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766 CHAPTER 34 1954 Guide towers and wetted filters they .all have a constant resistance during their operation. On the basis of their important characteristics wet collectors may be classified according to Table 2. FILTERS Filters for industrial air and gas cleaning consist primarily of cloth dust collectors and the ultra or absolute type of filter. Cloth dust collectors are generally constructed in the form of housings with cloth envelopes drawn over wire screen frames or with vertical cloth Table 1. Inebtial Separator Characteristics ` Efpicienct Pabticle Size Range In Micbons ., >100 100-10 <10 Resist ance (Pressure DrOp) Description and Remarks Settling and baffle chambers Large diameter cyclones Refined design centrifugals Integral rotor type Slotted scroll fan Medium High High High High Low Low Low A large chamber to reduce velocity to permit settling. Auxiliary baffles are sometimes used to improve performance Requires large space Medium Low to Low Low . Considered as single body over 3 ft in diam eter Gases and material must be dry. Air-tight dust bins or continuous removal of mate rial must be provided. Space require ments moderate High Medium Medium to low Available as multiple small diameter tubes, scroll shaped bodies, and multiple louyeiB Gases and material must be dry. Air-tight dust bins or continuous removal of mate rial must be provided. Space require ments moderate High Medium See remarks Rotor acts as air mover and separating ele- to low ment ., Usually limited to dry granular materials and dry gases. Low space requirements No external pressure loss involved but po'rer requirements are higher than usual ex hauster Medium to low Low See remarks Separation takes place in fan scroll No external loss involved but power reQUU' ments are higher than usual exhauster bags or tubes. Cloth filters depend to a great extent on the accumulated dust layer, for their effectiveness. Their basic or initial efficiency depends upon the type of fabric employed; felted media such as wool gives higher initial removal than woven fabrics. The increase in resistance to air flow, resulting from the accumulated dust layer, is related to rate of flow, con centration, and characteristics of the contaminant. Removal of the col lected material is accomplished through periodic shaking by single mecha nisms or combinations of them employing rapping, or through reversal of air flow, or by means of high velocity reverse air jets. With shaking or rapping, air flow is not constant and filtering velocities are usually he'0 under 6 fpm. With reverse jets or reverse air flow, substantially constan air flow is maintained and filtering velocities are generally higher (up 35 fpm). Filtering velocities in all fabric collectors are highly dependen upon the aerosol, fume, or dust being collected. The ultra or absolute type of filter8 may consist of pleated cellulose as- T a b l e 2. C h a r a c t e r is t ic s o f W e t C o lle c to r s Air Cleaning 767