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TABLE 2 Classes of Particulate Filters Classes of Filters Initial AP (> Rated Capacity (In Inches) Typical Size (In Inches) Typical Rated Flow (In CFM) NBS Atmospheric AFI Stain Artificial Approximate Efficiency Weight Element (In Per Cent) (In Per Cent) Cost Viscose (oiled) Fibrous, Impingement (Common Furnoce Filter) Viscose (oiled) Metallic, Impingement **Dry, Fibrous Low-Medium Efficiency .08-. 12 WG 24 x 24 x 2 .08-.12 WG 24 x 24 x 2 24 x 24 x 29 .20*.4 WG 24 x 24 x 11 2500 2500 2000 5-12 5-12 50-55 65-75 65-75 100* .25-2.00 $8-512 $25-$35 **Dry, Fibrous Medium Efficiency 24 x 24 x 29 .20-.50WG 24 x 24 x 11 Vi 2000 80-85 100* $25-$40 *Dry, Fibrous High Efficiency 24 x 24 x 29 .35-.60 WG 24 x 24 x 11'/, 2000 90-95 100* $25-545 Dry, Fibrous Hospital Type .30-.50 WG 24 x 24 x 11 Vi 1000 100* 100* $50-590 Dry, Fibrous, High Inter ception High Efficiency Particulate Air Filter .70-1.2 WG 24 x 24 x 11 Vi 1000 100* 100* $60-$100 'Essentially 100 per cent--Evaluation method not sufficiently critical to detect penetration. ''NOTE: To identify these classes better, the efficiencies reported are comparable to those for commercial electrostatic precipitators on the atmospheric oir NBS dust spot lest. It should be noted that manufac turers report filter efficiencies vari ous ways. Hence, a full understand ing of type of filter and the filter efficiency is necessary to obtain a satisfactory system. Shortcomings of a fibrous partic ulate filter are: 1) It adds resistance to the exist ing ventilation system; 2) It must be replaced about once a year; 3) It requires rela&ely large space. .2? Electrostatic preciptators use a high intensity electrical field to charge particles, which are collected on an opposite charged surface. The uoits vary in size from less than 100 cfm to thousands of cfm. A good average cost of a 500 cfm unit would be approximately $500. The collecting efficiency would be equal to the previously mentioned dry fibrous medium-type filters. The units offer excellent efficiency at low pressure drops. However, in gen 33 eral, the initial cost is high, frequent maintenance is required, and it is not uncommon to generate ozone and oxides of nitrogen. tant that they be selected with ut most care to ensure that proper protection is afforded.--End. Mechanical separators range from simple settling chambers, which em ploy gravitational forces, to sophis ticated devices such as centrifugal separators. Usually dry-type me. chanical devices have poor removal efficiencies on particles less than 10 microns in size. Excellent efficiences are obtained by combining mechani cal separators with a wet method, such as a venturi-type scrubber. Due to the numerous combinations of mechanical separators, their cost varies depending upon the applica tion. Usually, they require less maintenance than other type separ ators. The most likely use for a drytype separator is pre-filtration for high efficiency filters. In situations where it is not feasi ble to create and maintain engineer ing controls, proper respiratory pro tective devices must be provided and used. Because there are many types of such devices, it is impor REFERENCES ASHRAE Guide and Data Book, Systems 1970. American Society of Heating, Refrigerating and Air-Con ditioning Engineers, Inc., 345 47th St., New York 10017. Industrial Ventilation, 11th edition. American Conference of Governmen tal Industrial Hygienists. Committee on Industrial Ventilation, Box 453, Lansing, MI 48902. Air Pollution Control Manual. Part II--Control Equipment. 1968. Ameri can Industrial Hygiene Association, 210 Haddon Ave., Westmont, NJ 08108. Industrial Health Engineering, Brandt, Allen D. 1947. John Wiley and Sons, Inc., 605 Third Ave., New York 10016. Fundamentals of Industrial Hygiene, edited by Olishifski, J. B. and McElroy, F. E. 1971. National Safety Council, 425 N. Michigan Ave., Chicaeo 60611. 8004 1215