Document ypvYgjBgqwwno2nwg7Xdyvrgn

250 W. N. WITHERIDGE Figure 6. Rotary centrifugal separator. (Courtesy American Air Filter Co., Louisv Figure 7. Rotary centrifugal ecrubber. (Courtesy American Air Filter Co., Louisy-i within practical limits.' "Throwawayir"units ' are'"discarded when the"aif sistance becomes too great. Large industrial installations are prpvij|` manual or automatic arrangements for dislodging the "filter cake" and<(f~ the pressure drop to a point near its initial value. It is best to provide.s(| of pressure-indicating instrument connected across the filter to give - i-i >. w AIB CLEANING 251 fp^iresistance that would reduce air-flow capacity or increase power con- Mmability is likely to increase as the accumulated deposit increases velocities by decreasing the pore sizes. At the same time, dust ily is limited by the permissible increase in resistance. Therefore, rt+filt^s^shpuld be specified in terms of initial and final pressure hn^sabihty in weight per unit area or volume of medium, and ^Indli'nalfseparation percentage or degree. ^f^e11||ou'gh filters used for light., duty in, air conditioning ranges from m&b iwiater at velocities fromi 200 to 500 linear feet per minute, based ns.^areB"Pjepented to the air stream. .Industrial dust and fume filters, or "^mpdper-ate. usually at':iesiStahces..fromy.2;'itp., 5 inches of water at f^^pOl^pilliO'lmear feet per minute. Hemeoh^ 'has reviewed the char- fui .these- filters in some detail. The new "absolute" filters of feltlike ^rejMiSppsedSof paper pulp and asbestos perform at velocities and resist^gm^'jruble tothose occurring with cloth arrestors. t'Tnie'Ti'fe or period of service of an accumulating separator, such ^femends upon the upper limit of resistance to air flow through the ?)J||l||ids not only on the structure of the collecting medium, but also s^aPe the particles in the deposit and the concentration of ^^^peir stream. QuaiM^otancy of a filter may -be expressed in terms of the accumulated ^bp||tityvper square foot of area of the medium .that will result in a u'dim'iia-e'oE resistance from the initial value to the maximum value at the vplumeifdf air flow. |thfier.is the cleanable type, its service life depends in part on the ;usroj"}!$ resistance to a value low enough to give a workable margin |!li^aTuwa^je maximum pressure drop. Some media cannot be returned to 'TMMtfl|i^istance because of the tenacity of collected particles. Eventually, |jjjj|p||y cleaning is so slight, or the interval between cleanings so WW^MB.es less expensive to install a new filter. |&-vxT A. IMPINGEMENT FILTERS |aggfl^panel filters for air-conditioning or ventilating-systems have ^^^^|t|practically every material that can be formed into fibers, wires g;fri>#ated or expanded sheets, granules or beads. They remove par- 'TMUtream by a combination of screening, straipjpg, or sieving; jBgra&n^ntj and surface attractive forces, chiefly r electrostatic. ^^^^Mgn"'u'suaTly"'emphasizes 8tfaining*and''`iiapngemewTand, ffiMl^hibit best performance against coarse particles, and those ^^^pfHajies that are easy to capture in filters. ' *> are available in metals, such as steel, bronze, and aly- ^^^hmetals, such as glass, cellulose, rubber, animal hair, and as- liyiBon, Plant and Process Ventilation. Industrial Press, New York, 1955..,