2013 ASHRAE Annual Conference
Low temperature refrigeration storage equipment in the biotechnology industry typically uses cascade refrigeration to achieve evaporating temperatures of -80 °C (-112 °F) or below. Current systems utilize multiple compressors leading to high energy consumption. Equipment operating costs contribute significantly to the total operating costs of biotechnology companies and therefore motivate the development of more efficient alternatives for low temperature refrigeration. This paper describes a single compressor R-23/R-134a mixed refrigerant cycle that has been designed to extract a load of 0.256 kW (873.5 Btu/hr) from a conditioned space at -80 °C (-112 °F). The designed system compresses a mixture of the gaseous refrigerants to a high pressure and then condenses the R-134a in a water-cooled separator while the R-23 remains in vapor phase. The stream of liquid R-134a is expanded to the suction pressure and is used to condense the R-23 that remains 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 refine the design of a mixed refrigerant test apparatus. Theoretical analysis of the prototype system predicts that it will reach an evaporating 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 refrigerant apparatus 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 be altered to increase the amount of R-23.