Study On Performance Of Thermal & Electronic Expansion Valve In Cryogenic Refrigeration System

In the field of food processing,pharmaceutical and industrial production,cryogenic is widely applied,such as frozen storage and transportation of high value-added seafood,storage and transportation of biological medicine production,the preservation of low temperature biological products.The normal and efficient operation of the refrigeration system are the guarantee of providing stable and standard of low temperature environment.In addition to the compressor,evaporator and condenser,throttle mechanism is an important part of a vapor compression refrigeration system.Its main functions include to throttle depressurization,adjust cooling flow and maintain reasonable superheat.The operation and compatibility of the throttling device affects the performance and stability of the refrigeration system.At present,the commonly used throttling device in cryogenic refrigeration system is thermal and electronic expansion valve.Compared with thermal expansion valve,electronic expansion valve has incomparable advantages due to the different of mechanical structure and working principle,such as rapid response and accurate adjustment,etc.In this paper,the pulse width modulation control of the electronic expansion valve was proposed,and the fuzzy control algorithm was used to optimize and selected the more suitable control method under the low temperature condition.It will provide certain reference for the future control of the electronic expansion valve of the cryogenic refrigeration system.The main contents and conclusions in this paper were listed as follows:(1)To build the experimental apparatus of hardware structures.Thermal expansion valve and electronic expansion valve in low stage of cascade refrigeration system were selected.The data acquisition system hardware platform,including the selection of PLC,temperature/pressure sensor and mass flow meter and the communication between PLC and upper machine LabVIEW based on OPC protocol were established.The electronic expansion valve control system hardware platform,including the selection of PLC and temperature/pressure sensor and the communication between PLC and upper machine LabVIEW based on OPC protocol was also built.(2)To design experimental apparatus control system software design.Cryogenic refrigeration control system software was designed and developed.A supervisory control and data acquisition of the refrigeration system was realized.Electronic expansion valve control system software was developed.Pulse width modeling(PWM)for electromagnetic electronic expansion valve was realized.Electronic expansion valve control system was also optimized.Based on the nonlinear feature of refrigeration system,the use of fuzzy control to the electronic expansion valve control system was optimized and fuzzy logical control for valve opening was realized to satisfy the requirement of the evaporator outlet superheat control.(3)Experimental study on the electronic expansion valve under cryogenic conditions.First,on the basis of system stability and systemic,the optimum scope of duty ratio was determined to 30% ~ 50%.Through the experiments,the control performance of the two sets of designed electronic expansion valve controller has been tested and verified.Two experiments respectively was carried out,including tracking response performance of target superheat and inhibition performance of external disturbances.The experimental results showed that in the target tracking performance the overshoot of the fuzzy logic controller was 5.1% lower than that of the PWM control,the average time was 220 s,the average time was 80 s longer,the average IAE was 0.55 lower,and average ITAE was 1295.2 smaller than that of the PWM control.In terms of disturbance rejection performance,the overshoot of the fuzzy logic controller is 1.2% lower than that of the PWM control,the average adjustment time was 150 s,the average rising time was 75 s longer,average IAE was 0.71 lower,and average ITAE was 1474.7 smaller.So the fuzzy controller had obvious advantages.(4)Experimental study on the comparison between electronic and thermal expansion valve.Contrast experiment of superheat control performance under cryogenic conditions between electronic and thermal expansion valve was carried out under-50?,-55? and-60? three different working conditions and variable load conditions.The experimental results showed that compared with the thermal expansion valve,the electronic expansion valve with fuzzy controller had a higher average evaporation temperature of 5.8 ?,and 1.73 h faster at average cooling time.The change of superheat was more stable and smooth,compressor exhaust temperature and pressure was lower.Finally,under the three operating conditions,the average COP was 10.5% higher than that of the system of thermal expansion valve.In the variable load conditions,the average maximum fluctuation of superheat using electronic expansion valve was 1.75 K,the average adjustment time was 510 s,and it was 3.5K and 735 s respectively using thermal expansion valve.In addition the low temperature stage compressor exhaust temperature and evaporator temperature change were more intense compared with the using thermal expansion valve.Hence using electronic expansion valve on the system stability and system performance have obvious advantages,more suitable for the ultra-low temperature cold storage refrigeration system.