Cotton-wool, glass-fibre fabric, pleated paper of various types, foamed polyurethane, cellular polythene and other materials are used for the construction of dry filters. As with viscous filters, there are cell-type and automatic roll-type filters available. The construction of these, in broad outline, is very similar to that of viscous filters. A certain amount of viscous, gel-like substance is sometimes applied to glass-fibre fill to help dust retention and it is common practice to increase the density of packing towards the downstream end of the filter to improve the removal of the smaller sized particles and make the filter effective through its entire depth. In general, efficiency is improved by increasing the
Surface area of the material offered to the airstream. In cell-types this is achieved by using pads of material placed obliquely across the airstream (Figure 17.5). An alternative to this, which can be made to yield very high efficiencies, is to use a system of pleating. This is best achieved with glass fibre paper, although other material is also used, and gives high efficiencies. Figure 17.6 shows an arrangement of pleating.
When a very large amount of material is used in a filter, the efficiency becomes very high and the filter is termed an ‘absolute’ filter. No filter is truly absolute but almost any desired efficiency could be achieved if sufficient filtering fabric were used; this would be associated with a high pressure drop, but one way of achieving a high efficiency without the penalty of wasted energy is to use a very large filter, that is, one with a very low face velocity.
Automatic dry-fabric filters consist of an upper roll of clean fabric wound downwards across the airstream. The dirtied material is then rewound into a roll at the bottom of the unit. Figure 17.7 shows this arrangement. The advantage of this type of filter is the low
Fig. 17.5 Dry filter.
Maintenance cost when compared with the cell-type of dry filter. Removing a dirty roll and replacing it with a clean one at relatively infrequent intervals is a cleaner task and is, therefore, easier and cheaper than changing dirty cells. The disadvantage is that the efficiency is not so high. This is overcome to some extent by using a denser material and by introducing V-form or S-shaped changes of direction in the fabric as it crosses the airstream, thus presenting more surface area. The rolls, as in automatic viscous filters, are motorised, and the geared-down electric driving motors are operated from time switches. As with all filter installations there is a risk that the filtering medium may be by-passed by dirty air. This must not be allowed.
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