Fabric Filters

Design and application of fabric filters are covered in various references.23-31

The Industrial Gas Cleaning Institute Inc. (IGCI) defines a baghouse as follows: “A fabric filter is one in which the dust-bearing gas is passed unidi­rectionally through a fabric in such a manner that the dust particles are re­tained on the dirty gas side of the fabric, while the cleaned gas passes through the fabric to the clean gas side, where it is removed by natural and/or mechan­ical means.” Simply, fabric filter collectors remove particulates from gas streams by impaction, impingement, and diffusion. Large particles will not pass through the weave of the filter and are sieved out. Medium-sized particles are caught by impingement on the filter fibers. Small particles diffuse and are caught by the filter fibers due to Brownian movement. The main mechanisms for fabric filters are impingement and diffusion. If large particles have not set­tled out ahead of the filter, it is common to add an inertial separator or cy­clone to remove the coarse particles and reduce the dust loading to the filter.

The separation of dust from the gas stream consists of passing the dusty gas through a porous, flexible layer of textile material called a filter element. These filter elements, which are bag-shaped, are placed in structural enclo —

Sures called baghouses or fabric filter collectors. In addition to supporting the filter elements, the baghouse contains baffle plates for directing airflow into or out of these hoppers, a type of cleaning mechanism to remove the particulate from the filters, and a dust hopper for the collection and drainage of the dust. As dust accumulates on the filter, the pressure drop across the filter increases. Dust buildup on the filter is removed periodically by vigorous cleaning of the filter to maintain the pressure drop in a satisfactory way.

This type of bag-cleaning method is a fundamental characteristic of this type of collector. Terminology in the fabric filter field is not totally consistent or comprehensive. Table 13.2 presents acceptable definitions for common fab­ric filter terminology.

Important technical features of fabric filter collectors are listed in Table 13.3. A brief description of each characteristic follows. Detailed descriptions of fabric filters are presented in the literature.23-31»33

Baghouses may operate under negative or positive pressures. Negative pres­sure baghouses operate upstream of the fan and must be designed to withstand the maximum head developed by the fan. This could correspond to the case in which all the inlet dampers are closed and the fan is operating under cold conditions. Al­though the pressure drop across the filter unit may only be 50-200 mm WG, the compartment walls may have to be designed to 1000 mm WG. The compartment must be designed to minimize in-leakage air so that the fan does not have to be oversized. If the dust-air mixture can become explosive, the unit must be designed to resist a positive pressure that is determined from dust-explosion tests. Venting requirements for the baghouse to limit pressure buildup can be determined from a publication by Schofield.34 The allowable positive pressure for baghouse design must be specified.

Positive pressure baghouses are common for large structural units in the metallurgical industry. These units have to be designed only to withstand the pressure loss across the baghouse. They also have the advantage that an open grating can be used around the outside of the compartments, which allow’s ambient air to cool the exhaust gas.

Fabrication of baghouses can be of modular design, factory-welded subas­semblies, or structural design. The selection of the type of fabrication depends on size of unit, transportation requirements, site location, physical location of unit at plant (e. g., on roof), and materials of construction.

Modular designs have become very popular because of the high-quality workmanship, which can be controlled in the manufacturing plant, and the ease of erection in the field. These units can be complete, factory-assembled units consisting of bolted or welded components. The units may include the dust hopper, timer, cleaning accessories, supporting legs, bags, and internal baffles. For some installations, the units can be lifted directly into a base and the legs bolted in place. Large volumes can be handled using a multiplicity of units connected in parallel. The dusty inlet duct and clean outlet ducts may be connected to a common internal plenum or to external ductwork manifolds with inlet and outlet connections on each.

Factory-welded subassemblies are based on separate fabrication of bag housing, dust hopper, inlet plenum, outlet plenums, etc., to form the largest subassemblies that can be shipped over the road. Connections are made to ad­jacent components by welding or bolting at the job site. Bags can be already


Cleaning method: The physical principle used to dislodge and remove the collected dust from the fabric.

Cleaning mechanism: The specific mechanical or pneumatic system used to clean the fabric.

Cleaning sections: The number of segments into which the cleaning mechanism is divided.

(An intermittent baghouse would have a single section, while automatic or continuous baghouses would have two or more sections.)

Compartments: The number of rooms in a continuous baghouse that can be entered for maintenance while the remainder of the baghouse is operating (may contain one or more cleaning sections).

Cleaning methods

Intermittent: Periodic cleaning, with total flow’ interruption.

Automatic: “Continuous automatic” cleaning, w’ithout total flow interruption.

Continuous: “Continuous automatic” cleaning and maintenance possible, without total flow interruption.

Filtering fabric area

Gross fabric: The total effective area of filter fabric installed in the baghouse.

Net fabric: The gross fabric area less the area of fabric being continuously or periodically cleaned.

Filter-cleaning operations

Cleaning cycle: The total elapsed time from commencing to clean a section of the baghouse to commencing to clean that same section again.

Cleaning interval: The elapsed time from commencing to clean a section of the baghouse to beginning to clean the next section. (The cleaning interval times the number of sections equals the cleaning


Cleaning period: The total elapsed time that a section of the baghouse is off-stream for cleaning. (This time increment determines the total availability of section fabric for filtration use.}

Filter ratings

Air-to-cloth ratio: The unit capacity of a fabric filter, i. e., the total volume of gas in actual m Vs divided by the gross or net fabric area in m2.

Operational problems

Dusting: Dust passing through the fabric upon initial start-up or immediately following cleaning.

Bleeding: Dust continuously passing through the fabric during normal operation.

Blinding: A situation wherein fabric and dust cake permeability cannot be maintained. The collected dust so adheres to or is so embedded in the fabric substrate that it cannot be removed by the cleaning method employed. This is evidenced by continually increasing pressure drop.

Fabric Filters


подпись: descriptionTABLE 13.3 Technical Features of Fabric Filter Collectors


Operating conditions Fabrication techniques Collector configuration Type of cleaning Cleaning requirements Type of fabric Filter element

подпись: operating conditions fabrication techniques collector configuration type of cleaning cleaning requirements type of fabric filter elementPositive pressure/negative (section) pressure

Modular design/factory-welded subassemblies/structurai

Single compartment/multicompartment

Mechanical shaking/reverse air/pulse jet


Woven/non woven

Bags or tubular/pocket or envelope/cartridgc

Installed in the bag housing with all cleaning accessories factory piped and fac­tory wired. If the bags are factory installed, field welding is precluded.

Structural baghouses are limited to very large baghouses. All structural components are fabricated as separate panels with formed flanges. The panels are bolted or welded in the field. These flanged joints serve as stiffeners to the hoppers, plenum, tube sheets, and baghouse enclosures.

The collector configuration can be single compartment or multicompart­ment. Multicompartment designs are used where it is important to maintain a relatively constant pressure drop during collector operations. Multicompart­ment design allows compartments to be taken off-line for cleaning or mainte­nance with the collector system still in operation.

The three major types of filter cleaning are mechanical shaking, reverse air, and pulsejet. Mechanical shaking is used to clean woven fabrics because of their good resistance to vigorous shaking. The cloth may be fabricated into long tubes called bags or envelopes. Bags are hung vertically, with the closed end fas­tened at the top to a motor-driven shaker bar and the open end attached to the tube sheet for the entry of the dust-laden gas. The dirty gas passes through the fabric from inside to outside, with the dust deposited on the inside surface. The envelope type is mounted over a metal spacer or screen, with the open ends at­tached to a tube sheet at one side of the unit. The dirty gas passes from outside to inside and deposits the dust on the outside surface. For cleaning either of the preceding units, the airflow must be interrupted to permit the dislodged dust to fall to the hopper. The use of multicompartment design with enough capacity so that one unit can be shut down provides for continuous filtering operation.

Reverse airflow for filter cleaning was initially developed for delicate fabrics, such as glass cloth, that are used for their special chemical or heat — resisting properties. Reverse air is now applied to all types of fabrics for a wide range of applications. The prime advantage of reverse air is the long bag life that can be obtained for a well-operating system. The reverse-air sys­tem has a disadvantage of requiring another fan (reverse-air fan) and numer­ous air-regulating dampers. If the static pressure for the reverse-air fan is not selected correctly, the system can become inoperative since the bags cannot be cleaned because of high pressure drops.

Pulse-jet filters are commonly used for both dusts and fumes. These filters have the gas flow from outside to inside through internally supported bags of felted rather than woven fabric. Dust is deposited on the outside of the bag. The
clean gas flows up and out the top of the compartment. A tube sheet located at the top of the bag separates the dirty gas from the clean gas and prevents the dirty gas from escaping between the bags to the clean side. The felted fabric is used to keep the pressure drop at a reasonable level. Pulse-jet filters operate at higher air-to — cloth ratios than reverse-air or mechanical-shacking types (i. e., typically two to four times higher). This higher air-to-cloth ratio results in the dust particles’ pene­trating deeper into the material of the bag and necessitating a more effective method of bag cleaning. The cleaning technique used is a pulse-jet technique, which consists of a pulse of air introduced into the open top of the filter bag. The pulsejet expands the bag suddenly, and the dust which has accumulated on the outside of the bag is dislodged and collected in the hopper below. This pulse clean­ing operation is carried out with the unit on-line or off-line, depending on such factors as filter bag material, application, and the basic design principles utilized. The pulse can be a traditional high-pressure type (650 kPa) or a newer develop­ment using a low-pressure pulse (250 kPa).

The cleaning requirements can be continuous, automatic, or intermittent. Continuous cleaning requires a multicompartment unit. A typical cleaning cy­cle is to take one compartment off-line to clean. For some applications, on-line cleaning can be used with pulse-jet filters. In steady-state operation, cleaning may be done on a timed cycle. Automatic controls can be installed so that the cleaning mechanism is activated only when the pressure drop across the col­lector exceeds a certain setpoint. This automatic control using pressure drop can result in significant improvements in bag life. An intermittent cleaning c> — cle is applicable when the process can be stopped for a few minutes to clean the bags. A typical application would be for a plant that operates only on a day shift, in which the collector could be cleaned at the end of the shift.

The properties of the filter fabrics must be understood and properly speci­fied to ensure successful operation. Final selection should be made by test and by an economic analysis that examines original cost against maintenance ex­penses. Important fabric properties are permeability, mechanical strength, sol­ids retention, corrosion resistance, heat resistance, cleanability, and dimensional stability.28 Fabrics may be felted or woven. Felted fabrics are formed by compressing fibers under high pressure, and they are relatively thick. Woven fabrics are composed of twisted yarns that are woven into geo­metric patterns having various spacings between the yarns and having a spe­cific surface finish. The permeability of these fabrics depends on the type of fiber, the tightness of the twist, the size of the yarn, the type and tightness of weave, and the type of surface finish.

The filter elements can be tubular bags, pocket filters, or cartridges. Bags are the most common filter elements and come in standard sizes and specified bag length-to-djameter ratios. Europe is the home of the “pocket” or enve­lope-type filter. This device is characterized by especially large filter areas per unit of installed collector space. This minimal space advantage has favored ap­plication in Europe. Cartridge-type filters, which are used primarily for low inlet dust loadings, employ pleated filter cartridges. These filters have a large filtering area per unit volume. They use a lower filter velocity, which improves air cleaner performance by lowering penetration and reducing differential pressure, thus resulting in lower operating energy requirements.30’31 Figure 13.16 shows typical bag, pocket, and cartridge filter elements and units.








Fabric Filters


подпись: cartridgeBag Pocket or envelope

FIGURE 13.16 Typical bag, pocket, and cartridge filters.

Table 13.4 lists the physical and chemical properties of common filter media. Many other fabrics are available for special applications. These other fabrics are more expensive and should be considered only if the com­mon types of fabric are clearly not suitable for the proposed service. These special applications should be discussed with specialists from filter media firms.

The electrostatic properties of both the dust and the collecting fabric have an important effect on the filtration and cleaning process. Frederick35 has proposed that the optimal performance of collection by fabric media is dependent on relat­ing the relative triboelectric properties of dusts and fabrics and then utilizing these data with other characteristics to specify filter fabrics. Table 13.5 presents the electrostatic charging order of filter Fibers. Dusts may be classified in a similar tri­boelectric series. The charge polarity and magnitude developed on either the dust or the fiber are a function of the processing conditions and the materials them­selves. In the dust-cleaning process, the charge dissipation rate is a very important property. Stainless steel fibers have been woven into filter cloth for dissipating the electrostatic charges and for protection from fires and dust explosions. ’6 Penny5’ has examined the possibility of depositing the dust in a relatively porous layer or filter cake using charged particles. Some tests indicated a marked reduction in pressure drop. Another approach is to apply an electric field across the filter to en­hance dust-collection efficiency. Further work is required to establish how these effects can be incorporated into a more efficient design of a dust collector.

A major market which has developed for fabric filters is for the control ot flue-gas fly ash in the utility industry. This market is primarily at the expense of electrostaric precipitators. Fabric filters have the inherent advantage of op­erating at a high level of collection efficiency for a wide range of dust and gas conditions.

Pertinent literature on fabric filter applications covers topics from status re­ports to detailed descriptions of operation and maintenance problems and the use of a pulse-jet fabric filter. Leith, Gibson, and First’8 observed that filter resistance diminished by a factor of 4 and penetration by a factor of 2 when the gas inlet was moved from bottom to top, with all else remaining constant. It also appears that the top inlet design results in all bags operating at a constant face velocity, while the bottom inlet design results in the bag face velocity decreasing with time.

Physical characteristics

Relative resistance to attack by




























Low cosi’


















Low cost









Easy to clean
































































Poor resistance to abrasion Expensive


















Poor resistance to moisture










“Except phenol and formic acid •’Except phenol.

Except heated acetone.

‘’Except SO;.

Source: Stern2′

In the late 1970s, early development work in the field of fabric filter tech­nology involved the application of filters at high air-to-cloth ratios on metal­lurgical fume. Goodfellow, Geren, and Foord39 reviewed the status of the development work in the field and presented pertinent operating data from full-scale installations. The conventional approach for this application has been to use shaker or reverse-air-type fabric filters at a filtration velocity of about 0.75-0.9 m/min. Early experience with pulse-cleaned fabric filters on metallurgical fume led to “blinding” and “bleeding” problems; as a conse­quence, there was very little activity for many years. In the 1970s, new syn­thetic fabrics and finishing techniques were developed; testing showed that mechanical shaking and reverse-air cleaning could be operated at ratios as


Relative charge of generation


+ 20

Silicon-treated glass (filament and spun)

+ 1.5

Woven wool felt

+ 11

Nylon (spun)

+ 7 to + 10

Cotton (sateen)

+ 6

Orion (filament)

+ 4

Dacron (filament)


Dynel (spun)


Orion (spun)

-5 to -14

Dacron (spun)




Polypropylene (filament)

— 1.3





Polyethylene (filament and spun)


Source: Frederick3′

High as 1.25-1.8 m/min for low grain loadings. Technology has also devel­oped for the successful operation of pulse-jet units for fume application;;. Ad­vantages of the successful application of high-ratio technology include significant capital and operating cost savings and significant reductions in space layout requirements for the units. Pulse-jet units are now widely ac­cepted by industry for metallurgical fumes.

The information supplied to the manufacturer should be as detailed as possible and cover the following factors:

1. Process description and operations

2. Emission sources to be controlled

3. Gas flow rate, moisture content, and temperature

4. Chemical and physical characteristics of the gas

5. Chemical and physical characteristics of the solids

6. Solids flow rate

7. Performance

8. Preliminary layout drawings

Figure 13.17 shows isometrics of mechanical shaking, reverse air, cartridge, and pulse-jet types of bag filter units.

The selection and sizing of fabric filters are complex issues because of the many variables and the range of applications. The selection depends primarily on judgment based on experience. It is common for the user or the engineer to size the fabric filter based on first-hand knowledge or indepth experience from simi­lar operations. The equipment manufacturer may also size the equipment. If no good experience exists, it may be necessary to operate a pilot test filter to collect the necessary sizing data. Dennis, Cass, and Hall40 have presented a quantitative

Outlet Shaker mechanism Fabric filters

Hopper Inlet manifold


Reverse Air Ducr

Outlet manifold

Reverse I vQlS5’ air fan Fabric Inlet filters manifold


Mechanical shaking


Reverse air


Fabric Filters Fabric Filters

Clean air outlet

Dirty air inlet


Screw conveyor

Compressed air

Surge tank


Blow pipes Eductor tubes

Clean air outlet


Fabric Filters

Filter bags with venturi








Dirty air inlet Cartridge




FIGURE 13.17 Isometrics of four types of bag filter units.


Fabric Filters Fabric Filters

Relationship between local filtration velocities and the distribution of fabric dust loadings. This relationship provides a rational basis for predicting filter system behavior. Comparison between predicted and experimental performance showed excellent agreement for glass fabric-fly ash systems. The developed equations are in metric units. Since no formal system exists for reporting fabric filtration pa­rameters in the metric system, Table 13.6 presents filtration parameters in the common British units and metric equivalents.

The sizing parameter for fabric filters is the ratio of gas flow rate (m3/s) to the area of filtering media (m2). This ratio is termed the air-to-cloth ratio. The expression Filtration velocity is used synonymously with air-to-cloth ratio for describing fabric filters. For example, an air-to-cloth ratio of 1.5:1 (1.5 m3/s per m2) is equivalent to a filtration velocity of 1.5 m/s. Data are available in the literature on filtration velocities for different industrial application.23’33 Selection of a filter starts with a detailed specification. The bids, as prescribed by the vendors, should first be analyzed to ensure that they comply with the basic design criteria and that the proposed units are suitable for the dust-col­lection application. Unless first-hand knowledge is available, filter testing should be carried out either at the vendor’s pilot plant or a field pilot plant. The procedure to establish cloth area from the testing program is outlined by Kraus.29

These are the three major performance criteria for a filter:

1. Collection efficiency

2. Total energy input requirements

3. Costs

Collection efficiency is the single most important parameter in the perfor­mance of a filter. Based on typical operational efficiencies for various gas —

Filtration parameter Metric




Filter resistance N/m2

In. H20

J in. water =2,50 N/m2

Filter drag (N min)/m3

(in. HzO minVft

(1 in. water minj/ft

= 817 (N mm/m ’!

Velocity m/min


1 ft/mm = 0.305 m/mm

Volume flow m3/min


1 ft’/min = 0.0283 m*/


Fabric area m2


1 ft2 = 0.093 m2

Areal density g/m2


1 Lb/ft2 = 4882 g/mz

Areal density g/m2


1 oz/yd2 = 33.9 g/m2

Specific resistance (N min)/(g m)

(in. H20 mm ft)/lb

(1 in. water min ft)/lb

= 0.167 N min/g m

Coefficient dust concentration g/m3


1 grain/ft3 = 2.29 g/m-

Source: Dennis, Cass, and Hall40

Cleaning devices as a function of particle sizes below 2 microns in diameter, the fabric filter has the highest collection efficiency for particulates less than

0. 1 micron in diameter.

For high-performance filters, it is common to use penetration (P) rather than efficiency (tj) , where penetration is defined as the outlet dust concentra­tion divided by the inlet dust concentration. The efficiency (tj) is defined as follows:


подпись: (13.69)= 1_P= 1- Outlet dust concentration ^ I inlet dust concentration

Penetration or efficiency may be determined for an operating filter by simultaneous measurements of inlet and outlet dust concentrations using appropriate stack sampling techniques. Efficiencies can be consid­ered as instantaneous or cumulative since efficiencies change with the quantity of dust captured by the filter and the filtering time. Filtering ability is provided by the fabric initially, until a filter cake builds up and provides added filtering ability. A new, unused fabric filter has a collec­tion efficiency in the range of 50-75% for submicron particulate. As the dust loading increases, the efficiency increases. After a short period of time, the efficiency increases up to 90%. Usually, after an hour or so, the overall collection efficiency will be at the 99% level. After a period of cyclic filtration and cleaning, the overall collection efficiency will ex­ceed 99.9%.

Total energy input requirements are primarily a function of the operating pressure drop for the filter. The pressure drop relationship for a reverse-air or a mechanical shaker baghouse is given by the following equation.40

AP = K’2Cy2t, {13.70)


Ap = operating pressure drop across filter (mm WG)

K’2 = specific dust/fabric filter resistance coefficient C, = inlet dust concentration (g/m3)

V = average filtering velocity (cm/s) T = operating time between cleaning cycles (s).

This equation shows that the operating pressure drop is proportional to the square of the filtering velocity. For a fixed set of operating conditions, increas­ing filtering velocity to reduce the size of the collector will result in increases in pressure drop, fan power costs, and penetration and probably reduction in bag life.

For pulse-jet filters, the operating pressure drop is given approximately by the following equation:30

Ap = 2CjAp~2R~°-s, (13.71)


Ap = operating pressure drop across filter (mm WG)

C, = inlet dust concentration (g/m3) P = reservoir compressed air pressure (Pa)

R — reservoir pulse rate (pulses per bag per minute).

As with mechanical shaker and reverse-air baghouses, the operating pres­sure drop is approximately proportional to their square of the filtering ve­locity.

Costs for filters vary, depending on their size and the kind and ar­rangement of fabric and cleaning apparatus. Historically, the lowest pur­chase cost plus installation cost has been the determining factor for baghouse selection. A simple comparison shows that the present value of the operation and maintenance costs for the filter over 10 years is greater than the purchase plus installation cost. The cost of the filtering media is a major portion of the initial expense as well as of long-term operating costs. The life of fabrics on dry dusts is expected to be of the order of one to four years. Conway and Jacenty41 have proposed that a careful economic evalu­ation should be carried out to examine the lifetime cost of a filter installa­tion. The specifications should determine the purchase, installation, operation, and maintenance costs of the system. The best engineering and economic decision can be reached by properly identifying and evaluating all of the baghouse installation’s life costs through use of the life-cycle costing technique.

Life-cycle costing is a method of optimizing equipment selection and min­imizing cost for a project by measuring and comparing all of the costs of a project and translating them into today’s dollars. Chapter 15 covers life-cycle costing in more detail. It provides a technique for comparing several feasible solution alternatives by putting all of the costs on a common basis, and it al­lows the designer to choose an optimum.

Life-cycle cost of a filter installation = total purchase cost

+ installation cost + operating cost + cost of capital

— salvage value

— depreciation value

— tax consideration

Each factor should be determined individually to determine its relevance and significance. High-Efficiency Particulate Air (HEPA Filters)

By definition, a HEPA filter is a throw-away, extended-medium, drv-type filter having the following characteristics:

1. A minimum particle removal efficiency of 99.97% for 0.3 micron particles.

2. A maximum resistance, when clean, of 25 mm WG when operated at rated airflow capacity

3. A rigid casing extending the full depth of the medium

Common air filters used in conventional air-cleaning applications are un­able to decontaminate air to the levels required to meet the maximum permissi­ble concentration values that have been established for radioactive substances in air. The lowest threshold limit values specified for most chemical contaminants in air are at least two orders of magnitude higher than the maximum permissi­ble concentration of any radioactive material. HEPA filters must be used to meet these very low levels. For these components to meet their specified performance, all the system components, such as ductwork, dampers, etc., must meet stan­dards of design and installation substantially higher than those for the nonnu­clear industry. Burchsted, Fuller, and Kahn42 cover the design, construction, and testing of very high efficiency particulate air-cleaning systems.

HEPA filters consist of a filter pack sealed into a case. The filter pack or core is made by pleating a continuous web of fiberglass paper back and forth over corrugated separators. The filter pack is sealed into a full-depth wood or steel casing using a elastomeric sealant. Gasketing is a critical item to ensure that the filter passes the air leakage tests.

The decision to use prefilters must be determined for each application oil the basis of total air-cleaning system costs and the risks associated with expos­ing HEPA filters to the environment without protection. The effect of install­ing prefilters may range from being insignificant to increasing the life of the HEPA filter by a factor of 2 or 3. As a guideline, prefilters should be used if particles are larger than 1 or 2 microns in diameter or if dust concentration is greater than 23 mg/m3.

The primary factor in filter replacement is pressure drop. Typically, HEPA filters are replaced when the pressure drop exceeds 100 mm WG. By specifica­tion, HEPA filters must have sufficient structural pressure to withstand a con­tinuously applied overpressure of 250 mm WG or higher for at least 15 minutes without visible damage or loss of efficiency.

HEPA filters are one of the components of the total air-cleaning system. Burchsted, Fuller, and Kahn42 describes the design requirements in detail for

The complete air-cleaning system. Design considerations for the system include environmental factors (zoning, airborne particulates and gas, moisture, heat and hot air, corrosion, vibration), operational factors (operating mode. Hltrr — change frequency, building supply filters, prefilters, underrating, uniformity of airflow), system considerations (components, type of system), and emergency factors (shock and overpressure, fire and hot air, power and equipment oi. r- age, air-cleaning system layout).

For HEPA filters, the major design parameter is the system airflow, which can be used to establish the size and number of HEPA filters required. The penetration, resistance, and airflow are determined by the manufacturer be­fore the filter is shipped from the factory.43 Table 13.7 gives the standard sizes for HEPA filters.