Document kDR2oXGXr11mEZ8wVj1xbreZB

814 CHAPTER 37 1953 Guide Table 3. Properties, op Monofluorotbichloromethane (F-ll) Sat. Temp. F Abs. Press. Lb peb Sq In. Volume Enthalpy and Entropy Taken From --40 F Enthalpy Entropy 25 F Superheat 50 F Superheat Liquid Vapor Liquid Vapor Liquid Vapor En thalpy En tropy En En thalpy tropy 0 2.59 0.01020 13.700 7.81 90.4 0.0178 0.1975 93.9 0.2049 97.4 0.2120 5 2.96 0.01024 12.100 -8.81 91.2 0.0200 0.1974 94.7 0.2047 97:2 0.2117 10 3.38 0.01028 10.700 9.82 92.0 0.0222 0.1973 95.5 0.2045 99.0 0.2114 15 . 3.85 0.01032. 9.530 10.80 92.8 0.0243 0.1971 96.3 0.2043 99.8 0.2111 20 4.36 0.01036 8.490 11.90 93.7 0.0264 0.1970 97.2 0.2041 100.7 C.2109 25 4.94 0.01040 7.580 12.90 94.5 0.0286 0.1969 98.0 0.2039 101.5 0.2107 30 5.57 0.01045 6.770 13.90 95.3 0.0307 0.1969 98.8 0.2038 102.3 0.2105 35 6.27 0.01049 6.080 14.90 96.1 0.0328 0.1968 99.6 0.2037 103.1 0.2103 40 7.03 0.01053 5.460 16.00 96.8 0.0349 0.1968 100.3 0.2036 103.8 0.2101 45 7.88 0.01057 4.920 17.00 97.6 0.0370 0.1967 101.1 0.2035 104.6 0.2099 50 8.79 0.01062 4.440 18.10 98.4 0.0391 0.1967 101.9 0.2034 105.4 0.2098 55 9.80 0.01066 4.020 19.10 99.2 0.0412 0.1967 102.7 0.2033 106.2 0.2097 60 10.90 0.01071 3.640 20.20 100.0 0.0432 0.1967 103.5 0.2033 107.0 0.2096 65 12.10 0.01076 3.300 21:30 100.8 0.0453 0.1967 104.3 0.2032 107.8 0.2094 70 13.40 0:01081 3.000 22.40 101.5 0.0473 0.1967 105.0 0.2032 108.5 0.2093 75 14.80 0.01086 21740 23.50 102.2 0.0493 0.1967 105.7 0.2031- *109.2 0.2092 80 16.30 0.01091 2.500 24.50 102.9 0.0513 0.1966 106.4 0.2030 109.9 0.2090 85 17.90 0.01096 2.280 25.60 103.6 0.0533 0.1966 107.1 0.2029 110.6 0.2089 90 19.70 0.01101 2.090 26.70 104.4 0.0553 0.1966 107.9 0.2028 111.4 0.2088 95 21.60 0.01106 1.918 27.80 105.1 0.0573 0.1966 108.6 0.2028 112.1 0.2087 100 23.60 0.01111 1.761 28.90 105.7 0.0593 0.1965 109.2 0.2027 112.7 0.2085 105 25.90 0.01116 1.620 30.10 106.4 0.0613 0.1965 109.9 0.2026 113.4 0.2084 Values of v, and vd 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 pB 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, hm, is equal to the enthalpy of the saturated liquid at the entrance state, hi,, and can therefore be read from the table. Thus, ftf, " Am ^ ^rd -- (1 l) (Avd Afj) (51 r, X -- (fta Afd) t (ft,d " Afd) where hi, = enthalpy of saturated liquid at entrance to expansion valve. hm = enthalpy of mixture. Avd = 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; and 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 V~ Heat of Compression TAdded to Gas -- Compressor------ ' Heat Added to Refrigerant by Substance Cooled Cold Out Evaporator or Cooler ^Expansion Valve for Reducing Pressure -vHigh Pressure Superheated Gas Condenser Cold In Heat Taken from Refrigerant by Condensing Medium Hot Out 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