The Simulation Of Multistage Auto-cascade Refrigeration Systems

The auto-cascade refrigeration(ACR)system is driven by a single compressor,obtaining low temperature by multistage separation of zeotropic mixture.However,the increasing of cascade series often lead to a complex system structure with a lower efficiency.At present,the mainly difficulty of deep cold auto-cascade refrigeration system lies in the study of mixture ratio of the mixed refrigerants and system process design.The calculation of the equilibrium of the mixed refrigerant in the thermal cycle is the key to solve these difficulties.This research stimulated the multistage auto-cascade refrigeration system on the innovative use of Aspen Plus software.Simulating traditional three stage ACR system using three combinations of(R600a/R23/R14,R290/R23/R14,R134a/R23/R14)Zeotropic mixture based on PR state equation,the calculation results were compared and demonstrated with relevant literatures' experimental results.The simulation results were in agreement with the experimental data in optimal ratio of mixed refrigerant and system performance.Moreover,a new three stage ACR system using environmental protection refrigerant R600/R290/R170 was designed and stimulated based on RKS state equation.After the calculation of multiple operating conditions,a better three stage ACR system under water cooling condition was obtained with the mass fraction of 50/20/30,operating at low evaporating temperature of-93? at Coefficient of Performance(COP)of 0.238.A four stage ACR system using mixed refrigerant R290/R23/R14/R50 was designed,operating in deep cold zone with a small cooling capacity.And PR state equation was applied to mixture property estimation.The evaporating temperature,temperature of suction and discharge of compressor,heat exchanger and COP of system were analyzed in the stimulation of four stage ACR system with different mixture ratio and different gas-liquid separation temperature.The optimal four stage ACR system with the mass fraction of 51/12.2/25/11.8 can operate at low temperature of-143? at COP of 0.15 with 18.9% exergy efficiency.Using the theory of exergy analysis,the available energy loss of each component and the exergic efficiency of the multistage ACR system were calculated.The results showed that the proportion of the component exergy loss was as follows: compressor,heat exchangers,throttles and mixers.Accordingly reducing the exergy loss of the heat exchangers was the main direction to optimize the multistage ACR system.Water cooling condition and appropriate ratio of mixed refrigerant and gas-liquid separation temperatures can reduce the exergy loss in heat exchangers,thereby improving the exergic efficiency of the system.