Long-term hydraulic performance of dehumidifying heat-exchangers with and without hydrophilic coatings

This experimental study investigated the long-term wetting characteristics of dehumidifying finned tube heat-exchangers (coils). The coils were subjec ted to up to 1000 cycles in which they were alternately dipped into distill ed water for 15 minutes and then dried with a fan for 45 minutes. Wetting d urability was determined for six coils (three uncoated, two with proprietar y coatings, and one with a zinc paint on the downstream face), and with dif ferent fin press oils. The wettability evaluation consisted of periodic win d tunnel tests of fully wet coils under dehumidification and measurement of the advancing and receding contact angles by periodically removing small f in samples from the coils. One coating maintained the receding contact angl e below 17 degrees throughout the cycle tests. The advancing contact angles were much higher than the receding contact angle. The best coating maintai ned the wet/dry pressure drop ratio below 1.34 throughout the 1000 cycles. The wet/dry pressure drop ratio could be correlated as a function of recedi ng contact angle. An effect of oil was observed on uncoated coils in the in itial 100 cycles but vanished after 500 cycles.