| In vapour compression refrigeration system,the control of superheat in the evaporator is not only related to the stable operation of the system,but also affects the heat exchange efficiency of the evaporator and the system’s coefficient of performance.If the degree of superheat is overheat,the evaporator heat transfer area can not be maximized,heat transfer efficiency is low,affecting the energy efficiency of the system.If too little,the refrigerant at the evaporator outlet contains liquid,which causes the phenomenon of“liquid shock”of the compressor and affects the stable operation of the system.Therefore,the control of superheat has become one of the important research directions of vapor compression refrigeration system.Since the actual vapor compression refrigeration system will change with the load change of the system and the real-time operating condition of the system,the evaporator superheat exists with extremely strong nonlinear characteristics.The PID control method used in air conditioning can not automatically adjust the control parameters according to the dynamic operation of the system,and its control precision is not high.And fuzzy control can solve this problem well.Fuzzy control can be well applied to nonlinear control system instead of accurate mathematical model.At the same time,Fuzzy control has both dynamic response performance and system stability.In this paper,PI control and fuzzy PI controller are designed for evaporator superheat control.The principle and design process of fuzzy control are introduced.The fuzzy PI controller controls the opening of expansion valve by adjusting the proportional gain and integral time in real time.Combined with the existing system dynamic model,the control effects of the two control methods are simulated and analyzed.The fuzzy control was used to adjust the superheat in this paper and the work has been carried out.(1)Based on the existing evaporative compression refrigeration system model,the dynamic influence of pulse step change of expansion valve on superheat was studied.(2)The PI controller is established and the attenuation curve method was employed to adjust the parameters of the controller.Besides,the parameters of PI controller were optimized.(3)The principle of fuzzy control is introduced in detail and the fuzzy control model is built.(4)When the superheat degree value,rotating speed of compressor,evaporator inlet wind speed and the evaporator inlet air temperature were changed respectively,in order to control the evaporator superheat,the PI model and the fuzzy control model are simulated respectively and compared.The simulation results show that the vapor compression refrigeration system based on fuzzy control is better than PI control.The refrigeration system with the fuzzy control responds quickly and has good stability,strong anti-jamming capability. |
PDF Full Text Request