Azeotrope of HFC-125 and HFC-143a as an Alternative for R-502

R-502 has been the primary refrigerant for low temperature commercial refrigeration applications for many years. The impending phase-out of CFCs dictates its elimination in the very near future. The identification of alternative(s) for R-502 therefore becomes a high priority. This paper addresses the factors to be evaluated in selecting the best replacement and identifies an azeotropic mixture of HFC-125 and HFC-143a as a leading candidate which satisfies the selection criteria. HISTORICAL PERSPECTIVE Early references indicated that HCFC-22 was introduced as a low temperature refrigerant to supplement CFC-12. One source (1) states that R-22 was developed for reciprocating compressor applications below -25 F which is typical for frozen food applications. At that time, most compressors were of the open drive configuration and therefore less sensitive to the lower vapor heat capacity of HCFC-22. The compressor motors were air cooled rather than refrigerant cooled as most hermetic compressors are today. Furthermore, the motor heat was not added to the refrigerant, which kept the discharge temperatures at a more acceptable level. Water cooled condensers were much more common in the early days of retail food frozen food systems which resulted in lower condensing temperatures which also · tended to maintain lower discharge gas temperatures. In the 1950s, hermetic compressors combined with air cooled condensers started to predominate refrigeration systems. HCFC-22 was marginally acceptable with open drive compressors and water cooled systems where the condensing temperature rarely exceeded 105 F. Air cooled systems might have succeeded with generously sized condensers but the use of hermetic compressors placed an excessive burden on simple refrigeration systems. The change to a typical condensing temperature of 120 F combined with cooling a 75 percent isentropic efficient compressor would increase the discharge temperature from approximately 250 F to well above 350 F. Techniques developed to deal with the. excessive temperatures generated in the compression process included multistage compression systems and/or liquid injection. These enhancements led to increased cost and introduced their own reliability problems as the overall system complexity was increased. A patent was issued to A.F. Benning in 1953 (2) for an azeotrope of HCFC-22 and CFC-115. The patent did not address any specific application and for several years there did not seem to be any commercial interest in this azeotropic refrigerant. In 1961 the product was introduced commercially as R-502. Compressor manufacturers quickly endorsed its use for low temperature applications (3).