| For the liquid refrigerant defrosting system(LRDS),it refers to the method of defrosting the frosting air cooler with the liquid refrigerant in the high-pressure liquid reservoir as the heat source.Refrigerating and defrosting process are at the same time for LRDS,so this can reduce the fluctuation of storage temperature and ensure the quality of food storage.The liquid refrigerant can get subcooled from the cooling capacity of frost layer,which can improve the efficiency of refrigeration system.During defrosting,the temperature of the liquid refrigerant is not high and the damage to the air cooler is small.LRDS is partial defrosting method,which can increase the defrosting frequency,reduce the fin spacing and the volume of the air cooler and decrease the amount of refrigerant charging.However,the control process of the system is complex,and there is no complete control theory.Therefore,this subject explores the defrosting process under the condition of partial frosting,and hopes to further reduce the fluctuation of cold storage temperature by optimizing the structure and control process of the refrigeration equipment.Only by doing so,can the LRDS expand the application scope and accelerate the promotion.In the simulation stage,a complete refrigeration process(frosting process)model and a defrosting process model were established.Refrigeration process model includes: compressor model,condenser model,thermal expansion valve model,air cooler model and pipe model.The defrosting process model is divided into preheating stage,defrosting stage,defrosting water gasification stage and dry heating stage.The relationship between frosting mass and frosting thickness with time is obtained through the simulation.The characteristics of four stages of defrosting process were analyzed and reasonable control scheme was developed to provide theoretical basis for experimental research.During the preparation stage of the experiment,the total heat transfer coefficient of the cold storage was measured by the leakage cooling experiment,which was 20.34129 W(m K).At the same time,the cooling capacity of the cold storage system was measured.The total thermal capacity of the cold storage system was solved first,and then the cooling capacity was calculated according to the heat balance equation.The advantage of this method is that it requires less equipment and gets continuous curve of cooling capacity.For the calculation of the total thermal capacity of the cold storage,three methods are proposed.Study on device structure optimization.First of all,the air cooler is transformed.Heat insulation cotton is added to the outer cladding board of the air cooler.The outlet of the air cooler is equipped with louver,and the air return outlet is equipped with air return cover and electric air valve.During defrosting,the fan closes,louver is natural prolapse,and stop the electric wind valve at the same time,so that the air cooler can form enclosed space,quicken defrosting rate,and reduce the influence of the heat to cold storage temperature.After defrosting,the fan opens,the louver is blown up,and the electric air valve starts.This subject also studies the distributor and header by experiments,to see which is more suitable for the LRDS.The results showed that when the cold storage temperature was-5℃,using the distributor,defrosting time was 11.7min and the maximum temperature rise was 5.5℃.However,using the header,the defrosting time was 9.6min and the maximum temperature rise was 4℃.Although the distributor can improve the heat transfer performance,its throttling effect will reduce the defrosting refrigerant pressure and temperature.In a nutshell,the header is better than the distributor for LRDS.Study on control process.This subject mainly carries on the control research about the defrosting beginning,defrosting ending,the time difference of two air coolers putting into operation and drainage time(fan delay opening).(1)The wind pressure difference control is adopted at the beginning of defrosting.When the difference of the wind pressure is less than 50 Pa,the defrosting process starts to be on.For the LRDS under the condition of partial defrosting,the specific value of the air pressure difference should be set according to the actual requirements.(2)Returning air temperature control is adopted after defrosting.The defrosting experiment was carried out at the environmental temperature was 22℃ and the frost mass was 3kg.The results showed that when the cold storage temperature were 0℃,5℃,10℃,15℃,20℃ and 25℃,the returning air temperature were 3.0℃,4.0℃ and 5.0℃,6.4℃,7.4℃ and 10℃ after defrosting.(3)In order to prevent the air coolers forming the same frost thickness,leading to the control disorder of the defrosting process,it is necessary to stagger the first defrosting time.Manual control is used for the first time.After the two air coolers alternate refrigeration and defrosting,the wind pressure difference defrosting can be used.(4)The defrosting experiment was carried out at the environmental temperature was 22℃ and the cold storage temperature was-25℃.The results showed that the fan delay opening,after defrosting,could prevent the temperature rising rapidly.When the inside temperature of the air cooler dropped equal to the cold storage temperature,it is the best time to open the fan.Therefore,the fluctuation of the cold storage temperature was little.In frost mass of 1kg,2kg and 3kg respectively,the rise value of cold storage temperature decreased from 6.8 ℃,6.1 ℃ and 6.3 ℃ to 3.9 ℃,4.2 ℃ and 5.0 ℃ respectively.For fan delay opening,the returning air temperature of the compressor is higher than the air cooler,with a certain degree of superheat to prevent wet compression of the compressor.Study on application range.When the defrosting experiment was carried out at the cold storage temperature was-5℃,-15℃ and-20℃ respectively,the cold storage temperature rose 0.7℃,2.1℃ and 4.6℃ respectively.With the decrease of temperature,the cooling effect of a single cold air becomes worse during defrosting,and it is difficult to maintain the cold storage temperature.However,the lower cold storage temperature is,the less impact of temperature fluctuations on the food is.Therefore,the LRDS can be applied to the constant temperature and humidity machine under the condition of air conditioning and used in small cold storage system(high temperature cold storage and low temperature cold storage).It can also be used in quick freezing machine that requires continuous refrigeration. |
PDF Full Text Request