A summary statement of the position of industrial refrigeration with respect to refrigerants is that ammonia is likely to continue to be dominant in the field, but there is a need for refrigerants of low toxicity to supplement ammonia and to take the place immediately of R-502 and in the future R-22. While it is a suitable replacement for R-12, R-134a will be limited in its applications in industrial refrigeration, so the need remains for a halocarbon appropriate for low-temperature refrigeration. The industry has been forced to turn to mixtures or blends.
Refrigerant mixtures or blends are often characterized as azeotropic mixtures or nonazeotropic (or correctly, zeotropic) mixtures. Azeotropic blends are precise mixtures of substances that have properties differing from either of the two constituents. The diagram of the temperature-pressure-concentration relationship of an ideal zeotropic mixture might appear as shown in Figure 12.2. In a zeotropic mixture, the concentration of the two substances in the vapor is different from that in the liquid at a given pressure and temperature. There are certain applications where the properties of a zeotrope are advantageous, such as in the autocascade system for ultra low temperatures as explored in Chapter 21. For conventional industrial refrigeration systems, however, the zeotrope of Fig. 12.2 has the drawback that if a leakage from a system occurs that is in vapor form, for example, the composition of the lost vapor will be different than the original charge. Replacing the lost refrigerant thus requires some analysis of the concentration of the refrigerant mixture remaining in the system. Another charcteristic of the zeotrope when boiling or condensing is that its temperature changes, in contrast to a single refrigerant whose temperature remains constant during a constant-pressure evaporation or condensation. This characteristic could be an advantage on some systems which are designed for this behavior, but it is not the standard expectation in an industrial refrigeration system.