Analysis Of Variable Load Operation Characteristics Of Single Refrigerant Cascade Refrigeration System

The cold storage with overlapping refrigeration system can meet the operation needs of cold storage in different seasons,but when the temperature difference between indoor and outdoor cold storage is large and the refrigerating load fluctuation is large,the conventional overlapping refrigeration system can not ensure the long-term stable and efficient operation of cold storage,which is not conducive to energy conservation and environmental protection.In order to increase the temperature range of efficient and stable operation of refrigeration system,improve the operation performance of the system under variable working conditions and loads.In this paper,a superimposed refrigeration system with the same refrigerant in high temperature and low temperature stages is designed,and the system can realize the switching between single-stage refrigeration cycle mode and superimposed refrigeration cycle mode based on the matching of operating conditions and operating loads.In high temperature cold storage,cold season or cold storage low refrigeration load operation mode of single-stage refrigeration cycle operation,in low temperature cold storage,hot season or cold storage high refrigeration load operation mode of overlapping refrigeration cycle operation.By comparing the properties of various refrigerants,R410 A refrigerant is selected as the single refrigerant in the overlapping circulation system of this subject.Based on the existing research results,the theoretical analysis of single stage refrigeration cycle and overlapping refrigeration cycle under different working conditions and different loads is carried out.The results show that with the increase of evaporation temperature,the COP of the single-stage cycle and the overlapping cycle increases,and the increase rate of the former is greater than that of the latter.With the increase of refrigeration load,COP of single-stage cycle and overlapping cycle increases,and the increase rate of the latter is greater than that of the former.The system performance test bench was built to realize the switching operation of single stage refrigeration cycle and overlapping refrigeration cycle,and the performance of the two cycles was studied.The experimental results show that the exhaust temperature of the cascade refrigeration cycle is much lower than that of the single stage refrigeration cycle under the same temperature and refrigeration load.For the overlapping refrigeration cycle,the same refrigeration load corresponds to multiple sets of high and low temperature compressor frequency,and there is an optimal high and low temperature operating frequency to maximize the COP of the system.With the increase of evaporation temperature,the COP of both the overlapping refrigeration cycle and the single-stage refrigeration cycle increases,and the COP of the single-stage refrigeration cycle increases at a large rate,and there is an evaporation temperature switching point with the optimal system COP as the control objective.When the refrigeration load is 3k W,4k W,5k W,6k W,7k W,8k W,9k W,10 k W,the evaporation temperature switching point is-26.8℃,-21.8℃,-16.3℃,-12.5℃,-9.5℃,-6.6℃,-2.7℃,0.9℃,respectively.With the increase of refrigeration load,the COP of both the cascade refrigeration cycle and the single-stage refrigeration cycle increases,and the COP of the cascade refrigeration cycle increases at a higher rate,indicating the optimal COP switching point of the system.When the evaporation temperature is-5℃,-10℃,-15℃,-20℃,-25℃ and-30℃,the corresponding switching points of refrigeration load are 2.6k W,3.8k W,5.2KW,7.1k W,8.4k W and 9.4k W respectively.In this study,the COP of the optimal system was taken as the control objective,and the switching strategy of single stage refrigeration cycle and superposition refrigeration cycle was designed under the two different conditions of evaporation temperature and refrigeration load,respectively,which provided a technical solution for the cold storage to be in high efficiency and energy saving operation for a long time in the actual operation.