Experimental investigation of dropwise condensation on hydrophobic heat exchangers part I: Dimpled-sheets

Vapor-compression desalination is a reliable and robust desalination technology that benefits from high heat transfer coefficients in evaporators/condensers. In this study, heat transfer coefficients were measured in vertical dimpled-sheet heat exchangers (0.762-mm-thick naval brass 464 or 0.203-mm-thick copper), some of which were bare (filmwise condensation) and others were coated with a thin layer of passive electroless Ni-P-PTFE (dropwise condensation). The heat exchanger sheets were mounted in a sealed two-chamber apparatus with condensing saturated steam on one side and boiling saturated water on the other. Shearing steam on the hydrophobic condensing surface enhanced the overall heat transfer coefficient by 1.6 times and forced convection on the boiling side increased it by an additional 1.4 times. Adding a 0.635-mu m-thick layer of hydrophobic Ni-P-PTFE coating to naval brass 464 that was roughened on the boiling-side surface with sand-blasting increased the heat transfer coefficient by 4.3 times. Adding PTFE boiling stones to the boiling side of the hydrophobic copper plates increased the heat transfer coefficient by 1.15 times. The maximum overall heat transfer coefficient measured was U=184 kW/(m(2).degrees C), which occurred with 0.203-mm-thick copper (k=400 W/(m.degrees C)) coated with 0.635-mu m-thick Ni-P-PTFE and PTFE boiling stones as a dynamic nucleation agent in the bulk liquid (P=722 kPa, T=166 degrees C, v(steam)=0.49 m/s, R approximate to 1 kg shearing steam/kg condensate, v(liq)=1.57 m/s, Delta T=0.20 degrees C). Published by Elsevier B.V.