Performance Analysis Of Electronic Expansion Valve

As one of the key components of refrigeration system,expansion valve has a direct impact on the energy consumption of cold storage.At present,thermal expansion valve is mainly used in small cold storage,which is widely used because of its low cost and satisfactory control performance.Because of the hysteresis of the system and the complexity of the mechanical structure of the thermal expansion valve,it can not make a rapid and accurate response to the drastic changes of working conditions,which will lead to the unnecessary increase of the system energy consumption.At the same time,the control of the thermal expansion valve is self-contained,and the control variables of the thermal expansion valve are independent of the refrigeration system,which can not achieve the purpose of coordination with the overall operation of the system.it also brings difficulties to the intelligent control of the cold storage.With the development of Internet of things and big data,the management of cold storage will be more accurate and intelligent in the future.As one of the most critical components of refrigeration system,the real-time acquisition of the operating parameters of expansion valve(inlet temperature and pressure,outlet temperature and pressure,opening,flow,etc.)becomes particularly important in the control of cold storage.However,there are congenital problems in the thermal expansion valve,which makes it difficult to acquire and control parameters accurately.On the contrary,the electronic expansion valve fundamentally solves the above problems.This paper compares the control performance of the thermal expansion valve and the electronic expansion valve and their influence on the refrigeration system.Based on the analysis of the experimental data,the empirical formula of the flow rate of the electronic expansion valve is obtained.Finally,according to the flow formula,two prospects for the future control and management of cold storage are put forward.The research contents and conclusions are as follows:1?The expansion valve performance test platform was built.The test-bed can meet the requirements of various expansion valves for testing 0-10 kw cooling capacity.The 150 millimeter thick polyurethane foam board is used in the cold storage of the experimental stand to ensure that the leakage heat of the storage tank is reduced and the influence of environmental leakage heat on the system test is reduced.The compressor adopts four parallel bitzer semi-hermetic compressors,which can be divided into four grades to adjust the compressor load.At the same time,three compressors are used with frequency converter to reduce the impact of machine start on the compressor,and increase the stability of starting the compressor and compressor life.Using a 160m2 heat exchange area fan,it can completely adapt to the test of the cooling capacity within 10 kW,and a large area heat exchanger can solve the problem of insufficient heat exchange,reduce the temperature difference of heat exchange,and improve the accuracy of the test experiment.Under the condition of fully considering the product quality of thermal expansion valve,Danfoss expansion valve is used to ensure the persuasiveness of comparative experiment.The data monitoring system of the experimental platform can accurately collect the status information of the pipeline before and after the expansion valve in seconds.Mx100 standard software can complete the direct export and specialized analysis of experimental data.2?In the aspect of expansion valve performance,the influence of expansion valve control parameters(such as superheat)on its control performance and system performance is studied.It is found that the superheat regulation of EEV is faster than that of TXV,the range of superheat fluctuation is smaller,and the stability of EEV regulation is better than that of TXV;without considering the regulation time,the final regulation results of two expansion valves are equal in the small cold storage system with small delay;the superheat fluctuation of TXV is in direct proportion to the storage temperature,and in inverse proportion to the initial setting value of superheat;For the thermal expansion valve,the adaptability experiment of superheat is also carried out,and the change of superheat setting value under different working conditions is analyzed.It is found that the minimum setting value of superheat is 10 ? for low-temperature cold processing warehouse,because the cold warehouse passes through high temperature and low temperature successively,so as to prevent wet compression of compressor;for lowtemperature cold storage(only cold storage)or cold storage with little temperature change,the minimum setting value of superheat is 5 ? to prevent wet compression of compressor;considering cop and cooling capacity,low-temperature cold storage If the superheat is too low,cop will not be significantly improved.If the superheat is reduced in high temperature cold storage,cop will be significantly improved.However,considering the wet compression,the superheat setting can meet the requirements of high temperature and low temperature It is suggested that low temperature is 5 ? and high temperature is 10 ?;the regulation adaptability of TXV is much worse than that of EEV.When the working condition changes,the superheat degree of TXV system will change when it works stably,and when there is no artificial regulation,the cop of the system will fluctuate in uncertainty,generally the situation will worsen,and the refrigerating capacity will not change much compared with that of EEV system.3?The experimental analysis of the flow characteristics of the electronic expansion valve,and the establishment of two empirical formulas to calculate the outflow coefficient,and the error analysis is made,It provides help for future cold storage management and experimental simulation.Empirical formula of power-law flow:(4-42),(4-43),(4-44),Table 4-2;empirical formula of polynomial flow:(4-42),(4-43),(4-46),table 4-3;The calculation accuracy of power-law empirical formula is higher than that of polynomial empirical formula,and the error is within 5%,but the parameters include viscosity ratio,which consumes more calculation resources.The empirical formula in polynomial form has low accuracy,error less than 10%,less input parameters and less calculation resources.4?Based on the empirical formula of flow rate of electronic expansion valve,the application prospect of two aspects in the control and management of cold storage is put forward.1)Calculate the cold working time based on the heat load.The schedule calculation is more accurate and reasonable,but the remaining time calculation error is large,only in the last stage of cold processing time prediction has a certain accuracy.2)The coordinate of Electronic expansion valve opening and compressor frequency.The machine set of the simulation experiment is a semi closed piston small refrigerator unit.The simulation results provide reference for the future frequency control and energy saving optimization of the refrigerator.The simulation results show that the combined control of EEV and compressor can improve the stability and performance of the system,especially under the conditions of low EEV opening,low flow and low load of expansion valve,the COP of the system can be significantly improved by using variable frequency compressor,which is several times of the independent uncoordinated control system.