The first step in reviewing design criteria is determining the flow rate of the gas being filtered
by the baghouse, which is measured in cubic meters (cubic feet) per minute. The gas volume to be treated is set by the process exhaust, but the filtration velocity or air-to-cloth ratio is
determined by the baghouse vendor's design. The air-to-cloth ratio that is finally chosen
depends on specific design features including fabric type, fibers used for the fabric, bag cleaning
mechanism, and the total number of compartments, to mention a few. Figure 1 depicts a
number of these design features. A thorough review of baghouse design plans should consider
the following factors.
Physical and chemical properties of the dust are extremely important for selecting the fabric
that will be used. These include size, type, shape, and density of dust; average and maximum
concentrations; chemical and physical properties such as abrasiveness, explosiveness, electrostatic
charge, and agglomerating tendencies. For example, abrasive dusts will deteriorate fabrics
such as cotton or glass very quickly. If the dust has an electrostatic charge, the fabric
choice must be compatible to provide maximum particle collection yet still be able to be
cleaned without damaging the bags.
Predicting the gas flow rate is essential for good baghouse design. The average and maximum
flow rate, temperature, moisture content, chemical properties such as dew point, corrosiveness,
and combustibility should be identified prior to the final design. If the baghouse is
going to be installed on an existing source, a stack test could be performed by the industrial
facility to determine the process gas stream properties. If the baghouse is being installed on a
new source, data from a similar plant or operation may be used, but the baghouse should be
designed conservatively (large amount of bags, additional compartments, etc.). Sometimes, heavy dust concentrations are handled by using a baghouse in conjunction with a cyclone precleaner,
instead of building a larger baghouse. Once the gas stream properties are known, the
designers will be able to determine if the baghouse will require extras such as shell insulation,
special bag treatments, or corrosion-proof coatings on structural components.
Fabric construction design features are then chosen. The design engineers must determine
the following: woven or felt filters, filter thickness, fiber size, fiber density, filter treatments
such as napping, resin and heat setting, and special coatings. Once dust and gas stream properties
have been determined, filter choice and special treatment of the filter can be properly
made. For example, if the process exhaust from a coal-fired boiler is 400°F (204°C), with a
fairly high sulfur oxide concentration, the best choice might be to go with woven glass bags
that are coated with silicon graphite or other lubricating material such as Teflon.
Along with choosing the filter type the designer must select the appropriate fiber type. Fibers
typically used include cotton, nylon, fiberglass, Teflon, Nomex, Ryton, etc. The design should
include a fiber choice dictated by any gas stream properties that would limit the life of the bag.
(See Lesson 4 for typical fabrics and fibers used for bags.) For more information about fabric
construction, see McKenna and Turner (1989).
Proper air-to-cloth (A/C) ratio is the key parameter for proper design. As stated previously,
reverse-air fabric filters have the lowest A/C ratios, then shakers, and pulse-jet baghouses have
the highest. For more information about air-to-cloth ratios, see McKenna and Turner (1989).
Once the bag material is selected, the bag cleaning methods must be properly matched with
the chosen bags. The cost of the bag, filter construction, and the normal operating pressure
drop across the baghouse help dictate which cleaning method is most appropriate. For example,
if felted Nomex bags are chosen for gas stream conditions that are high in temperature and
somewhat alkaline, pulse-jet cleaning would most likely be used.
The ratio of filtering time to cleaning time is the measure of the percent of time the filters
are performing. This general, “rule-of-thumb” ratio should be at least 10:1 or greater
(McKenna and Furlong 1992). For example, if the bags need shaking for 2 minutes every 15
minutes they are on-line, the baghouse should be enlarged to handle this heavy dust concentration
from the process. If bags are cleaned too frequently, their life will be greatly reduced.
Cleaning and filtering stress is very important to minimize bag failures. The amount of flexing
and creasing to the fabric must be matched with the cleaning mechanism and the A/C ratio;
reverse-air is the gentlest, shaking and pulse-jet place the most vigorous stress on the fabric.
For example, it would probably not be advisable to use woven glass bags on a shaker baghouse.
These bags would normally not last very long due to the great stress on them during the
cleaning cycle. However, fiberglass bags are used on reverse-air baghouses that use shakeand-
deflate cleaning. Also, some heavy woven glass bags (16 to 20 oz) are used on pulse-jet
units (which also have high cleaning stress).
Bag spacing is very important for good operation and ease of maintenance. Bag spacing
affects the velocity at which the flue gas moves through the baghouse compartment. If bags
are spaced too close together, the gas velocity would be high because there is very little area
between the bags for the gas stream to pass through. Settling of dust particles during bag
cleaning would become difficult at high velocities. Therefore, it is preferable to space bags far enough apart to minimize this potential problem but not so far apart as to increase the size of
the baghouse shell and associated costs.
For pulse-jet baghouses, bag spacing is important to prevent bag abrasion. Bag-to-bag abrasion
can occur at the bottom of the bags because the bags are attached to the tube sheet only at
their tops which allows them to hang freely. Slight bows in the bag support cages or a slight
warping in the tube sheet can cause bag-to-bag contact at the bottom of the bags.
Finally, access for bag inspection and replacement is important. For example, in a reverse-air
unit, sufficient space between bags should be used so that maintenance personnel can check
each bag visually for holes. The bag can either be replaced or a cap can be placed on the tube
sheet opening to seal off the bag until it is later changed. The bag layout should allow the bag
maintenance technician to reach all the bags from the walkway. One measure of bag accessibility
is called bag reach and is the maximum number of rows from the nearest walkway.
There is no single value for bag reach, but typical units have a value of 3 or 4.
The compartment design should allow for proper cleaning of bags. The design should
include an extra compartment to allow for reserve capacity and inspection and maintenance of
broken bags. Shaker and reverse-air cleaning baghouses that are used in continuous operation
require an extra compartment for cleaning bags while the other compartments are still on-line
filtering. Compartmentalized pulse-jet units are frequently being used on municipal solid
waste and hazardous waste incinerators for controlling particulate and acid gas emissions.
The design of baghouse dampers (also called baghouse valves) is important. Reverse-air baghouses
use inlet and outlet dampers for gas filtering and bag cleaning sequences. As described
in Lesson 2, during the filtering mode, the compartment’s outlet gas damper and inlet dampers
are both open. During the cleaning sequence, the outlet damper is closed to block the flow of
gas through the compartment. The reverse-air damper is then opened to allow the air for bag
cleaning to enter the compartment.
Dampers are occasionally installed in by-pass ducts. By-pass ducts, which allow the gas
stream to by-pass the baghouse completely, are a means of preventing significant damage to
the bags and/or baghouse. Dampers in by-pass ducts are opened when the pressure drop across
the baghouse or the gas temperature becomes too high. However, many state regulatory agencies
have outlawed the use of baghouse by-pass ducts and dampers to prevent the release of
unabated particulate emissions into the atmosphere.
Space and cost requirements are also considered in the design. Baghouses require a good
deal of installation space; initial costs, and operating and maintenance costs can be high. Bag
replacement per year can average between 25 and 50% of the original number installed, particularly
if the unit is operated continuously and required to meet emission limits less than 0.010
gr/dscf. This can be very expensive if the bags are made of Teflon which are approximately
$100 for a 5-inch, 9-foot long bag, or Gore-tex which are approximately $140 for a 6-inch, 12-
foot long bag.
The emission regulations in terms of grain-loading and opacity requirements will ultimately
play an important role in the final design decisions. Baghouses usually have a collection
efficiency of greater than 99%.Many emission regulations (and permit limits) require that
industrial facilities meet opacity limits of less than 10% for six minutes, thus requiring the
baghouse to operate continuously at optimum performance.
by the baghouse, which is measured in cubic meters (cubic feet) per minute. The gas volume to be treated is set by the process exhaust, but the filtration velocity or air-to-cloth ratio is
determined by the baghouse vendor's design. The air-to-cloth ratio that is finally chosen
depends on specific design features including fabric type, fibers used for the fabric, bag cleaning
mechanism, and the total number of compartments, to mention a few. Figure 1 depicts a
number of these design features. A thorough review of baghouse design plans should consider
the following factors.
Figure 1. Design considerations for a pulse-jet baghouse
Physical and chemical properties of the dust are extremely important for selecting the fabric
that will be used. These include size, type, shape, and density of dust; average and maximum
concentrations; chemical and physical properties such as abrasiveness, explosiveness, electrostatic
charge, and agglomerating tendencies. For example, abrasive dusts will deteriorate fabrics
such as cotton or glass very quickly. If the dust has an electrostatic charge, the fabric
choice must be compatible to provide maximum particle collection yet still be able to be
cleaned without damaging the bags.
Predicting the gas flow rate is essential for good baghouse design. The average and maximum
flow rate, temperature, moisture content, chemical properties such as dew point, corrosiveness,
and combustibility should be identified prior to the final design. If the baghouse is
going to be installed on an existing source, a stack test could be performed by the industrial
facility to determine the process gas stream properties. If the baghouse is being installed on a
new source, data from a similar plant or operation may be used, but the baghouse should be
designed conservatively (large amount of bags, additional compartments, etc.). Sometimes, heavy dust concentrations are handled by using a baghouse in conjunction with a cyclone precleaner,
instead of building a larger baghouse. Once the gas stream properties are known, the
designers will be able to determine if the baghouse will require extras such as shell insulation,
special bag treatments, or corrosion-proof coatings on structural components.
Fabric construction design features are then chosen. The design engineers must determine
the following: woven or felt filters, filter thickness, fiber size, fiber density, filter treatments
such as napping, resin and heat setting, and special coatings. Once dust and gas stream properties
have been determined, filter choice and special treatment of the filter can be properly
made. For example, if the process exhaust from a coal-fired boiler is 400°F (204°C), with a
fairly high sulfur oxide concentration, the best choice might be to go with woven glass bags
that are coated with silicon graphite or other lubricating material such as Teflon.
Along with choosing the filter type the designer must select the appropriate fiber type. Fibers
typically used include cotton, nylon, fiberglass, Teflon, Nomex, Ryton, etc. The design should
include a fiber choice dictated by any gas stream properties that would limit the life of the bag.
(See Lesson 4 for typical fabrics and fibers used for bags.) For more information about fabric
construction, see McKenna and Turner (1989).
Proper air-to-cloth (A/C) ratio is the key parameter for proper design. As stated previously,
reverse-air fabric filters have the lowest A/C ratios, then shakers, and pulse-jet baghouses have
the highest. For more information about air-to-cloth ratios, see McKenna and Turner (1989).
Once the bag material is selected, the bag cleaning methods must be properly matched with
the chosen bags. The cost of the bag, filter construction, and the normal operating pressure
drop across the baghouse help dictate which cleaning method is most appropriate. For example,
if felted Nomex bags are chosen for gas stream conditions that are high in temperature and
somewhat alkaline, pulse-jet cleaning would most likely be used.
The ratio of filtering time to cleaning time is the measure of the percent of time the filters
are performing. This general, “rule-of-thumb” ratio should be at least 10:1 or greater
(McKenna and Furlong 1992). For example, if the bags need shaking for 2 minutes every 15
minutes they are on-line, the baghouse should be enlarged to handle this heavy dust concentration
from the process. If bags are cleaned too frequently, their life will be greatly reduced.
Cleaning and filtering stress is very important to minimize bag failures. The amount of flexing
and creasing to the fabric must be matched with the cleaning mechanism and the A/C ratio;
reverse-air is the gentlest, shaking and pulse-jet place the most vigorous stress on the fabric.
For example, it would probably not be advisable to use woven glass bags on a shaker baghouse.
These bags would normally not last very long due to the great stress on them during the
cleaning cycle. However, fiberglass bags are used on reverse-air baghouses that use shakeand-
deflate cleaning. Also, some heavy woven glass bags (16 to 20 oz) are used on pulse-jet
units (which also have high cleaning stress).
Bag spacing is very important for good operation and ease of maintenance. Bag spacing
affects the velocity at which the flue gas moves through the baghouse compartment. If bags
are spaced too close together, the gas velocity would be high because there is very little area
between the bags for the gas stream to pass through. Settling of dust particles during bag
cleaning would become difficult at high velocities. Therefore, it is preferable to space bags far enough apart to minimize this potential problem but not so far apart as to increase the size of
the baghouse shell and associated costs.
For pulse-jet baghouses, bag spacing is important to prevent bag abrasion. Bag-to-bag abrasion
can occur at the bottom of the bags because the bags are attached to the tube sheet only at
their tops which allows them to hang freely. Slight bows in the bag support cages or a slight
warping in the tube sheet can cause bag-to-bag contact at the bottom of the bags.
Finally, access for bag inspection and replacement is important. For example, in a reverse-air
unit, sufficient space between bags should be used so that maintenance personnel can check
each bag visually for holes. The bag can either be replaced or a cap can be placed on the tube
sheet opening to seal off the bag until it is later changed. The bag layout should allow the bag
maintenance technician to reach all the bags from the walkway. One measure of bag accessibility
is called bag reach and is the maximum number of rows from the nearest walkway.
There is no single value for bag reach, but typical units have a value of 3 or 4.
The compartment design should allow for proper cleaning of bags. The design should
include an extra compartment to allow for reserve capacity and inspection and maintenance of
broken bags. Shaker and reverse-air cleaning baghouses that are used in continuous operation
require an extra compartment for cleaning bags while the other compartments are still on-line
filtering. Compartmentalized pulse-jet units are frequently being used on municipal solid
waste and hazardous waste incinerators for controlling particulate and acid gas emissions.
The design of baghouse dampers (also called baghouse valves) is important. Reverse-air baghouses
use inlet and outlet dampers for gas filtering and bag cleaning sequences. As described
in Lesson 2, during the filtering mode, the compartment’s outlet gas damper and inlet dampers
are both open. During the cleaning sequence, the outlet damper is closed to block the flow of
gas through the compartment. The reverse-air damper is then opened to allow the air for bag
cleaning to enter the compartment.
Dampers are occasionally installed in by-pass ducts. By-pass ducts, which allow the gas
stream to by-pass the baghouse completely, are a means of preventing significant damage to
the bags and/or baghouse. Dampers in by-pass ducts are opened when the pressure drop across
the baghouse or the gas temperature becomes too high. However, many state regulatory agencies
have outlawed the use of baghouse by-pass ducts and dampers to prevent the release of
unabated particulate emissions into the atmosphere.
Space and cost requirements are also considered in the design. Baghouses require a good
deal of installation space; initial costs, and operating and maintenance costs can be high. Bag
replacement per year can average between 25 and 50% of the original number installed, particularly
if the unit is operated continuously and required to meet emission limits less than 0.010
gr/dscf. This can be very expensive if the bags are made of Teflon which are approximately
$100 for a 5-inch, 9-foot long bag, or Gore-tex which are approximately $140 for a 6-inch, 12-
foot long bag.
The emission regulations in terms of grain-loading and opacity requirements will ultimately
play an important role in the final design decisions. Baghouses usually have a collection
efficiency of greater than 99%.Many emission regulations (and permit limits) require that
industrial facilities meet opacity limits of less than 10% for six minutes, thus requiring the
baghouse to operate continuously at optimum performance.
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