Effect Of Temperature Difference Of Air-cooled Heat Exchanger On Frost

In this paper,the frost layer growth characteristics of finned tube evaporator and supercooler were studied experimentally.First,install the finned tube heat exchanger and the surface cooler as the frost experimental system in the wind tunnel experiment section of the low temperature closed-loop single experiment section.By studying the effect of heat exchange temperature difference on the growth characteristics of the frost layer under the condition of constant fan frequency,the following conclusions are obtained:(1)The frost layer thickness of about 3 rows of tubes in front of the evaporator at the initial head wind speeds of 3,4,5 m/s all show a three-stage mode change of "rapid growthalmost unchanged-slow decrease".The "almost unchanged" period is denser than the "rapid growth" period.During the "slow decrease" period,the frost layer on the front of the evaporator is more likely to be collapsed by the wind into the heat exchanger when the temperature difference is large.The evaporator and the supercooler are at the initial oncoming wind speeds of 2m/s,2 and 3m/s respectively,and the frost layer thickness increases with the frost time at different heat exchange temperature differences.(2)The frost amount of the evaporator and the surface cooler almost increase linearly with the frost time,and the growth rate increases with the increase of the heat exchange temperature difference.Combined with the "slowly decreasing" frost layer thickness in the front row of the evaporator,it means that a part of the water vapor in the humid air is crushed after increasing the frost layer thickness,and the other part of the water vapor increases the frost layer density.(3)At the initial oncoming wind speeds of 2m/s,2 and 3m/s,the evaporator and the supercooler respectively,when the heat exchange temperature difference is different,the oncoming wind speed gradually decreases with the frost time.At the initial oncoming wind speeds of 3,4,and 5 m/s,the evaporators oncoming wind speeds at different heat exchange temperature differences first showed "rapid decline",then "first rise and then decline",and then "slow decay".(4)Under the condition that the frequency of the fixed fan is constant for the evaporator and the surface cooler,the cooling capacity will increase first and then decrease with the frost time at different heat exchange temperature differences.The cooling capacity of the evaporator showed a downward trend during the "slow decrease" of the frost layer thickness of the front row of tubes,which further explained that the frost layer of the rows of tubes in front of the heat exchanger was collapsed and blown to the rows of tubes behind the heat exchanger,which generally caused The air circulation area of the evaporator is reduced,the air volume is decreased,and the cooling capacity is decreased.The change rate of the cooling capacity of the surface cooler is affected by the difference in the growth rate of the frost layer.(5)The heat transfer coefficients of the evaporator and the surface cooler at the initial oncoming wind speeds of 2m/s,2 and 3m/s,respectively,at different heat exchange temperature differences first increase with frost time and then decrease approximately linearly.trend.At the initial head wind speeds of 3,4,and 5 m/s,the heat transfer coefficient of the evaporator at different heat transfer temperature differences with frosting time showed a "first increase-then decrease-slowly decrease" three-stage mode change.During the period of "slow decrease",it shows that the effect of increasing the heat transfer coefficient caused by the reduction of the frost layer thickness of the front row of the evaporator and the reduction of wind speed and the increase of the heat transfer thermal resistance of the rear row of the frost layer entering the heat exchanger The effect of reducing the heat transfer coefficient is almost offset.The heat transfer coefficient of the surface cooler is more affected by the air-side heat transfer coefficient when it decreases.(6)The evaporators defrosting water and accumulated frost volume increase with the increase in heat exchange temperature when the fixed fan frequency is unchanged.This is due to the large change in the "slowly decreasing" period of the frost layer thickness.The thermal temperature difference will increase the frost crystals being crushed into the evaporator.The defrosting water and the accumulated frost amount of the surface cooler increase first and then decrease with the heat exchange temperature difference under the condition that the fixed fan frequency is unchanged.This is because the frost rate is fast and the frost period changes when the heat exchange temperature difference is large.short.(7)The frost running time per decay of 1m wind speed of the evaporator and surface cooler under the condition of constant fan frequency is gradually reduced as the heat exchange temperature difference increases.When the evaporators heat exchange tempera ture difference increased from 3℃ to 7℃ at the initial oncoming wind speeds of 2,3,4,and 5m/s,the frost duration lasted an average of 17.7% per 1m of wind speed attenuation.When the surface coolers heat exchange temperature difference increased from 3 ℃ to 7℃ at the initial oncoming wind speed of 2 and 3m/s,the frost duration lasted an average reduction of 74.5% per 1m of wind speed attenuation.Comparing the finned tube evaporator and the surface cooler,it was found that at a temperature of-18°C and a relative humidity of 80%,the frost layer structure formed by the front row tubes of the large-piece heat exchanger is easy to be crushed into when the initial wind speed exceeds 2m/s Inside the heat exchanger,you can delay the frost cycle and reduce the number of defrosts.