Book - School of Science and Technology

Filter cleaningAir filtration 489When heavily charged with dirt, the resistance of most filters rises sharply, thus reducingair flow, and hence ventilation rate. The cleaning of filters is achieved in a variety of waysas follows:Washable filters. These, consisting of a foam plastic element, are contained in metalframes with wire retaining grids as shown in Figure 17.12. The material is flameretardant and, when fouled, is washed in detergent and reused. The media have a longlife.Brush filters. Eliminating the necessity for any special cleaning, these consist of `fluebrush'elements contained in a segmented frame as shown in Figure 17.13. The elementof hair, nylon, steel or nickel silver wire is removed for either vacuum or other simplecleaning and washed before replacement. The media have an indefinite life.Fabric filters. In panel or wedge form, as in Figure 17.14, these are mounted in frames andwhen dirty are thrown away. In another type, the filter material is of a glass fibre orother suitable base, and is supplied in rolls. The roll is horizontal and is graduallywound from one spool onto another on the principle of a camera film, see Figure 17.15;this movement is achieved by motor drive controlled from a pressure differential switchacross the filter, or from a timing device. Fresh filtering medium is unrolled only as it isrequired. The medium may pass over open mesh drums, as shown in the diagram, or beflat and held in place by retaining bars on both upstream and downstream faces. Suchequipment may, alternatively, operate with the rolls vertical but the edge sealingarrangements are then less effective and the drive may be less positive due to mechanicalproblems.This type of filter will, in theory, require no labour for maintenance, excepting at thelong intervals of possibly three to six months for the changing of a complete roll.Figure 17.12 Washable filter (ACE)