Document nNDZEonrKX1wdqQ8Mq2B8xwaR

720. CHAPTER 39 1948. Guide.: other considerations which make such a system desirable. In the first place, where a toxic refrigerant is undesirable or cannot be used because of fire or other risks, especially in densely populated areas, the brine, can be cooled in an isolated room or building and can then be circulated through the air conditioning equipment. This arrangement eliminates any possibility of direct contact between the air and refrigerant. Equipment Selection The selection of proper refrigeration equipment for any air conditioning, 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). Fig. 11. Compressor and Coil Performance 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. Current general practice is outlined in Table 11. . 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 36). The reciprocating compressor in the built-up central system (see Chapter 43) covers the widest range of application since 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. r Centrifugal compressors are used for large installations, and usually where the indirect system is required. The driving mechanism can be a Refrigeration 721 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. 7 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 evaporating 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. 11. 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 12 illustrate two types of conditioned enclosures having the same total load of 148,000 Btu per hour, but with two different Table 12. Typical Operating Conditions for Two Types of Load Type or Enclosure Load, Btu per Hour Sensible Latent Total Ratio Sensible- to Total Air Entering Con. Operating Balance Point F Ifc* Per Cent BW Evaporator Temp F Deg Condenser ' Pressure Lb per 8q In. Per Cent RauiiiMfi Heat 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 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. 11 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 temperature is lowered to 34.4 F as shown in point B of Fig. 11. In.order to ,obtain the 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 the amount of pressure drop expressed in Fahrenheit degrees. ABBREVIATIONS AND SYMBOLS IN CHAPTER cop = coefficient of performance, ratio of refrigerating effect to the heat equivalent of the compressor work. CVE = conventional volumetric efficiency. d - internal diameter in inches. Ht = cooling load in tons. Ad = enthalpy of vapor at condition of discharge from compressor. Afc ~ enthalpy of liquid at discharge from compressor. . Aid = enthalpy of liquid at discharge of expansion valve. Afs = enthalpy of liquid at entrance to expansion valve.