Study On The Performance Of R134a-CO₂ Cascade Cycle Refrigeration Equipment

A company in Guilin received an order from a logistics park to build a multi-temperature cold storage,which needs to meet the needs of different storage temperatures.At present,the refrigerants of refrigeration equipment are mostly CFCs,HCFCs and their compounds.Excessive emissions will cause ozone holes and accelerate global warming.The CO2 cascade cycle refrigeration system can not only meet the needs of multi-storage temperature,but also be more energy-saving and environmentally friendly.In this paper,the performance study of CO2 complex cycle refrigeration equipment is carried out mainly through thermodynamic theoretical analysis,numerical simulation analysis and comparative analysis of experimental test data collection.Firstly,a thermodynamic analysis model was established to compare the thermodynamic analysis of four different hightemperature grade work gases to compare the effects of changes in evaporating temperature and condensing temperature of different high-temperature grade work gases on system cooling capacity,mass flow ratio,COP and compressor refrigerant gas outlet temperature in order to determine the best high-temperature grade work gases;Secondly,computational fluid dynamics software(CFD)is used to establish a three-dimensional model of a single refrigerant pipe,a horizontal shell-and-tube condenser and a low-temperature cold storage,to analyze the intrinsic frequency,vibration pattern and maximum deformation of the refrigerant pipe by the influence of pipe material,fluid pressure inside the pipe and fluid-solid coupling,to increase the influence of the folded plate on the heat transfer performance of the horizontal shell-andtube condenser and the temperature and velocity fields inside the cold storage system under stable operation.The effect of the folding plate on the heat transfer performance of the horizontal shell and tube condenser and the temperature and velocity fields inside the cold storage system under stable operation;Again,the proposed construction site of the supermarket was measured in the field and the overall design of the cold storage and equipment selection was carried out to complete the construction of the cold storage;finally,the completed cold storage of the supermarket was tested and the experimental test data collection was carried out,and the collected data were compared with the simulation results to verify the performance of the R134a-CO2 compound cycle refrigeration equipment.The following conclusions were drawn:(1)Thermodynamic analysis results show that under the same refrigeration capacity NH3CO2 and R134a-CO2 stacked cycle refrigeration system refrigerant charge less and COP and unit cooling capacity is larger,its performance is better than the other two refrigerant,but taking into account the NH3-CO2 high temperature compressor exhaust temperature,while NH3 has a strong toxicity(B2),and this set of CO2 stacked cycle refrigeration system is for the use of supermarkets,once the NH3 leakage will cause greater harm,so the best hightemperature level of the system to choose R134a refrigeration refrigerant.(2)The results of numerical simulation show that the intrinsic frequency and maximum deformation of the refrigerant pipe will be affected by different pipes,different pressures and fluid-solid coupling,and the intrinsic frequencies of the modal analysis of the three cases are greater than the working frequency of the refrigerant pipe and the difference is large,so no resonance will occur;The horizontal shell and tube condenser shell process can effectively improve the fluid distribution after adding the folding plate,so that the condensation effect can be increased by 60%;the temperature inside the cold storage reaches-31℃ after the system runs for 10min,which meets the temperature requirement of low temperature cold storage,and the internal temperature is evenly distributed and kept stable,the average wind speed in the central area of the cold storage is 0.37m/s,which is smaller to facilitate food storage and reduce dry consumption.(3)Data acquisition comparison results show that the temperature at the measurement point inside the cold storage and the numerical simulation of the temperature change trend is similar,in the same measurement point,the measured value and the simulated value only exist less than 10%of the smaller error,indicating that the numerical simulation results are more accurate,the temperature inside the library to meet the requirements of low-temperature storage temperature;The increase in evaporating temperature of R134a-CO2 compound cycle refrigeration system will increase the COP value and the increase in condensing temperature will decrease the COP value;the refrigeration capacity meets the design requirements.(4)From the above theoretical analysis,numerical simulation analysis and actual data collection comparison analysis,it can be concluded that R134a-CO2 complex cycle refrigeration equipment has good feasibility and can be used as a supermarket multi-store temperature cold storage refrigeration system.