Research On Frosting Control Of Evaporator Used In Mobile Air-Conditioning System

As people become more and more attention to the requirements of automobile comfortableness,how to effectively control the frosting phenomenon in evaporator used in mobile air-conditioning system becomes a common problem needed to be solved both for the refrigeration industry and the automobile industry.Although lots of works have been done on the frosting mechanism in heat exchanger,most research based on finned tube heat exchanger does not apply to parallel flow heat exchanger which is more compactness and poorer drainage performance.Under theses backgrounds,law of frosting and its control mode in parallel flow heat exchanger used in R134 a MAC system are studied.The main contents and conclusions are as follows:1)The frosting characteristics of parallel flow heat exchanger are experimental studied on AC performance test bench.The frosting process is meticulous divided into five stages based on the tight relationship between evaporating pressure and the frost layer growth rule.After the date analysis of frosting process under different operating conditions,we reveal the different effects of inlet air temperature,relative humidity and wind speed influence the parallel flow evaporator frosting law.Inlet air relative humidity,inlet air temperature,inlet air relative humidity and inlet air speed are respectively the most influential factors correspond to frost thickness,frost amount,refrigerating capacity and air pressure drop.2)The surface temperature distribution of parallel flow evaporator under condensation and frosting conditions are experimental studied.The result shows that position of the second lowest temperature on the evaporator surface would change frequently under condensation conditions,while the lowest temperature point is relatively stable.Under the frosting conditions,the position of lowest temperature would also change irregularity.These are the main reason leads to the malfunction in the use of defrosting start-time prediction based on surface temperature.3)Critical frosting evaporating pressure range was experimental identified,and a frosting control mode which can make the evaporator frost-free and continuous working have been put forward.There exists a unified critical frosting evaporating pressure between 160~165kPa.G.The evaporator can be frost-free and high performance by using the electronic expansion valve to control the evaporating pressure within the scope of the above pressure.In the meanwhile,it founds that a 5% increase of valve opening degree would enhance 2.5kPa evaporating pressure averagely.The MAC system needs 66 s to be steady again if a 5%~20% increase of valve opening degree was using to adjust the evaporating pressure.4)A defrosting start-time prediction method based on evaporating temperature was put forward and its validation on predicting frost degree was experimental verified.The average relative error between defrosting time determined by using the above method and proper defrosting time is just only 7.2%,and it is between 0%~-15%.On the premise of allowing frost occurs on part of evaporator surface,it can effectively control the frosting range effectively and extend the cooling period by using the new defrosting start-time prediction method.