Document zekgg6qjpo8VMMgrGrLzxD03

ii i W ii~ -t... 236 W. N. WITHERIDGE ''Subscreen sizes" require well-engineered equipment to remove them frof or gas, and the difficulty of separation increases as particle size decreases;'-! micron", particles, which include many varieties of solid'fumes, demand thl effective separating equipment, exceedingly well engineered and incorpd| features that often result in high-cost installations. The determination of particle-size distribution is necessary for correc. cation of air-cleaning equipment. This subject is treated in Chapter VII aif in Volume III Terminal Settling Velocity. A highly useful concept in the air-cleaninj_ is the terminal velocity or constant settling velocity reached by a particlfeil under the influence of gravity. A particle less than about 1000 /* in size w|| with a velocity at which the resistance to its motion offered by the fluid ba the gravitational force tending to accelerate its motion toward the earth.' ;; Foley4 presented convincing reasons for expressing particle size, masl shape by means of terminal velocity when required to predict the perfofn of inertial separators against solid particles. In fact, it was noted that tell velocity already has been adopted in some European countries for specify^ performance of gas-cleaning systems. The behavior of particles in a moving gas can be measured dire1' such devices as the Roller Air Elutriator, described in Volume III. Particle Shape. Although terminal settling velocity integrates particlj with size, density, and porosity, it does not describe completely the beh$p particles that deviate strongly from the spherical, in the direction ei or of discs or flakes. It is well known that a solid object of irregular sha; to present its largest cross section perpendicular to' its direction of mif through a fluid. Accordingly, the orientation of a particle during its approacf filter medium can influence the probability of its capture. The best ; example is the ease with which lint can be separated in home air condition relatively open filter media. Fibers may acquire a spinning or rotating tendency, while dis&9-<)J may move with an oscillating or sideslipping action. Particles of less shape will exhibit combinations of spin and sideslip, and these complex mov| may be helpful in the process of filtration. It is suggested, therefore, thatiif|| velocity specifications, be supplemented with information on particle .Stiail the petfpfmBhhd^Mf.filit6fF-'''a|id'Tn'ertia'l devices are compare'dr!ih!S product-deveIo|ra^i1|studie8.-` y.wg Fractionatio^l Air-borne Materials. An estimate of the charactel| borne matter can Bfe'-made by examining the materials charged into 6f:p| by"''aTbWSsi!~CH8i'fs`TSffcWfaFy,"how'eV'ef,"tb "account"'fbr"any"tendwii process to fractionate such materials, resulting in a distribution of size5^, IfHchemical species, as suspended in the air, that is different from that form. *R. B. Foley, Trans. Am. Soc. Mech. Engrs., 69, 101-108 (1947). AIR CLEANING 237 TvfcVi^qhoi- "'Vong ducts between a process and the air-treating equipment is ' ^thp.nature of heterogeneous suspensions. Gravitation, conden- ^^ffly/^i^o.oeulation, frictional charging, and even leakage, all contribute ,,'aihu'f5faciibnation of the suspended matter. Bs^^iriiMle to assume that fractionation will occur in one way or another, Sffi'"y},fj^'rlt'in.tP..the contrary should be supported by convincing evidence. of contaminated air entering a cleaning system should be of samples taken directly from the air stream at that point. Specification of Performance ^JSicf'fiSiiSof the performance of air- or gas-cleaning equipment on a given ^jifeni^jui^^feolude most or all of the following factors: ^Siot!riiITqf,niaterial retained or discharged (to be discussed in this section). TT^IjyTibution of particles retained or discharged. .Ar^ij^mng capacity in cubic feet per minute. ;t|&H'y'at a specified location. or. pressure drop through the equipment, ifcduirement in cubic feet or in floor area. tfyj&efgr holding capacity for contaminant. 'liraf)specified conditions of service. SMpiaf 'air cleaning is the recovery of suspended materials, it. is ||llf|w the percentage or amount of material collected or retained in S^JiMihe-other-handrin-air-pollution-control-and air conditioning, the |Perial collected is only incidental information. If the object is to liter quality for supply to occupied spaces, or of lower nuisance 33 -'discharge outdoors, then performance should be specified in terms Tju^mtwif-icontaminant in the outgoing air stream. ' ^i.'^TcrncnT.of the degree of cleaning usually is done with sampling devices fg?ifsM0a|ur,e separating and collecting systems with the highest .possible 1y'^^fe|t.herefore apparent that performance of an air- or gas-cleaning ^Kn.the performance of the sampling device.used in determining plamount of contaminants at the inlet and outlet of the cleaning ||||j|ly, the method of testing should be specified in a statement of P^^^;i't-is-not-standard4n-the-:field,-witli4tnown. .capability, the m. sSfrshould be described well enough to be duplicated. (A thorough mlpand gas-sampling equipment is presented in Volume III.) fe. A. EFFICIENCY OR EFFECTIVENESS fejsljumfidersto..o..d..that the word "efficiency," in the field of air or gas ^ ^npjlrefer to the proportion of energy used effectively in the cleaning iJiiead^it is used almost exclusively to indicate the proportion of matter from tSie air stream, without regard to the amount of power required. .fcZ* I' ik