Study On R134a/R410A Variable Refrigerant Flow Cascade Refrigeration System

Cascade refrigeration system can avoid the limitation of pressure ratio,uneven oil returning,excessive cylinder volume,and other problems compared with single-stage and two-stage compression refrigeration systems.It also reduces adverse effects of evaporation pressure on the system.There are few studies in refrigerating field in the current research of cascade refrigeration system.Especially,there are many studies in production of ultra-low temperature environment.Therefore,small-scale cascade refrigeration system applied in refrigerating field has great development space.In this paper,a small-scale variable refrigerant flow cascade refrigeration system is designed and studied.It consumes less energy and does not need expansion containers,so its structure is simpler and more compact.The using of variable frequency compressor greatly improves system performance.These excellent characteristics provide a reference for expanding the application range of cascade refrigeration system.This subject is mainly carried out according to the steps of simulation analysis,calculation and selection,experimental platform construction and experimental research.The designing of R134a/R410A variable refrigerant flow cascade refrigeration system adopts fixed frequency compressor and throttling by capillary in high-temperature,and variable frequency compressor and throttling by electronic expansion valve（EEV）were used in low-temperature.The effects of evaporation temperature（T₀）,condensation temperature（T_k）,intermediate temperature（T_m）,displacement ratio（Q_v）and heat transfer temperature difference（△T）on COP of system performance coefficient were studied.And the relationship between these parameters was also studied through theoretical analysis of thermal performance of each stage of the system cycle.The results show that elevated evaporation temperature or reduced condensation temperature will increase COP and decrease displacement ratio.COP increases first and then decreases with the increasing of intermediate temperature.Optimum intermediate temperature makes the system most efficient.The larger heat transfer temperature difference is,the greater theoretical energy consumption is and COP will be smaller.In addition,main components and auxiliary equipment are designed,calculated and selected in turn.The use of electrical control systems and data measuring instruments are also described in process of building the experimental platform.The influence of evaporation temperature,compressor frequency and displacement ratio on system performance is analyzed in the aspect of experimental research.Maintaining condensation temperature at 40 ^oC,evaporation temperature at-34 ^oC to-30 ^oC,and low-temperature compressor frequency at 50 to 65 Hz.Different parameters were adjusted in order to study the trend of system energy consumption and efficiency.Within the temperature range studied,COP increased by 0.3%on average for every 1 ^oC increase in evaporation temperature.Energy consumption of low-temperature decreases by25.1%and it decreases by 23.2%only when the temperature changes from-34 ^oC to-32 ^oC.With the increase of frequency,COP increases first and then decreases.COP reaches a maximum value of 1.43 When low-temperature compressor frequency is 59 Hz and EEV opening reaches 85%.The displacement ratio increased,and the growth rate increased from12.8%to 14.4%.With the increase of displacement ratio,COP increases first and then decreases.It can also be seen from analysis that the influence of opening of EEV on displacement ratio is positively correlated with compressor frequency.To sum up,the R134a/R410A variable flow cascade refrigeration system is compact in structure and low in cost.It also has high refrigeration efficiency even in a severe environment（condensation temperature is 40 ^oC）.The experimental results show that there is an optimal low-temperature frequency to maximize the system efficiency,which shows that the use of frequency conversion technology can improve system efficiency to a greater extent.Comparing with simulation analysis,it can be seen that in actual operation state,low-temperature needs a larger refrigerant flow in order to achieve higher efficiency.The lower evaporation temperature is,the higher energy consumption of low-temperature is.Therefore,the unit has higher efficiency and better energy saving effect when applied in the field above-32 ^oC.At the same time,improving low-temperature power consumption is also the key to improve the performance of the cascade refrigeration unit.