Performance Optimization Study On Auto-cascade Refrigeration System Using Hydrocarbon Refrigerants

Compared with the traditional multi-stage compression and cascade refrigeration systems,auto-cascade refrigeration（ACR）systems can achieve low-temperatures from-40℃to-160℃using only a single compression and a set of zeotropic mixed refrigerants.ACR systems have the application advantages of simple system configuration,wide refrigeration temperature zone,stable operation and low manufacturing cost.Under the background of international policies on energy conservation and carbon reduction,some hydrocarbons（HCs）have become suitable alternatives to conventional refrigerants and have been increasingly used in small-and medium-sized ACR systems because of their natural environmental advantages of zero ozone depletion potential（ODP）and very low global warming potential（GWP）.Based on the research status of ACR systems at home and abroad,this paper introduces the working principle,the selection principle of refrigerant components,the selection of refrigerant physical properties and phase equilibrium calculation methods,and the exergy analysis theory of ACR system.In order to realize the low-temperature for different production requirements,the mathematical and thermodynamic model of the main components of ACR system were established.On this basis,Aspen Plus software was used to simulate and calculate the two-stage,three-stage and four-stage ACR systems.For the different-stage ACR systems using R134a,R23 and R14,the simulation studies of HCs refrigerant substitution and performance optimization were carried out.Based on the two-stage ACR system using R23/R134a,R170 and R1150 were used as alternative refrigerants for R23,respectively.R600 and R600a were used as alternative refrigerants for R134a,respectively.The results show that the two-stage ACR system has the best refrigerant ratio to achieve the optimal COP(COP_max).When the evaporator-outlet temperature T_e,out was lower than-45℃(the condenser-outlet temperature T_c,out was 30℃)or T_c,out was higher than 25℃(T_e,out was-50℃),the COP_max of R170/R600 was higher than that of R23/R134a,and its COP_max advantage increased with the increase of T_c,out or the decrease of T_e,out.Among the components of two-stage ACR system,the heat exchanger had the highest proportion of exergy loss,more than 50%.The exergy loss ratios of throttles and compressor were similar,about 20%,respectively.Based on the three-stage ACR system using R134a/R23/R14,R290,R600,R600a and R1270 were used as alternative refrigerants for R134a.And R170 was used as alternative refrigerants for R23.Besides,a regenerator-enhanced three-stage ACR（RACR）system was proposed.The results show that the COP,thermodynamical perfectness and exergy efficiency of the three-stage RACR system were higher than those of the three-stage ACR system,and the RACR performance advantages increased with the increase of T_e,out.Increasing the ratio of R14 significantly improved the energy efficiency of the three-stage RACR system,but led to an increase in the discharge temperature(T_dis).In order to obtain a low-temperature of-100℃,R600/R170/R14（mass fraction was 0.24/0.12/0.64）was the best alternative to R134a/R23/R14 in the water-cooled three-stage RACR system.Based on the research conclusions of two-and three-stage ACR systems,a four-stage RACR system using R600/R170/R14/R50 was proposed to achieve low-temperatures below-140℃.The results show that increasing the ratio of R170 and reducing the ratio of R14 or R50 could significantly improve the energy efficiency of the system.But the former increased T_dis slightly,while the latter improved T_dis.The high-boiling-point refrigerant components R290,R600,R600a,and R1270 were used in the four-stage RACR systems,respectively.The minimum gas-phase fraction before throttling(x_4min)of the four systems in stable operation could optimize the energy efficiency of the system.Among them,R600/R170/R14/R50(mass fraction was 0.38/0.20/0.17/0.25 and x_4min was 0.12)performed best.Adjusting the gas-liquid separation temperatures at different stages was proposed to improve the system energy efficiency to achieve-142℃.Thereinto,appropriately increasing the second-stage gas-liquid separation temperature had the best effect on system performance improvement.