Study On The Performances Of CO₂ Transcritical Two-stage Compression Refrigeration System

The large emissions of Freon refrigerants are one of the important factors contributing to the global warming and ozone depletion.As an environmentally sound refrigerant,CO2 can replace existing refrigerants and alleviate the global environment caused by refrigerant emissions.The damage caused.Based on the local climatic conditions in Tianjin,this paper designs and builds a CO2 transcritical two-stage compression refrigeration system and conducts in-depth research on its performance.In this paper,the cyclic characteristics of CO2 transcritical two-stage compression refrigeration system is studied.The combination of thermodynamic theory analysis and simulation is adopted,and the experimental platform is built.Theoretical analysis and simulation mainly focus on the system's evaporation temperature,gas cooler outlet temperature,compressor efficiency,main liquid path supercooling,gas cooler outlet pressure,high pressure stage compressor suction superheat,low pressure stage compressor suction superheat.The influence of other parameters.on the COP value and exhaust temperature of the CO2 transcritical two-stage compression refrigeration system,summarizing the key factors affecting the performance of the system is as follows:(1)When the evaporating temperature is changed within the range of-40 to 0?,the COP of the CO2 transcritical two-stage compression refrigeration system is 23%higher than the COP of the CO2 transcritical single-stage compression refrigeration system,and the evaporating temperature is increased by 5?.The COP rising rate of CO2 transcritical two-stage compression refrigeration system can reach up to 13.70%.When the gas cooler outlet temperature is changed within the range of 32-41?,the coefficient of performance of the CO2 transcritical two-stage compression refrigeration system is higher than the performance coefficient of the CO2 transcritical single-stage compression refrigeration system by more than 20%,and the gas cooler For every 1? increase in outlet temperature,the COP of the CO2 transcritical two-stage compression refrigeration system can be reduced by up to 9.12%;(2)When the efficiency of the high-pressure compressor of the system is changed in the CO2 transcritical two-stage compression refrigeration system,the COP rise rate of the system is significantly higher than the COP rise rate of the system when changing the efficiency of the low-pressure compressor of the system.When the compressor efficiency is 0.7,the COP of the CO2 transcritical two-stage compression refrigeration system is higher than the COP of the CO2 transcritical single-stage compression refrigeration system by more than 26.07%;(3)For every 1? increase in the degree of subcooling of the main liquid path,the COP rise of the CO2 transcritical two-stage compression refrigeration system does not exceed 0.7%,and the selection of the primary liquid path supercooling is preferably 2-5?;(4)CO2 transcritical single-stage compression refrigeration system and CO2 transcritical two-stage compression refrigeration system have different gas cooler outlet pressures when obtaining optimal COP,and the corresponding relationship can be calculated by Equations 3-1 and Equations 3-2;(5)The high-pressure stage suction superheat is increased by 21? from 0.1?,the total power consumption decreases by no more than 1%,the system COP rises by no more than 1%,but the high-pressure compressor's exhaust temperature rises by 43.49%.The suction superheat of the high-pressure compressor should be controlled at 5?8?;(6)The process of increasing the suction superheat of the low-pressure compressor from 0.1? to 21? will cause the system power consumption to rise by more than 9%,the compressor discharge temperature to rise by more than 23%,and the system COP to drop by more than 10%.The system's return air superheat should be controlled at 5?8?.Generally speaking,the lower the cooling temperature,the higher the efficiency of the compressor with evaporation temperature,and the reasonable supercooling degree of the main liquid passage and the suction superheating degree of the high and low pressure compressor can enhance the COP of the system on the premise of ensuring the smooth operation of the CO2 transcritical two-stage compression refrigeration system.By comparing and analyzing some of the data in this paper with those obtained by predecessors,it is found that the data in this paper have high accuracy.