Document 7M1oEJ5JR54db6OxnG490aY3R

HEATING VENTILATINC AIR CONDITIONING GUIDE 1.944 cooling medium required is also less than for any other system with the exception of the centrifugal compressor, which uses the same amount. Centrifugal compressors are used for large installations, and usually where the indirect system is required. The driving mechanism can be steam turbine or electric motor. The steam jet system is used where steam is available and cooling water can be had in large quantities! It will be noted by referring to Fig. 4 that all systems using compressors have a common characteristic and that is, that the capacity varies with the evaporating temperature. Not only can the equipment be selected to produce a given result but the performance can be predicted under varying load conditions by the simple expedient of using the variable of CHAPTER 25. REFRIGERATION obtain the same capacity, a larger condensing unit is used. This illus tration assumes zero pressure dropi through the suction line. The pres sure drop can be taken into account by shifting the compressor per formance curves by the amount of pressure drop expressed in degrees Fahrenheit. THE REVERSE CYCLE In heating by the reverse refrigeration cycle energy is absorbed in an evaporator from some available source of heat, pumped to a higher tem perature and delivered to a condenser4. The heat from the condenser is used for heating purposes. The compressor acts as a heat pump whose fundamental function is to raise the potential of the heat. The theoretical .Fig. 10 Compressor and Coil Performance evaporating temperature as the hbscissa and the load or capacity as the ordinate in a series of curves: Manufacturers, of compressors and cooling coils furnish performance data for apparatus that can be plotted in the form of curves similar to those shown in Fig. 10. The performance of a compressor is plotted as a series of curves, each curve being drawn for a given condensing pressure. The performance of a direct expansion coil at two different air velocities is plotted on the same graph. The operating point will be, of course, where the two curves cross. Data given in Table 11 illustrate two types'of conditioned enclosures having the same total load of 148,000 Btu per hour, but with two different ratios of sensible to total heat. In the case of the office with a ratio of 82 per cent sensible to total heat, the operating point A in Fig. 10 is found to be 42.2 F evaporating temperature with a face velocity of 500 fpm. In the case of the restaurant, with a ratio of 69.5 per cent sensible to total heat, the air velocity is lowered to 300 fpm and the evaporating tem perature is lowered to 34.4 F as shown in point B of Fig. 10. In order to 490 . ratio of the heat delivered to the work of compression is given in Equa tion 1. where T\ = absolute temperature of evaporator. Tt = absolute temperature of condenser. (1) Thus, with a small spread of temperature between the evaporator and the condenser, 6 or 8 times as much heat may be obtained theoretically, and 3 to 5 times practically, as the work introduced. There are a number < Cooling Homes, A Field for Refrigeration, by A. R. Stevenson, presented at the symposium of the Refrigeration with Gas Committee of the American Gas Association, April 20, 1926. The Heat Pump. An Economical Method of Producing Low-grade Heat from Electricity, by T. G. N. Haldane (Electric Review Vol. 105, p. 1161-1162, December 27. 1929, and I. E. E. Journal, Vol. 68, p. 666-675, June, 1930). Edison Building Heated and Cooled by Electricity, by H. L. Doolittle {Power, Vol. 74, p. 384, September 8, 1931). House Heating by Pump with 5 to 1 Pick-up Ratio, by Gilbert Wilkes and R. E. 'Marbury {Electrical World, Vol. 100; p. 828, December 17, 1932). An All Electric Heating, Cooling and Air Conditioning System, by Philip Sporn and D. W. McLenegan (A.S.H.V.E. Transactions, Vol. 41, 1935, p. 307). Using the Reversed Cycle Refrigerating Principle for a Self-Contained Heating and Cooling Unit, by Henry L. Galson (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, October, 1935, p. 497). Heating by Reversed Refrigeration, by A. J. Lawless {Heating, Piping and Air Conditioning, August, p. 473, September, p; 519, 1940). 6 491