A Study on the Use of a Hydrophobic Coating to Enhance Condensate Drainage on a Heat Pump Heat Exchanger

Frost accumulation on heat pump heat exchangers operating in the heating mode have received much attention from researchers in recent years. For frost formation conditions on heat pumps, the frosting process begins when water vapor condenses onto the cold heat exchanger surface from the ambient air stream and supercools for some period before nucleating to ice. The frozen droplets serve as sites for the growth of frost which builds on the heat exchanger during the frosting cycle. The frost is eliminated from the heat exchangers during the defrost cycle. As the frost melts, it drains from the heat exchanger. After the completion of the defrost cycle, some liquid water is left on the heat exchanger surface. The retention of the condensate after a defrost cycle means additional liquid water is available on the heat exchanger for re-freezing during the next frosting cycle. One way to help drain condensate during the defrost cycle and reduce retention of water in the heat exchanger is to apply low energy coatings to the surface of the tubes and fins. The purpose of this paper was to investigate the effect of hydrophobic coatings to shed water and reduce retention of condensate in a fin and tube heat exchanger. A review of the literature revealed very few references on the use of hydrophobic coatings to prevent liquid water from remaining on a heat exchanger surface. Polymer chemists have employed the use of the contact angle of a liquid droplet resting on a surface and the tilt release angle of the droplet as the surface is tilted to quantify adhesion of the liquid droplet. Low surface energy coatings, such as poly tetra-fluoroethyelene (Teflon) typically have very low tilt angles and consequently, water droplets slide off of these type surfaces very easily. Conversely, bare metals have a high surface energy and water adheres quite readily to them. Benchtop experiments were conducted with copper and aluminum samples that were coated with a hydrophobic polymer as well as untreated samples. Contact angles for sessile droplets and tilt release angles were determined for both groups. Contact angles were 10 to 15° greater (105 - 110°) for the treated versus untreated metal surfaces for the sessile droplets. Tilt release angles varied from 15 to 50° for the treated surfaces. Water droplets did not move on the bare metal surfaces even at a 90° tilt angle which indicated high adhesion to the surface. Next, a small section was cut from a single row split-fin and tube heat exchanger. This section was cleaned and sealed to prevent water from entering the severed piping. The section was submerged in a pan of water, withdrawn, and allowed to “drip dry” for one minute before being weighed. After ten repetitions, the section was sprayed with a hydrophobic polymer and the tests repeated. Results showed a 38% reduction in the amount of water retained in the treated versus untreated heat exchanger section. Results of these tests show promise for application to heat pump systems as well as systems where the rapid removal of water from the heat exchanger is desirable.