Document G1D15wgm6ekd6gkqg7ebybaq

806 . " CHAPTER 36 } .;> , 1953 Guide .: I* ~' minimutn.dry-bulb temperature possible.without rdehumidifleatiort,. Fahr enheit degrees. . -v idpi = dew-point of air entering coil, Fahrenheit degrees. <dpi = dew-point of air leaving coil, Fahrenheit degrees. t, = average refrigerant temperature, Fahrenheit degrees. .<. "-average, temperature of external surface. of-coil,, Fahrenheit degrees: : Atm '= mean,-temperature, difference between .fluid in-coil and; air passing ;over coil, Fahrenheit degrees. Note: Atm is usually the logarithmic mean. At0 -- logarithmic mean- temperature difference between air and coil surface. U = overall coefficient of heat transfer,.Btu per (hour) (square foot of exter nal coil surface) (Fahrenheit degrees temperature difference between fluid in coil and air flowing over coil). V = water velocity, feet per second. - . .^ Z.= a constant-for use in Equation 8 obtained by plotting on - logarithmiccoordinates G against values, of/0. Note: Numerical subscripts refer to condition entering and1 leaving, respectively. : REFERENCES - 1 Reheating by Means of Refrigerant Compressor Discharge GaS, by S. F. Nicoll (A.S.H.V.E. Transactions, Vol. 47, 1941, p. 239). * Rational Development and Rating of Extended Air Cooling Surface, by H. B. Pownall (:Refrigerating Engineering, October, 1935, p. 211). 3 Performance of Surface-Coil Dehumidifiers for Comfort Air Conditioning, by G. L. Tuve and L. G. Seigel (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 523). 4 The Effect of Turbulence Promoters on Heat Transfer Coefficients for Water Flowing in Horizontal Tubes, by L. G. Seigel (A.S.H.V.E. Transactions, Vol. 52, 1946, p. 197). , 5 Maximum Rate of Heat Transfer with Minimum Loss of Energy, by Z. Nagoaka and A. Watanabe (Proceedings, International Congress on Refrigeration, 7th Congress, Vol. 3, No. 16, pp. 221-245, 1937). CHAPTER 37 REFRIGERATION Refrigeration Theory: Definitions. and Basic Concepts, Refrigerants, Vapor Compression Refrigeration Cycles, Clearance and Volumetric Efficiency, Complex Refrigeration Cycles, Air Cycle, Steam Jet, Absorption and Ice Systems, Heat Pump; Basic Refrigeration Equipment: Compression Machines, Condensers, Evaporators and Coolers; Refrigeration Controls, Piping and Accessories; Equipment -Characteristics and Selection WITH the increasing use of all-year comfort air conditioning instal lations, the importance of refrigeration to the air conditioning engineer has been greatly magnified. The details of equipment operation, mainte nance and design remain problems for the refrigeration engineer, but the air conditioning engineer does retain a responsibility to the customer which, requires on his part some knowledge of the different refrigeration cycles and the relative merits of each. In order to assist in meeting this need, the present chapter has been divided into four parts, the first covering the fundamental technical relationships which govern the selection and analysis of an operating cycle, the next two presenting brief discussions of basic refrigerating equipment and auxiliaries, and the last, information on selec tion criteria. REFRIGERATION THEORY Definitions and Basic Concepts The ton of refrigeration is a quantity unit which originated in the days when harvested ice was the principal source of summer cooling. By defi nition the ton is the cooling effect realized when one ton of 32 F ice melts to water at 32 F; since the latent heat of fusion of ice is 144 Btu per pound, the ton represents a unit cooling effect of 144 X 2,000 = 288,000 Btu. In common practice the ton is usually considered a rate (rather than quantity) unit, and is taken as 288,000 Btu per day (24 hours), or 12,000 Btu per hour, or 200 Btu per minute. Thus for air conditioning calculations, the size of the requisite refrigeration machine, expressed in tons, can be obtained by dividing the heat gain of the structure, expressed in Btu per hour, by 12,000. In equation form: where H, = (Btu per hour heat gain) + 12,000 (1) Ht = load in tons.. The working substance, or refrigerant, is the fluid which carries heat through the refrigeration cycle from the evaporator, where heat enters the refrigerant, to the condenser where the heat is discharged to some cooling medium. The great majority of modern refrigeration systems use a liquefi able vapor as the working substance. By altering the pressure of the refrigerant its boiling temperature is changed, allowing the material to boil in the evaporator at a temperature sufficiently lower than that of the con ditioned space, to insure maintenance of an effective heat transfer rate from 807