Baghouse Control Particulate Pollution

Baghouses Control
Particulate Poll-ution
The baghouse filter is one of the simplest technologies developed over the
years for containment of airborne industrial waste products. As more and
more industries have come to view such
containment as a public responsibility,
baghouse technology has matured and
available equipment has become more
efficient, more specialized and, in some
cases, more complex.
Present and anticipated particulate
control regulations have placed more
by William Gregg and James W. Griffin and more emphasis on bag-type dust
collectors. Based on sophisticated application of well-known principles, bag
collectors are replacing or supplementing scrubber and electrostatic precipitation technologies, especially in the
newly emphasized capture of ultra fine
particulate material and in control of
acid gases.
Properly applied bag filters can capture extremely fine particle size dust
and still operate at reasonable pressure
drops. They collect dry particulate in
dry form. This differs from a scrubber
‘I system where water must be supplied
and disposed of, and liquidsolid separation is then necessary.
Today’s bag filters have improved
fibers and filter media, which permits
operation over broader temperature
ranges and in more corrosive environments to capture finer particulate.
It is now recognized that finer par-
Baghouse technology
is Jinding approval
in the capture of
ultra Jine particulate
and in control
of acid gases.
An inside view of a baghouse shows
numerous bag filters capable of
capturing extremely fine particle
size dust.
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ticulate can pose significant health
risks, and can degrade the quality of
the atmosphere far more than coarser
particles. The PM- 10 revision from
TSP Federal Standard requires control
of particles in the range of 10 microns
and below.
In the past, regulations were based
on capturing a certain percentage of the
total mass of emissions. It did not matter if coarse or fine particles were captured, as long as a specified amount
were captured. Today’s revised standard puts a dual burden on the user:
to determine what part of emissions are
10 microns or below, and to provide
efficient collection.
The PM-LO revised TSP Federal
Standard is just one example of the new
breed of occupational and environmental legislation addressing the hazards of ultra fine particulate. The 1990
Clean Air Act gives added incentive for
continued review of the filtration process, and new OSHA regulations taking
effect in 1992 will apply across the
board, even in non-production areas of
the workplace.
What is a baghouse?
Since most dust collection filters
have a tube-like or bag-like form and
hang vertically from an overhead support, the term “baghouse” is often applied to this type of dust collection sys-
-
.
tem. Dust collector, fabric filter and bag
filter are other common terms. A
baghouse involves a protective supporting structure, the “house,” in which the
dust filters or bags are suspended. The
baghouse also includes arrangements
for distributing dust laden air to the
bags, for drawing off clean air exhaust,
for cleaning the filter elements and removing the collected dust.
An industrial or utility baghouse dust
collector will vary in size in direct relation to the volumetric rate of air or gas
flow. They can range in size from seven
bags to several thousand filter bags.
Large baghouse systems are often composed of many individual dust collector
modules manifolded together with inlet
(dirty gas) and outlet (clean gas)
ductwork.
Clean air A
Air is cleaned
as it passes through filter bag
Types of baghouse dust collectors
Dust collectors use two basic types
of filter geometry: dust may be collected
on the inside surface of the filter bag
or on the outside of the bag.
Once dust has accumulated on the
surface of the filter there are three principal ways to clean it, and dust collectors are classified by the cleaning
method: shaker, reverse air or reverse
gas, and pulse jet.
In general, the shaker type dust collector uses synthetic or natural fiber
bags. Reverse-air collectors often handle higher temperature applications, using woven fiberglass media. Pulse jet
collectors operate in a range from low
to high temperature, using natural fiber,
synthetic, fiberglass, and even ceramic
filter media for low, high and very high
temperature operation. With the ceramic media and an appropriate design,
a pulse jet collector can be used at tem6igure 1. Incoming dirty gas enters the shaker baghouse dust collector and passes
peratures above 1500°F.
Operation of a fabric filter depends through the filter bags from the inside out.
on the buildup of a suitable filter cake
or dust cake, which adds to the effi- filter bags from the inside out. The tu- Reverse air filter
The reverse air filter, also called a
ciency of the filter media by trapping bular bags hang from a tube or cell
still more dust. While cleaning arrange- plate, which in turn is suspended from backwash, re-pressure, or collapsements must remove a substantial por- the shaking or rapping mechanism. reverse type, collects air inside the filtion of the dust cake to minimize pres- Dust is captured on the inside surface ter, similar to the shaker. As with the
sure loss, it is important not to remove of the bag, while clean air goes through shaker, dirty air or gas passes through
all of it. Over cleaning permits dust the housing and out a stack or manifold the tubular filter bags and dust is captured on the inside surface of the bag
seepage and accelerates the rise in pres- duct. See Figure 1.
Excess buildup of dust on the bags while clean air goes on to be discharged.
sure drop when new dust is deposited
within the filter media instead of being is eliminated when, at a predetermined See Figure 2.
This type of filter is cleaned by the
point (by timing cycle or pressure
trapped on the existing cake.
drop), airflow is stopped and the shak- back flow of air or gas. Air flow to the
ing mechanism is activated. The tubu- baghouse is stopped at intervals. A supShaker dust collector
The shaker, which collects dust inside lar filter collapses, dislodging its accu- ply of clean air is introduced on the
the filter bag, is still used for many ap- mulated cake or dust. The dust falls by reverse or clean side of the filter to
plications. Incoming dirty gas enters the gravity into a collection hopper and the change the direction of flow. Collected
dust is dislodged and falls into the hopdust collector and passes through the filter bag goes back into service.
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Dust is dislodged by short,
rapid bursts of compressed air.
per by gravity. Since back pressure induced by the reverse flow tends to collapse the filter bags, which would effectively stop the reverse flow, the bags
are held open by reinforcing rings.
Pulse-jet collector
Pulse jet dust collectors trap particulate material on the outside of the filter.
Air enters the system and passes from
the outside through to the inside of the
filter bag, which is kept open by a support cage on its clean inside. See Figure
3. The bag is suspended from an overhead cell plate. Dust is trapped on the
filter fabric - in this case on the outside of the bag -while clean gas or air
passes through the cell plate and away
to the atmosphere.
In the pulse-jet collector, bag cleaning
is usually an on-line process. Incoming
air flow typically is not stopped. Instead, dust is dislodged by short, rapid
bursts of compressed air introduced at
the mouth of the tubular filter bag. The
burst of air momentarily interrupts normal air flow and reverse-inflates the filter element. The sudden burst of compressed air also pulls more air from surrounding regions into the bag equal to
two or three times the compressed air.
The combination of compressed and induced air rapidly inflates the bag, snapping it open and dislodging the ac-
Clean air
Six
-wire
distenders
per bag
(
Reverse air
bag cleaning
laden air
Figure 2. The reverse air filter captures dust on the inside surface of the bag while
clean air goes on to be discharged.
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cumulated dust.
Pulse jet collectors use compressed
air ranging from about 10 to 100 psig
to accomplish the bag cleaning, depending on the design.
Pulse jet collectors are used for collecting even heavy loadings of very fine
dust from kilns, dryers and fine pulverizers for applications such as dry chemicals, plastics, pigments, cement, raw
and processed minerals, carbon black
and coal dust collection in boiler plants.
A variation of the pulse jet collector
is the cartridge filter collector, which
uses a pleated filter cartridge similar
to an air filter in an automobile. The
cartridge filter collector can be more
compact than bag filters. The pleated
filter construction enables more filter
surface area to be placed in a given collector volume than the bag or tube construction. However, slower rates of filtration must be used with such cartridge
filters, partially offsetting the greater
surface advantage. Also, since a number
of dusts will plug or fill the pleats of
these filters, such collectors are often
inappropriate.
The typical cartridge filter uses cellulose fiber filter media. Cellulose synthetic fiber blends also are available.
The cellulose fiber restricts operating
temperatures to 150°F in most cases.
In some instances, filter elements fabricated from bag filter fabric are used,
but these have far less filter area than
the paper media elements.
Pulse-jet technology (high energy) is
often more cost-effective than shaker
(medium energy) or reverse-gas (low energy) cleaning, even for fine particulate
filtration. This is because pulse-jet collectors operate at higher filtration rates.
This translates into smaller housings
when compared to shaker or reverse-air
filters of the same capacity. As a result,
this technology is coming into use in
some of the applications traditionally
dedicated to the other types.
Filter fabrics
Fabric filter dust collectors rely on
all their components to perform effectively. Filter fabrics vary according to
particulate capture size, temperature
range, corrosion resistance and their
ability to meet anticipated factors such
as dust, loading emission requirements
and upset conditions.
Most fabrics used for bag filters are
synthetic, man-made fibers. Others include fiberglass, glass felt and ceramicbased materials. The synthetics include
polyester, polypropylene, acrylic,
polyaramid fiber and some recent developments including polyphenylene
sulfide and P84. These fabrics can be
woven or non-woven (felted).
Generally, but with exceptions,
shaker and reverse-air filters use woven
fabrics while pulse-jets use felted fabrics. Many of these fabrics can be
treated to add anti-static, acid-resistant,
flame-retardant or other characteristics
to meet special requirements and the
design of a particular collector.
Polyester fabric is the lowest in cost
but is limited to temperatures below
275°F under ideal conditions. Moisture
or acid attack can cause it to lose its
strength at temperatures as low as
200°F. This type of filter bag is most
often selected for industrial filtration
applications.
Acrylic homopolymer fiber is recommended for use up to 285°F in dry heat
and 260°F in moist heat. It resists hydrolysis and acid attack. Acrylic is often
used where temperatures do not exceed
260°F.
Woven fiberglass is the dominant filter bag material for higher temperature
applications up to 550°F. It is inexpensive and offers acid resistance, but is
fragile and generally requires slower filtration rates than felt filter media for
the same application.
Fiberglass filter media are available
with additives such as graphite and finishes like polytetrafluoroethylene
(PTFE), which can improve operating
characteristics and bag life, but also
raise cost.
The polyaramid fiber has thermal stability up to 375°F and is chosen for
many high temperature applications. It
must be applied with caution, however,
since it will not hold up where the gas
stream is moist or acidic.
Polyphenylene sulfide also offers a
thermal stability to 375°F and resists
most chemicals, but it is subject to oxidation above 15 percent oxygen.
P84 offers high filtration efficiency
and a temperature limitation of 500°F.
It is subject to hydrolytic attack and
is usually applied in specific problem
situations.
Glass felt is a filter media for high
temperature applications up to 500°F.
It is fairly expensive and fragile, requiring careful handling.
PTFE fibers and a blend of PTFE
with glass fibers are highly resistant to
chemical attack and unaffected by temperatures up to 450°F. They can be used Figure 3. Pulse-jet dust collectors trap particulate material on the Outside of the
for dust collection in extremely corro- filter as air passes from the outside to the inside of the filter bag. Clean gas or
air passes through the cell plate and away to the atmosphere.
sive process gas streams.
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