Fabric types for dust collection

A variety of fabrics styles are employed in dust collection, these being chosen according to the physical forces that will be imposed by the filter and the quantity and abrasive nature of the dust being collected. As a consequence, for reverse air and pulsejet filters needlefelts are preferred whereas for shake filters both woven and needlefelt structures are used. By comparison for cartridge filters spunbonded, meltblown and electrospun fabrics are more likely to be found. 

Primarily because of their good resistance to frequent and aggressive flexing during the cleaning phase, filter sleeves made from woven fabrics are 

Weft count 175 tex (woollen spun) (multifilament) 120 tex (cotton spun) 

As some reverse air filters also employ shake or vibration mechanisms, it is not uncommon to find the same fabric in both types of filter. This notwithstanding, conventional reverse air filter sleeves are most likely to feature multifllament yarns in the warp combined with staple fibre yams in the weft. On this occasion the twill weave will be replaced by a 5- or 6-shaft warp satin construction, which is designed to facilitate superior dust release from the smooth multifilament warp yarns during the less aggressive cleaning phase. Particle capture will be further enhanced if the fabric is subjected to a raising or napping process on the reverse side. A typical polyester construction, as may be found in a reverse air filter, is also shown in Table 7.3 for comparison with the previous construction. 

Whilst needle felts are produced to countless specifications in order to address the nature, volume and velocity of the dust being collected, the type of filter employed, to meet a standard test requirement or an equipment manufacturer s specification, their area densities will generally fall between 340 and 640 gm the greater the dust load and intensity of cleaning, the greater is the area density. 

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See reference 36 for discussion of various types of dust collection equipment. Fabric filters will be used in order to recover all the fines in a dry state. These should recover 99% of the material larger than 0.2 microns.36 A continuous-envelope fabric filter will be used, since this seems to have the lowest annual costs. For this filter ft3 of surface area is required for about each 2 ft3/min of air. 

Fabric-filter systems, commonly called bag-filter or bag-house systems, are dust-collection systems in which dustladen air is passed through a bag-type filter. The bag collects the dust in layers on its surface and the dust layer itself effectively becomes the filter medium. Because the bag s pores are usually much larger than those of the dust-particle layer that forms, the initial efficiency is very low. However, it improves once an adequate dust-layer forms. Therefore, the potential for dust penetration of the filter media is extremely low except during the initial period after startup, bag change, or during the fabric-cleaning, or blow-down, cycle. 

The frequency of cleaning depends on the nature and concentration of the dust. Typical cleaning intervals vary from about 2 to 15 minutes. However, the cleaning action of the pulse is so effective that the dust layer may be completely removed from the surface of the fabric. Consequently, the fabric itself must serve as the principal filter media for a substantial part of the filtration cycle, which decreases cleaning efficiency. Because of this, woven fabrics are unsuitable for use in these devices and felt-type fabrics are used instead. With felt filters, although the bulk of the dust is still removed, an adequate level of dust collection is provided by the fabric until the dust layer reforms. 

Many types of particulate collection devices are available commercially (see Table 53.2). Each operates on a different principle for accomplishing removal of particulates from the gas stream. Four basic types are common in drying systems (1) the drying vessel itself (in the case of vessel dryers), (2) cyclones, (3) bag filters, and (4) wet scrubbers. Electrostatic precipitators (ESPs) are not used widely in drying installations in spite of their low-pressure drop and high collection performance. The initial cost of purchase and construction is high. For this reason, the emphasis in Section 53.3 will be on the three most widely used devices, e.g., cyclones, fabric filters, and wet scrubbers. For a concise discussion of various types of solid-gas separation equipment and guidelines for selection of dust collectors,... 

Fabric filters (baghouscs) represent a second accepted method for separating particles from a flue gas stream. In a baghouse, the dusty gas flows into and through a number of filter bags, and the particles are retained on the fabric. Different types are available to collect various kinds of dust with high efficiency. 

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