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