Study On Performance Of Two-Stage Compression And Cascade Compression System With Single Working Fluid

In the food refrigeration industry,when the evaporation temperature decreases,the single-stage compression refrigeration system may not be able to meet the operating conditions of high pressure ratio.At this time,cascade refrigeration system or two-stage compression refrigeration system is chosen.In practical application,the two-stage compression refrigeration system can reduce the clearance volume of compressor,reduce the compression ratio of each stage,decrease the exhaust temperature of compressor,and increase the performance and stability of the system.The cascade refrigeration system has the characteristics of wide refrigeration range,low refrigeration temperature,high reliability and simple structure,but the key problem of the two refrigeration systems is load variation.When converting,it can not be effectively regulated,but can only be achieved by starting and stopping compressors,resulting in a part of energy waste.Therefore,facing the current energy and environmental problems,this subject mainly studies the operation characteristics and performance analysis of refrigeration system in the process of load change under low temperature environment.Based on the existing experimental conditions,R410 A is used as refrigerant to study the two-stage compression refrigeration system and cascade refrigeration system with incomplete cooling in the middle of primary throttle,in which the cascade refrigeration system has high and low temperatures.R410 A is used in all grade refrigerants.In this paper,the thermodynamic mathematical model of cascade refrigeration system and two-stage compression refrigeration system is established and solved by using MATLAB mathematical modeling software.It is found that under the condition of condensation temperature 30℃ and refrigeration capacity 12 kW,evaporation temperature is-45℃,and the temperature difference is 1℃.The intermediate temperature is controlled from-11.6℃ to-0.16℃.The exhaust temperature of the high pressure compressor of the two-stage compression refrigeration system with incomplete cooling in the middle of cascade and primary throttle decreases,the mass flow rate of refrigerant increases,and the COP of the system performance coefficient presents.There is a trend of increasing first and then decreasing.There is an optimum intermediate temperature to achieve the best performance of the system.But compared with the incomplete cooling refrigeration system with one throttle,the performance parameters of cascade refrigeration system change obviously,and the optimum COP is 11.1%.Therefore,reasonable selection of intermediate temperature and refrigeration mode can achieve the goal of energy saving and highefficiency.Controlling the evaporation temperature from-35 ℃ to-45 ℃,the COP of refrigeration system decreases continuously.The COP of cascade refrigeration system decreases by 18.22%,the COP of incomplete cooling in the middle of primary throttle decreases by 20.27%.The COP of cascade refrigeration system is always higher than that of primary throttle refrigeration system and its descending speed is lower.Therefore,cascade refrigeration system has good cryogenic performance.In the aspect of experimental research,a comprehensive experimental platform is designed and built to realize the switching between cascade refrigeration system and two-stage compression refrigeration system with incomplete cooling in the middle of primary throttle.The frequency of low-pressure compressor is fixed,and the frequency of high-pressure compressor is changed to control the intermediate temperature.The operation state of the regulating system is optimal.When the condensing temperature is 30℃,the evaporation temperature is lowered(-35/-40/-45℃).The operating characteristics of the two refrigeration systems were studied by taking the temperature and the frequency increase of the high-pressure compressor(40~80Hz adjusted every 5Hz)as variables.The experimental results show that reducing evaporation temperature and increasing the frequency of high-pressure compressor can decrease the intermediate temperature and pressure of the system,increase the suction and exhaust temperature of high-pressure compressor,and increase the compression ratio,and the degree of change of cascade refrigeration system is obvious.When only reducing evaporation temperature,the COP of the system decreases continuously,and the COP of cascade refrigeration system is always higher than once.The maximum difference of COP between the two systems is 0.9 when the throttle intermediate incomplete cooling and the evaporation temperature is-35 ℃.When the evaporation temperature is-35/-40/-45℃,the COP of the system increases first and then decreases when the high-pressure compressor frequency of the high-pressure refrigeration system with incomplete cooling in the throttle intermediate is 55/60/55 Hz,the optimum intermediate temperature is 8.3℃/2℃/-3.3℃,and cascade.When the high-temperature compressor frequency of the refrigeration system is 55/55/55 Hz,the optimum intermediate temperature is-13.5℃/-15.84℃/-19.37℃.At this time,the refrigeration system achieves the maximum COP,which indicates that there exists the optimum intermediate temperature to make the system work best.From the comparison results of simulation and experiment,it is concluded that the cascade refrigeration system has the advantages of high efficiency,simple structure and stable operation under the same low temperature demand.This paper can provide theoretical and experimental basis for the performance optimization of refrigeration system to improve the use space of cascaderefrigeration system and promote the popularization of single refrigerant cascade refrigeration system.