Silver Surfaces for Dropwise Condensation of Steam
O'neill, Gary Alan
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https://hdl.handle.net/2142/72140
Description
Title
Silver Surfaces for Dropwise Condensation of Steam
Author(s)
O'neill, Gary Alan
Issue Date
1984
Department of Study
Chemical Engineering
Discipline
Chemical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Chemical
Abstract
The condensation of atmospheric pressure steam on copper coated with electroplated and vapor deposited silver was studied. It was found that, with a thick enough coating, silver will promote dropwise condensation without the intentional introduction of an additional promotor. For electroplating, the critical silver thickness depended on the electroplating bath used and the method of surface preparation prior to plating. Six combinations of four plating baths and two surface preparation techniques were studied. The minimum thickness which was able to promote dropwise condensation was 500 (ANGSTROM). Vapor deposited silver over copper also promoted dropwise condensation. The minimum required vapor deposit thickness was not found, however, it is not more than 3,000 (ANGSTROM).
For runs which were dropwise, the range of temperature driving force ((DELTA)T) observed was 0.9(DEGREES)F to 42.3(DEGREES)F. Heat fluxes as high as 490,000 BTU/hr.sq.ft. were obtained. While the surfaces with total filmwise condensation did not produce a steam side heat transfer coefficient (h) greater than 4,210 BTU/hr.sq.ft.(DEGREES)F, values of h as high as 42,650 BTU/hr.sq.ft.(DEGREES)F were obtained on the dropwise surfaces.
The lifetime of dropwise condensation on electroplated silver was also found to depend on the electroplating bath used and surface preparation prior to plating. One surface promoted nearly perfect dropwise condensation for 2,400 hours. A portion of this surface was still condensing dropwise even after 3,450 hours of continuous condensation.
Auger electron spectroscopy, Electron Spectroscopy for Chemical Analysis (ESCA) and scanning electron microscopy were used to analyze the condensing surfaces. Surfaces which were dropwise had larger amounts of silver and carbon, and smaller amounts of copper and oxygen, then surfaces which were filmwise. A variable, which is the ratio of several surface concentrations, was introduced which correlated all heat transfer and Auger data even on surfaces which had mixed (part dropwise and part filmwise) condensation. ESCA and Auger analyses verified that the increased carbon on dropwise surfaces was being adsorbed during steam condensation.
A Nusselt type derivation for filmwise condensation on a vertical disc is also presented.
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