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

Abstract

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 μm. 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·°C) (0.203-mm-thick copper plate, round-shaped vertical grooves, 2.54-μm-thick lead-free Ni–P–PTFE, P = 722 kPa, T = 160 °C, Δ T = 0.20 °C, saturated liquid velocity v liq = 1.57 m/s, shearing steam v steam = 0.23 m/s, and flow ratio R ≈ 0.6 kg shearing steam/kg condensate).