Analytical screening of alternatives for R-502 in low-temperature refrigerating applications

摘要

R-502 is used as the working fluid of low-temperature refrigerating systems with evaporating temperatures as low as (minus)40(degrees)F ((minus)40(degrees)C). It was selected as the refrigerant of choice for these applications because it is both nonflammable and nontoxic and has an acceptable compressor discharge temperature when used in a high-efficiency vapor-compression system. Replacement fluids are going to be needed for use in place of R-502, however, because of the provisions of the Montreal Protocol. R-502 is an azeotropic blend of R-22 and R-115, and R-115 is a chlorofluorocarbon (CFC) that is contributing to the destruction of stratospheric ozone. The provisions of the Montreal Protocol currently require a total phase-out of the production of all fully halogenated CFCs, including R-115, by the year 2000. Consequently, replacement fluids will be needed for new and existing equipment. This study was initiated to identify other possible fluids that can be used either in new or retrofit applications for low-temperature refrigerant. It includes an evaluation of predicted cycle efficiency, system capacity, pressure ratio, fluid flammability, and compressor discharge temperature. Eight different chemical compounds are considered for use as pure refrigerants as well as components in binary and ternary mixtures: R-134a, R-134, R-125, R-143a, R-152a,R-32, R-124, and R-22. An exhaustive study of the composition of blends was performed to determine the mass fractions of each component in the mixtures that are most promising on the basis of efficiency, flammability, low discharge temperature, and acceptable capacity and pressure ratio. Several blends are identified for further laboratory and analytical evaluation.