Page 12 - dust collection systems

fibers, bonded together by chemical, mechanical, heat or
solvent treatment. A nonwoven fabric, fibrous matl, is often

called needlefelt or felt. Most bags are either completely or

partially made by weaving since nonwoven fabrics are

generally attached to a woven base called a scrim.

Fabric filtration can be subdivided into two

mechanisms/modes: cake filtration and noncake/depth

filtration.

Cake Filtration

In a woven fabric filter, particles get collected by a ‘sieving’

mechanism in which particles too large to pass through the

mesh of the fabric are caught and retained on the surface of

the filter. The caught particles gradually build up a cake on

the fabric surface so that the labyrinthine nature of the gas

flow path continually increases while the effective mesh size

decreases. Some particles escape through the filter until the

cake is formed. The collection efficiency of the filter will

therefore tend to improve with use. However, the pressure

drop across the filter increases. Hence, optimum operating

cake layer/thickness is a trade-off between collection

efficiency and pressure drop and regular cleaning is essential

to maintain the pressure drop at an operational level.

As collection efficiency during the initial cake construction

period is lower than after a uniform cake has been

established, operation without a dust cake is undesirable and

is minimized