Fiber filters, viscous oils

Air filters operate under conditions of streamline flow as indicated by the streamlines drawn around a cylindrical fiber. It was assumed that capture of a particle takes place if any contact is made during its movement around the fiber. Once captured, the particle is not re-entrained in the air stream and deposited deeper in the bed. Support for this assumption has been found by using an atomized suspension of Staphylococcus albus and spores of Bacillus subtilisP Nevertheless, some fiber filters are treated with viscous oils, presumably to make capture more positive and to reduce re-entrainment. 

Panel filters may use either viscous or dry filter media. Viscous filters are so called because the filter medium is coated with a tacky liquid of high viscosity (e.g., mineral oil and adhesives) to retain the dust. The filter pad consists of an assembly of coarse fibers (now usually metal, glass, or plastic). Because the fibers are coarse and the media are highlv porous, resistance to air flow is low and high filtration velocities can be used. 

Dry filters are usually deeper than viscous filters. The dry filter media use finer fibers and have much smaller pores than the viscous media and need not rely on an oil coating to retain collected dust. Because of their greater resistance to air flow, dry filters must use lower filtration velocities to avoid excessive pressure drops. Hence, dry media must have larger surface areas and are usually pleated or arranged in the form of pockets (Fig. 17-64), generally sheets of cellulose pulp, cotton, felt, or spun glass. 

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