Study On The Effect Of The Condensation Evaporator On The Performance Of The Cascade Refrigeration System

The thesis mainly focuses on R404A/CO2 cascade refrigeration cycle system. Firstly,the research progress on cascade refrigeration cycle system is introduced, especially on NH3/CO2 and R290/CO2 cascade refrigeration cycle system. Due to its good heat transfer performance and environmental friendly, the cascade refrigeration cycle system using CO2 as low-temperature working medium have been attracted more and more attention in refrigeration and air-conditioning industry, and have more and more practical application cases.Secondly, R404A/CO2 cascade refrigeration cycle system is analyzed in thermodynamic; the main points of its theoretical analysis and comparison are evaporating temperature of CO2, condensing temperature of R404 A, cascade temperature difference(heat-transfer temperature difference of condensing evaporator), liquid-suction heat exchanger, discharge gas cooler etc. The results show that, at certain working conditions,the higher evaporating temperature, the lower condensing temperature, the smaller cascade temperature difference, the better the coefficient of performance of the system; By the use of liquid-suction heat exchanger, the coefficient of performance of R404 A high-temperature cycle is increased and that of CO2 low-temperature cycle is decreased, what is more,compared to the coefficient of performance of the system without using liquid-suction heat exchanger simultaneously in high- and low-temperature cycle, the lower condensing temperature of CO2, the better that of the system with using liquid-suction heat exchanger simultaneously in high- and low-temperature cycle; With the help of discharge gas cooler in CO2 low-temperature cycle, the heat load of condensing evaporator is reduced, thus the mass flow rate(or charges) of refrigerant, power consumption of compressor and the heat load of condenser in R404 A high-temperature cycle are reduced.Thirdly, influences of heat-transfer temperature difference, degree of supercooling and superheat, pressure drop within the condensing evaporator on the COP of R404A/CO2 cascade refrigeration cycle system are analyzed theoretically. It is found that, at definite working conditions, the smaller heat-transfer temperature, the greater the outlet superheat degree of R404 A and the outlet supercooling degree of CO2, the lower pressure drop of R404 A and CO2 side, the better the COP of the system.Fourthly, the cascade refrigeration system experiment device, which consisted of R404 A and CO2 compressor, condenser, condensing evaporator, freezer, cooling water tank,oil separator and throttling components etc., as well as temperature, pressure and flow ratemeasure and data acquisition system, was designed and constructed. Installation and debugging, safety protection and operation steps of experiment device for leak detection,vacuum, charging refrigerant was carried out, which provide conditions for subsequent experiment.Finally, effects of heat-transfer temperature, the outlet superheat degree of R404 A, the outlet supercooling degree of CO2, the lower pressure drop of R404 A and CO2 side within the condensing evaporator on the COP of R404A/CO2 cascade refrigeration system are analyzed by using experimental data. It is concluded that, at different working conditions,the lower heat-transfer temperature difference and the outlet superheat degree of R404 A,the higher the inlet temperature of CO2(Accordingly, the higher the evaporating temperature of CO2) in the condensing evaporator, the better the COP of the system which was measured in the experiment.