The halocarbon in the high-pressure group of refrigerants in Fig.12.5, R-23, is a candidate for the low-temperature circuit of the cascade system which will be examined in greater detail in Chapter 21. R-23 may replace R-503, which was a popular low-temperature refrigerant but also an azeotropic mixture, composed of 59.9% R-13 and 40.1% R-23. Since R-13 is a CFC, R-503 has been phased out. R-23 now remains as a single refrigerant adaptable to low-temperature circuits. Water is fairly soluble in R-23, which is an advantage because R-23 is not extremely susceptible to separation and freezing during low-temperature operation. The solubility of oil in R-23 may not be as good as it was in R-503, but this problem is currently being addressed. In one retrofit where R-503 was replaced by R-23, the time for drawing down the temperature was increased. Furthermore, the- R-23 discharge temperature from the compressor was high 132°C (270°F)—necessitating the injection of a controlled amount of liquid refrigerant at the suction of the compressor.
Lately, an azeotropic blend of R-23 and R-116 (hexofluoroethane) has been explored. Preliminary tests suggest the use of polyol ester (POE) oils to achieve lubricant/refrigerant miscibility. This refrigerant offers lower compressor discharge temperatures than R-23.