Experimental investigation of dropwise condensation on hydrophobic heat exchangers. Part II: Effect of coatings and surface geometry

This is Part II of an experimental investigation of hydrophobic heat exchangers. Two plates were studied: (a) 0.127-mm-thick titanium grade 2 and (b) 0.203-mm-thick copper. Titanium plate had round-dimpled spacers. Copper had either round-dimpled spacers or round-shaped vertical-grooved spacers. Titanium was bare but copper had electroless Ni-P-PTFE hydrophobic coating. Two chemical compositions of the hydrophobic coating were employed: lead-containing and lead-free. For some studies, the coating thickness was varied from 0.635 to 127 mu m. To measure the overall heat transfer coefficient, the plates were mounted in a sealed two-chamber apparatus with condensing saturated steam on one side and forced-convective boiling liquid water on the other. The best overall heat transfer coefficient was U =240 kW/(m(2).degrees C) (0.203-mm-thick copper plate, round-shaped vertical grooves, 2.54-mu m-thick lead-free Ni-P-PTFE, P = 722 kPa, T = 160 degrees C, Delta T = 0.20 degrees C, saturated liquid velocity v(liq) = 1.57 m/s, shearing steam v(steam) = 0.23 m/s, and flow ratio R approximate to 160 degrees C, shearing steam/kg condensate). Published by Elsevier B.V.