| The successful operation of the refrigeration system is largely at the cost of energy,and refrigerants always have more or less problems due to various reasons,so some measures need to be taken to improve the efficiency of the refrigeration system and reduce the use of refrigerants.In this paper,the indirect cooling refrigeration system is combined with gravity recirculation for liquid supply,so that the system can not only reduce the amount of refrigerants,but also make the evaporator supply more liquid and increase the heat transfer coefficient of the evaporator,so as to achieve a good heat transfer effect.Before the experiment,the recirculation evaporator suitable for indirect cooling refrigeration system is designed and calculated.The designed evaporator is submerged serpentine tube evaporator.The calculated results are as follows:the outer diameter of evaporation tube is 12mm,the number of evaporation tube is 19,the box body is600mm×600mm×600mm,the row number of evaporation tube is 4,the outer diameter of collector tube is 25mm,and the heat exchange area of the evaporator is 1.51.Then,according to the structural parameters of the evaporator,MATLAB software is used to simulate the calculation of the liquid supply height to build an experimental bench.The calculation of the liquid supply height is mainly based on the inequalities caused by the frictional resistance pressure drop,acceleration pressure drop,gravity pressure drop and local resistance pressure drop caused by the evaporation tube and pipeline connected by the evaporator and the gas-liquid separator,the liquid supply height is 1800mm.The experimental conditions are as follows:the frequency of compressor is 50Hz,the condensation temperature is 30℃,the volume flow rate of secondary refrigerant is 1250L/h,the storage temperature is-12℃,-9℃,-6℃,-3℃,0℃,3℃and 6℃,and the frequency of air cooler is 20Hz,30Hz,40Hz and 50Hz.In this paper,the effects of different storage temperature and the frequency of air cooler on cooling capacity,the heat transfer coefficient of evaporator,the heat transfer coefficient of air cooler,the heat transfer temperature difference of air cooler,system total power consumption and system COP in the refrigerant recirculation indirect cooling system are studied.In addition,the cooling capacity and COP of refrigerant recycling indirect cooling refrigeration system under different working conditions are simulated to verify the accuracy of the simulation model.After that,the changes of the two liquid supply modes of gravity recirculation and direct expansion in the indirect cooling refrigeration system under different working conditions are compared and studied.The experimental results are as follows:(1)With the increase of the storage temperature,the heat transfer coefficient,heat transfer temperature difference of air cooler and the cooling capacity of refrigerant recirculation indirect cooling refrigeration system with the same frequency of air cooler all increase.When the storage temperature increases from the lowest temperature of-12℃to the highest temperature of 6℃,the average heat transfer coefficient of the air cooler increases by 15.87W/(m~2·K),while the heat transfer temperature difference of the air cooler is relatively small,all below 3℃.When the storage temperature decreases by 18℃,the heat transfer temperature difference of air cooler decreases by 0.67℃on average.(2)When the frequency of air cooler is maintained at a certain value,as the storage temperature increases,the heat transfer coefficient of evaporator in the refrigerant recirculation indirect cooling refrigeration system will show a downward trend.When the storage temperature changes from-12℃to 6℃,the heat transfer coefficient of evaporator decreases by 99.36W/(m~2·K)on average.(3)As the storage temperature increases,the total power consumption of the refrigerant recirculation indirect cooling refrigeration system will increase accordingly,and the total power consumption will increase by approximately 23.72W when the storage temperature increases by 1℃.However,its increase is small relative to the increase in cooling capacity,which causes the COP of the system to increase as the storage temperature increases.When the storage temperature increases from-12℃to 6℃and the frequency of air cooler are 50Hz,40Hz,30Hz and 20Hz,the system COP increased by 0.78,0.82,0.84and 0.87 respectively.(4)When the storage temperature remains constant,the heat transfer coefficient of the air cooler increases with the increase of the frequency of air cooler,but the increase gradually decreases in the refrigerant recycling indirect cooling refrigeration system.When the frequency of air cooler is 30Hz,40Hz and 50Hz,the heat transfer coefficient of the air cooler is increased by 1.85%,1.82%,and 1.25%,respectively,compared with the heat transfer coefficient of the air cooler when the frequency of air cooler is 20Hz,30Hz and40Hz.The heat transfer temperature difference of air cooler in the system decreases with the increase of the frequency of air cooler,and the larger the frequency of air cooler,the greater the decrease.Because the heat exchange area of the air cooler is fixed,the cooling capacity of the system increases at first and then decreases with the increase of the frequency of air cooler.When the frequency of air cooler is 30Hz,the cooling capacity reaches the maximum,and the maximum value can reach 5134.77W,which provides a certain reference value for the same refrigeration system when designing the air cooler.(5)The total power consumption of the system will increase with the increase of the frequency of air cooler,and the system COP first increases and then decreases with the increase of the frequency of air cooler under the combined action of cooling capacity and total power consumption.When the system COP reaches the maximum,the frequency of air cooler is 30Hz,and the corresponding system COP is 4.47%,10.16%and 27.05%higher than the system COP corresponding to the frequency of air cooler of 20Hz,40Hz and 50Hz,respectively.(6)A simulation study on the cooling capacity and system COP of the refrigerant recirculation indirect cooling refrigeration system under the set operating conditions.The relative errors between the experimental values and simulated values are within±10%and±15%,respectively.The average errors are 2.37%and 2.78%,respectively,and the standard errors are 0.11%and 0.30%,respectively.It can be seen that the simulation model established in this paper meets its accuracy requirements and provides a certain degree of theoretical basis for the performance research of the refrigerant recirculation indirect cooling refrigeration system in the future.(7)With the increase of the storage temperature,the cooling capacity and total power consumption of indirect cooling system with gravity recirculation and direct expansion increase.And the cooling capacity and total power consumption of the system with gravity recirculation are larger than those of the direct expansion system,and the lower the storage temperature,the more obvious the difference between them.(8)For any frequency of air cooler,the system COP of the refrigerant recirculation indirect cooling refrigeration system and the direct expansion indirect cooling refrigeration system is positively correlated with the storage temperature.The system COP of the refrigerant recirculation indirect cooling refrigeration system is significantly higher than that of the direct expansion refrigeration system.When the storage temperature is lower,the difference between them is more obvious.When the storage temperature is 6°C,the system COP of the two liquid supply modes is almost equal.When the frequency of air cooler is50Hz,the system COP of the indirect cooling refrigeration system under the gravity recirculation liquid supply mode is 74.55%,10.10%,7.41%,4.80%,4.20%,2.50%and2.35%higher than the COP of the direct expansion liquid supply mode.This shows that the gravity recirculation liquid supply method at low temperature has more advantages than the direct expansion liquid supply method in the indirect cooling refrigeration system. |
