STEAM CONDENSATION ON CHROMIUM SURFACES (DROPWISE, FILMWISE)

The conditions under which steam will condense filmwise and the conditions under which it will condense dropwise on chromium surfaces were carefully determined. Steam condensation was carried out at atmospheric pressure on the surface of a vertical 1.0 inch diameter disk made of either shiny chromium electroplated over copper or polished 99.9% pure metallic chromium formed by powder metallurgy.;The mode of condensation was found to be quite sensitive to the cleanliness of the test surface. Commercially plated chromium surfaces not degreased prior to condensation experiments yielded temporary dropwise condensation lasting about 75 hours. Identical chromium surfaces degreased prior to condensation experiments yielded only filmwise condensation.;Dropwise condensation on electroplated chromium was promoted by exposure to noncondensible gases during condensation at low steam boilup rates. Under "low" boilup rate conditions, the amount of steam produced on the boilup was less than could be condensed at the existing cooling rate. Thus, the surface was steam-deficient, allowing it to contact noncondensibles. Unfortunately, no stable enhancement of heat transfer rates accompanied this dropwise condensation, because of the detrimental effects of noncondensibles on heat transfer. A hysteresis was observed between heat flux and boilup rate with enhanced heat fluxes and dropwise condensation possible along one path and filmwise condensation occurring along the other. This dropwise condensation with heat fluxes above those expected for filmwise condensation was not stable, and disturbances to the condensation system could cause the dropwise condensation to revert to filmwise condensation.;A monolayer of the promoter, oleic acid, applied to an electroplated chromium surface yielded temporary dropwise condensation for 24 hours. Four PPM of oleic acid added directly to the vapor phase promoted dropwise condensation on electroplated chromium for over a thousand hours. And, oleic acid vacuum impregnated into porous electroplated and metallic chromium promoted dropwise condensation for over 200 hours and over 3000 hours respectively. (Abstract shortened with permission of author.);Clean steam condensed filmwise on both clean electroplated and solid chromium surfaces under the same conditions where it condensed dropwise on gold and silver surfaces. Neither the thickness of electroplated chromium or the addition of an electroplated nickel undercoat altered the filmwise mode of condensation.