Fabric filters reverse-flow cleaned

Fabric-filter designs fall into three types, depending on the method of cleaning used (1) shaker-cleaned, (2) reverse-flow-cleaned, and (3) reverse-pulse-cleaned. 

Reverse-flow-cleaned filters are similar to the shaker-cleaned design, except the shaker mechanism is eliminated. As with shaker-cleaned filters, compartments are taken off-line sequentially for cleaning. The primary use of reverse-flow cleaning is in units using fiberglass-fabric bags at temperatures above 150°C (300°F). 

Many large fabric filters are self-cleaning. For some, agitation or motion shakes off the collected material and cleans the filters. Some use reverse air flow to knock material loose. During a cleaning cycle and cleaning action, exhaust system air must divert from the exhaust system to an alternate collector. 

Two principal types of fabric are adaptable to filter use woven fabrics, which are used in shaker and reverse-flow filters and felts, which are used in reverse-pulse filters. The felts made from synthetic fibers are needle felts (i.e., felted on a needle loom) and are normally reinforced with a woven insert. The physical properties and air permeabilities of some typical woven and felt filter fabrics are presented in Tables 17-6 and 17-7. The air permeability of a filter fabric is defined as the flow rate of air in cubic feet per minute (at 70°F, 1 atm) that will pass through 1 ft2 of clean fabric under an applied differential pressure of Vt in water. The resistance coefficient KF of the clean fabric is defined by the equation in Table 17-6, which may be used to calculate the value of KF from the air permeability. If Ap, is taken as 0.5 in water, t as 0.0181 cP (the viscosity of air at 70°F and 1 atm), and Vj as the air permeability, then //, = 27.8/air permeability. 

Fabric Filters. Fabric fillers normally use either woven or felt type cloth in die common beg-filler configuration. The fabric is made into bags 6-10 in. in diameter and 6-40 fl long. The air flow can be in either direction through the bag wnll. The dust is periodically removed from the bag either by a shaking mechanism or hy a reverse flow palse-jet. Means are provided in either style so that only a fraction of (he total number of bags are being cleaned at any one time. 

FIGURE 53.3 Fabric filters (a) large, multicompartment fabric filter cleaned by mechanical shaking. (Courtesy of Wheelabrator-Shint.) (b) Envelope-type fabric filter cleaned by reverse flow. (Courtesy of Shinto Dust Collector.)... 

In a filter, the dusty gases are passed through a filter medium, which retains the particles to the extent provided by the medium chosen. Media include sand gravel beds natural, synthetic, or glass fiber woven mats woven fabrics or felts. The most cotmnon are woven fabrics, which are cleaned by shaking, intermittent reverse flow, continuous reverse flow, or air pulses. 

Cleaning by reverse-flow pulses of high pressure air is normally applied to fabric filters and to pleated cartridge collectors alike. The high pressure pulses may be created by a pressure blower or come directly from a compressed air supply. In the latter case the type is normally called a pulse-jet filter. 

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