The Investigation On The Performance Of Frozen Equipment Using Sub-critical CO₂ Low Temperature Cascade Cycle

The development of agricultural industrialization has accelerated the rapid development of the refrigeration and refrigeration industry.As the initial link in the cold chain logistics,cold storage has a direct impact on the quality of food storage.In China,most of the cold storages were built in the early stage of reform and opening up,and there are widespread problems of aging equipment,poor cooling performance,and high power consumption.In recent years,environmental protection and energy conservation and emission reduction have become the general direction of the development of the refrigeration industry.The cold storage refrigeration system is facing a new generation.At present,China's CO₂ cold storage is developing rapidly.The CO₂ cascade refrigeration system can provide-40°C temperature to store foods.It is suitable for most cold storages,especially low temperature storage,which not only saves energy but also is more environmentally friendly.In this paper,the performance of sub-critical CO₂ low temperature cascade refrigeration equipment is studied by thermodynamic analysis,numerical simulation and experimental research.Established a thermodynamic analysis model to compare the performance of three different low-temperature cascade refrigeration systems using CO₂natural working fluids,and analyze the influence of parameters such as intermediate condensation temperature and evaporation temperature on system COP,power consumption,exergy loss,enthalpy efficiency and heat exchanger area.The CFD simulation software was used to establish the 3D model of the cold storage,and the temperature distribution and velocity distribution under the normal working of the cold storage were analyzed.The feasibility of the cascade refrigeration system applied to the low temperature freezing warehouse was obtained by research.Aiming at the frosting phenomenon inside the cold storage,a hot-gas defrosting method in which the double evaporators are alternately operated was proposed.The influence of the defrosting method on the temperature field of the cold storage was studied through the previously established cold storage model.Finally,the experimental platform of the frozen equipment using sub-critical CO₂ low temperature cascade cycle was designed,and the experimental data were obtained by changing the parameters of the experimental conditions.The performance of the frozen equipment using sub-critical CO₂ low temperature cascade cycle was analyzed by comparing the experimental data with the simulation results.The main conclusions of this paper as follow:?1?The results of thermodynamic analysis show that the intermediate condensation temperature and evaporation temperature are the key factors affecting the performance of the system.There is an optimal intermediate condensation temperature in the cascade system,and the COP increases with the increase of evaporation temperature.The COP of the R290/CO₂ and R32/CO₂ cascade refrigeration systems is superior to the R404A/CO₂cascade refrigeration system.The enthalpy loss of the cascade system increases with the increase of the evaporation temperature and results in the enthalpy efficiency decreases and the heat exchanger area also decreases.At the evaporation temperature of-40°C,the power consumption of the R404A/CO₂ system is 6.35%and 5.37%,which are higher than that of the R290/CO₂ and R32/CO₂ systems,respectively.The R290/CO₂ system has a exergy efficiency of 0.416,which is higher than that of the R32/CO₂ system?0.411?and the R404A/CO₂ system?0.372?.The comparison shows that R290 is the best high temperature cycle refrigerant of CO₂ cascade refrigeration system among the three refrigerants.?2?The numerical simulation results show that the cold storage quickly reaches the normal temperature of the low temperature storage after the system works,the temperature distribution inside the storage is uniform,and the wind speed in the central area of the stored food is small,which is beneficial to reduce the dry consumption of food.The simulation results in the defrost process proves that the alternating hot-gas defrost process of the double evaporators had little effect on the temperature field in the cold storage,and the temperature distribution in the central area of the storage was still uniform.?3?Results from experimental data analysis show that the system COP increases with the increase of evaporation temperature and decreases with the increase of condensation temperature.The CO₂ compressor has low efficiency,which causes the actual performance of the system to have a certain gap with the simulation results.The actual temperature distribution in the cold storage is consistent with the simulated temperature distribution.The experimental results of the defrosting process showed that the COP and cooling capacity of the system decreased by 14.52%and 17.90%,respectively,due to defrost using part of the refrigerant gas.However,during the defrost process,the temperature of the cold storage rises within an acceptable range compared to the electric frosting process.?4?From the above theoretical analysis and experimental research,it can be concluded that the frozen equipment using sub-critical CO₂ low temperature cascade cycle has good feasibility,and the CO₂ natural low temperature cascade system can be used as a cold storage refrigeration system.