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720 CHAPTER 32 , 1953 Guide : The Flow of Air ln Ducts, by E. Kemler (Healing and Ventilating, May 1936, P-38). ' . : Total, Static and Velocity Pressure, by A. A. Berestneff (Heating, Piping and Air Conditioning, March 1932, p. 195). PRESSURE LOSS IN ELBOWS Experimental Investigation of Velocity Distributions Downstreapn of Single Duct Bends, by John R. Weske (National Advisory Committee for Aeronautics Technical Note 1471, January, 1948). u Investigations of the Flow in Curved Ducts at Large Reynolds Numbers, by John R. Weske (Journal of Applied Mechanics, December, 1948, pp. 344-348. Flow of Fluids in Curved Passages, by J. Eustice (Engineering, Vol. 120, 1925, p. 604). ... New Data for the Design of Elbows in Duct Systems, by Loring Wirt (General Electric Review, Vol. 30, June 1927, pp. 286-296). ASHVE Research Report No. 1216--Effect of Vanes in deducing Pressure Loss in Elbows in Seven-Inch Square Ventilating Duct, by M. C. Stuart, C. F. Warner, and W. C. Roberts (ASHVE Transactions, Vol. 48, 1942, pp. 409-424). :. Pressure Loss in Elbows and Duct Branches, by Andrew Vazsonyi-(ASAfE Trans actions, April 1944, pp. 177-183). ASHVE Research Report No. 1329--Friction Equivalents for Round, Square and Rectangular Ducts, by R. GV Huebscher (ASHVE Transactions, Vol. 54, 1948, p. 101). ...... .. Loss in 90-Degree Pipe Bends of Constant Circular Cross Section, by Albert Hofmann (Transactions of the Hydraulic Institute of the Munich Technical University Bulletin 3, 1929, ASME 1935, pp. 29-41). Pressure Losses in Rectangular Elbows, by R. D. Madison and J. R. Parker (ASME Transactions, AER 58-2, April 1936, pp. 167-176). ASHVE Research Report No. 1211--Pressure Loss Caused by Elbows in EightInch Round Ventilating Duct, by M. C. Stuart, C. F. Warner and W. C. Roberts (ASHVE Transactions, Vol. 48, 1942, pp, 335-350). . Friction of Air in Elbows, by A. I. Brown (Power Plant Engineering. August 15, 1932, p.630). .1 Loss of Pressure Due to Elbows in the Transmission of Air Through Pipes or Ducts, by Frank L. Busey (ASHVE Transactions, Vol. 19, 1913, pp. 366r376). - The Resistance to Flow of Air at Bends and in Straight Airways, by W. E. Cooke and I. C. F. Statham- (Institution of'Mining Engineers Transactions, Vol. 76-77, June 11, 1929, pp. 188-212). ''An Investigation of Pressure Losses in Air Duct Elbows, by Oliver E. Parker (Northeastern University thesis, May 28, 1934). Investigation of Air Flow in Right Angle Elbows in a Rectangular Duct, by Charles H. McLellan and Walter A. Bartlett, Jr. (National Advisory Committee for Aeronautics, Advanced Restricted Report L-328, October, 1941). DUCT DESIGN A Rational Method of Duct Design, by L. G. Miller (ASHVE Transactions, Vol. 43, 1937, p. 71). MISCELLANEOUS Performance Tests of Asbestos Insulating Air Ducts, by R. H. Heilman and R. A. McArthur (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 197). Aluminum in Heating, Ventilating and Air Conditioning, Reynolds Metals Co., Louisville, Ky. CHAPTER 33 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' psi, 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) centrifuged or radial flow in which the air flows radially through the impeller within a scroll type housing, and (2) axial flow ini 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 vaneaxiaf 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 sures, speed, power input, mechanical and static efficiency, at a stated density. These terms are defined by the National Association of Fan Manufacturers1 as follows: 1. Volume handled by a fan is the number of cubic feet of air per minute expressed at fan outlet conditions. 721