The Study Of Compressors Displacement Ratio In Cascade Refrigeration System

As the single-stage vapor compression is limited by the compression ratio and some other factors,the evaporation temperature is generally higher than-30?;when we need lower temperature we need to use two-stage compression or cascade refrigeration system,in order to overcome a series of problems caused by the high compression ratio such as the high exhaust temperature,decrease of volume coefficient and the volume efficiency,decline of coefficient of performance.Therefore,the two-stage compression system and cascade refrigeration system have a good prospect in low temperature refrigeration.The main contents of this paper are as follows:First,compared with the present situation of two-stage compression and cascade refrigeration system,it can be seen that fixed frequency compressor is widely used in the two-stage compression and cascade cycle refrigeration system thus the displacement is constant.When the system operating conditions change,off design conditions,it will reduce the system coefficient of performance;at the same time in the system composed by fixed frequency compressor,the system target temperature usually maintained by the on-off of the compressors,resulting in frequent start which will increased power consumption and shorten the service life of the system.In the view of these problems,we conclude the conclusion that the cascade cycle system has some advantages by comparing the performance of coefficient and the system structure between the four kinds of two-stage compression refrigeration system and the cascade cycle refrigeration system.Second,the theoretical analysis of the cascade refrigeration cycle show that: When the working condition is constant,as the displacement ratio(the ratio of the compressor inlet volume flow rates between the high temperature cycle and the low temperature cycle)increase the intermediate temperature increases,while the system COP increases first and then decreases,we can find an optimal system displacement ratio that makes COP the maximum;when the displacement ratio is kept constant,the intermediate temperature decreases with the decrease of the evaporation temperature of the system.With the increase of the evaporation temperature of the system,the optimal displacement ratio corresponding to the maximum COP increases gradually.The evaporation temperature changes 10 ?,and the corresponding optimal displacement ratio ranges about 0.1.Third,through theoretical analysis,we designed a variable flow two-stage compression system and cascade cycle experimental system,R410 A refrigerant will be used in the high and low temperature systems;thus we can turn the cascade system to the two-stage compression system through the opening of the pipeline valve.The experimental system chooses two invert scroll compressors with electronic expansion valve,through the PLC and compressor driver to control the operation of the system this will adjust the inlet volume flow to make the system COP in the optimal state.Fourth,the experiment select evaporation temperature-40 ? ~-45 ?,the condensation temperature is 30 ?,in the process of the experiment,we keep the frequency of the low temperature compressor in 60 Hz constant,then we adjust the frequency of the high temperature compressor for 3Hz each time,we can record the measurement data after the operation of the system be stable for at least 15 minutes.The refrigerating capacity can be measured by the heat balance method,and the coefficient of performance of the system under different working conditions is calculated.Based on the theoretical analysis,system design and experimental study of the cascade refrigeration cycle,the experimental system can improve the refrigeration efficiency and reduce the temperature fluctuation.At the same time,it can provide theoretical and data basis for the research between cascade cycle and two-stage compression system.