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780 CHAPTER 33 1956 Guide ings, drives and motors outside the gas stream, centrifugal fans are usually used and in arrangements 1, 2, 4, or 8 (Fig. 6). The exception is in the case of fans handling explosive mixtures, where propeller or axial fans are per missible when built with explosion-proof motors and non-ferrous wheels. Fans handling hot air or gas are generally of the centrifugal types and follow arrangements 1 or 8 so that bearings, drives and motors are remote from the heat. If of double inlet type, they have inlet boxes and long shafts to keep the bearings outside the stream. Propeller or axial types are rare when the temperature exceeds 150 F. With temperatures above 600 F special heat resistant materials may be required for the rotors. In addition to a remote location for the bearings, external cooling is fre quently required when the temperature exceeds 200 to 250 F. This is often obtained by use of air cooled or water cooled jackets. With anti friction bearings a heat radiating disk or a small circulating impeller between the fan and the nearest bearing is sometimes sufficient. Oil lu brication rather than grease lubrication is commonly recommended. REFERENCES 1 A.S.M.E. Test Code for Fans (American Society of Mechanical Engineers, PTC 11--1946). * Standards, Definitions and Terms in Use by the Fan and Blower Industry (National Association of Fan Manufacturers, Bulletin No. 110, 1952, pp. 2-5). 1 Standard Test Code for Centrifugal and Axial Fans (A.S.H.V.E. and the Na tional Association of Fan Manufacturers, NAFM Bulletin No. 110, 1952, p. 18). 4 Sound Measurement Test Code for Centrifugal and Axial Fans (National Asso ciation of Fan Manufacturers, Bulletin No. 110, 1952, p. 33). Noise Ratings of Ventilating Fans, by W. H. Hoppmann II and Fred Lager (A.S.H.V.E. Transac tions, Vol. 51, 1945, p. 271). E Fan Laws Simplify Performance Calculations, by R. D. Moyer (Power, August 1946). The Centrifugal Fan, by Frank L. Busey (A.S.H.V.E. Transactions, Vol. 21, 1915, p. 43). 7 Energy Transfer Between a Fluid and a Rotor for Pump and Turbine Machinery, by Sanford A. Moss, Chester W. Smith and William R. Foote (American Society of Mechanical Engineers Transactions, 1941). 8 The Characteristics of 78 Related Airfoil Sections from Tests in the Variable Density Wind Tunnel, by Eastman N. Jacobs, Kenneth E. Ward and Robert M. Pinkerton (National Advisory Committee for Aviation Report No. 460). Aerodynamic Characteristics of a Large Number of Airfoils Tested in the Variable Density Wind Tunnel, by Robert M. Pinkerton and Harry Greenberg (National Advisory Committee for Aeronautics Report No. 678). 8 The Specific Characteristics of Fans, by M. C. Stuart and J. B. Lusk (A.S.H.V.E. Transactions, Vol. 43,1937, p. 57). 10 The Axial Flow Fan and Its Place in Ventilation, by W. R. Heath and A. E. Criqui (A.S.H.V.E. Transactions, Vol. 50,1944).. " Mine Ventilation, by J. J. Walsh (A.S.H.V.E. Transactions, Vol. 23, 1917, p. 659). Mine Ventilation and Its Relation to Health and Safety, by D. Harrington (A.S.H.V.E. Transactions, Vol. 51, 1945, p. 243). 17 Comfort Cooling with Attic Ventilating Fans, by G. B. Helmrich and G. H. Tuttle (A.S.H.V.E. Transactions, Vol. 40, 1934, p. 155). A.S.H.V.E. Research Report No. 979--Study of Summer Cooling in the Research Residence for. the Sum mer of 1933, by A. P. Kratz and S. Konzo (A.S.H.V.E. Transactions, Vol. 40,1934, p. 167). A.S.H.V.E. Research Report No. 1198--The Effect of Attic Fan Operation on the Cooling of a Structure, by W. A. Hinton and A. F. Poor (A.S.H.V.E. Transac tions, Vol. 48, 1942, p. 145). The Installation and Use of Attic Fans, by W-HBadgett (Agricultural and Mechanical College of Texas, Bulletin No. 52, 1940). Some Effects of Attic Fan Operation on Comfort, by W. A. Hinton and W. G. Wanamaker (A.S.H.V.E. Transactions, Vol. 50,1944, p. 371). BIBLIOGRAPHY The Centrifugal Pumps and Blowers, by A. H. Church (John Wiley & Sons). r an Engineering, Buffalo Forge Co. FTahnesT, hbeyorTyhaenoddoPreerfBoarmumaneciesteorf, AJxr.ia(lMFclGowrawFa-Hnsil,l)b. y Curt Keller, Adapted by Lionel S. Marks and John R. Weske (McGraw-Hill). CHAPTER 34 AIR CLEANING Atmospheric Air Cleaners: Airborne Particulate Matter, Viscous Impingement Filters, Dry Air Filters, Electronic Air Cleaners, Air Filter Performance, Selection, Maintenance, Installation, Adsorption of Vapors; Industrial Air and Gas Cleaners: Degree of Cleaning, Selection, Types, Application, Inertial Separators, Scrubbers, Wet Collectors, Filters, Electronic Precipita tors, Adsorbers, Absorbers, Combustion Devices AIR cleaning devices remove contaminants from an air or gas stream. They are~available in a wide range of designs to meet various air cleaning requirements. Degree of removal required, quantity and char acteristics of the contaminant to be removed, and conditions of the air or gas stream will have a bearing on the device selected for a given applica tion. Definitions and a discussion of contaminant characteristics are given in Chapter 8, together with some consideration of their origin. Air cleaning devices are divided in this chapter into two basic groups: Atmospheric Air Cleaners, described in Part I, and Industrial Air and Gas . Cleaners, described in Part, II. Atmospheric Air Cleaners are ordinarily used to remove particulates such as are found in outside air, and are employed in ventilation, air conditioning, and heating systems where dust content seldom' exceeds 4 grains per 1000 cu ft of air. Industrial Air and Gas Cleaners are ordinarily used for the heavier concentrations encountered in local exhaust ventilation where the particu late content (loading) ranges from 100 to 20,000 grains per 1000 cu ft of air. Because of these heavier loadings, atmospheric air cleaners can seldom be used for the control of process aerosols in industrial applications. PART I--ATMOSPHERIC AIR CLEANERS Conventional air filters and electronic air cleaners are installed in air handling systems to remove dusts. These dusts constitute a mixture of particle sizes within the classification of temporary and permanent im purities listed in Fig. 1, Chapter 8, including bacteria, pollens, house dusts, and similar allergens which motivate attacks on persons of allergic sensitivity.1-! Since the purpose of the filter or cleaner is to free the air of as much existing contamination as practicable, the degree of air cleanliness required should influence the choice of apparatus. Atmospheric dusts are .mixtures of particles in all sizes. The removal of these particles and fractions becomes progressively difficult as the particle size decreases. Smoke particles are of major importance in many applications. Air cleaners will justify their cost through a reduction in housekeeping expense in the 781