GENERAL The action of wet collectors is applicable not only to particulates@ but to all aerosols@ while filters may be applied to dry aerosols or dusts. Aerosols can be classified as mechanical aerosols@ including dust and spray@ and condensed aerosols@ including fumes (solids) and mist (liquids). Mechanical aerosols are formed by size reduction (for example@ grinding) while condensed aerosols are formed by condensation of the vapor phase. The sizes of dispersed particles generally are related to the manner in which they are formed. These size ranges are approximate only. Condensed aerosols generally tend to agglomerate into particles of large size@ that may actually exceed the size of many industrial dusts. Filtrations secure good removal efficiencies@ and the total annual operating costs are moderate. When filters are used@ the deposition of particulates accumulates on the filtering medium and inevitably the porosity of the medium is reduced. Eventually@ the flow of gases is restricted so as to require either (I) removal of the deposit@ in order that the medium can be reused@ or (2) discarding of the clogged filter and replacement with a new medium. Wet collectors are used for all types of aerosols and have the decided advantage that they may be used for absorption of contaminant gases as well. In lieu of a filter medium@ the solids collect in the liquid phase@ and - the concentration of these solids must be regulated. Although some types of scrubbers can tolerate a larger amount of solids in the spray liquid@ in all scrubbers solids built up in this liquid must be removed periodically@ and on occasion continuously.