3617 -- An Analytical Screening of Alternatives for R502 in Low-Temperature Refrigerating Applications ✓ Most Recent

R-502 is used as the working fluid of low-temperature refrigerating systems with evaporating temperatures as low as -40 deg F (-40 deg 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 vapour-compression system. Replacement fluids are going to be needed for use in place of R502, however, because of the provisions of the Montreal Protocol. R502 is an azeotropic blend of R22 and R115, and R115 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, included R115, by the year 2000. Consequently, replacement fluids will be needed for new and existing equipment. One alternative is to use R22, but this requires major design and hardware changes in order to keep the compressor discharge temperature below the point where the lubricant begins to breakdown. This can be done with staged compression using liquid reinjection or other modifications to the compressor, but it is expensive, particularly for adapting existing equipment. This study was initiated to identify other possible fluids that can be used either in new or retrofit applications for low-temperature refrigeration. 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 - R134a, R134, R125, R143a, R152a, R32, R124, and R22. 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 evaluations.

KEYWORDS: R502, low temperature, refrigeration, refrigerants, R134a, R134, R125, R143a, R152a, R32, R124, R22, R115, mixtures, azeotropic, fluorocarbons, calculating, cyclic, efficiency