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824 CHAPTER 31 1958 Guide DUCT DESIGN ASHVB Research Report No. 1050--A Rational Method of Duct Design, by L. G. Miller. (ASHVE Transactions, Vol. 43, 1937, p. 71). Modern Thinking Applied to Duct Design, by Kirby Walker, (Healing, Piping & Air Conditioning, March 1949, p. 85; April 1949, p. 104; May 1949, p. 91; July 1949, p. 95;Septeinber 1949, p. 96; November 1949, p. 97; March 1950, p. 97 and May 1950, P- 94). . .. Pointers on Air Conditioning Duct Design and Installation (Healing and V entilat- ing's Reference Section, Heating and Ventilating, October 1951). Economical Duct Layouts, by Peter Franck (Heating, Piping & Air Conditioning, December 1952, p. 100). The Design of Aluminum Duct Systems, by F. W. Hutchinson, 1954 (Kaiser Alu minum & Chemical Sales, Inc., Oakland, Cal.). Nomograph Simplifies Duct-Design, by R. W. Ruppert, HPAC Data Sheet (Heating, Piping & Air Conditioning, July 1955, p. 127). To Size Ducts Adequately Select the Right Pressure Drop, by V. J. Turecamo (Heating, Piping & Air Conditioning, April 1956, p. 110). MISCELLANEOUS Performance Tests of Asbestos Insulating Air Ducts, by R. H, Heilman and R. A. McArthur (ASHVE Transactions, Vol. 44, 1938, p. 197). CHAPTER 32 FANS Types, Fan Performance, Fan Laws, Fan Performance Curves, System Characteristics, Fan Arrangements, Fan Control, Motive Power, Fan Selection, Fan Installation, Fan Applications IN HEATING, ventilating and air conditioning practice, the devices used to produce air flow are variously known as fans, blowers, exhausters or propellers. The A.S.M.E. Test Code1 limits fans to those in which the fluid density change does not exceed 7 percent (one psi at atmospheric pressure) and labels as compressors those devices operating beyond that pressure range. Since air conditioning rarely requires pressures of over -Jpsi, all such devices will be known as fans and the air will be considered non-compressible. Types Fans are divided into two general classifications: (1) centrifugal or radial flow in which the air flows radially, through the impeller within a scroll type housing, and (2) axial flow in which the air flows axially through the impeller within a cylinder or ring. Centrifugal fans are further subdivided into types denoted by the curva ture or slope , of the impeller blades, the angle of which largely determines the operating characteristics. For a given output, a forward inclination of blade indicates a relatively low speed of operation, and a backward in clination, a relatively high speed of operation. Many intermediate forms are also found. . Axial flow fans are subdivided into types differentiated mainly by their enclosures and refinements of impellers and appurtenances. All types vary in shape, number and angles of blades; ratios of hub diameter to im peller diameter; materials and methods of fabrication, depending upon de sign and preference of manufacturer. Tubeaxial and vaneaxial fans, usu ally used against appreciable resistance, commonly have relatively large hubs and helical blades (the angle varies radially along the blade). The blades may be of uniform thickness, either flat or cambered, and either cast or made of plates; or they may be of air foil sections, either cast or of double thickness sheet. Streamlining of both impeller and enclosure is common practice. Vaneaxial fans incorporate guide vanes to modify per formance and increase efficiency. Propeller fans customarily used for free delivery, or against low resistance, also are found with a variety of blade conformations, but are simple in construction. They are merely mounted within a plate or ring. The fan nomenclature in Fig. 1 has been standardized by the National Association of Fan Manufacturers,2 FAN PERFORMANCE Fan performance is a statement of volume, total pressures, static pres ses, speed, power input, mechanical and static efficiency, at a stated density. These terms are defined by the National Association of Fan Manufacturers1 as follows: J- Volume handled by a fan is the number of cubic feet of air per minute expressed at fan outlet conditions. 825