EPA-452/F-03-024
Air Pollution Control Technology
Fact Sheet
Name of Technology:
Fabric Filter- Mechanical Shaker Cleaned Type
- Mechanical Shaker Cleaned Type with Sonic Horn Enhancement
(also referred to as Baghouses)
Type of Technology: Control Device - Capture/Disposal
Applicable Pollutants: Particulate Matter (PM), including particulate matter less than or equal to 10
micrometers (• m) in aerodynamic diameter (PM10), particulate matter less than or equal to 2.5 • m in
aerodynamic diameter (PM2 5), and hazardous air pollutants (HAPs) that are in particulate form, such as most
metals (except mercury is the notable exception, as a significant portion of emissions are in the form of
elemental vapor).
Achievable Emission Limits/Reductions:
Typical new equipment design efficiencies are between 99 and 99.9%. Older existing equipment have a
range of actual operating efficiencies of 95 to 99.9%. Several factors determine fabric filter collection
efficiency. These include gas filtration velocity, particle characteristics, fabric characteristics, and cleaning
mechanism. In general, collection efficiency increases with increasing filtration velocity and particle size.
For a given combination of filter design and dust, the effluent particle concentration from a fabricfilter is nearly
constant, whereas the overall efficiency is more likely to vary with particulate loading. For this reason, fabric
filters can be considered to be constant outlet devices rather than constant efficiency devices. Constant
effluent concentration is achieved because at any given time, part of the fabric filter is being cleaned. As a
result of the cleaning mechanisms used in fabric filters, the collection efficiency is constantly changing. Each
cleaning cycle removes at least some of the filter cake and loosens particles which remain on the filter. When
filtration resumes, the filtering capability has been reduced because of the lost filter cake and loose particles
are pushed through the filter by the flow of gas. As particles are captured, the efficiency increases until the
next cleaning cycle. Average collection efficiencies for fabric filters are usually determined from tests that
cover a number of cleaning cycles at a constant inlet loading. (EPA, 1998a)
Applicable Source Type: Point
Typical Industrial Applications:
Fabric filters can perform very effectively in many different applications. Common applications of fabric filter
systems with mechanical shaker cleaning are presented in Table 1, however, fabric filters can be used in most
any process where dust is generated and can be collected and ducted to a central location. Sonic horn
enhancement of mechanical shaker cleaning is generally used for applications with dense particulates such
as utility boilers, metal processing, and mineral products.
EPA-CICA Fact Sheet
Fabric Filter
Mechanical Shaker Cleaned Type
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Table 1. Typical Industrial Applications of Shaker Cleaned Fabric Filters (EPA, 1998a)
Application Source Category Code (SCC)
Utility Boilers (Coal) 1-01-002...003
Non-Ferrous Metals Processing
(Primary and Secondary):
Copper 3-03-005,3-04-002
Lead 3-03-010,3-04-004
Zinc 3-03-030,3-04-008
Aluminum 3-03-000...002
3-04-001
Other metals production 3-03-011...014
3-04-005...006
3-04-010...022
Ferrous Metals Processing:
Coke 3-03-003...004
Ferroalloy Production 3-03-006...007
Iron and Steel Production 3-03-008...009
Gray Iron Foundries 3-04-003
Steel Foundries 3-04-007,-009
Mineral Products:
Cement Manufacturing 3-05-006...007
Coal Cleaning 3-05-010
Stone Quarrying and Processing 3-05-020
Other 3-05-003...999
Asphalt Manufacture 3-05-001...002
Emission Stream Characteristics:
a. Air Flow: Baghouses are separated into two groups, standard and custom, which are further
separated into low, medium, and high capacity. Standard baghouses are factory-built, off the shelf
units. They may handle from less than 0.10 to more than 50 standard cubic meters per second
(sm3/sec) ("hundreds" to more than 100,000 standard cubic feet per minute (scfm)). Custom
baghouses are designed for specific applications and are built to the specifications prescribed by
the customer. These units are generally much larger than standard units, i.e., from 50 to over 500
snf/sec (100,000 to over 1,000,000 scfm). (EPA, 1998b)
b. Temperature: Typically, gas temperatures up to about 260-C (500-F), with surges to about 290-C
(550-F), can be accommodated with the appropriate fabric material. Spray coolers or dilution air
can be used to lower the temperature of the pollutant stream. This prevents the temperature limits
of the fabric from being exceeded. Lowering the temperature, however, increases the humidity of
the pollutant stream. Therefore, the minimum temperature of the pollutant stream must remain
above the dew point of any condensable in the stream. The baghouse and associated ductwork
should be insulated and possibly heated if condensation may occur. (EPA, 1998b)
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c. Pollutant Loading: Typical inlet concentrations to baghouses are 1 to 23 grams per cubic meter
(g/m3) (0.5 to 10 grains per cubic foot (gr/ft3)), but in extreme cases, inlet conditions may vary
between 0.1 to more than 230 g/m3 (0.05 to more than 100 gr/ft3). (EPA, 1998b)
d. Other Considerations: Moisture and corrosives content are the major gas stream characteristics
requiring design consideration. Standard fabric filters can be used in pressure or vacuum service,
but only within the range of about ±640 millimeters of water column (25 inches of water column).
Well-designed and operated baghouses have been shown to be capable of reducing overall
particulate emissions to less than 0.05 gr/m3 (0.010 gr/ft3), and in a number of cases, to as low as
0.002 to 0.011 g/m3 (0.001 to 0.005 gr/ft3). (AWMA, 1992)
Emission Stream Pretreatment Requirements:
Because of the wide variety of filtertypes available to the designer, it is not usually required to pretreat a waste
stream's inlet temperature. However, in some high temperature applications, the cost of high temperature-
resistant bags must be weighed against the cost of cooling the inlet temperature with spray coolers or dilution
air (EPA, 1998b). When much of the pollutant loading consists of relatively large particles, mechanical
collectors such as cyclones may be used to reduce the load on the fabric filter, especially at high inlet
concentrations (EPA, 1998b).
Cost Information:
Cost estimates, expressed in 2002 dollars, are presented below for mechanical shaker cleaned fabric filters
and for sonic horn enhancement. Both the shaker cleaned and sonic horn cost estimates assume a
conventional design under typical operating conditions. The costs do not include auxiliary equipment such
as fans and ductwork.
The costs for shaker cleaned systems are generated using EPA's cost-estimating spreadsheet for fabric filters
(EPA, 1998b). The cost estimate forsonic horn enhancement is obtained from the manufacturer quote given
in the OAQPS Control Cost Manual (EPA, 1998b). Sonic horns are presented as an incremental cost to the
capital cost for a shaker cleaned system. The operational and maintenance (O&M) cost for shaker cleaned
systems are reduced by 1% to 3% with the sonic horn enhancement.
Costs are primarily driven by the waste stream volumetric flow rate and pollutant loading. In general, a small
unit controlling a low pollutant loading will not be as cost effective as a large unit controlling a high pollutant
loading. The costs presented are forflow rates of 470 m3/sec (1,000,000 scfm) and 1.0 m3/sec (2,000 scfm),
respectively, and a pollutant loading of 9 g/m3 (4.0 gr/ft3).
Pollutants that require an unusually high level of control or that require the fabric filter bags or the unit itself
to be constructed of special materials, such as Gore-Tex or stainless steel, will increase the costs of the
system (EPA, 1998b). The additional costs for controlling more complex waste streams are not reflected in
the estimates given below. For these types of systems, the capital cost could increase by as much as 30%
and the O&M cost could increase by as much as 7%.
a. Capital Cost: $17,000 to $153,000 per nf/sec ($8 to $72 per scfm)
$1,000 to $1,300 per nf/sec ($0.51 to $0.61 per scfm), additional cost for
sonic horns
b. O&M Cost: $9,300 to $51,000 per nf/sec ($4 to $24 per scfm), annually
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c. Annualized Cost: $11,000 to $95,000 per nf/sec ($5 to $45 per scfm), annually
d. Cost Effectiveness: $41 to $334 per metric ton ($37 to $303 per short ton)
Theory of Operation:
In a fabric filter, flue gas is passed through a tightly woven or felted fabric, causing PM in the flue gas to be
collected on the fabric by sieving and other mechanisms. Fabric filters may be in the form of sheets,
cartridges, or bags, with a number of the individual fabric filter units housed together in a group. Bags are
most common type of fabric filter. The dust cake that forms on the filter from the collected PM can
significantly increase collection efficiency. Fabric filters are frequently referred to as baghouses because the
fabric is usually configured in cylindrical bags. Bags may be 6 to 9 m (20 to 30 ft) long and 13 to 31
centimeters (cm) (5 to 12 inches) in diameter. Groups of bags are placed in isolable compartments to allow
cleaning of the bags or replacement of some of the bags without shutting down the entire fabric filter.
(STAPPA/ALAPCO, 1996)
Operating conditions are important determinants of the choice of fabric. Some fabrics (e.g., polyolefins,
nylons, acrylics, polyesters) are useful only at relatively low temperatures of 95 to 150-C (200 to 300-F). For
high-temperature flue gas streams, more thermally stable fabrics such as fiberglass, Teflon®, or Nomex® must
be used (STAPPA/ALAPCO, 1996).
Practical application of fabric filters requires the use of a large fabric area in order to avoid an unacceptable
pressure drop across the fabric. Baghouse size for a particular unit is determined by the choice of air-to-cloth
ratio, or the ratio of volumetric air flow to cloth area. The selection of air-to-cloth ratio depends on the
particulate loading and characteristics, and the cleaning method used. A high particulate loading will require
the use of a larger baghouse in order to avoid forming too heavy a dust cake, which would result in an
excessive pressure drop As an example, a baghouse for a 250 megawatt (MW) utility boiler may have 5,000
separate bags with a total fabric area approaching 46,500 m2 (500,000 square feet). (ICAC, 1999)
Determinants of baghouse performance include the fabric chosen, the cleaning frequency and methods, and
the particulate characteristics. Fabrics can be chosen which will intercept a greater fraction of particulate, and
some fabrics are coated with a membrane with very fine openings for enhanced removal of submicron
particulate. Such fabrics tend to be more expensive. Cleaning intensity and frequency are important variables
in determining removal efficiency. Because the dust cake can provide a significant fraction of the fine
particulate removal capability of a fabric, cleaning which is too frequent or too intense will lower the removal
efficiency. On the other hand, if removal is too infrequent ortoo ineffective, then the baghouse pressure drop
will become too high. (ICAC, 1999)
Mechanical shaking has been a popular cleaning method for many years because of its simplicity as well as
its effectiveness. In typical operation, dusty gas enters an inlet pipe to the shaker cleaned fabric filter and very
large particles are removed from the stream when they strike the baffle plate in the inlet duct and fall into the
hopper. The particulate-laden gas is drawn from beneath a cell plate in the floor and into the filter bags. The
gas proceeds from the inside of the bags to the outside and through the outlet pipe. The particles are
collected on the inside surface of the bags and a filter cake accumulates. In mechanical shaking units, the
tops of bags are attached to a shaker bar, which is moved briskly (usually in a horizontal direction) to clean
the bags. The shaker bars are operated by mechanical motors or by hand, in applications where cleaning is
not required frequently. (EPA, 1998a)
The vibration cleaning method is similar to mechanical shaking units. It utilizes a pneumatically driven high
frequency, low amplitude vibration of the bag frame to clean the bags. This method has limited application
due to its low cleaning energy and smaller baghouse design (Billings, 1970).
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Sonic horns are increasingly being used to enhance the collection efficiency of mechanical shaker and
reverse-air fabric filters (AWMA, 1992). Sonic horns utilize compressed air to vibrate a metal diaphragm,
producing a low frequency sound wave from the horn bell. The number of horns required is determined by
fabric area and the number of baghouse compartments. Typically, 1 to 4 horns per compartment operating
at 150 to 200 hertz are required. Compressed air to power the horns is supplied at 275 to 620 kiloPascals
(kPa) (40 to 90 pounds per square inch gage (psig)). Sonic horns activate for approximately 10 to 30 seconds
during each cleaning cycle (Carr, 1984) .
Sonic horn cleaning significantly reduces the residual dust load on the bags. This decreases the pressure
drop across the filter fabric by 20 to 60%. It also lessens the mechanical stress on the bags, resulting in
longer operational life (Carr, 1984). As stated previously, this can decrease the O&M cost by 1 to 3%,
annually. Baghouse compartments are easily retrofitted with sonic horns. Sonic assistance is frequently used
with fabric filters at coal-burning utilities (EPA, 1998a).
Advantages:
Fabric filters in general provide high collection efficiencies on both coarse and fine (submicron) particulates.
They are relatively insensitive to fluctuations in gas stream conditions. Efficiency and pressure drop are
relatively unaffected by large changes in inlet dust loadings for continuously cleaned filters. Filter outlet air
is very clean and may be recirculated within the plant in many cases (for energy conservation). Collected
material is collected dry for subsequent processing or disposal. Corrosion and rusting of components are
usually not problems. Operation is relatively simple. Unlike electrostatic precipitators, fabric filter systems
do not require the use of high voltage, therefore, maintenance is simplified and flammable dust may be
collected with proper care. The use of selected fibrous or granular filter aids (precoating) permits the high-
efficiency collection of submicron smokes and gaseous contaminants. Filter collectors are available in a large
number of configurations, resulting in a range of dimensions and inlet and outlet flange locations to suit
installation requirements. (AWMA, 1992)
Disadvantages:
Temperatures much in excess of 290-C (550-F) require special refractory mineral or metallic fabrics, which
can be expensive. Certain dusts may require fabric treatments to reduce dust seepage, or in other cases,
assist in the removal of the collected dust. Concentrations of some dusts in the collector, approximately 50
g/m3 (22 gr/ft3), may represent a fire or explosion hazard if a spark or flame is accidentally admitted. Fabrics
can burn if readily oxidizable dust is being collected. Fabric filters have relatively high maintenance
requirements (e.g., periodic bag replacement). Fabric life may be shortened at elevated temperatures and
in the presence of acid or alkaline particulate or gas constituents. They cannot be operated in moist
environments; hygroscopic materials, condensation of moisture, or tarry adhesive components may cause
crusty caking or plugging of the fabric or require special additives. Respiratory protection for maintenance
personnel may be required when replacing fabric. Medium pressure drop is required, typically in the range
of 100 to 250 mm of water column (4 to 10 inches of water column). (AWMA, 1992)
Other Considerations:
Fabric filters are useful for collecting particles with resistivities either too low or too high for collection with
electrostatic precipitators. Fabric filters therefore may be good candidates for collecting fly ash from low-sulfur
coals orfly ash containing high unburned carbon levels, which respectively have high and low resistivities, and
thus are relatively difficult to collect with electrostatic precipitators. (STAPPA/ALAPCO, 1996)
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References:
AWMA, 1992. Air& Waste Management Association, Air Pollution Engineering Manual, Van Nostrand
Reinhold, New York.
EPA, 1998a. U.S. EPA, Office of Air Quality Planning and Standards, "Stationary Source Control
Techniques Document for Fine Particulate Matter," EPA-452/R-97-001, Research Triangle Park, NC.,
October.
EPA, 1998b. U.S. EPA, Office of Air Quality Planning and Standards, "OAQPS Control Cost Manual,"
Fifth Edition, Chapter 5, EPA 453/B-96-001, Research Triangle Park, NC. December.
Billings, 1970. Billings, Charles, et al, Handbook of Fabric Filter Technology Volume I: Fabric Filter
Systems Study, GCA Corp., Bedford MA, December.
Carr, 1984. Carr, R. C. and W. B. Smith, Fabric Filter Technology for Utility Coal-Fired Power Plants, Part
V: Development and Evaluation of Bag Cleaning Methods in Utility Baghouses, J. Air Pollution Control
Assoc., 34(5):584, May.
ICAC, 1999. Institute of Clean Air Companies internet web page www.icac.com, Control Technology
Information - Fabric Filters, page last updated January 11, 1999.
STAPPA/ALAPCO, 1996. State and Territorial Air Pollution Program Administrators and Association of
Local Air Pollution Control Officials, "Controlling Particulate Matter Under the Clean Air Act: A Menu of
Options," July.
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