Full Description

Low temperature refrigeration storage equipment in the biotechnology industry typically uses cascade refrigeration toachieve evaporating temperatures of -80 C (-112 F) or below. Current systems utilize multiple compressors leading tohigh energy consumption. Equipment operating costs contribute significantly to the total operating costs of biotechnologycompanies and therefore motivate the development of more efficient alternatives for low temperature refrigeration. Thispaper describes a single compressor R-23/R-134a mixed refrigerant cycle that has been designed to extract a load of 0.256kW (873.5 Btu/hr) from a conditioned space at -80 C (-112 F). The designed system compresses a mixture of thegaseous refrigerants to a high pressure and then condenses the R-134a in a water-cooled separator while the R-23 remainsin vapor phase. The stream of liquid R-134a is expanded to the suction pressure and is used to condense the R-23 thatremains in vapor phase, operating much like an inter-stage heat exchanger in a cascade cycle. The condensed stream of R-23 then expands to the suction pressure and enters a low-temperature evaporator, where it absorbs energy from the load.

A model of the cycle is developed based upon first and second law principles of thermodynamics and used to refinethe design of a mixed refrigerant test apparatus. Theoretical analysis of the prototype system predicts that it will reach anevaporating temperature of -78.6 C (-109.5 F) when it operates with a mixture of 33.4% R-23 and 66.6% R-134a by mass.In experiments conducted using the same condensing temperature and mixture composition the mixed refrigerantapparatus reached an evaporating temperature of -75.0 C (-103 F), corresponding closely to the predicted temperature of-78.6 C (-109.5 F). To reach the desired evaporating temperature of -80 C (-112 F) the refrigerant mixture must bealtered to increase the amount of R-23.

Citation: ASHRAE Conference Papers, Denver, CO