Dust Filter Fabric Materials

Fabric filter bags are generally round tubes, made of woven fabric or nonwoven fabric. Information about important properties, characteristics and use of various fabric materials is given in this chapter.
 
Inlet gas characteristics are extremely important in selecting the material for the filtration medium. 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.
 
Filter fabric is manufactured from various materials, which provide different beneficial characteristics. Following selection chart (Source: GE Energy) summarizes some of the common fabric materials and conditions they are most suited to handle.

Fabric Material

Maximum Continuous Operating Temp. °C

Abrasion

Filtration Properties

Alkalines

Mineral Acids

Polyester

135°C

Excellent

Excellent

Fair

Fair

Acrylic

130°C

Good

Good

Fair

Good

PPS (Ryton®)

190°C

Good

Excellent

Excellent

Excellent

Aramid(Nomex®)

204°C

Excellent

Excellent

Good

Fair

P84

180-260°C

Fair

Excellent

Fair

Good

Fiberglass

260°C

Fair

Fair

Fair

Poor

PTFE(Teflon®)

260°C

Good

Fair

Excellent

Excellent

Following table summarizes important characteristics and use of various fabric materials

Polyester

Polyesters are among the most widely used fabrics for general applications. The primary damaging agents are water (hydrolysis) and concentrated sulfuric, nitric, and carbolic acids.

Acrylic

These synthetic fibers offer good hydrolytic resistance over a limited temperature range, 127°C for continuous and 135°C for surge application. Acrylic fibers are used in the manufacture of ferrous and nonferrous metals, carbon black, cement, lime, and fertilizers. They are also used extensively in wet filtration applications.

PPS [Ryton®]

Developed in 1973, the first commercial fibres from PPS (poly phenylene sulphide) appeared in the early 1980s with the introduction of Ryton® by Phillips Fibers Corp. and subsequently by Toyobo as Procon® and by Toray as Torcon®. It has excellent resistance to both acids and alkalis, which makes it very useful in combustion-control applications. Its early applications have been on industrial coal-fired boilers, waste-to-energy incineration (with and without spray dryers), titanium dioxide, and installations where Nomex does not perform well due to chemical or hydrolytic attack. It will hydrolyze, but only at temperatures above 190°C.

Aramid [Nomex®]

It has excellent thermal stability, shrinking less than 1% at 177°C. The fiber is flame resistant, but when impregnated with combustible dusts, will support combustion that will melt and destroy the fabric. Unacceptably short bag life will result where sulfur oxides (SOx) and moisture are present and frequent dew point excursions occur, such as in coal-fired boilers. Some acid-retardant finishes have been developed for Nomex, but have been found to improve bag life by no more than 50%, leaving most bag life cycles unacceptably short.

P84

P84 is an aromatic polymer fiber produced in felt form only. The unique shape of the fiber produces improved capture efficiency characteristics. Composites are made from this material to take advantage of the superior filtration characteristics of P84 while reducing its cost. Any of the felted materials can be combined with P84 to produce a fabric composite that exhibits the characteristics of both materials.

Fiberglass (Glass Fibers)

Most fiberglass fabrics are woven from minute 0.0038 mm (0.00015 inch) filaments. Many variations of yarn construction, fabric weaves, and fabric finishes are available. It is also produced in a felted form. Fiberglass has the highest operating temperature range available in conventional fabrics. Above 260°C, the fiberglass itself is not directly damaged, but the finish which provides yarn-to-yarn lubrication begins to vaporize, resulting in accelerated mechanical wear of the glass fibers. Fiberglass is noncombustible, has zero moisture absorption (cannot hydrolyze), has excellent dimensional stability, and has reasonably good strength characteristics. Woven glass fabrics have high tensile strength characteristics but relatively low flex strength, especially in the fill (circumference) direction of the bag, and low abrasion resistance. Care must be taken to minimize flexing and rubbing. Fiberglass fabrics have relatively good resistance to acids, but impurities in the glass fibers are attacked by hydrofluoric, concentrated sulfuric, and hot phosphoric acids. They also have poor resistance to hot solutions of weak alkalis, acid anhydrides, and metallic oxides. For these reasons, glass fabrics should not be operated below the acid dew point. Fiberglass fabrics are used extensively with coal-fired boilers and high temperature metals applications.

PTFE (Teflon®)

Teflon® is unique among synthetics in its ability to resist chemical attack across the entire pH range throughout its operating temperature range of 232°C continuous. This fluorocarbon fiber is non-adhesive, has zero moisture absorption, and is unaffected by mildew or ultraviolet light. The primary shortcomings of Teflon® are its high cost and relatively poor abrasion resistance. Applications of Teflon® include coal-fired boilers, waste-to-energy incinerators, carbon black, titanium dioxide, primary and secondary smelting operations, and chemical processing.

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