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 theMullen burst test
to compare strengths of new and used bags, can indicate the deterioration in strength of
used bags.

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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.

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MIT Flex

TheMIT 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.

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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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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
MIT Flex
Mullen Burst Strength
Tensile Strength

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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 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 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 fromchemical 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.

                                        Typical fabrics used for bags

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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 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.

                                   Summary of pretreatment processes

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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).

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

                                            Table 1. Typical fabrics used for bags

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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 1).

Figure 1. 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.

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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 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 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 2).

Figure 2. Twill weave patterns (2/1 and 3/1)

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 3).

Figure 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.

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). 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. Sieving (on a woven filter)

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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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