An azeotropic mixture, in contrast to the zeotropic mixture, has a temperature-pressure-concentration diagram where the saturated vapor and saturated liquid lines coincide at a range of concentrations, as shown in Fig. 12.3. At the point or region where the saturated vapor and liquid lines merge, the mixture of the two substances behaves with the properties of a single substance, having properties different from either of its constituents.
Even combinations that are azeotropic at certain concentrations at one pressure may not be perfectly azeotropic at another pressure, as Figure 12.4 shows. The low-pressure temperature-concentration at low pressure, which is that of Figure 12.3 may change as the pressure increases. Usually the azeotropic region shifts toward the high concentration of the low- temperature boiler (material A in this case). Even the azeotrope R-502, which has been used successfully for many years, was subject to fractional distillation at certain operating pressures.
Most of the new substitute halocarbons that have become available in the past few years are blends, most of which are near-azeotropic, but a few are labeled azeotropic, such as R-507 which will be used as an example in this book. As an example of a near-azotropic refrigerant, R-404a will be chosen. In contrast to single refrigerants and azeotropes, the saturation tables (see Appendix) show a slightly different pressure for the vapor compared to the liquid at a given temperature.
Table 12.3 lists the major blends intended as substitutes for R-502 and/or R-22.