Document ddBmZ6Okk7RbERLR5Mj8Ykz6

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 Capacity Tons 0 to 5 5 to 25 25 to 50 50 to 400 400 and Over Table 10. Basis of Equipment Selection Majority Used Some Used Few Used Unit systems in con Unit central systems Built up central sys ditioned space. using duct distribu tems. tion. Built up central sys tems using reciprocating compressors. Unit central systems using duct distribu tion. Unit systems in con ditioned1 space. Built up systems using absorption and adsorption systems. Built up central sys Built up central sys Central systems tems usingreciprocat- tems using centri using adsorption ing compressors. fugal compressors. systems. Built up central sys tems using reciprocating compressors. Built up central sys tems using steam jet and centrifugal com pressors. Built up central sys Built up central sys tems using centri tems using steam jet. fugal compressors. 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 tempterature. 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. 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. Table 11. Typical Operating Conditions for Two Types of Load Ttpb op Enclosure Load, Btu per Hour Sensible ' Intent Total Ratio Sensible TO Total Air Entering Coil Operating Balance Point Per Cent R.H. Evaporator Temp Deg F Condenser Pressure Lb per' . So In. Per Cent Sensible 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 82.1 504 CHAPTER 25. REFRIGERATION 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 obtain the same capacity, a larger condensing unit is used. This illus tration assumes zero pressure drop 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. Fig. 10. Compressor and Coil Performance 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 condenser. 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 ratio of the heat delivered to the work of compression is given in Equa tion 1. where T, T, - r, (0 Ti = absolute temperature of evaporator. 7j = absolute temperature of condenser. . Thus, with a small spread of temperature between the evaporator and the condenser, 6 or 8 times as much heat may be obtained theoretically, 505