Book - School of Science and Technology

492 Fans and air treatment equipmentOil coatedfilter screenCorrugated filterelementsPositivedisplacementoil pumpOil strainerOil reservoirFigure 17.18 Automatic viscous roller type filter (Ozonair)AgglomeratorplatesIoniserPrincipleFiltercellsBagfilterFigure 17.19 Electrostatic filter (AAF)Electrostatic filter. In this type of filter the dirt collects on the earthed plates and itsremoval is accomplished by washing with hot-water jets; adequate drainage is thereforenecessary. This may be done by hand or automatically, the latter being preferred.Another type uses oiled plates to increase the dust retention properties. After cleaning,a period of drying is necessary before the filter can be put backinto use. Pre-filtering isrecommended and consideration should be given to an after-filter for use when theelectrostatic section is being cleaned or during failure.Another pattern, which is more compact and requires less maintenance, allows thedust particles to collect on the plates in a thickening layer to be swept off by the airstream as agglomerated flakes on to a roll or bag filter which is an integral part ofthe filter assembly (see Figure 17.19).For clearance of fog only, the electrostatic and absolute filters are effective, or acombination of electrostatic plus fabric. Excessive collection of moisture from fogmay cause an electrostatic filter to cut-out due to short circuit. A special heater maybe used ahead of the filter to ensure that air entering is dry, thus avoiding this trouble.