Effect of condenser tube material on heat transfer during dropwise condensation of steam

Abstract

Overall heat-transfer measurements were made while condensing steam on horizontal tubes of copper, brass, aluminium and stainless steel, promoted with dioctadecyl disulphide. A second set of measurements was made when the brass, aluminium and stainless steel tubes had been copper-plated to a thickness of 9 ± 1 μm. After tube-wall resistances (based on uniform radial conduction) had been subtracted, the overall heat-transfer coefficients for the copper, brass, copper-plated brass, copper-plated aluminium and copper-plated steel tubes were essentially the same. The overall coefficients for the unplated aluminium and steel tubes were markedly lower. Recently reported [1] differences in the results for different tube materials are considered to be due to differences in promoter effectiveness on the different surfaces rather than the thermal properties of the tube material. Visual observation indicated mixed or film condensation for the aluminium and stainless steel surfaces and ideal dropwise condensation on the copper, brass and copper-plated surfaces. In the case of the copper, brass and copper-plated tubes, the results were, within experimental error, consistant with established vapour-side heat-transfer coefficients for dropwise condensation. All tests were conducted at pressures near to atmospheric. The non-condensing gas content of the steam was minimised by pre-boiling and possible effects of remaining traces of gas obviated by vapour cross flow over the condenser tube. In all cases measurements were repeated on different days with excellent reproducibility. The heat-transfer rate obtained from the mass flow rate and temperature rise of the coolant was in good agreement with that obtained by measuring the condensation rate.