Document MML7vKqjoEbjQd9n8p8Ejgaqj

810 CHAPTER 36 1951 Guide Capaott Tom 0 to 5 5 to 25 25 to 50 50 to 400 400 and Over Table 8. Babib of Equifment Selection Majority Used Boats Used Few Used Unit systems in con* Unit central systems Built up central sys- ditioned space. using , duct distri .terns. bution. 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. 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. Built up central sys tems using recipro cating compres sors. Built up central systerns using steam jet and centrifugal compressors. Built up central sys Built up central sys tems using centrif tems using steam ugal compressors. jet. 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. 12 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 evaporat ing 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. 14. 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 Table 9. Typical Operating Conditions for Two Types of Load Tttr or . Enclosure Load, Btu per Hour Sensible Latent Totai Ratio Sen sible TO Total Air Entering Coil F. Deg, . Per Cent R.H. Operating Balance Point. Evapo rator Temp F Deg Con denser Per Cent Pressure Sensible Lb per Heat 8qln., 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 Refrigeration 811 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. 14 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- Fio. 14. Compressor and Coil Performance ture is lowered to 34.4 F as shown in point B of Fig, 14. 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 (CP) = coefficient of performance, ratio of refrigeratingeffect to the heat equivalent of the compressor work, (CVE) - clearance volumetric efficiency. d . internal diameter in inches. H.= cooling load in tons. . As - enthalpy of vapor at condition of. discharge from compressor, hi, * enthalpy of liquid at discharge from compressor, Ah " enthalpy of liquid at discharge of expansion valve, Ai = enthalpy of liquid at entrance to expansion valve, . Am enthalpy of mixture, At - enthalpy of saturated vapor. Ard " enthalpy of saturated vapor at discharge of valve or compressor, . A.. - enthalpy of saturated vapor at.state s entering compressor, hp - horsepower: Pd - pressure of saturated liquid and vapor at discharge of compressor.