Fourteen polymer coatings were evaluated for their ability to promote and sustain dropwise condensation of steam. Nine of the coatings employed a fluoropolymer as a major constituent; four employed hydrocarbons and one a silicone. Each coating was applied to 25-mm-square by approximately 1-mm-thick metal substrates of brass, copper, copper–nickel, and titanium. While exposed to steam at atmospheric pressure, each coating was visually evaluated for its ability to promote dropwise condensation. Observations were also conducted over a period of 22,000 hr. Hardness and adhesion tests were performed on selected specimens. On the basis of sustained performance, six coatings were selected for application to the outside of 19-mm-dia copper tubes in order to perform a heat transfer evaluation. These tubes were mounted horizontally in a separate apparatus through which steam flowed vertically downward. Steam-side heat transfer coefficients were inferred from overall measurements. Test results indicate that the steam-side heat transfer coefficient can be increased by a factor of five to eight through the use of polymer coatings to promote dropwise condensation.

No abstract

The film condensation heat transfer performance of six externally finned copper tubes has been evaluated. All tubes had rectangular-shaped fins with a height and thickness of 1 mm. The spacing between fins was 0.5, 1.0, 1.5, 2.0, 4.0, and 9.0 mm. Data were also obtained for a smooth tube whose outside diameter of 19.0 mm was equal to the diameter at the base of the fins for all of the finned tubes. Tests were performed both at atmospheric pressure and under vacuum (∼ 11.3 kPa). Steam flowed vertically downward with a velocity of approximately 1 and 2 m/s at atmospheric pressure and under vacuum, respectively. The smooth tube was fitted with wall thermocouples for the evaluation of the water-side heat transfer coefficient. This was used, subsequently, to determine the steam-side heat transfer coefficient for the finned tubes for which only overall measurements were made. Strenuous efforts were made to obtain high-accuracy data; in particular, the coolant temperature rise was determined by both quartz-crystal thermometers and a 10-junction thermopile. The two temperature-rise measurements always agreed to within ± 0.03 K. Care was taken to avoid errors due to the presence of noncondensing gases and to ensure that filmwise condensation conditions prevailed over the entire tube throughout all tests. The steam-side heat transfer coefficient for the smooth tube agreed closely with values found by other recent workers. Maximum steam-side enhancement was found for the tube with a fin spacing of 1.5 mm. At this fin spacing, the heat transfer enhancement ratios were around 3.6 and 5.2 for low-pressure and atmospheric pressure runs, respectively.

Heat transfer measurements were made at near-atmospheric pressure on a smooth tube, on 24 integral-fin tubes having machined, rectangular-shaped fins, and on a commercial integral-fin tube. All tubes were made of copper. The vapor flowed vertically downward with a nominal velocity of 0.4 m/s. Vapor-side heat transfer coefficients were determined with a typical uncertainty of ± 7 percent using a “modified Wilson plot” technique. The vapor-side heat transfer coefficient of the integral-fin tubes (based upon the outside surface area of the smooth tube) was enhanced considerably more than the surface area enhancement provided by the fins. Heat transfer enhancements (for the same vapor-to-wall temperature difference) up to around 7 were measured for a corresponding area enhancement of only 3.9. The optimum fin spacing was found to lie between 0.2 and 0.5 mm, depending upon fin thickness and height. The data were compared with those of other investigations and with several existing theoretical models. Visual observations of condensate drainage patterns from the finned tubes were also made.

No abstract