The magnitudes of the evaporating and condensing pressures at the expected operating temperatures strongly affect the choice of refrigerant. Figure 12.5 shows the saturation pressures as functions of temperature for most of the refrigerants that are listed in Table 12.2. It is desirable that the operating pressures be low enough to use pipe and vessels of standard wall thicknesses. On the other hand, pressures below atmospheric have the disadvantage of drawing in air along with water vapor should there be any leaks. If a leak in the system does exist, it is usually preferable for refrigerant to leak out rather than for air to leak in. The graph in Fig. 12.5 explains why R-23, ethane, and carbon dioxide are usually reserved for low-temperature systems, and why R-134a might be chosen in the future for systems in which the condensing temperature is high, such as in an industrial heat pump.
Carbon dioxide (R-744) possesses some thermodynamic characteristics that make it advantageous in certain applications. The pressure-enthalpy diagram of carbon dioxide is shown in Fig. 12.6 and demonstrates that solid carbon dioxide, or dry ice, sublimes from a solid to a vapor at -78.4°C (-109.1 °F) when the pressure is atmospheric. At pressures above 518.1 kPa (75.1 psia), carbon dioxide does not solidify and can only exist in liquid and/or vapor phases.