Issues affecting the reliability of dynamic dip testing as a method to assess the condensate drainage behavior from the air-side surface of dehumidifying heat exchangers

For years dynamic dip testing has been working as a simple, fast, and easy method to assess the condensate drainage behavior for dehumidifying heat exchangers, because it was found that a heat exchanger holding more water in a dip test also tends to hold more condensate in a wind-tunnel experiment. This work addresses several issues which can affect the reliability of this method but have been previously overlooked. Dip test results as well as retention visualization for 22 heat exchanger specimens with different configurations and surface wettability are reported and discussed with relevant dip test and wind-tunnel experimental data available in the literature. The data demonstrate that when dealing with round-tube heat exchangers with unusual wettability, dip test can sometimes give an evaluation which is counter to the results in the wind-tunnel. In terms of surface wettability, receding contact angle (θ_R) is found to be the primary factor affecting a dip test, while the contact angle hysteresis (θ_A - θ_R) becomes more important in a condensing environment. It has also been observed that dip test measurement is very sensitive to the "dipping rate", or the speed at which a specimen is withdrawn from the water reservoir. The differences in condensation and drainage mechanisms between wind-tunnel experiments and dip tests are elaborated in this paper, and a proper and effective use of dynamic dip testing is recommended.