Document RjybVkOxXzk9dZ8pYzDVrBQjv

542 CHAPTER 38 1959 Guide Refrigeration 543: With the ideal Carnot cycle operating as a heat pump, the co efficient of performance is <cp)-^r~r O) The Carnot cycle coefficient of performance for both a re frigerating machine and a heat pump increases as the spread between the evaporator and the condenser temperatures de creases. In general, the same is true for an actual system operating as either a refrigerating machine or a heat pump. Refrigerants A desirable refrigerant should possess chemical, physical, and thermodynamic properties that permit its efficient ap plication in refrigerating systems. In addition', when the vol ume of the charge is large, there should be little or no danger to health or to property in case of its escape. Thermodynamically, a material for use as a refrigerant should have a large latent heat of vaporisation since it is this heat quantity--subject to minor variations--that constitutes the working effectiveness of the refrigerant. Further, since the work required to compress a vapor increases rapidly with the pressure ratio, the thermodynamic characteristics of the fluid should be such that the required low-to-high temperature range can be achieved with only a moderate change in ratio. A further consideration, from the standpoint of practical operating effectiveness, is that the suction pressure should not be below atmospheric (to prevent leakage of air into the refrigerant lines) nor should the condenser pressure be ex cessively high (to prevent need for extra-heavy construc tion). The specific volume-specific enthalpy relationship is also important because some materials would have such low density, when in vapor form, that impractical compressor displacements would be needed to handle the suction vapor. Properties of refrigerants are usually given either in tabu lar or graphic form. In contrast to the temperature-entropy, plotting which is used almost exclusively in steam-power work, refrigeration problems are usually referred to a pres sure-enthalpy chart. The advantage of pressure-enthalpy plotting in that linear distances on the-chart correspond to. energy gains or losses, and the two types of processes, con stant-pressure and constant-enthalpy, that occur most fre quently in refrigeration cycles, can both be represented by straight vertical or horizontal lines, figs. 1 and 2 present pressure-enthalpy charts for- dichlorodifluoromethane, .CCl*F, and monochlorodifluoromethane, CHC1F*, respec tively. Although tabular arrangements of refrigerant proper ties require interpolation between values, they have the ad vantage of an accuracy greater than that obtainable from a chart. Tables 1, 2, 3, and 4 give the thermodynamic proper ties of four of the more common refrigerants used in airconditioning installations: Refrigerant* 12,dichlorodifluoro methane ; Refrigerant 22, monochlorodifluoromethane; Refrigerant 11, trichloromonofluoromethane; and Refriger ant 113, trichlorotrifiuoroethane. The first two of these are commonly used in reciprocating compressors while the last two are commonly used in centrifugal machines Referring to Table 1, the first column gives the range of saturation temperatures likely to occur in practice. The sec ond column gives the saturation pressure expressed in pounds per square inch absolute corresponding to a given tempera ture, while the next six columns give the three fundamental specific properties, volume, enthalpy, and entropy, of the saturated liquid and saturated vapor, respectively. The last * ASHE desxfistioaa (or tbse rdricenntt. four columns give values of enthalpy and entropy for gases; with 25 deg and with 50 deg of superheat', note particularly that the column heading SO F superheat means, nottbat thegas is at a temperature of 50 F, but that its temperature ex ceeds by 50 deg the saturation temperature corresponding toits actual pressure. Tims, CCUF, vapor at 380 psig and 91 F possesses 50 deg of superheat, since its saturation tempera ture corresponding to 52.7 psia is 41 F. The tabular arrangements of refrigerant properties are lit erally for saturated or superheated materials only. In many cases, however, the engineer must work with subcooled liq uids. With an accuracy sufficient for all practical purposes, , the specific volume and the enthalpy of any subcooled refrig erant can be taken as equal to the values read from the tables for a saturated liquid at the same temperature. Thus, if CCUF. at 121 pstft and 40 F is passing through a pipe, its volume and enthalpy can be determined from Table 1 as 0.0116 cu ft per pound and 17.0 Btu per pound. Frequently it is necessary to determine the properties of a toet vapor or of a mixture of liquid with some added vapor, such as is found at the discharge from an expansion valve. This can be done from the tables by noting that the specific enthalpy of the mixture must be equal to that of the saturated liquid, plus a fraction of the latent heat of vaporization equal to the fraction of refrigerant that is present in vapor form. -Consider, for example, CCUF, with a quality (the percent in vapor form) of 30 percent; the enthalpy of thfe material would be equal to where A. - k, + 0.30 (A, - A,) . (4) Km " specific enthalpy of the mixture. hf " specific enthalpy of the liquid. A, -- specific enthalpy of the saturated vapor. Values of hf and A,,for use in Equation 4 can be obtained directly or by calculation from the tables of properties of re frigerants. By a reversal of this same procedure the tabular data ran be used to determine the state of a mixture leaving an ex pansion valve. Consider a valve to which saturated liquid at pressure pt 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 mixture, hg,, is equal to the enthalpy of the saturated liquid at the entrance state, hu, and can therefore be read from the table. Thus, A/. = A. = Aw - (1 - 1) (Aw - hi4) (5) or, where x " (km -- hft) (Aw -- A/*) (6) hf, enthalpy of saturated liquid at entrance to expansion valve. km " enthalpy of mixture. Aw = enthalpy of saturated vapor at discharge. hf4 " enthalpy of liquid at discharge. x * proportion of liquid io 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