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952 CHAPTER 36 1958 Guide Table 9. Basis op Equipment Selection Capacity Tons Majority Used Some Used Fev Used 0 to 10 Unit systems in con Unit central systems Built up central sys ditioned space. using duct distri tems. bution. 10 to 25 Unit central systems using duct distri bution. Unit systems in con ditioned space. Built up central sys tems using recipro cating compres sors, adsorption, and absorption sys tems. 25 to 100 Built up central sys tems using recipro cating compres sors. Unit central systems using duct distri bution. Central systems using adsorption systems or centrifugal re frigeration. 100 to 200 Built up central sys tems using recipro cating compres sors. Built up central sys Built up central sys tems using absorp tems using steam tion and centrifugal jet. compressors. 200 and Over Built up central sys tems using centrif ugal compressors. Built up central sys tems using steam jet or absorption systems. Built up central sys tems using recipro cating compres sors. application may be made on the basis of the magnitude of the load. Cur rent general practice is outlined in Table 9. Unit Of packaged systems, consisting of a reciprocating compressor, condenser, evaporator, and fans, are generally used in the smaller sized jobs where electric power is available, as they are manufactured complete, ready to install, and are the most economical (see Chapter 25). The reciprocating compressor in the built-up central system (see Chapter 29) covers the widest range of application since it is applicable to Refrigeration 953 either the direct expansion or indirect systems, and can be driven by steam or gas engines, or by electric motors. The quantity of condensing 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 a 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. 19 that all systems using compressors have a common characteristic, namely, 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 vary ing load conditions by the simple expedient of using the variable of evapo- Table 10. Typical Operating Conditions for Two Types of Load Type op Enclosure Load, Btu per Hour Sensible Latent Total Ratio Sen sible TO . Total Con.Air Entering Operating Balance-Point F .Deg Per Cent R.H. Evapo rator Temp F Deg Con denser Per Cent Pressure Sensible Lb per Heat Sq In. Restaurant.... 103,000 45,000 148,000 0.695 82 45 34.4 123 69.9 Office... 121,000 27,000 148,000 0.820 82 45 42.2 100 82.1 rating temperature as-the abscissa, 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. 21. The performance of a compressor is plotted as a senes 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 10 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 2 percent sensible to total heat, the operating point A in Fig. 21 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 percent sensible to total heat, the air velocity is lowered to 300 fpm, and the evaporating tempera ture is lowered to 34.4 F as shown in point B of Fig. 21. In order to obtain he same capacity, a larger condensing unit is used. This illustration assumes zero pressure drop through the suction line. The pressure drop can be taken into account by shifting the compressor performance curves by e amount of pressure drop expressed in Fahrenheit degrees. REFERENCES Nn Refrigeration, by Paul C. Scofield (Refrigerating Engineering, Vol. 57, D> June 1949, p. 558). A T^^?''ca^'on an<t Economy of Steam Jet Refrigeration to Air Conditioning, by Uumford and A. A. Markson (A.S.H.V.E. Transactions, Vol. 44,1938, p. 33).