Document Rj5aaG2z9RQQgqrKm2XO0zZz8

882 CHAPTER 37 1956 Guide that the power required by the reciprocating compressor increases rapidly with increase in condenser temperature, while the power curve for the centrifugal compressor is relatively flat. It is also evident that the, ca pacity of the steam jet compressor is independent of condenser temperature until a certain point is reached, where it drops to zero. As previously stated, steam jet equipment requires more, condensing water than other types of compression systems. Consequently, steam jet systems are well, suited to those applications where condensing water is cheap, or where condensing water is rather high in temperature. The selection of proper refrigeration equipment for any air conditioning Table 8. Basis of Equipment Selection Capacity Tons Majority Used Sous Used Few Used 0 to 5 ; Unit systems in con ditioned space. Unit central systems using duct distri bution. Built up central sys tems. 5 to 25 Built up central sys tems using recipro cating compres sors. Unit central systems using duct distri bution. Unit systems in con ditioned space. Built up systems us ing absorption and adsorption sys tems. 25 to 50 Built up central sys tems using recipro cating . compres sors. Built up central sys tems using centrif ugal compressors. Central systems us ing adsorption sys tems. 50 to 400 Built up central sys tems using recipro cating compres sors. Built up central sys tems using steam jet and centrifugal compressors. Built up central sys Built up central sys 400 and Over tems using centrif tems using steam ugal compressors. jet. job is of utmost importance for satisfactory results. The most important factors in the selection of the equipment are: 1. Loads (as determined by the conditions of the space to be cooled). 2. Economics (both initial and operating costs). 3. Codes (local safety codes must be adhered to and influence the type of system to be used). A broad division of equipment to be used for a particular installation or application may be made on the basis of the magnitude of the load. Cur rent general practice is outlined in Table 8. Unit or 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 26). The reciprocating, compressor in the built-up central system (see Refrigeration 883 Chapter .30) covers the widest range of application ance it is applicable to 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. 14 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 3 ui i/t z 3 Fig. 16. Compressor and Coil Performance 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 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. 16. 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 9 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 percent sensible to total heat, the operating point A in Fig. 16 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-