Performance of dehumidifying heat exchangers with and without wetting coatings

Limited previous work has shown that use of special hydrophilic coatings will provide lower air pressure drop in finned tube heat exchangers operated under dehumidifying conditions. However, no detailed work has been reported on the effect of different coating types, or different fin surface geometrics on the wet pressure drop. In this study, wind tunnel tests were performed on three different fin geometrics (wavy, lanced and louver) under wet and dry conditions. All dehumidification tests were done for fully wet surface conditions. For each geometry, the tests were performed on uncoated and coated heat exchangers. For all three fin geometries, the wet-to-dry pressure drop ratio was 1.2 at 2.5 m/s frontal air velocity. The coatings have no influence on the wet or dry heat transfer coefficient. However, the wet surface heat transfer coefficient was 10 to 30 percent less than the dry heat transfer coefficient, depending on the particular fin geometry. The effect of the fin press oil on wet pressure drop was also studied. If the oil contains a surfactant, good temporary wetting can he obtained on an uncoated surface; however, this effect is quickly degraded as the oil is washed from the surface during,wet operation. This work also provides a critical assessment of data reduction methods for wet surface operation, including calculation of the fin efficiency.