6.I9 VI.I.4: CHOICE OF REFRIGERANTS The choice of refrigerant has a significant effect on the system's performance, since the enthalpies in the equ~tions above and the allowable temperature ranges of operation are determined by refrigerant properties. A complete discussion of the effect of these properties on the design and evaluation of heat pump systems can be found in references (6.I2), (6.I3), (6.I4), (6.I5), (6. Ill. The freezing point of the refrigerant must be lower than the evaporator temperature (T 4 or T 1 ); otherwise solid material precipitates out in the evaporator. The critical temperature of the refrigerant (associated with the 'critical state' shown in Figure 6.Il) must be above the operating temperature of part of the external radiator, if condensation is to occur. This sets a maximum on the outlet temperature T 3 , and thus on T 2 as well. The heat capacity of the refrigerant should be high, so that a low mass flow will carry a large load of power to the radiator. This property is reflected in the enthalpy values of the fluid. If the system leaks, the rate of leakage increases directly with pressure and:inversely with the molecular weight of the fluid. This favors refrigerants with high molecular weights. The physical and chemical properties of the refrigerant also affect the ease of leak detection. A high vapor density is desirable. It packs a given mass flow into smaller volume, allowing smaller pipes and compressors. Low viscosity is also advantageous, since it reduces friction losses. The refrigerant should also be chemically stable. The operating pressures of the Rankine heat pump system should be higher than the pressures outside it, so that leaks can only cause refrigerant leakage outward. Contamination of the heat pump fluid lessens the systems performance and poses difficult decontamination problems. Safety considerations suggest that the refrigerant should not be flammable or toxic, even in low concentrations. The use of this Rankine heat pump system in a space colony lends emphasis to certain desirable properties. Specifically, the
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