Impact Of Interim Running On Heat Flow Rate And Pressure Drop Of Finned-Tube Heat Exchangers

Finned tube heat exchangers are widely used in family, business and industry air condition system. The key to keep the high efficiency of the air condition system is the heat transfer performance of the finned tube heat exchangers. In the initial stages the finned tube heat exchangers can transfer heat at high efficiency. After used some time the finned tube heat exchangers can not keep this high efficient heat transfer performance. That's to say that the long-term heat transfer performance of the finned tube heat exchangers is in recession. This paper wants to declare the effect of the interim operation of the air conditioner on the finned tube heat exchangers during the long-term operation of the air conditioners.According to the market research an air condition runs about 842.9 hours a year. Then we can calculate that the on-off-time of the air conditioner is 1214. An aluminum fin-and-tube heat exchanger and a copper fin-and-tube heat exchanger are studied. Both of their fin surfaces are hydrophilic treated. Both heat exchangers are tested 4800 times to simulate the effect of 4-year run. To simulate the cooling and heating cycles two steps are used. First the tested heat exchangers are immersed in cold water for one minute till the temperature of the fin surface fall to 5℃. The second step the tested heat exchangers are put in a heating box for three minutes till the temperature of the fin surface rise to 27℃.Heat transfer performance and pressure drop facilities are tested at various air velocity every 300 cycles to detect how the heat transfer performance and pressure drop of the tested heat exchangers affected by the running times. An experimental rig is designed to detect the data of the air-side heat transfer performance. The rig comprises three systems: the water loop, the air flow loop and the data management system. During each experiment the heat transfer performance of the tested fin-and-tube heat exchangers are detected at four various air velocities. There are all one hundred and thirty six experiments. The result concluded from the experimental data shows the heat transfer performance drops obviously, the heat flow rate attenuates and the pressure drop rises. These effects are obvious at lower air velocity. And these effects on aluminum fin-and-tube heat exchangers exceed the effects on copper fin-and-tube heat exchangers. In case of the air inlet velocity is 0.5～2.0m/s the heat transfer performance of the aluminum fin-and-tube heat exchangers drops about 9.9%～36.3% and the pressure drop of the aluminum fin-and-tube heat exchangers drops farthest 7.0%. And in this case the heat transfer performance of the copper fin-and-tube heat exchangers drops about 7.3%～19.8% and the pressure drop of the copper fin-and-tube heat exchangers drops farthest 9.7%. At higher velocity the heat flow rate enhances with the cycles adding.