changes which do make up fruit ripening are : seed maturation, abscission,
production of volatile compounds, development of wax on skin and changes in ;colour,
respiration rate, rate of ethylene production, tissue permeability, composition of pectin and
carbohydrates ,organic acids and protein. (Pratt, 1975)
2.4.2 Pathological Infection
Pathogens are one of the major causes of deterioration of fruits and vegetables. When they
infest any food material, they destroy and make it not pleasing to the sight.
(Bachmann and Earles, 2000) disclosed that crops destined for storage should be as free as
possible from skin breaks, bruises, spots, rots, decay, and other deterioration (Olosunde, 2006)
also said that insects and pests can cause considerable damage of fruits and vegetables through
either complete removal of the fruits or feeding on them, thus causing skin breaks which may
facilitate entry of decay organism.
2.4.3 Mechanical Injuries
The injuries that are visible on fruits and vegetables are caused by mishandling or other
cause which leads to cracks, bruises, cuts or abrasion which makes the produce not attractive and
also less marketable.
Aworh, (1988) disclosed that impact bruising of tomatoes results in higher respiration and
ethylene production rates, increased damage and lower levels of titratable and ascorbic acid,
which can alter taste and nutritive value.
25
Olosunde, (2006) also disclosed that mechanical damage can also accelerate the rate of
water loss from produce, bruising damages the surface organization of the tissue and allows a
much greater flow of gaseous material through the damaged area.
2.4.4 Evaporation of Water
Evaporative loss from the surface of fruits and vegetable has an effect on the quality of the
produce. The higher the rate of evaporation, the lower the moisture content and shelf life of the
agricultural produce.
Weight loss results from moisture loss via evaporation of water from the tissues when the
fruits and vegetables are attempting to be in equilibrium with the environment with the
environment which is usually at lower water activity.
2.5 Post- Harvest Changes in Quality of Fruits and Vegetables
Changes do occur during post-harvest operations for fruits and vegetables which leads
to decrease in their shelf life which it on the long run leads to decrease in the quantity
supplied for consumption and for export market.
Dzivama, (2000) described the common and notable changes that do occur during postharvest in the quality of fruits and vegetables which include:
i
Colour Change
ii
Loss of weight
iii
Change in the firmness.
iv
Change in total soluble solids
26
2.5.1 Colour Change
Fruits ripening process continues even after harvesting which could be an important factor
to be noted during post-harvest operations. Wilson et al., (2005) disclosed that immature or over
mature produce may not last as long in storage as that picked at proper maturity.
Colour is the most obvious change that occurs in many fruits and vegetables and this a
major criterion that most consumers uses to determine whether the fruit is ripe , unripe ,over-ripe
or spoiled and the assessment of colour change is done by comparing the colour of produce
under investigation against a standard colour chart (Dzivama, 2000).
2.5.2 Loss of weight
Most fresh produce contains from 65 to 95 percent water when harvested (FAO, 1989).
Water is an important constituent of most fruits and vegetables and it adds up to the total weight.
Losses of water will definite reduce the weight. When the harvested produce loses 5 or 10
percent of its fresh weight, it begins to wilt and soon becomes unusable (FAO, 1989).
The loss of weight comprises of both respiratory and evaporative losses. The former, which
occurs as a result of respiration, depends mainly on the temperature of the surrounding air. The
latter occurs as a result of water vapour deficit of the environment compared with that of the
produce (Dennis, 1979).
FAO, (1989) disclosed that the faster the surrounding air moves over fresh produce the
quicker water is lost. Air movement through produce is essential to remove the heat of
respiration, but the rate of movement must be kept as low as possible.
27
2.5.3 Fruit firmness
Ripening of fruits has a direct relationship with the fruit firmness and since respiration
continues even after harvest the fruits have the tendency of become over-ripen.
Dzivama,( 2000) declared that as a result of continued chemical activity within the fruits
tissues even after harvest after which it becomes over-ripe and soft which makes any factor
that can slow down the rate of respiration will automatically slow down the fruit firmness
change which can be achieved by storing at low temperature.
2.5.4 Change in total soluble solid
During ripening ,carbohydrate are broken down into simpler unit particularly the conversion of
starch to sugar ,giving the fruits its characteristics sweet taste on ripening and the degree of
ripening can be measured by measuring the sugar content in an extracted fruit juice (Dzivama,
2000).
2.6 Principles of Evaporative Cooling
2.6.1 Evaporative Cooling with Psychrometric Chart
According to Rusten, (1985) cooling through the evaporation of water is an ancient and
effective way of cooling water. He further disclosed that this was the method been used by plant
and animal to reduce their temperature. He gave the conditions at which evaporative cooling
would take place which are stated below:
(1) Temperatures are high
(2) Humidity is Low
(3) Water can be spared for its use
28
(4) Air movement is available (from wind to electric fan)
Also he disclosed that the change of liquid stage to vapour requires the addition of energy or heat.
The energy that is added to water to change it to vapour comes from the environment, thus making the
environment cooler.
Therefore, the use of the psychrometric chart is of great importance in order to discover whether
evaporative cooling has taken place. Air conditions can be quickly characterized by using a special graph
called a psychrometric chart. Properties on the chart include dry-bulb and wet-bulb temperatures, relative
humidity, humidity ratio, specific volume, dew point temperature, and enthalpy Beiler, (2009).
When considering water evaporating into air, the wet-bulb temperature, as compared to the
air's dry-bulb temperature, is a measure of the potential for evaporative cooling. The greater the difference
between temperatures, the greater the evaporative cooling effect. When the temperatures are the same, no
net evaporation of water in air occurs, thus there is no cooling effect (Wikipedia.com).
Therefore for optimum cooling efficiency using the evaporative cooling technique temperature and
the relative humidity measurement is needed to be taken and the psychrometric chart defines these
variables at various stages.
2.6.2 Factors affecting rate of evaporation
Evaporative cooling results in reduction of temperature an increase in relative humidity
(Olosunde, 2006).It is necessary to understand the factors that can limit the efficiency of the
system from producing the intended results.
There are four major factors that affect the rate of evaporation which was analysed by
(Rusten, 1985).He later added that though they are discussed separately but it is important to
29
keep in mind that they all interact with each other to influence the overall rate of evaporation,
and therefore the rate of cooling.
The factors discussed by (Rusten, 1985) include:
(1) Air Temperatures:
Evaporation occurs when water is absorbs sufficient energy to change from liquid to gas.
Air with a relatively high temperature will be able to stimulate the evaporative process and also
be capable of holding a great quantity of water vapour. Therefore, areas with high temperatures
will have a high rate of evaporation and more cooling will occur. With lower temperature, less
water vapour can be held and less evaporation and cooling will take place.
(2) Air Movement (Velocity)
Air movement either natural (wind) or artificial (fan) is an important factor that influences
the rate of evaporation. As water evaporates from wet surface, it raises the humidity of the air
that is closest to the water surface (moist area) .If the humid air remains in place, the rate of
evaporation will start to slow down as the humidity rises. On the other hand if the humid air near
the water surface is constantly being moved away and replaced with drier air, the rate of
evaporation will either increase or remain constant.
(3) Surface Area
The area of the evaporating surface is another important factor that affects the rate of
evaporation. The greater the surface area from which the water evaporates, the greater the rate of
evaporation.
30
(4) Relative Humidity of the Air
This is the measurement of the amount of water vapour in the air as a percentage of the
maximum quantity that the air is capable of holding at a specific temperature. When the relative
humidity of the air is low, this means that only a portion of the total quantity of water which the
air is capable of holding is being held. Under this condition, the air is capable of taking
additional moisture, hence with all other conditions favourable, the rate of evaporation will be
higher, and thus the efficiency of the evaporative cooling system is expected to be higher.
2.7 Methods of evaporative cooling
(Rusten, 1985) specified that there are two main methods of evaporative cooling namely
(1) Direct evaporative cooling (2) Indirect evaporative cooling
(1) Direct Evaporative Cooling:
This is a method by which air is passed through a media that is flooded with water .The latent
heat associated with the vaporizing of the water cools and humidifies the air streams which now
allows the moist and cool air to move to its intended direction. (Sellers, 2004)
Sanjeev, (2008) disclosed that direct evaporative cooling has the following major
limitations:
1) The increase in humidity of air may be undesirable.
2) The lowest temperature obtainable is the wet-bulb temperature of the outside air,
31
3) The high concentration and precipitation of salts in water deposit on the pads and the other
parts, which causes blockage, and corrosion, and requires frequent cleaning, replacement, and
servicing.
(2) Indirect Evaporative cooling:
A heat exchanger is combined with an evaporative cooler and the common approach used is
the passes return/exhaust air through an evaporative cooling process and then to an air-to air heat
exchanger which in turn cools the air, another approach is the use of a cooling tower to cool a
water circuit in an evaporative manner through a coil to a cool air stream (Sellers, 2004)
Sanjeev, (2008) also said indirect cooling differs from direct cooling in the sense that in
indirect cooling the process air cools by the evaporation of water. But there is no direct contact
of water and process air. Instead a secondary airstream is used for evaporation of water. So the
moisture content of process air remains the same
2.8 Forms of direct evaporative cooling
Dzivama, (2000) did a study on the forms of evaporative cooling process and below are his
findings;
There are two forms of in which the evaporative cooling principle can be applied. The
difference is based on the means of providing the air movement across/through the moist
materials. These are the passive and non-passive forms. The passive form of evaporative cooling
relies on the natural wind velocity, to provide the means of air movement across/through
32
the
moist surface to effect evaporation. This form can be constructed on the farm, for short term on
farm storage while the non- passive form uses a fan to provide air movement.
2.8.1 Passive-direct evaporative cooling system
Construction and design varies but the general principles are the same. The main components
include:
i) The cabinets where the produce is stored.
ii) The absorbent material used to expose the water to the moving air
iii) An overhead tank/through through which the water seeps down on to and wet the
absorbent material
The absorbent material covering the cabinet absorbs water from the tank on top of the
cabinets, the entire cloth that was used as cabinet is soaked in water and the air moves past the
wet cloth and evaporation occurs. As long as evaporation takes place, the contents of the cabinet
will be kept at a temperature lower than that of the environment and the temperature reduction
obtained in this type of cooler ranged from 5˚C to 10˚C.
Different researches have been done by researchers. Rusten,(1985), Susanta and Khurdiya,
(1986), Olosunde, (2007), Sushmita et al.,(2008) have designed various forms of coolers.
33
2.8.2 Non- passive direct evaporative cooling system
This cooling system uses a small fan and a water pump which is powered by electricity. The
products are kept in storage cabins inside the coolers, Absorbent material which receives the
water and expose it to evaporation with the help of the fan which draws air through the pad and a
overhead tank which is constantly supplying water to the absorbent material.
Materials used as the absorbent materials are hessian materials, cotton waste and celdek and
the body frame is made of wood. The pad and the fan are directly opposite to each other.
2.9 Energy changes during evaporative cooling
2.9.1 Vapour transmission through materials
The rate of water vapour transmission is based on Fick’s Law which is expressed as:
W = - µdP
A
dx
2.1
Where:
W= weight of water vapour transmitted (g)
A= area (m2)
Ө = time (hrs)
X= distance along path (m)
34
µ =permeability (g.m/m2.hr.KPA)
Fick’s equation may be integrated from x=0 to L and from P1 to P2 to give
W = µ A Ө (ΔP/L)
2.12
W = total weight of vapour transmitted (g)
A = area of cross-section of flow path (m2)
Ө = time of transmission (hr)
ΔP = Partial pressure difference between ends flow path (KPa)
L = length of pad or thickness of material (m)
µ = average permeability of material (g.m/m2.hr.KPa)
For convenience in evaluating combined materials, permeability can be expressed as a
coefficient of transmissions M, as:
W = M A ӨΔP
2.13
Where M = permeace in g/hr/m2 per vapour difference in KPa.
Permeance, like conductance is relative to any given material or a combination of materials.
Permeability, like conductivity relates to the property of a substance and is numerically equal to
the Permeance for the unit thickness.
35
Resistance to vapour flow provided by sheet or board is the reciprocal of the permeance .The
overall vapour resistances of a combination of materials. As in a wall is the sum of the resistance
s of the components .The overall Permeance of a wall may be found in a way similar to the
calculation of the overall coefficient of thermal resistance.
That is Mtotal = (
1
+ 1
M1
M2
+
1 + ....... + 1
M3........................ ..Mn
)-1
2.14
2.9.2 Heat and Mass Balance at Pad-end
The heat and mass balance can be derived as follows:
(i)
Heat balance for the pad end
The mass of air is passing through the pad of volume PAPT at anytime dt is equal to
Equals to:
Ma = (ρaVa + ρaVaW0) PAPp dt
2.15
Where
Ma = Mass of moist air, kg
Pp = Porosity of the pad, in decimal
Ρa = Density of air, kg/m3
Va = Velocity of air, m/s
W0 = Humidity ratio of the air, kg of water/kg of dry air
PA = Effective pad surface area, m2, approximated by effective evaporator surface
Area expressed as PA =AT × PE (Earle, 1983)
AT = Total pad surface area, m2
PE = Pad material efficiency approximated to fin efficiency as in evaporator and considered as the
porosity of the material.
Dt = time, sec, it takes for the air to pass through the pad thickness PT, express as PT/VA
36
The enthalpy of the air flowing through the pad at any time dt is equal to:
ha = ( ρaVa + ρaVaW0Cv)T0PAPPPT/VA
2.16
Where:
ha= enthalpy of moist air, KJ/Kg
T0 = outside air temperature, ˚C
Ca = specific heat capacity of the air, KJ/kg˚C
CV = specific heat capacity of water vapour, KJ/kg˚C
The change in the enthalpy of the air as it passes through the pad thickness due to the void spaces or
porosity of the pad in time dt, is equal to
hc = ( ρaVa + ρaVaW0Cv) ×(dT/dPT)) × (PAPPPT/VA)
2.17
where
hc= change in enthalpy of the air with the respect to the change in pad thickness , dPT
dT/dPT = change in the temperature of the air after passing through the pad of thickness dPT
The change in the enthalpy of the air per unit pad thickness is due to the convective heat transfer
from the air to the pad, required for the evaporation of the water from the pad .This can be
represented by the Newton’s law of cooling in time dt, as :
q= h1(T0-TP) PAPPPT/TA
2.18
where:
q = rate of heat transfer from the air to the pad, kJ/s.
h1= convective eat transfer coefficient, w/m2 ˚C( KJ/m2˚C)
Tp = temperature of the air after passing through the pad, ˚C
37
This change in the sensible heat is equal to the change in the enthalpy of the air after passing
through the pad through the pad, therefore equating equations
dP/dPT = h1( T0 –TP)/ (ρaVaCa + ρaVaW0CV)
2.19
ii) Mass balance for the pad- end.
The mass transfer from the pad of a unit thickness, m, to the air, is due to the concentration
difference or partial vapour pressure difference between the free air streams and the boundary
layer of the pad .The rate of evaporation could be expressed as;
MT = hDρaVa(Hp-H0) PAPpPT/VA = (hDρaVaMw)/(R0Tabs) × (Pvs – P`va)PAPPPT/VA
Where:
MT= mass of water evaporated by the air from the pad, kg/s.
hD= mass transfer coefficient, m/s.
H0= concentration of water vapour in the outside free stream, kg/m3
Hp = concentration of water vapour in the boundary layer of the pad, kg/m3
Pvs = saturation vapour pressure at the wet-bulb temperature, kg/m2
Pva= partial vapour pressure of the water vapour in the unsaturated air stream, kg/m2
Mw= molecular weight of water
R0= universal gas constant, 8315kJ/ kg ˚K mole.
38
2.20
Tabs= absolute temperature, calculated as the average temperature between the dry bulb and
wet bulb, °K
The heat required to evaporate the water from the pad is dt is equal to;
Q = MThfg =hfghDMwρaVa)/(RoTabs) × (Pvs-Pva) PAPPPT/VA
2.21
Where;
Q= heat required to evaporate the water from the pad, kJ
hfg= heat of vapourization, Kj/kg, which is expressed as
hfg = 2.503× 106 – 2.38×103 ( Tabs-273.16), for temperature equals to 273.16<338.723
(Brooker et al., 1992)
At equilibrium, the total change in enthalpy of the air is equal to the heat required for the
evaporation of the water from the pad thus equating equation 2.19 and 2.21 as;
h1(TO-Tp)/(ρaVaCa + ρaVaWoCv) = hfghDMwρaVa)/(RoTabs)×(Pvs-Pva)PPPPPT/VA 2.22
We have;
TP= TO - (hfgMw(Pvs- Pva))
×
ρ2aV2a(Ca+WoCv) × PPPPPT
ROTabs ρaCa(sc/pr)2/3
2.23
VA
Where; h1/hD=ρaCa(sc/pr)2/3, the ratio of the convective heat transfer to that of the mass
transfer coefficient.
The relative humidity passing through the pad could be predicted by representing the
evaporative cooling on a psychometric chat; after calculating the temperature TP. When the air
39
passes through the pad, it is cooled adiabatically and it follows along with the wet- bulb
temperature line on the psychometric chart.
2.10 Cooling pad material
There is transfer of heat from the pad material during evaporation and during this process
water is been evaporated.
The cooling capacity of a system is independent on the amount of air flow and its saturation
which in turn depends on the characteristics of the pad, air velocity through the pad and the water
flow rate (Thakur and Dhimgra, 1983).
Evaporation from the wetted pad affected by some factors which are wind, temperature,
surface area , humidity, air velocity ,water flow rate and thickness. The amount of water that the
air can evaporate from the pad depends on the rate of saturation and the temperature of the
air(Olosunde, 2006).The lower the relative humidity the higher the rate of evaporation and thus
the more the cooling takes place(Dvizama, 2000).
Various materials have been used as pad ranging from, palm tree leaves, hessian cloths,
,aspen wood, jute, cotton materials, perforated clay blocks mad some other materials based on
the functionality , costing and availability.
Dvizama (2000) tested luffa (aegyptica), stem variety sponge and jute material for the use of
pads in an evaporative cooler. During the experiment it was discovered that jute pad had the
highest efficiency with thickness of 60mm compared the other used pad materials.
Olosunde (2006) tested three materials namely jute, hessian and cotton waste and after
series of experiment, jute pad also had the highest efficiency.
40
Special Review of an Evaporative Cooler by Fabiyi (2010)
Fabiyi (2010) constructed an evaporative cooling device for preserving fruits and vegetable.
He constructed a device that makes use of water inside a plastic tank placed at the top of a
stand. The water is made to flow into the chamber through gravity.
The chamber consisted of a pad end comprising a jute bag incorporated in between particle
board with the other end exposed. The water flows through connected pipes which have been
pierced, into the pad material. The pad material is soaked and air movement through the pad
cools the produce stored in the device.
Fabiyi also incorporated a suction fan which is expected to cause the air flow through the
device.
Areas of modification
The particle board will be painted to prevent attack from termite and other organisms. It is
to also reduce heat conduction by convection and radiation..
A stand will be made to make the device a movable one so that it can be turned around to
suite the air movement and direction at any point in time. The stand will have tires at the base,
four tires to be precise.
The alternating current suction fan in the device will be replaced with a direct current
suction fan as the availability of electricity in most farms is not reliable. The direct current
supply of about 12v will be adequate to power the suction fan.
41
CHAPTER THREE
MATERIALS AND METHODS
3.1
Design of the evaporative cooling device
3.1.1
Principles of operation
The fundamental principle of evaporation being always accompanied by cooling is
employed in this design.
An enclosure in which the stuff to be cooled is stored has two of its sides made of pads
which constantly receives water drops coming from a reservoir placed on top. The wetted pads
have suction fans placed in their fronts, to draw in air from the enclosure, through the pads.
The design of the evaporative cooler is based on the principle of evaporation being always
accompanied by a cooling effect to its surrounding.
It is an enclosed system. Air is allowed to pass through the pad while a suction fan
centrally located draws in air through the pad. Water drips into the jute pad at a constant rate
through a water distribution system. As the water drips into the pad the suction fan draws warm
air through the wetted pad .During this process, the warm air passes through the wetted pad and
cooling occurs as a result of the water being evaporated. A temperature difference of about 10°C
is attained.
The fan requirement for an evaporative cooler is listed below, as given in ASAE (1988)
“Exhaust fans should have freely operating pressure louvers on their exhaust side to prevent unwanted air
exchange when fans are not operating
42
I.
Guard fans to prevent accidents .Use woven wire mesh screen placed within 100mm of
moving parts.
II.
Fans should be tested and rated according to air movement and control association, Inc
(AMCA) standard 210
III.
Pad should cool air to within 2c of the wet bulb temperature at a pressure loss not
exceeding 0.015kPa
IV.
The pad is normally run continuously along the side or end of the house opposite the
ventilation fans. Vertical pad height should not exceed 2.5 m nor be less than 0.5 m for
uniform water flow
V.
Vertical pads must be well mounted and secured to prevent sagging .Pads should be easy to
install and replace
VI.
VII.
Construct any air inlet so it may be readily covered without removing the pads
A horizontal pad can be irrigated at a rate close to the cooling system evaporative
requirement. Maximum recommended flow rate is 0.21L/s.m of pads area lower rates can
be achieved by intermittent operation of the pad irrigation system.
VIII.
IX.
Screen the water returned to the pump to filter out pad fibbers and other debris
As water evaporates the salt concentration is increased. In area than have water with high
minerals content a bleed off system is necessary to prevent mineral precipitation in the pad
X.
For small components (less than 30m floor area). Where mechanical ventilation or
evaporative cooling is installed, use the following design
Criteria: Evaporative cooler fan capacity per unit floor area 0.08 m/s.m”
This design should be capable of handling 1.05m3 preservation of vegetable for a minimum drop
in temperature of 10°C.
43
3.2 Materials for construction
John Krigger (2004) designed a prototype of the evaporative cooler with a wooden cabin of
rectangular cross section with a suction fan and a pump with a single –faced pad system using
jute pad. The design is modified by using two-sided padded cooler without a pump using jute
bag as the absorbent material.
The materials used are cheap and readily available. As shown in Figure 3.2 the evaporative
cooler in this study consist of:
(a)
Suction fan(alternating current)
(b)
Pad end
(c)
Water reservoir(150ltr)
(d)
Pipe network
MATERIALS FOR MODIFICATION
(a) Angle iron(2 full length)
(b) Metal plate(3ft by 8ft)
(c) Tyres(80mm diameter)
(d) Suction fan(direct current)
(e) Direct current supply(60Ah)
(f) Connecting wires(2.5mm)
(g) A switch
The pad was installed on both sides of the cabin and the suction fan was centrally located at the
opposite side of the main entrance of the cooler. The pipe network is connected to the water tank. The
pipe network allows the dripping of water into the pipe to the pad. Excess water is drained.
44
3.3 Features of the pad
3.3.1 Pad-end
The pad is held in place by a wooden frame work and a wire mesh which covers both sides of
the wooden frame. A rectangular large hole which allows air movement into the pad constitutes
the cross-section of the wire mesh. A thickness of 60mm was used based on the experiment
carried out by John Krigger (2004) where it was noted that this gives the highest efficiency..
A groove was created in the 2×4 thick wood that was used for easy removal of the wire mesh
for easy changing of the pads when there is need for a change .A small copper wire is used to
hold the pad together at the top to prevent sagging of the pad. A hole was cut on the 2×2 thick
wood which allowed water to drip from the horizontal pipe unto the jute pad. A hole was also cut
at the bottom to allow excess water to drip out of the pad.
The inside of the frame work was covered with 12mm particle board which was painted with
silver paint to reduce the effect of moisture on the particle board.
3.3.2 Water Distribution Network
The water distribution network consists of pipe network, an overhead tank of about 150 litres and
a bottom channel to take the excess water. The pipe network consists of a valve which was used
to regulate the flow rate.
The water is being pumped by gravity as the stand used is 1.7m in height compared to the
height of the cooler which is of 1.5m. The horizontal pipes which are layed on the cur region of
the 2×2 wood constitutes of holes which allows water to drip into the pad and at the end of the
pad is a stopper which prevent the water to be wasted.
45
3.3.3 Storage Cabin
The main frame of the cabin was constructed with 2×4 thick hard wood. The wall, roof and
floor are constructed with 12mm particle board which was painted with silver paint which helps
to reduce the effect of moisture. The interior of the cabin was divided into two sections by a wire
mesh. The shelves are of dimensions 800×500 mm and are reinforced at the edges by 50 mm soft
wood. The shelves are allowed to slide in and out for easy access and the removal of the produce.
The dimension of the storage cabin is 1000×700×1500 mm.
3.3.4 Fan position
A negative pressure is needed to be created inside the cabin which is a function of the pad
and the fan, when this happens, air at a higher pressure rushes into the system through the pad
.For proper air circulation, the fan was located at the central position directly opposite the door
which is air tight which now allows air to be drawn from the pad area which in turn draws the
cool air and expel the humidified air out.
3.3.5 Pad material selection
The selection on the type of pad used in the design was based on the following conditions:
i Porosity
ii Water absorption/ evaporation rate of the material
iii Availability
iv Cost
v Ease of construction
46
Based on the following aforementioned requirements, for the pad, Jute bag material was
considered favourable.
ƒ
Jute bag Material
It is a product of natural fibre. It has pore spaces which are carefully woven between the strings. In
the northern part of the country it is usually used for packaging onions which are transported down to the
southern part of the country. Older pads are better than new ones because they are more porous than the
new ones.
3.4
ƒ
MAJOR MODIFICATION REQUIREMENTS
The angle iron
The evaporative cooling device will be placed on a stand that will comprise of four tyres. The stand
will be made up of frames to give it good support. This angle iron will form these frames on all the
four sides of the device. The stand becomes necessary because air movement is not constant to a
particular direction, this will make it easy to move the system and turn it to any desired direction.
ƒ
The metal plate
A complete size of a metal sheet is used to form the base of the stand. The base was brazed on two
sides at the middle of the metal plate; this is done in order to prevent the device from digging through
the base because of the weight of the device
ƒ
The suction fan
A suction fan is similar to a humidifier. A fan blows a fine mist of water into the air. If the air is not
too humid, the water evaporates, absorbing heat from the air, allowing the misting fan to work as an
air conditioner. A suction fan may be used outdoors, especially in a dry climate. A direct current type
47
of fan has been selected to humidify the system, this is because of non regular supply of electricity;
especially in the rural areas and most farmers cannot afford the cost of an alternating current
generator. A 12volts supply is expected to be delivered from the direct current supply. The battery
can be recharged but it will take a longer period before it can run down.
ƒ
The switch
The switch will play a role of terminating the supply of power to the fan when it is no longer needed.
The switch is connected to the positive terminal of the battery and that of the fan. A switch to be used
is expected to be capable of withstanding the current flow that will pass through the terminals; a
lower quality switch will give way when current passes through them.
The materials used for modification are presented in the following table
Table 3.1...table of materials
Dimensions
Notes
Metal plate
Number
used
1
3ft by 8ft
02
Angle iron
2
2 by 2inch, 18ft long
03
Tyres
4
80mm in diameter
04
1
150 litres
05
Plastic
drum
Paint
1
1 bucket
06
Battery
1
60Ah
07
Connecting 1
wires
Switch
1
It makes up the
major base area
Used to form the
frame of the base
For mobility of the
system
It is the water
reservoir
To reduce the
effects of moisture
To supply direct
current to the fan
To connect battery
to fan
To cut-off and on,
the supply current
Serial number
Materials
01
08
2.5mm core
Electrical wiring
type
48
3.5 Experimental methods and procedures
3.5.1 No-load test of the evaporating cooling system
A no-load test was conducted on the system to see the effect of the evaporation that is
expected to take place whether the process is effective or not to determine its efficiency before
being loaded with the agricultural produce that will be stored. This is computed by measuring
temperature difference and the relative humidity.
3.5.1.1 Temperature and Humidity measurement.
The difference between the internal and external temperatures will determine whether
evaporation is effective for the system. The temperature readings are taken using the dry and wet
bulb thermometer.
Understanding evaporative cooling performance requires an understanding of psychometrics.
Evaporative cooling performance is dynamic due to changes in external temperature and
humidity level. Under typical operating conditions, an evaporative cooler can cool air by some
27 Celsius degrees (80 Fahrenheit degrees). A residential cooler should cool air to within 3–4 C°
(5–7 F°) of the wet-bulb temperature.
It is simple to predict cooler performance from standard weather report information. Because
weather reports usually contain the dew point and relative humidity, but not the wet-bulb
temperature, a Psychometric chart must be used to identify the wet bulb temperature. Once the
wet bulb temperature and the dry bulb temperature are identified the cooling performance or
leaving air temperature of the cooler may be determined:
TLA = TDB – ((TDB – TWB) x E)
49
TLA = Leaving Air Temp
TDB = Dry Bulb Temp
TWB = Wet Bulb Temp
E = Efficiency of the evaporative media.
Evaporative media efficiency usually runs between 80% to 90% and the evaporation efficiency
drops very little over time. Typical aspen pads used in residential evaporative coolers offer
around 85% efficiency
However, either of two methods can be used to estimate performance:
•
Use a Psychrometric chart to calculate wet bulb temperature, and then add 6–8 F° as described
above.
•
Use a rule of thumb which estimates that the wet bulb temperature is approximately equal to the
ambient temperature, minus one third of the difference between the ambient temperature and the
dew point. As before, add 6–8 F° as described above.
But in this experiment, the psychrometric chat has been adopted because it is more reliable
Some examples clarify this relationship:
•
At 32 °C (90 °F) and 15% relative humidity, air may be cooled to nearly 16 °C (61 °F). The dew
point for these conditions is 2 °C (36 °F).
•
At 32 °C (90 °F) and 50% relative humidity, air may be cooled to about 24 °C (75 °F). The dew
point for these conditions is 20 °C (68 °F).
•
At 40 °C (104 °F) and 15% relative humidity, air may be cooled to nearly 21 °C (70 °F). The dew
point for these conditions is 8 °C (46 °F).
50
The psychrometric chart which is a thermodynamic graph of the thermodynamic properties
of moist air at a constant pressure (often equated to an elevation relative to sea
level)(wikipedia.com).The relative humidity is then gotten using the psychometric chart which
has reading for both the dry and wet bulb temperature.
The effectiveness of the jute pad is based on the cooling efficiency. The saturation efficiency
(SE) of the cooler for the jute bag used was calculated using the formula by (Harris, 1987)
SE = T1(db) – T2(db)
T1(db) – T1(wb)
Where:
3.1
T1(db) = dry –bulb outdoor temperature, °C
T2(db) = dry- - bulb cooler temperature, °C
T1(wb) = wet-bulb outdoor temperature, °C
3.5.2 Load test of the Evaporative cooling System
The evaporative cooling efficiency of the cooler when loaded with tomatoes and celosia
spp. was evaluated. The quality assessment of the two produce was estimated and the crops
selected was based on the low shelf life.
The weight of the produce after some days were estimated and compared to their initial
weight.
51
3.5.3.1 Physiological Weight Loss
The difference in the weight was estimated by storing the produce both in the cooler and in
the ambient. This was done for some period of time to discover the effect of storing the two
produce both in the ambient and in the cooler and to discover the efficiency using the
evaporative cooler.
3.5.3.2 Colour Changes
The change in the colour of the produce was also noted both in the cooler and in the
ambient. The colour changes discovered was based basically on the physical appearance of the
vegetables.
52
CHAPTER FOUR
4.0
RESULTS AND DISCUSSION
4.1 Results
4.1.1 No load test of the evaporative cooler
Temperature Readings
The temperature values from dry and wet bulb thermometers both in and out of the
evaporative cooler are presented below.
Table 4.1: No Load Temperature Readings Within The Storage Chamber starting On
May 30, 2011 at 12.00 Noon running for five consecutive days
Runs (days) DB(°C) WB(°C)
1
26
19
2
27
19
3
27
20
4
28
19
5
27
18
•
DB = Dry bulb Temperature
•
WB =Wet Bulb Temperature
53
Table 4.2: No Load Temperature Readings Outside The Storage Chamber starting On
May 30, 2011 at 12.00 Noon for five consecutive days
Runs (days) Dry bulb (°C) Wet bulb (°C)
1
31
25
2
31
26
3
29
26
4
32
27
5
30
26
4.1.3 Cooling Efficiency
The cooling efficiency was also calculated when not on load based on formula by (Harris,
1987)
SE = T1(db) – T2(db)
4.1
T1(db) – T1(wb)
Where:
SE = Cooling efficiency
T1(db) = dry –bulb outdoor temperature, °C
T2(db) = dry- - bulb cooler temperature, °C
T1(wb) = wet-bulb outdoor temperature, °C
54
Table 4.2.1; average weekly cooling efficiencies without products
Runs(days) Cooling Efficiency (%)
1
83.3
2
80.0
3
66.7
4
80.0
5
75.0
The average cooling efficiency of the evaporative cooler is 85.15%.
Cooling Efficiency (%)
90
80
cooling efficiency
70
60
50
40
Cooling Efficiency (%)
30
20
10
0
1
2
3
days
4
5
Figure 4.1`; variation of cooler’s efficiency with days
55
Pictures of cooler before modification
Plate4.1a; front view showing internal chamber
plate4.2a;side view
Pictures of the cooler after modification
Plate4.1b; Front view
plate4.2b; side view
Plate4.3 Water distribution channel
56
4.2 Discussion
4.2.1 Assessment of the quality of stored products
4.2.1.1Physiological Weight Loss
The change in the weight of the samples both stored in the ambient and the cooler was
estimated .This was done for a total of nine days after which the percentage weight was
estimated using the formula below.
Percentage Weight loss = Original Weight – New weight
Original Weight
Table4.3; percentage weight losses in tomatoes
No. of days
Jute (%)
Controlled (%)
0
0
0
3
0.21
0.29
5
0.32
0.91
7
0.71
1.39
9
0.83
1.72
57
× 100
% Weight loss for Tomatoes
% Weight Loss
2
1.5
1
Jute (%)
0.5
Controlled (%)
0
1
5
9
Days
FIG 4.3 Percentage Weight Losses for Tomatoes
Table 4.3.1; percentage weight losses of Celosia spp.
no. of days
Jute (%)
Controlled (%)
0
0
0
3
0.28
0.75
5
0.55
1.31
7
1.19
1.63
9
1.32
1.88
58
2
1.8
1.6
percentage weight loss(%)
1.4
1.2
1
Jute (%)
0.8
Controlled (%)
0.6
0.4
0.2
0
0
2
4
Days
6
8
10
Fig 4.4; Percentage Weight Losses for Celosia spp.
4.2.1.2 Colour Changes
The colour changes noticed with the product stored in the ambient was most evident.
The tomatoes colour changed from the reddish colour and some parts turned to yellow and later
to black .The Celosia Spp. stored in the ambient started changing its colour on the 3rd day of the
experiment run until there was a total change on the 7th day .But the two samples stored in the
cooler still retained their colour with little significant change within the test period. Plate 4.1
shows the vegetable in the storage chamber
4.2.1.3 Firmness
The change in the firmness was much noticed in the tomatoes because of its shape. The
tomatoes stored in the cooler still retained their firmness, but those placed in ambient condition
have started to lose their firmness after the 3rd day and after the 7th day most of the tomatoes have
started deteriorating.
59
For the tomatoes stored in the chamber, lose in firmness was noticed on the 8th day;
although weight losses occurred on daily basis. Deterioration in the tomatoes was noticed on the
12th day.
Plate 4.4 Storage Chamber with Vegetable and in ambient
Tomatoes in ambient on the first day Tomatoes in the chamber on the first day
State of tomatoes in ambient after 3days
State of tomatoes in the chamber after 11days.
60
CHAPTER FIVE
5.0
5.1
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
This study was based on the principles of evaporation which causes cooling such that warm
dry air is cooled and humidified by passing it through a jute bag.
This study is also to provide an alternative storage device for vegetables because of their high
perish ability. It can then be concluded based on the results obtained from this study that the
evaporative device in this case is suitable as temporary storage device recommended for farmers
willing to preserve their freshly harvested vegetables for some days before they market them.
Jute bag was used in the construction (fan and pad system). Cooled dry air is passed into the
storage chamber where the vegetables are stored. The assumption is based that the dry cool air
will reduce or totally remove the effects of the heat load of the store thereby providing a
favourable condition for the preservation of the vegetables. The average drop in the temperature
during the no-load test is about 9°C.
The cooling efficiency of the cooler was estimated on no-load condition. The efficiency of
the cooler during the no-load test was averagely 85.15 %.The products to be stored was divided
into two for both the cooler and the controlled in order to determine the effects of the cooler
using physical phenomena like the weight , colour and firmness.
The percentage weight loss of the vegetables was much in the ambient compared to those
stored in the cooler. The colour changes noticed in the vegetables stored in the ambient was
greater compared to the ones stored in the cooler.
The change in the firmness of the vegetable stored in the cooler was negligible when been
compared to the ones stored in the ambient.
61
5.2 Recommendations
I will want to recommend that for further study on this work a different type of wood
should used as walls and the ceiling instead of particle board which has a high rate of water
absorption and which is easily subjected to spoilage after sometime.
I also recommend that for the future design of this type of cooler, a design should be for a
water reservoir of reduced height with an adequate flow rate such that water can easily be
conveyed into the reservoir by an individual of average height.
62
REFERENCES
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Pads in Evaporative Coolers. Agricultural Mechanization in Asia Africa and Latin America Vol.
26 26(2) ,Pp 52-54
Ajibola, O.O. (1991). Storage Facilities and Requirements for Fruits and Vegetables. Paper
presented at the Nigeria Society of Engineers Course on Designing , Construction and
Maintenance of Food Storage System. Pp. 9-11
ASHRAE (2003) Evaporative Cooling System. American Society of Heating and Refrigeration
and Air Conditioning.www.ashare.com
Carrasco, A.D. (1987) Evaluation of Direct Evaporative Roof-Spray Cooling System
.Proceedings on Improving Building Systems in Hot and Humid Climates, Houston ,Texas US.,
September 15-16 Pp.1-4
Dvizama, A. U. (2000).Performance Evaluation of an Active Cooling System for the Storage of
Fruits and Vegetables. Ph.D. Thesis ,University of Ibadan , Ibadan.
FAO/SIDA .(1986) .Farm Structures in Tropical Climates , 6 FAO/SIDA . Rome.
Harris, N.C. (1987). Modern Air Conditioning Practice, 3rd edition, McGraw-Hill
Book Co., New York.
IITA,
Nigeria
Food
Consumption
and
Nutrition
Survey
2001-2003.
An
FG/IITA/USAID/UNICEF/USDA Report, 2004
Mordi, J.I.,and Olorunda, A.O. (2003) Effect of Evaporative Cooler Environment on the Visual
Qualities and Storage Life of Fresh Tomatoes. J. Food Sci. Technol. 40(6): 587-591.
63
Olosunde, W.A. (2006) .Performance Evaluation of Absorbent Materials in the Evaporative
Cooling System for the Storage of Fruits and Vegetable M.Sc thesis, Department Of Agricultural
Engineering, University of Ibadan, Ibadan.
Rastavorski, A. (1981). Heat Balance in Potato Store Centre for Agricultural Publication and
Documentation. Wageningen, Pp210
Roy, S.K.and Khurdiya, D.S. (1986) cited in Dash S.K. paper, presented at training course on
‘Zero Energy Cool Chamber’ held at I.A.R.I. New Delhi, 8-10 Nov., 2000.
Rusten, D.W (1985) principles and practices of evaporative cooling, Mcgregor publishers,
london, p45
Sanni, L.A (1999). Development of Evaporative Cooling Storage System for Vegetable Crops
.M.Sc. project report, Department of Agricultural Engineering, Obafemi Awolowo University,
Ile-Ife, Nigeria
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Sushmita, M.D. , Hemant, D., and Radhacharan, V. (2008): Vegetables in Evaporative Cool
Chamber and in Ambient, Macmillan Publ. Ltd.; London and Basingstoke, Pp 1 -10.
Shiundu, K.M. (2002) .Role of African Leafy Vegetables (ALVs) in Alleviating Food and
Nutrition Insecurity in Africa. AJFNS; 2: (2) 96 – 97.
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Am Diet Assoc 96 (10): 1027–39
Tiwari, G.N. and Alok Srivastava, (1983) Experiment Validation of a Thermal Model of an
Evaporative Cooling System, all books store. Pp1-10
64
Wilson, C.L. , El-Ghaouth, A. ,Wisniewski, M.E., (1999) .Prospecting in Nature’s Storehouse for
Biopesticides Conference Magistra Revista Maxicana de Fitopatologia 17,Pp 49-53
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www.evaprocool.com; accessed on March 29, 2011. At 1300 GMT
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www.dualheating .com; accessed on April 14, 2011. At 0300 GMT
65
APPENDICES
APPENDIX A: DAILY TEMPERATURE AND RELATIVE HUMIDITY READINGS
(NO LOAD)
Table B1 : Daily Temperature and Relative humidity Readings
Time
Ambient Condition
(hrs)
Tdb(°C)
Twb(°C)
Cooler Conditions
RH%
Tdb(°C)
Efficiency
RH(%)
(%)
8
27
17
36
18.5
86
85
9
27
18
41
20
82
78
10
28
17.5
34
19
86
86
11
29
19
39
21
82
80
12
30
20
40
22
83
80
13
31
21
41
22
91
90
14
31
21
37
23
84
80
15
30
19.5
42
21
87
86
16
30
21
39
22
91
89
DAY TWO
Time
Ambient Condition
(hrs)
Tdb(°C)
Twb(°C)
Cooler Conditions
RH%
Tdb(°C)
Efficiency
RH(%)
(%)
8
26
17.5
42
18
92
94
9
27
17
36
19
82
80
66
10
24
15
37
16.5
85
83
11
23
14
35
15
89
89
12
23
14
35
15
89
89
13
24
14.5
34
16
85
84
14
24.5
15
35
16.5
86
84
15
25
16
38
17
90
89
16
24
14.5
34
15
92
94
DAY THREE
Time
Ambient Condition
(hrs)
Tdb(°C)
Twb(°C)
Cooler Conditions
RH%
Efficiency
Tdb(°C)
RH(%)
(%)
8
26
17
40
17.5
94
94
9
26
17.5
42
18
92
88
10
27
17
36
19
82
80
11
27
18
41
18.5
92
94
12
28
19
42
20
89
88
13
29
18
34
20.5
80
94
14
30
20
39
22
83
80
15
29
19.5
41
21
93
84
16
28
18.5
39
20
87
84
DAY FOUR
Time
Ambient Condition
(hrs)
Tdb(°C)
8
28
Twb(°C)
19
Cooler Conditions
RH%
Tdb(°C)
42
22
67
RH(%)
75
Efficiency
(%)
67
9
28
19.5
45
21
87
82
10
29
20
43
21.5
87
83
11
30
21
45
22
90
89
12
31
21.5
43
23
87
84
13
32
22
42
24
84
80
14
33
22.5
41
25
80
84
15
32
23
46
24.5
88
83
16
31
22
45
23
90
89
DAY FIVE
Time
(hrs)
Ambient Condition
Tdb(°C)
Cooler Conditions
Twb(°C)
RH%
Tdb(°C)
RH(%)
Efficiency
(%)
8
27
18
41
19
90
89
9
28
18.5
40
19.5
90
89
10
28.5
19
40
21.5
79
82
11
29
20
43
21
90
88
12
30
21
45
22.5
87
83
13
31
21
40
23
83
89
14
31.5
22
43
23.5
87
90
15
30
21
44
22
90
89
16
30
21.5
47
22.5
90
88
68
APPENDIX B PHYSIOLOGICAL AND PERCENTAGE WEIGHT LOSS OF SAMPLES
Table D1 Physiological weight measurement of Tomatoes
Weight of samples grammes (g)
Days
Jute
Ambient
0
1784.3
1784.5
3
1780.8
1778.3
5
1778.5
1767.1
7
1772.1
1761.1
9
1770.1
175
Percentage Weight Loss
Days
Jute (%)
Ambient (%)
0
0
0
3
0.2
0.3
5
0.3
0.9
7
0.7
1.3
9
0.8
1.6
Table D2 Physiological weight measurement of Celosia spp.
Weight of samples grammes(g)
Days
Jute
69
Ambient
0
2749.5
2748.5
3
2740.2
2728.1
5
2730.6
2716.5
7
2717.5
2707.2
9
2710.8
2698.5
Percentage Weight Loss
Days
Jute (%)
Ambient (%)
0
0
0
3
0.3
0.7
5
0.6
1.2
7
1.2
1.5
9
1.4
1.8
70
PSYCHROMETRIC CHAR
T
71