Dropwise Condensation of Steam on Horizontal Corrugated Tubes Using an Organic Self-Assembled Monolayer Coating

Abstract

Steam condensation on corrugated tubes has been enhanced by using a hydrophobic coating to promote dropwise condensation. The coating was created with self-as-sembled-monolayers (SAMs) on gold and copper-nickel alloy surfaces. SAMs are formed by chemisorption of alkylthiols on these metal surfaces. The negligible thickness (10-15 A) of SAMs results in negligible heat transfer resistance. Moreover, only a minuscule amount of the organic material is needed to create these monolayers, so there is no possible contamination threat to the system from erosion of the coating. The coating was applied directly to Korodense corrugated tubes made of 90/10 copper-nickel, and to a previously gold-sputtered titanium Korodense tube. The quality of the drops on SAMs, based on visual observation, was found to be similar for the two surfaces, with the gold-sputtered titanium surface showing a slight superiority over the copper-nickel surface. When compared to filmwise condensation, the coating increased the condensation heat transfer coefficient by factors of 3.6 and 2.7, at a wall subcooling of about 16°C, for copper-nickel and gold-sputtered titanium Korodense tubes, respectively, under atmospheric operation (101 kPa). The respective enhancements under vacuum (10 kPa) conditions were 2.5 and 2.3 at a wall subcooling of about 6°C.