Study On Thermal Insulation Characteristic And Refrigeration System Performance Of -100? Cold Storage

In recent years,with the development of science,the development of refrigeration technology is also changing with each passing day,and the food industry has also developed rapidly.Because people are demanding higher and higher food quality and taste,so the refrigerators used for food storage have to meet higher temperature requirements.For example,tuna,for example,is one of the three most important nutritious fish in the world recommended by the International Nutrition Organization.To make tuna into raw fish fillets and have a good taste,a cold chain of-55 C is essential from catching tuna to delivering it to people's tables for people to eat.Therefore,develop a kind of efficient and energy-saving cold storage for food like tuna has become one of the research hotspots of various scholars in recent years.However,the ultra-low temperature cold storage is facing many technical difficulties at present.How to maintain the structural stability of the heat preservation system in ultra-low temperature environment is one of the biggest problems.In this paper,a new type of thermal insulation structure for ultra-low temperature cold storage is proposed,and theoretical and experimental studies on the refrigeration system of ultra-low temperature cold storage——three-stage cascade refrigeration system are carried out.The variation of refrigeration system parameters at different evaporation temperatures is studied,and the main factors affecting the variation of parameters are explored,and we try to find the methods to improve the performance of three-stage cascade refrigeration system.In the research of the performance of thermal insulation structure,the -100?cold storage consists of the storage body,the floor of the cold storage and the transition room.The transition room is set outside the door of the cold storage to reduce the heat exchange between the cold storage and the external environment.From outside to inside,the cold storage body is composed of 200 mm thick rigid polyurethane foam sandwich board,epoxy resin layer,150 mm thick perlite and 304 stainless steel integral steel plate,the lower layer of the cold storage floor is composed of a multilayer polystyrene high density foam,and the lower layer of the cold storage floor is bonded with the upper non-woven fabric layer through the middle epoxy resin layer.When choosing the material of enclosure structure and filling steel plate and storage body,it is fully considered that rigid polyurethane foam plastics and perlite have the characteristics of low density,low thermal conductivity,moisture-proof and fire-resistant,stable chemical property,low price,non-toxic and non-polluting.The steel plate selected for the inner layer of the cold storage is an integral whole,so there is no gap,under ultra-low temperature environment,it is very difficult to shrink,which minimizes the damage of the cracks in the cold storage panels caused by the shrinkage of the insulation structure of the ordinary cold storage.In theoretical research,the mathematical model of three-stage cascade refrigeration system is established by T-S diagram.The variation of COP with evaporation temperature of medium temperature stage and high temperature stage,optimal intermediate condensation temperature,exhaust temperature of compressors at all levels,compression power of compressors at all levels,COP and thermal perfection,rake efficiency and total rake loss,and the proportion of rake loss of each part with evaporation temperature were simulated.In the aspect of experiment,the stability of the envelope structure proposed in this experiment after repeated cooling and heating is studied,and the thermal insulation performance of the envelope structure is verified by observing the cooling and heating time of the cold storage.In addition,the variation of refrigeration capacity,COP,exhaust temperature of three compressors,suction and exhaust pressure of three compressors,and power consumption of compressors with evaporation temperature for three-stage cascade refrigeration system are studied.The experimental results show that:(1)When the ambient temperature is 25? and the temperature in the cold storage is heated to 50 ?,the leakage cooling coefficient of the enclosure structure of the cold storage is 7.2W/?.After ten cooling and heating processes,the leakage cooling coefficient of the enclosure structure increases by only 2.1%.It can be seen that the cold storage envelope structure proposed in this experiment has not changed greatly during the repeated heating and cooling process.This kind of envelope structure can overcome the damage of ultra-low temperature environment to the insulation body and has good thermal insulation performance.(2)It takes 12 hours to reduce the temperature in the cold storage from ambient temperature(25 ?)to -100.2?.During the process of reducing the temperature in the cold storage from 20 ? to -100.2?,the temperature in the transition room decreases from 25? to-1.7?.This shows that the cold leakage at the inner door of the cold storage is very small,the thermal insulation performance of the enclosure structure of the cold storage is good,and the transition room can also play a good role in reducing the cold leakage in the cold storage.(3)After the temperature in the cold storage drops to -100.2?,it recovers to 20.3? after 15 hours,but it still does not completely recover to the outside environment temperature,and the temperature recovery is slower and slower.All these verify that the insulation performance of the envelope structure design method proposed in this experiment is good.(4)With the decrease of the temperature in the cold storage,the refrigerant flow rate in the refrigeration system becomes slower,which leads to the decrease of the compressor's suction capacity and the decrease of the compressor's input power.The total power consumption of the three compressors and the refrigeration capacity of the three-stage cascade refrigeration system decrease with the decrease of the temperature in the cold storage.(5)The COP of the three-stage cascade refrigeration system decreases with the decrease of the temperature in the cold storage,from 0.47 of-60? to 0.27 of -100 ?,which decreases by 42.6%.In addition,the overall COP of the three-stage cascade refrigeration system is low,and the highest COP is only 0.47 under this experimental condition.