CO2를 2차 냉매로 사용하는 간접 냉동시스템의 성능특성에 관한 실험적 연구

Abstract

In indirect refrigeration system the evaporator does not absorb heat directly from the object to be cooled but through a secondary cold refrigerant. This system is mainly employed by large supermarkets and large cold storage. The reason is using direct refrigeration system will result in high pressure drop and refrigerant charge because it will need a large sized evaporator. In direct refrigeration system there are refrigerant leakages which increases refrigerant costs and also cause environmental problems if the refrigerant in use is not natural. For these reasons, indirect refrigeration system which uses secondary refrigerant is better for large supermarkets and large cold storage. However, the viscosity of secondary refrigerant used in existing indirect refrigeration systems increases with decreasing evaporation temperature, so it requires a bigger pump power. Thus, a secondary refrigerant with constant or decreasing viscosity while evaporation temperature decreases is better to reduce pump power. An ideal alternative is CO2. In this paper, an experimental analysis on the performance of an indirect refrigeration system which uses CO2 as secondary refrigerant and R404A as primary refrigerant is carried out and the data computed to aid further studies in this field. The analysis was performed in two ways. Firstly, the coefficient of performance (COP) in different operating conditions was analysed to find optimal design conditions in the system. Secondly, the exergy losses for the system components were analysed to help improve the performance of the components with most losses for an increase COP. Results indicated that, the COP of the indirect refrigeration system decreases with decreasing CO2 evaporating temperature thereby increasing R404A condensing temperature and CO2 cooler temperature difference. In addition, the COP does not change with change in mass flow rate of CO2. Finally, regardless of the operating conditions, the compressor contributed about 55% of the total exergy loss followed by the R404A condenser which was about 20%. Therefore, to improve the COP of the indirect refrigeration system, the compressor and R404A condenser must be designed to minimize exergy losses.