Book - School of Science and Technology

Air filtration 485Table 17.1 Typical mass of solids in the atmosphereLocalityTotal mass(mg/m 3 )Rural and suburban 0.05±0.5Metropolitan 0.1±1.0Industrial 0.2±5.0Factories or workrooms 0.5±10.0Table 17.2 Typical analysis of atmospheric dustAmount of solid (%)Range of particle size,diameter (mm)Number ofparticlesTotal massof particles30±10 0.005 2810±5 0.175 525±3 0.25 113±1 1.1 61±0.5 6.97 2Below 0.5 91.5 1particle size. These values will vary between locations and with the season; winter conditionsnormally producing the highest values.Necessity for air cleaningIf, in a mechanically-ventilated or air-conditioned building, air is blown in without somemeans for filtration, deposits of dust will be found to occur throughout the rooms and thesystem of ducts will in itself become coated with solid matter. Heater batteries, coolingcoils and fans will also become fouled so that in time the efficiency of the system as awhole will fall off at an increasing rate. Except in certain industrial applications, ventilationand air-conditioning systems therefore invariably include some means for filtrationof the air.The removal of the larger particles is, of course, a simple matter, since any mesh of fineenough aperture will arrest such particles. A plain mesh is however liable to becomeclogged very quickly, and hence is of little use for the purpose. The finer material and thesmokes are, however, much more difficult to arrest and yet it is these which are largelyresponsible for the staining of decorations, the soiling of shirts and garments and, to someextent, also, the bearing of harmful bacteria. Apart from outside air, recirculated aircarries fluff from carpets, blankets and clothes, dust from paper and brought in on shoesand, in an industrial application, any dust resulting from the process.The greater the degree of filtration, as a rule, the higher the cost of the equipment andthe greater the space occupied. The selection of the best filter for a particular applicationtherefore depends on whether great value is placed on a high degree of cleanliness or not.It is perhaps worthy of note that the staining on ceilings close to the point of air supply toa space, seen in many installations after a period of use, is more likely to be due tocontamination generated from within the space than to dirt in the outside air.