Theoretical Analysis And Experimental Research Of R404A/R23 Cascade Refrigeration System

Since the 21 st century,the development of our country entered the fast traffic lane,with the rapid development of industrial technology,refrigeration industry also closely follow the development of industrial technology.The application of refrigeration technology involves many industries,such as medical and health care,industrial manufacturing,food,scientific research,etc.Different industries have different temperature requirements.In order to meet the low temperature requirements of the industry,the cascade refrigeration system and doublestage compression refrigeration system which can produce lower temperature are proposed.The comparison of the two shows that the cascade refrigeration system has more advantages when the production temperature is-50 ℃ or below.With the enhancement of the concept of environmental protection at home and abroad,higher requirements are put forward for refrigerants,and emphasis is placed on the development and use of environmental protection and natural refrigerants.Freon(ODP is not zero)and high GWP refrigerants will gradually be replaced.Therefore,this topic selects R404A/R23 cascade refrigeration system as the research object,through the combination of theoretical and experimental research,analyzes the influence of system parameters in different working conditions on the refrigeration system and the operation of the unit under ultra-low temperature refrigeration temperature.In the theoretical study,mathematical models were established for each part of the system first.Exergy analysis was carried out to investigate the impact of system parameters(evaporation temperature,condensation temperature,intermediate temperature,heat transfer temperature difference of evaporation condenser,superheat and supercooling)on the cascade refrigeration system through thermal analysis and exergy analysis.In order to improve the thermal performance,a method of adding regenerator was put forward.In the experimental study,the influence of evaporation temperature and condensation temperature on exhaust temperature,compression ratio and refrigeration coefficient COP was analyzed by changing evaporation temperature and condensation temperature.And set ultra-low temperature(-60℃,-70℃),research and analysis unit at ultra-low temperature,refrigeration temperature accuracy and stability.Both the decrease of evaporation temperature and the increase of condensation temperature lead to higher energy consumption and exergic loss,while COP and exergic efficiency decrease,and the effect of evaporation temperature on thermal performance of the system is more significant.As the intermediate temperature increased,the total power consumption of the system and exergic loss decreased first and then increased,There is a minimum value where exergy efficiency of COP and exergic are optimal.Therefore,empirical formulas of optimal intermediate temperature and optimal COP(which are mainly affected by evaporation temperature,condensation temperature and intermediate heat transfer temperature difference)are fitted by mathematical means.The empirical formula error of the optimal intermediate temperature is less than 10%,which can provide effective data reference for the actual operating intermediate temperature of the unit.By studying the mass flow rate of refrigerant,it is concluded that the mass flow rate of high and low temperature refrigerant increases with the increase of intermediate temperature,and the increase range of high temperature refrigerant is more significant.The mass flow ratio of high and low refrigerant decreases with the increase of intermediate temperature,increases with the decrease of evaporation temperature,and increases with the increase of condensation temperature.Therefore,in order to reduce the mass flow ratio of high and low temperature,the condensation temperature should be reduced as far as possible and the evaporation temperature should be increased.Exergic loss of exergy is mainly concentrated in evaporative condenser,high temperature compression and high temperature throttling parts,which should be the main optimization direction to reduce irreversible exergic loss.Exergic efficiency had a negative impact on the thermal performance of the system and exergic efficiency,but exergic could prevent the compressor from inputting liquid(protecting the unit),so there should be an appropriate exergic in the design and use.Increasing exergic cooling can effectively increase thermal performance and exergic efficiency.To meet the need for overheating and increase exergic efficiency at the same time,an exergic regenerator is proposed,which is installed at high and low temperature levels.As the superheat increased(as did the supercooling),Exergic efficiency and COP of heat-recovery system increased at high temperature level while Exergic efficiency and COP of heat-recovery system decreased at low temperature level.Therefore,a regenerative device is only suitable for the stacked refrigeration system at high temperature level(R404A).In order to study the performance of the stack refrigeration unit,it takes 27 min for the unit to reach the target temperature(-60℃)and 57 min for the unit to reach the target temperature(-70℃)under the setting of ultra-low temperature.The inlet and outlet pressures of the lowtemperature compressor are stable at 1.22 bar and 11.1bar on average.The refrigeration temperature is basically stable at the set temperature;The thermal conductivity oil was connected with the heat sink equipment in series,and the evaporation temperature was set as-30℃ and-60℃ respectively.The results showed that the refrigeration effect of the unit was good,and the temperature fluctuation was less than 1℃,which fully met the temperature requirements of the equipment.cascade unit is good.