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Lesson 4

Fabric Filter Material

Goal

To familiarize you with the construction of fabric filter material, fibers used, and problems affecting fabric life.

Objectives

At the end of this lesson, you will be able to do the following:

1. Name two ways filters are constructed

2. List at least seven natural or synthetic fibers used to make filters and identify the conditions

under which they are used

3. Define five fabric treatment processes

4. Name three failure mechanisms that reduce filter life

5. Briefly describe four types of fabric tests that are used for troubleshooting bag problems

Filter Construction

Bag filters can be made of woven or nonwoven materials. Nonwoven materials can further be

divided as felted or membrane. Most bags are either completely or partially made by weaving

since nonwoven fabrics are generally attached to a woven base called a scrim. Woven filters

are made of yarn with a definite repeated pattern. Felted filters are composed of randomly

placed fibers compressed into a mat and attached to loosely woven backing material. A membrane filter is a special treatment where a thin, porous membrane (expanded polyfluorocarbon) is bonded to the scrim, or support fabric. Woven filters are generally used with low

energy cleaning methods such as shaking and reverse-air. Felted fabrics are usually used with

higher energy cleaning systems such as pulse-jet cleaning. Membrane filters were developed

in efforts to achieve high efficiency particle capture and to handle flue gas conditions where

high moisture and resulting high pressure drop problems frequently occur.
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Woven Filters

Woven filters have open spaces around the fibers. The weave design used will depend on

the intended application of the woven filter. The simplest weave is the plain weave. The

yarn is woven over and under to form a checkerboard pattern (Figure 4-1). This weave is

usually the tightest, having the smallest pore openings in the fabric. Consequently, it

retains particles very quickly. This weave is not frequently used, because the bags tend to

have a high pressure drop (even without any dust cake).

Figure 4-1. Plain weave or checkerboard

Other weaves include the twill and sateen (satin). In the twill weave, yarn is woven over

two and under one for a 2/1 twill and over three and under one for a 3/1 twill weave (see

Figure 4-2).

Figure 4-2. Twill weave patterns (2/1 and 3/1)
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The twill weave does not retain particles as well as the plain weave, but does not tend to

blind as fast. Bag blinding is a condition where the particles (dust) becomes embedded in

the filter over time and are not removed by the bag cleaning process. The twill weave

allows good flow rates through the filter and high resistance to abrasion. In the satin

weave, yarn is woven over one and under four in both directions. Satin weave does not

retain particles as well as the plain twill weave, but has the best (easiest) cake release

when the fabric is cleaned (Figure 4-3).

Figure 4-3. Sateen weave (satin weave)

Different weaving patterns increase or decrease the open spaces between the fibers. This

will affect both fabric strength and permeability. Fabric permeability affects the amount

of air passing through the filter at a specified pressure drop. A tight weave, for instance,

has low permeability and is better for the capture of small particles at the cost of increased

pressure drop.
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The true filtering surface for the woven filter is not the bag itself, but the dust layer or filter cake. The bag simply provides the surface for capture of larger particles. Particles are

collected by impaction or interception as the open areas in the weave are closed. This process is referred to as sieving (Figure 4-4). Some particles escape through the filter until the

cake is formed. Once the cake builds up, effective filtering will occur until the bag

becomes plugged and cleaning is required. At this point, the pressure drop will be exceedingly high and filtering will no longer be cost effective. The effective filtering time will

vary from approximately 15 to 20 minutes to as long as a number of hours, depending on

the concentration of particulate matter in the gas stream.

Figure 4-4. Sieving (on a woven filter)

Felted Filters

Felted filters are made by needle punching fibers onto a woven backing called a scrim.

The fibers are randomly placed as opposed to the definite repeated pattern of the woven

filter. The felts are attached to the scrim by chemical, heat, resin, or stitch-bonding

methods.

To collect fine particles, the felted filters depend to a lesser degree on the initial dust

deposits than do woven filters. The felted filters are generally 2 to 3 times thicker than

woven filters. Each individual randomly oriented fiber acts as a target for particle capture

by impaction and interception. Small particles can be collected on the outer surface of the

filter (Figure 4-5).
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Figure 4-5. Felted fabric filter

Felted filters are usually used in pulse-jet baghouses. A pulse-jet baghouse generally filters more air per cloth area (higher air-to-cloth ratio) than a shaker or reverse-air unit.

Felted bags should not be used in high humidity situations, especially if the particles are

hygroscopic (these particles have an affinity to absorb moisture and thus become sticky).

Clogging or blinding could result in such situations.

Fibers

The fibers used for fabric filters vary depending on the industrial application to be controlled.

Early filters were mostly made from natural fibers such as cotton or wool. These fibers are relatively inexpensive but have temperature limitations (< 212°F or 100°C) and only average

abrasion resistance. Cotton is readily available making it very popular for low temperature

simple applications. Wool withstands moisture very well and can be made into thick felt

easily.

Synthetic fibers are more widely used today than natural fibers because they can operate at

higher temperatures and better resist chemical attack. The synthetic fiber most often used for

high temperature application is fiberglass or glass fibers. Fiberglass is the generic substance

found in Fiberglas. For low temperature applications polypropylene is the most inexpensive

synthetic fiber and is used in many industrial applications such as foundries, coal crushers, and

food industries. Nylon is the most abrasion-resistant synthetic fiber, making it useful in applications filtering abrasive dusts. Polyesters such as Dacron fibers have good overall qualities to

resist acids, alkalines, and abrasion and are relatively inexpensive, making them useful for

many industrial processes such as smelters, foundries, and other metal industries.

Nomex fibers are widely used for fabric filter bags because of their resistance to relatively

high temperatures and to abrasion. Nomex is used for filtering dusts from cement coolers,

asphalt batch plants, ferroalloy furnaces, and coal dryers.

Other registered trademark fibers such as Teflon, Fiberglas, Ryton, and P84, as well as carbon

fibers can be used in very high temperature situations. Teflon has very good resistance to acid

attack (except fluorine) and can withstand continuous temperatures up to 445°F (230°C).
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Fiberglas or glass is often used in baghouses that handle very high temperatures (up to 500°F

or 260°C) for continuous operation. About 90% of the baghouses currently operating on coal

fired utility boilers use bags made with glass fibers (McKenna and Furlong 1992). Glass fibers

are usually lubricated in some fashion so they will slide over one another without breaking or

cutting during the cleaning cycle. Graphite is commonly used as a lubricant and will help

retain the upper service temperature limits. Glass fibers can break easily and require a very

gentle cleaning cycle. Ryton is a felted filter made from polyphenylene sulfide fibers generally

attached to a polyfluorocarbon scrim. Ryton can operate at high temperatures (350°F or

177°C) and shows good resistance to acids and alkalis. Fiberglas, Teflon, Nomex and Ryton

have been used to remove particulate emissions generated from industrial and utility coal-fired

boilers (Belba et al. 1992).

Another material used to make bags is Gore-tex membrane manufactured by W. G. Gore and

Associates, Inc. The Gore-tex membrane is an expanded polytetrafluoroethylene (PTFE)

membrane that is laminated with a variety of fibers such as Fiberglas, polyester, and Nomex to

produce felt and woven filters. Some test reports have indicated very good emission reduction

(99.9+%), low pressure drops, increased bag life and higher air-to-cloth ratios using this material in metal industries, chemical industries, food industries, and coal-fired boilers. However,

other fabrics have been able to obtain similar results.

Finally, for very high temperature applications (> 500°C), ceramic filters are now available

(McKenna and Turner 1989). These filters show promise for high temperature applications

such as using the filters ahead of boiler superheater tube sections to remove particles and

improve heat transfer in the boiler tubes.

Table 4-1 lists a number of typical fibers used for fabric filters. The properties of the listed

fibers include temperature limits, acid and alkali resistance, abrasion resistance, and relative

bag costs. Table 4-1 is only a general guide since bag filters can be made of two or more layers

of materials to achieve specific effects (i.e. strength, stability, filtering etc.) The cost (1992) of

a fiberglass bag 14 feet long and 6 inches in diameter is approximately $35 to $40. From Table

4-1 the price of a Teflon bag of the same size is approximately $115 to $135.
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Fabric Treatment

Fabrics are usually pretreated to improve their mechanical and dimensional stability. They can

be treated with silicone to give them better cake release properties. Natural fabrics (wool and

cotton) are usually preshrunk to eliminate bag shrinkage during operation. Both synthetic and

natural fabrics usually undergo processes such as calendering, napping, singeing, glazing, or

coating.

These processes increase fabric life, improve dimensional stability (so that the bags retain

their shape or fit after long use), and facilitate bag cleaning.

Calendering is the high pressure pressing of the fabric by rollers to flatten or smooth the

material. Calendering pushes the surface fibers down onto the body of the filter medium. This

is done to increase surface life and dimensional stability and to give a more uniform surface to

bag fabric.

Napping is the scraping of the filter surface across metal points or burrs on a revolving

cylinder. Napping raises the surface fibers, creating a "fuzz", that provides a large number of

Table 4-1. Typical fabrics used for bags

Generic

name

Fiber Maximum temperature Acid

resistance

Alkali

resistance

Flex

abrasion

resistance

Relative

cost

Continuous Surges

°F °C °F °C

Natural fiber

cellulose

Cotton 180 82 225 107 poor excellent average 0.4

Polyolefin Polypropylene190 88 200 93 excellent excellent good 0.5

Natural fiber

protein

Wool 200 93 250 121 good poor average 0.8

Polyamide Nylon 200 93 250 121 poor to fair excellent excellent 0.6

Acrylic Orlon® 240 116 260 127 very good fair average 0.7

Polyester Dacron® 275 135 325 163 good fair excellent 0.5

Aromatic

polyamide

Nomex® 400 204 425 218 fair very good very good 2.0

FluorocarbonTeflon® 450 232 500 260 excellent

except

poor for

fluorine

excellent

except

poor for

trifluoride,

chlorine,

and

molten

alkaline

metals

fair 6.7

Glass Fiberglas®

or glass

500 260 550 288 good poor poor to fair 1.0

Polymer P84® 450 232 500 260 good fair fair 2.5

Polymer Ryton® 375 191 450 232 excellent excellent good 2.5-4.0

Sources: McKenna and Turner 1989.

Greiner 1993.
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sites for particle collection by interception and diffusion. Fabrics used for collecting sticky or

oily dusts are occasionally napped to provide good collection and bag cleaning ease.

Singeing is done by passing the filter material over an open flame, removing any straggly surface fibers. This provides a more uniform surface.

Glazing is the high pressure pressing of the fiber at elevated temperatures. The fibers are fused

to the body of the filter medium. Glazing improves the mechanical stability of the filter and

helps reduce bag shrinkage that occurs from prolonged use.

Coating, or resin treating, involves immersing the filter material in natural or synthetic resin

such as polyvinyl chloride, cellulose acetate, or urea-phenol. This is done to lubricate the

woven fibers, or to provide high temperature durability or chemical resistance for various

fabric material. For example, glass bags are occasionally coated with Teflon or silicon graphite

to prevent abrasion during bag cleaning and aid in acid resistance. The Teflon coating is generally applied at 10% of finished weight level.

Bag Failure Mechanisms

Three failure mechanisms can shorten the operating life of a bag. They are related to thermal

durability, abrasion, and chemical attack.

The chief design variable is the upper temperature limit of the fabric, or thermal durability.

As shown in Table 4-1, fabrics have upper temperature limits which they can withstand continuously. The table also shows surge limits which are temperatures at which the baghouse can be

operated for short durations. Consult the fabric supplier for the length of time that the surge

temperature can be tolerated. The process exhaust temperature will determine which fabric

material should be used for dust collection. Exhaust gas cooling may be feasible, but the

exhaust gas must be kept hot enough to prevent moisture or acid from condensing on the bags.

Another problem frequently encountered in baghouse operation is abrasion. Bag abrasion can

result from bags rubbing against each other, from the type of bag cleaning used, or where dust

enters the bag and contacts the fabric material. For instance, in a shaker baghouse, vigorous

Table 4-2. Summary of pretreatment processes

Pretreatment Method Result Reason for use

Calendering High pressure

pressing by rollers

Flattens, smooths, or

decorates

Increases surface life

Increases dimensional stability

Provides more uniform fabric

surface

Napping Scraping across

metal points

Raises surface fibers Provides extra areas for

interception and diffusion

Singeing Passing over open

flame

Removes straggly

surface fibers

Provides uniform surface area

Glazing High pressure

pressing at elevated

temperatures

Fibers fused to filter

medium

Improves mechanical stability

Coating Immersing in natural

or synthetic resin

Lubricates woven

fibers

Provides high temperature

durability

Provides chemical resistance

for various fabric material

Source: McKenna and Greiner 1982.
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shaking may cause premature bag deterioration, particularly at the points where the bags are

attached. In pulse-jet units, the continual, slight motion of the bags against the supporting

cages can also seriously affect bag life. As a result, a 25% per year bag replacement rate is

common. This can be the single biggest maintenance problem associated with baghouses

(Greiner 1992).

Bag failure can also occur from chemical attack to the fabric. Changes in dust composition

and exhaust gas temperatures from industrial processes can greatly affect the bag material. If

the exhaust gas stream is lowered to its dew point (either water or acid dew point), the design

of the baghouse (fabric choice) may be completely inadequate. Proper fabric selection and

good process operating practices can help eliminate bag deterioration caused by chemical

attack. Lesson 6 discusses bag failures in more detail.

Fabric Testing

A number of standard ASTM tests can be conducted on bag filters either to verify the bag filter's conformity with purchase specifications or to use as a troubleshooting tool for problem

bag failures. As with all measurement techniques, the results of these bag tests are relative.

Often for these tests to be useful, they must be conducted over time in order to compare relative degradation. In addition, with some of the newer fabrics, some of these tests may not be

meaningful. These tests can be used to indicate bag strength and flow loss. Four of the standard tests performed are: permeability, MIT flex, Mullen burst strength, and tensile strength

(McKenna and Turner 1989). These tests can be conducted if the installed baghouse is having

problems with bag life or unusually high pressure drop.

Permeability

The permeability test is used to determine the amount of air that can flow through a given

cloth area. Permeability is defined in ASTM Standard D-737-69 as the volume of air that

can flow through one square foot of cloth at a pressure drop of no more than 0.5 in. w.g.

(125 Pa). Because air permeability is not a linear function of the pressure difference measured across fabric surfaces, the ASTM method prescribes that permeability tests be made

at a pressure drop of 0.5 in. w.g. (125 Pa). Certain fabrics may be too dense or too open to

maintain this pressure drop. In these cases, the ASTM method states that measured pressure drop be given in the test report.

The permeability of clean felts usually ranges between 15-35 ft/min (8-18 cm/s), while

lighter-weight woven materials have permeability values greater than 50 ft/min (25 cm/s).

Permeability can be measured on clean or dirty bags. Dirty bags are usually tested in the

"as received" state. They are then cleaned by vacuuming or washing and retested. These

measured values can be compared to the original clean permeability of the fabric to determine if bags that have been in service have become blinded. It is also possible that the

pores in the fabric will open wider after extended use, which is shown by permeability values higher than the original values. This condition, however, does not occur as frequently

as blinding.
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MIT Flex

The MIT Flex Test is used to measure the ability of fabrics to withstand self-abrasion from

flexing. This test method is described in ASTM Standard D-2176-69, which is the standard method for testing the endurance of paper with the MIT test apparatus.

The flex test has frequently been used to help determine the rate of deterioration of glass

bags used in baghouses installed on coal-fired utility boilers. This test also helps provide

insight into the effect of bag tensioning on bag life. Flex testing is occasionally performed

after exposing the fabric to heat and/or acid in order to simulate conditions in utility boiler

baghouses. The test cannot be done with a continuous dust load on the fabric, which limits

the comparison to actual field conditions.

Mullen Burst Strength

The Mullen burst strength test, described in ASTM Standard D-231, is designed to show

the relative total strength of fabrics to withstand pulsing or pressure.

For new glass fabrics, the Mullen burst test provides a good indication of whether the fabric has been weakened by the heat cleaning given the fabric before coating it with materials such as Teflon or silicon graphite.

Tensile Strength

The tensile strength test provides data on fabric stretch, elongation, and tear. This test

method is described in ASTM Standard D-1682-64 for breaking load and elongation of

textile fabrics.

Tensile strength varies, depending on fabric type and weight. Synthetic fabrics generally

tend to stretch or show greater elongation than natural fabrics. Glass materials usually

have high tensile strengths. The tensile test, used in combination with the Mullen burst test

to compare strengths of new and used bags, can indicate the deterioration in strength of

used bags.
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Review Exercise

1. Bag filters (bags) are made from ____________________ materials.

a. Woven

b. Felted

c. Membrane

d. All of the above

2. ____________________ filters are made from yarn with a definite repeated pattern.

3. The ____________________ and ____________________ weaves have better cake release than

the simple weave.

4. In a woven filter, the woven material is not the true filtering surface. The dust

____________________ provides the surface for filtering particles.

5. ____________________ filters are made by needle punching fibers onto a woven backing called a

scrim.

6. The layer of woven material used for strength and support of felted or membrane material is

referred to as the ____________________.

7. Two natural fibers used for fabric filters are ____________________ and

____________________.

8. Wool and cotton are inexpensive but are susceptible to failure at ____________________

____________________.

9. The fabric that is most often used in high temperature (> 200°C) industrial processes is:

a. Fiberglas

b. Nylon

c. Cotton

d. Polypropylene

10. True or False? Fabrics are pretreated to improve their mechanical and dimensional stability.

11. The filter surface of fabric material is sometimes scraped with metal points or burrs on a revolving

cylinder to create a "fuzz" on the material. This treatment is called:

a. Singeing

b. Glazing

c. Napping

d. Resin treating

12. True or False? Glass bags are occasionally coated with Teflon or silicon graphite to prevent

abrasion during bag cleaning.
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13. When fabric material is passed over an open flame to remove straggly fibers, the treatment is

called ____________________.

14. Failure mechanisms that shorten bag operating life are:

a. Abrasion

b. Temperature excursions

c. Chemical attack

d. Varying particle size in flue gas

e. a, b, and c only

15. True or False? The chief design variable for prolonged bag life is the upper temperature limit of

the bag.

16. The amount of air that can flow through a given cloth area is the ____________________ of the

cloth.
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Review Answers

1. d. All of the above

Bag filters (bags) are made from woven, felted, or membrane materials.

2. Woven

Woven filters are made from yarn with a definite repeated pattern.

3. Twill

Sateen

The twill and sateen weaves have better cake release than the simple weave.

4. Cake

In a woven filter, the woven material is not the true filtering surface. The dust cake provides the

surface for filtering particles.

5. Felted

Felted filters are made by needle punching fibers onto a woven backing called a scrim.

6. Scrim

The layer of woven material used for strength and support of felted or membrane material is

referred to as the scrim.

7. Wool

Cotton

Two natural fibers used for fabric filters are wool and cotton.

8. High temperature

Wool and cotton are inexpensive but are susceptible to failure at high temperature.

9. a. Fiberglas

The fabric that is most often used in high temperature (> 200°C) industrial processes is Fiberglas.

10. True

Fabrics are pretreated to improve their mechanical and dimensional stability.

11. c. Napping

The filter surface of fabric material is sometimes scraped with metal points or burrs on a revolving

cylinder to create a "fuzz" on the material. This treatment is called napping.

12. True

Glass bags are occasionally coated with Teflon or silicon graphite to prevent abrasion during bag

cleaning.

13. Singeing

When fabric material is passed over an open flame to remove straggly fibers, the treatment is

called singeing.
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14. e. a, b, and c only

Three failure mechanisms that shorten bag operating life are abrasion, temperature excursions, and

chemical attack.

15. True

The chief design variable for prolonged bag life is the upper temperature limit of the bag.

16. Permeability

The amount of air that can flow through a given cloth area is the permeability of the cloth.
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