Performance evaluation of natural refrigerant pairs R744/R717 and R744/R290 in cascade systems with IHX and economizer configurations for individual quick freezers

Abstract

This study presents a comprehensive analysis of the effectiveness of two different sub cooling arrangements in natural refrigerant cascade systems (CRS) R744/R717 and R744/R290 having solitary evaporator maintained at −45°C, which is an individual quick freezer (IQF) for seafood application. The assessed sub-cooling configurations, denoted as CRSI and CRSE, involve specific arrangements: CRSI incorporates an intermediate heat exchanger (IHX) within the high-temperature circuit (HTC), whereas CRSE integrates an economizer arrangement within a screw compressor in the HTC. Performance comparisons are drawn with a conventional R404A system under an ambient temperature of 40°C. The polytropic compressor equations are employed to model system performance under design conditions. The cooling load data from a seafood processing unit in Kochi, Kerala, India is utilized. It is found that for both R744/R717 and R744/R290 CRSs, the CRSE configuration shows a higher COP compared to the CRSI configuration. The CRSE setup with R744/R717 refrigerant had the best cooling COP of 1.06, while the second highest COP is found for CRSE R744/R290, reaching 1.03. The COP improvement obtained is 50.5 and 46.7% over the conventional R404A system. The heat recovery potential of the proposed CRSs is lower than the R404A system. R744/R717 in CRSI configuration have higher heat recovery potential than other CRSs. However, the overall (combined heating cooling) COP of R744/R717 in the CRSE configuration was found to be the highest and is 43% higher than the R404A system. The annual energy consumption (AEC) and total equivalent warming impact (TEWI) are also compared and it is found that adoption of R744/R717 CRSE can lower the AEC and TEWI in an IQF application by 33.6 and 68.8% respectively. Further, the economic analysis of the systems shows that although the CRS systems require a higher initial investment, the same can be mitigated by lower operating costs and reduced environmental penalty costs.