Document zQVomaNzmjbBX4j9O5EE61dzB

814 CHAPTER 37 1953 Guide Table 3. Properties op Monofluohotrichloromethane (F-ll) Enthalpy and Entropy Taken Fbom --40 F OAT. Temp. F Press. Lb peb Sq In. Liquid Enthalpy Entropy 25 F Superheat 50 F Superheat Vapor Liquid Vapor Liquid Vapor En thalpy En- En tropy thalpy En tropy 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 2.59 2.96 3.38 3.85 4.36 4.94 5.57 6.27 7.03 7.88 8.79 9.80 10.90 12.10 13.40 14.80 16.30 17.90 19.70 21.60 23.60 25.90 0.01020 0.01024 0.01028 0.01032 0.01036 0.01040 13.700 12.100 10.700 9.530 8.490 7.580 7.81 8.81 9.82 10.80 11.90 12.90 0.01045 0.01049 0.01053 0.01057 0.01062 6.770 6.080 5.460 4.920 4.440 13.90 14.90 16.00 17.00 18.10 0.01066 0.01071 0.01076 0.01081 0.01086 4.020 3.640 3.300 3.000 2i740 19.10 20.20 21.30 22.40 23.50 0.01091 0.01096 . 0.01101 0.01106 0.01111 0.01116 2.500 2.280 2.090 1.918 1.761 1.620 24.50 25.60 26.70 27.80 28.90 30.10 90.4 91.2 92.0 92.8 93.7 94.5 0.0178 0.0200 0.0222 0.0243 0.0264 0.0286. 0.1975 0.1974 0.1973 0.1971 0.1970 0.1969 93.9 94.7 95.5 96.3 97.2 98.0 95.3 0.0307 0.1969 98.8 96.1 0.0328 0.1968 99.6 96.8 0.0349 `0.1968 100.3 97.6 0.0370 0.1967 101.1 98.4 0.0391 0.1967 101.9 99.2 100.0 100.8 101.5 102.2 0.0412 0.0432 0.0453 0.0473 0.0493 0.1967 0.1987 0.1967 0.1967 0.1967 102.7 103.5 104.3 105.0 105.7 102.9 0.0513 103.6 0.0533 104.4 0.0553 105.1 0.0573 105.7 0.0593 106.4 ' 0.0613 0.1966 0.1966 0.1966 0.1966 0.1965 0.1965 106.4 107.1 107.9 108.6 109.2 109.9 0.2049 0.2047 0.2045 0.2043 0.2041 0.2039 97.4 97.2 99.0 99.8 100.7 101.5 0.2038 0.2037 0.2036 0.2035 0.2034 102.3 103.1 103.8 104.6 105.4 0.2033 0.2033 0.2032 0.2032 0.2031" 106.2 107.0 107.8 108.5 109.2 0.2030 0.2029 0.2028 0.2028 0.2027 0.2026 109.9 110.6 111.4 112.1 112.7 113.4 0.2120 0.2117 0.2114 0.2111 0.2109 0.2107 0.2105 0.2103 0.2101 0.2099 0.2098 0.2097 0.2096 0.2094 0.2093 0.2092 0.2090 0.2089 0.2088 0.2087 0.2085 0.2084 Values of v, and ud for use in Equation 4 can be obtained directly or by calculation from the tables of properties of refrigerants. By a reversal of this same procedure the tabular data can be used to determine the state of a mixture leaving an expansion valve. Consider a valve to which saturated liquid at pressure p, is admitted, and a mixture of saturated liquid and vapor at pressure pd is discharged. The quality of the material at discharge is then determined by making use of the fact that the expansion process is completely irreversible, is a throttling process, and hence, occurs without change in enthalpy. Thus, the enthalpy of the mix ture, ha,, is equal to the enthalpy of the saturated liquid at the entrance state, h,,, and can therefore be read from the table. Thus, -- hn -- AyJ (1 ) (^vd ^fd) (5) or, where I ~ (Ajb Afd) (^vd *" Afd) (6) hi, = enthalpy of saturated liquid at entrance to expansion valve. ha, -- enthalpy of mixture. hvd = enthalpy of saturated vapor at discharge. Aid = enthalpy of liquid at discharge. x = proportion of. liquid in the mixture, decimal. Vapor Compression Refrigeration Cycle Simple Cycle. The refrigerant cycle is the series of state changes (which occur, in the conditioning processes) needed to restore the refrigerant to a condition in which it will possess the ability to extract heat from the space to be cooled. For all compression-type systems the cycle consists of four processes: heat gain in the evaporator; pressure rise in the compressor; Refrigeration 815; heat loss in the condenser; pressure loss in the,expansion valve. The com pression process is accomplished at the expense of energy added to the compressor in the form of shaft, work,'and the expansion process could be, carried out, if the economics of the system would permit, in an expanding engine with consequent release of energy as shaft work. In ordinary sys tems, however, the additional first cost and maintenance costs of an expand ing engine so greatly exceed the advantage resulting from the work realized, that such engines are not used; arid the pressure reduction is allowed to occur irreversibly in an expansion valve. Basically, then, a refrigeration cycle consists of two heat transfer processes and two pressure change proc esses, no work entering into the heat transfer processes and---in the simple cycle--no heat transfer occurring during the pressure-change processes. ' The most common and least complicated type of refrigeration cycle is called the simple saturation cycle, and is shown diagrammatically in Fig. 3 and plotted upon pressure-enthalpy coordinates in Fig. 4. For this system, Low Pressure Saturated Gas Heat of Compression TAdded to Gas -- Compressor------ High Pressure Superheated Gas Hot In ' Heat Added to Refrigerant by Substance Cooled Cold Out Evaporator or Cooler ^Expansion Valve for Reducing Pressure Condenser pH Heat Taken from Refrigerant by Condensing Medium High Pressure Saturated Liquid Fig. 3. Mechanical Refrigeration System saturated vapor flows without gain or loss of heat from the evaporator to the suction of the compressor. During passage through the compressor the energy added as shaft work goes entirely to increase the enthalpy of the refrigerant, and the compression process, which is assumed to occur irreversibly and without external heat transfer, is characterized by constant entropy. Thus, the state of the superheated vapor, leaving the compressor can be determined from the tables of thermodynamic properties by noting the discharge pressure and fixing, also, the entropy of the saturated vapor at entrance to the compressor. Superheated vapor from the compressor flows to the condenser where de-superheating and condensation take place. From the condenser the re frigerant flows to the expansion valve, undergoes a constant-enthalpy pres sure reduction, and returns to the evaporator where it again removes a quantity of undesired heat. When the evaporator is arranged to permit direct cooling of room air by the refrigerant, the system is said to be of the direct expansion type, while a system in which the evaporating refrigerant cools water or brine, which in turn cools the air, is said to be indirect. Although many differences exist between most actual systems and that of the simple saturation cycle, this latter is, nonetheless, of great value in that it provides an extremely simple method of rapidly achieving an approximate