Characteristics Research On Room Air Conditioner With Small Diameter Fin-and-tube Heat Exchanger

Adopting smaller diameter fin-and-tube heat exchanger in room airconditioner can not only reduce material cost, but also can lower the refrigerantcharge, which can promote the use of environmental refrigerant-R290. When thetube diameter becomes smaller, the heat transfer characteristic of heat exchangeron the air side may change. While the researches about heat transfercharacteristic of heat exchanger on air side now are mainly to those with7mmdiameter or larger tubes. Therefore, it is necessary to study the heat transfercharacteristic of heat exchanger with smaller diameter tubes on the air side.When the tube diameter becomes smaller, the perssrure drop in the tube willincrease largely, which may lead the performance deterioration of heatexchanger. In order to guarantee the heat exchanger performance, the number offlow circuit should be increased, which will cause the uneven refrigerantdistribution. Thus, distributor is adopted to solve the problem, while theresearches on distributor are few and unsystematic. Systematic research ondistributor is needed to study distributor performance and give suggestion ondistributor structure modification. Although samller diameter fin-and-tube heat exchanger can reduce the material cost and refrigerant charge, the performanceof R290air conditioner with smaller diameter tubes are still unknown. Therefore,the performance of R290air conditioner with smaller diameter tubes should bestudied to confirm whether it is worthy to promote R290air conditioner withsmaller diameter tubes. This paper did a comprehensive research air conditionerwith smaller diameter tubes:1) The heat transfer characteristics of fin-and-tube heat exchangers with5mm diameter tubes under wet conditions were investigated by experiments on11louver fin-and-tube heat exchangers with different structures. The effects offin pitch and inlet relative humidity on air-side performance were analyzed. Theresults indicate that the effect of inlet relative humidity on the heat transfer rateis small, and heat transfer rate decreases with the increase of fin pitch, which ismore obvious than that of fin-and-tube heat exchanger with7mm or largerdiameter tubes. The water bridge was observed at the bottom of fin inexperiments; however, it did not occur on the fin with7mm or large diametertubes. A correlation for j factor was developed to predict the heat transferperformance of fin-and-tube heat exchanger with5mm diameter tubes. Theproposed correlation can describe85.7%of the test data within the deviation of±15%. The proposed correlation of j has a mean deviation of6.5%. Acomparison of j correlations of5mm fin-and-tube heat exchanger and largerfin-and-tube heat exchanger was made: only56.4%of the test data are withinthe deviation of±15%, when adopting j correlation of larger fin-and-tube heat exchanger to predect the performance of5mm fin-and-tube heat exchanger.2) This paper studied the performance of common distributors used in smalldiameter fin-and-tube heat exchangers and proposed the direction of structuremodification. Experimental results indicate the performance of jack-typedistributor is largely affected by installation angle. It should be installedvertically. The performance of cone-type distributor is affected by installationangle the least. It could be installed with the angle from0°to30°. Theperformance of reflect-type distributor is less affected by installation angle. Itcould be installed with the angel from0°to15°. Performances of all types ofdistributors get better when the flow rate increases. Taking influence of bothinstallation angle and flow rate on diffusion performance into conditeration,cone-type distributor performance is the best, followed by cone-type distributorand jack-type distributor. The direction of structure modification was proposedbased on the experiment and theory, and the validity of the direction wasconfirmed by experiment. The direction of structure modification is that makethe flow path smooth and make the cross area of mixture chamber small.3) The influence of refrigerant circuit arrangement on performance of5mmindoor unit and4mm outdoor unit was studied. In order to study theperformance of R290air conditioner with smaller diameter tubes, the indoor andoutdoor units with the optimal refrigerant circuit arrangements were matched.Compared to R22air conditioner with the rated refrigerating capacity of2600W,R290air conditioner with smaller diameter tubes has larger heat exchange capacity and higher COP under both cooling and heating condition, whichmeans that it is worthy to promote R290to be used in air conditioner withsmaller diameter tubes.