Design Simulation And Experimental Study On Refrigerant Circuit Of Evaporator Of Direct-Expansion Fresh Air Unit

Air conditioning energy consumption accounts for a large proportion of the entire building energy consumption.Finned tube heat exchangers are widely used in air conditioning systems.Improving the performance of finned tube heat exchangers will significantly reduce building energy consumption.Factors that have an important impact on the performance of finned tube heat exchangers include geometric parameters such as tube row spacing,tube diameter,fin form,and fin spacing.This paper mainly studies the influence of refrigerant flow path arrangement on the performance of finned tube evaporators of two types of direct expansion fresh air units.The geometric parameters such as the number of coils,spacing,and the number of copper tubes per row of tubes of the evaporator are determined through theoretical calculation.Based on this,two types of evaporator structures are proposed,one is the upstream and downstream arrangement,with downstream flow followed by countercurrent flow,and the other is the multi row tube full countercurrent type.The refrigerant flow path design and performance analysis of two types of evaporators were performed using the Coildesigner software.For the forward to reverse type,the upper and lower parts are designed with different refrigerant branch numbers and combined separately.The heat exchange performance,pressure drop performance,and flow distribution of different combinations are compared.Considering the economy and stability of the refrigeration system operation,the optimal design scheme is to set 12 refrigerant branches in the upper part and 6 refrigerant branches in the lower part,Subsequently,the performance of this scheme was compared with that of full countercurrent finned tube evaporators with different refrigerant flow branches,and 12 refrigerant branches were selected as the optimal scheme for full countercurrent evaporator.Subsequently,the heat transfer performance,pressure drop performance,refrigerant flow rate,and sensible heat ratio of the two evaporators were analyzed using different relative humidity and superheat as control conditions.Finally,the actual production of these two types of evaporator samples,using a direct expansion fresh air unit as a carrier,established an experimental platform to test the heat transfer capacity and energy efficiency of these two types of finned tube evaporators.According to the actual test results,it is found that under nominal operating conditions,the refrigeration capacity of the two evaporators gradually increases and then tends to flatten out when the compressor is put into 75%,60%,50%,and 40% of the operation;At high frequencies in the compressor,the dehumidification amount first increases to the inflection point and then gradually decreases,while at low frequencies,it gradually decreases;The EER value first increases gradually and then decreases;The cooling capacity of the forward and backward type is 4.7%,4.8%,4.4%,and 5% higher on average than that of the full reverse flow type;The dehumidification capacity of the forward and backward type is 9.1%,9%,5.8%,and 13.4% higher than that of the full reverse flow type on average;The EER values of the forward and backward type were 4.7%,4.8%,4.4%,and 4.5% higher on average than those of the full reverse flow type;Under dehumidification conditions,when the compressor frequency is put into operation at 100%,75%,50%,and 25%,and when the compressor is put into operation at more than 50%,the cooling capacity of both evaporators gradually increases and then tends to flatten out.When the compressor is put into operation at 25%,the cooling capacity basically remains unchanged;The dehumidification capacity of the two evaporators gradually increases when the compressor is put into operation at 100%,increases first and then decreases when the compressor is put into operation at 75% and 50%,and remains basically unchanged when the compressor is put into operation at 25%;When the compressor is put into operation for more than 50%,the EER value first increases,then tends to slow down,and then decreases.When the compressor is put into operation for 25%,the EER value continuously decreases;The cooling capacity of the forward and backward type is 5.8%,8.8%,9.3%,and 3.7% higher on average than that of the full reverse flow type;The dehumidification capacity of the forward flow and backward flow type is 10.3%,10.1%,7.6%,and 4% higher than that of the full reverse flow type on average;The EER values of the forward and backward type are on average 3.8%,4.9%,5.7%,and 0.6% higher than those of the full reverse flow type.