The use of an organic self-assembled monolayer coating to promote dropwise condensation of steam on horizontal tubes

Hydrophobic coatings have been created through self-assembled monolayers (SAMs) on gold, copper, and copper-nickel alloy surfaces that enhance stream condensation through dropwise condensation. The monolayer is formed by chemisorption of alkylthiols on these metal surfaces. Due to their negligible thickness (10-15 Angstrom). SAMs have negligible heat transfer resistance, and involve a minuscule amount of the organic material to pose any contamination problem to the system from erosion of the coating. The coating was applied directly to copper and 90/10 copper-nickel tubes, and to previously gold-sputtered aluminum tubes. The quality of the drops on SAMs, based on visual observation, was found to be similar for the three surfaces, with the gold surface showing a slight superiority. When compared to complete filmwise condensation, the SAM coating increased the condensation heat transfer coefficient by factors of 4 for gold-coated aluminum, and by about 5 for copper and copper-nickel tubes, under vacuum operation (10 kPa). The respective enhancements under atmospheric conditions were about 9 and 14. Comparatively, the heat transfer coefficient obtained with a bare gold surface (with no organic coating) was 2.5 times that of the filmwise condensation heat transfer coefficient under vacuum, and 3.4 at atmospheric conditions.