Qxd 942002 1238 pm page 546 546 heat transfer less

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Unformatted text preview: ociated with film condensation. Large heat transfer coefficients enable designers to achieve a specified heat transfer rate with a smaller surface area, and thus a smaller (and FIGURE 10–35 Dropwise condensation of steam on a vertical surface (from Hampson and Özisik, Ref. 11). ¸ cen58933_ch10.qxd 9/4/2002 12:38 PM Page 546 546 HEAT TRANSFER less expensive) condenser. Therefore, dropwise condensation is the preferred mode of condensation in heat transfer applications. The challenge in dropwise condensation is not to achieve it, but rather, to sustain it for prolonged periods of time. Dropwise condensation is achieved by adding a promoting chemical into the vapor, treating the surface with a promoter chemical, or coating the surface with a polymer such as teflon or a noble metal such as gold, silver, rhodium, palladium, or platinum. The promoters used include various waxes and fatty acids such as oleic, stearic, and linoic acids. They lose their effectiveness after a while, however, because of fouling, oxidation, and the removal of the promoter from the surface. It is possible to sustain dropwise condensation for over a year by the combined effects of surface coating and periodic injection of the promoter into the vapor. However, any gain in heat transfer must be weighed against the cost associated with sustaining dropwise condensation. Dropwise condensation has been studied experimentally for a number of surface–fluid combinations. Of these, the studies on the condensation of steam on copper surfaces has attracted the most attention because of their widespread use in steam power plants. P. Griffith (1983) recommends these simple correlations for dropwise condensation of steam on copper surfaces: hdropwise 51,104 2044Tsat , 255,310 22°C Tsat Tsat 100°C 100°C (10-36) (10-37) where Tsat is in °C and the heat transfer coefficient hdropwise is in W/m2 · °C. The very high heat transfer coefficients achievable with dropwise condensation are of little significance if the material of the condensing surface is not a good conductor like copper or...
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