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

Hydrophobic coatings have been created through self-assembled monolayers (S AMs) on gold, copper, and copper-nickel alloy surfaces that enhance stream condensation through dropwise condensation. The monolayer is formed by chem isorption of alkylthiols on these metal surfaces. Due to their negligible t hickness (10-15 Angstrom). SAMs have negligible heat transfer resistance, a nd involve a minuscule amount of the organic material to pose any contamina tion problem to the system from erosion of the coating. The coating was app lied directly to copper and 90/10 copper-nickel tubes, and to previously go ld-sputtered aluminum tubes. The quality of the drops on SAMs, based on vis ual observation, was found to be similar for the three surfaces, with the g old surface showing a slight superiority. When compared to complete filmwis e condensation, the SAM coating increased the condensation heat transfer co efficient by factors of 4 for gold-coated aluminum, and by about 5 for copp er and copper-nickel tubes, under vacuum operation (10 kPa). The respective enhancements under atmospheric conditions were about 9 and 14. Comparative ly, the heat transfer coefficient obtained with a bare gold surface (with n o organic coating) was 2.5 times that of the filmwise condensation heat tra nsfer coefficient under vacuum, and 3.4 at atmospheric conditions.