| In the process of the design refrigeration system,taking into account the compression ratio and system operating efficiency,the evaporate temperature of single-stage steam compression refrigeration cycle generally was above-30 degree.If the compression ratio continued to rise,the relative clearance volume of the compressor increased in the single-stage compression refrigeration system,the gas transmission coefficient would drop sharply,resulting in a significant reduction in the energy efficiency ratio of the system.To solve the problem of the operation of the refrigeration system under the large compression ratio,the researchers gave two sets of technical solutions.One was the use of two-stage compression refrigeration system,and the other was the use of cascade refrigeration system.Both of these schemes could effectively solve the problem of the efficiency reduction caused by the large compression ratio.From the current research situation,the two-stage compression refrigeration system still had its shortcomings in the application of technology,which could not solve the capacity problem of one machine two-stage compression refrigeration system and the oil return problem of the match group type two-stage compressor.The cascade refrigeration system consisted of two single-stage compressor refrigeration systems,the compressor independently operated,and solved the two issues of the two-stage compression refrigeration system effectively.In this paper,the cascade refrigeration system of the cold storage system was used as the research object,and carried out the research on the problem of the inter-stage matching.At present,in the domestic and foreign refrigeration system,the cascade system mostly used in fixed-frequency compressor,high and low temperature system flow ratio was fixed,generally according to the conditions and refrigerant characteristics to choose the best flow ratio.In the actual environment,the situation was transformed at any time,and the evaporation temperature would change according to different needs,even in some special cases,the refrigerant would be replaced,to maintain the best operating state of the system need to change the system flow ratio;In addition,the fixed-frequency compression refrigeration system usually through the shutdown to achieve a constant temperature,resulting in compressor starting and stopping frequently,and power consumption increased,reduced the life of the refrigeration system,on the other hand start-stop refrigeration would cause cold storage temperature large fluctuations,and caused serious food consumption.Aiming at the shortcomings of the current cold storage problem,this paper proposed a variable flow cascade refrigeration system.The high and low temperature stage adopted rotor type inverter compressor.Under the same working condition,by adjusting the throttle opening and the compressor frequency,changed the high and low temperature suction pressure,so as to find the best condensing evaporator pressure,reduced the condensing evaporator heat transfer temperature difference,and reduced the cold storage temperature fluctuations.Ultimately the paper achieved efficient operation of refrigeration systems and reduced the role of food dryness.In theory,the optimization of the cascade refrigeration system as the aim,the relationship between the high and low temperature flow ratio and the refrigeration coefficient COP at the optimum intermediate temperature of R410 A single refrigerant cascade refrigeration cycle was discussed theoretically.The results of theoretical analysis showed that: When the low temperature flow ratio increased,the intermediate temperature increases,the COP increased first and then decreased,and there was an optimal flow ratio to maximize the COP value;the effect of working temperature on COP would be greater than the influence of flow rate change.When the ambient temperature increased or need to reduce the temperature of the cold,increased the low temperature level flow.When adjust to the high temperature class flow to ensure that the cascade refrigeration system was at the best high Low temperature class flow ratio.On the basis of the theoretical analysis,the cascade refrigeration experiment system was designed and built.The system consisted of DC variable frequency rotor R410 A compressor,air-cooled condenser,brazed plate heat exchanger and electronic expansion valve.Testing the cooling capacity of the cascade refrigeration system was applied the heat balance method.A variable refrigerant flow refrigeration system is designed to perform a separate compressor refrigeration cycle and to switch to a cascade compression refrigeration cycle to meet the operational testing of different systems.Through theoretical modeling and experimental verification,this paper summarized the relationship between the refrigerant flow rates at high and low temperature system and the optimum temperature relationship of condensing evaporator,and the influence of different operating conditions on the system energy efficiency,when the system changed with the load and the working condition.The results showed that the higher the evaporation temperature,the lower the condensing temperature,and the smaller the temperature differences of the stack heat transfer,the smaller the suction heat of the compressor,the greater the degree of condensation undercooling,the greater the system’s COP.The main factors that affected the performance of the system were the evaporative temperature and the low temperature level system.The R410 A cascade refrigeration cycle system was more advantageous than the R410 A two-stage compression system in safe use,environmental protection and energy savings when providing low temperatures below-30 ℃.The influence of evaporation temperature and condensation temperature on the cascade refrigeration system was verified by the experimental data.The evaporation superheat degree of R410 A was controlled at 4 ~ 6 ℃,which was beneficial to the improvement of system performance. |
