Experimental evaluation of a cascade refrigeration system operating with R744/R134a.

This work reports a preliminary experimental investigation related to the operation of a subcritical cascade refrigeration system using the R744/R134a refrigerants, in a commercial application of stored food. The experimental facility is composed of an alternative variable speed compressor for the R744 and an electronic expansion valve (EEV) that promote the direct evaporation of the CO2 in the interior of a cold-room (2,3mx 2,6mx 2,5m) to maintain the internal air temperature at -25°C. The high temperature stage consist of an alternative compressor for the R134a, a thermostatic expansion valve (TEV) and an air-cooled condenser. A cascade heat exchanger that is a condenser for the CO2 system and an evaporator for the R134a system completes the apparatus. A total of three experimental results were performed in steady state conditions. The R134a system operated at the nominal frequency, 60 Hz, while the R744 system operated in three different frequencies: 60 Hz, 50 Hz and 40 Hz. This parameter was varied to verify the range of cooling capacity and performance, COP, of the cascade system. As conclusion, the cascade system operated in a flexible way, which represents a real application with variable demand of storage food. In addition, the results showed good agreement with the experimental data available in the literature.