The evaporating temperature which required for the low temperature freezing system is -50℃∼-30℃ range. Since it is difficult to keep the required capacity in a cabinet, it is advantageous to design the system by using cascade refrigeration system. Besides the research is needed to use carbon dioxide and ammonia because it is environment-friendly working fluid and has a high possibility for performance improvement. To investigate of performance characteristics of the R744-R717 cascade refrigeration system, the theoretical model was developed and performance analyzed according to cascade heat exchanger operating temperature.
As a result, optimal cascade R744 condensing temperature was -5℃ and maximum COP was 1.13 when the temperature difference of cascade heat exchanger was 5℃. Besides the research is needed to use R744 and R1234yf because it is environment-friendly refrigerant and has a high possibility for performance improvement. Especially, the R1234yf may be used without changing the R134a system.
As well, to investigate of performance characteristics of the R744-R1234yf and R744-R134a cascade refrigeration system, the theoretical model was developed and the performance analyzed according to cascade heat exchanger operating temperature. Maximum COP of R744-R1234yf system was 2.15 when the evaporation temperature of R1234yf was ?10℃. The other system was 2.64 when the evaporation temperature of R134a was ?15℃. In addition, the performance analysis of internal heat exchanger which is attached to the R744-R717 cascade refrigeration system operated with cascade condensing temperature. To compare the performance of cascade cycle with/without the internal heat exchanger, the operating condition was set to the optimal condition which has the maximum COP of the cascade cycle without internal heat exchanger. As a result, cooling capacity and COP increased by 2.16% and 1.85%, respectively, when the internal heat exchanger was attached into a low-temperature cycle. Besides, those increased by 2.62% and 1.69%, respectively when the internal heat exchanger attached both high- and low-temperature cycle.