Heat Chap10-034 - Chapter 10 Boiling and Condensation...

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Chapter 10 Boiling and Condensation Condensation Heat Transfer 10-34C Condensation is a vapor-to-liquid phase change process. It occurs when the temperature of a vapor is reduced below its saturation temperature T sat . This is usually done by bringing the vapor into contact with a solid surface whose temperature T s is below the saturation temperature T sat of the vapor. 10-35C In film condensation , the condensate wets the surface and forms a liquid film on the surface which slides down under the influence of gravity. The thickness of the liquid film increases in the flow direction as more vapor condenses on the film. This is how condensation normally occurs in practice. In dropwise condensation, the condensed vapor forms droplets on the surface instead of a continuous film, and the surface is covered by countless droplets of varying diameters. Dropwise condensation is a much more effective mechanism of heat transfer. 10-36C In condensate flow, the wetted perimeter is defined as the length of the surface-condensate interface at a cross-section of condensate flow. It differs from the ordinary perimeter in that the latter refers to the entire circumference of the condensate at some cross-section. 10-37C The modified latent heat of vaporization h fg * is the amount of heat released as a unit mass of vapor condenses at a specified temperature, plus the amount of heat released as the condensate is cooled further to some average temperature between T sat and T s . It is defined as h h C T T fg fg pl s * . ( ) = + - 0 68 sat where C pl is the specific heat of the liquid at the average film temperature. 10-38C During film condensation on a vertical plate, heat flux at the top will be higher since the thickness of the film at the top, and thus its thermal resistance, is lower. 10-39C Setting the heat transfer coefficient relations for a vertical tube of height L and a horizontal tube of diameter D equal to each other yields L D = 2 77 . , which implies that for a tube whose length is 2.77 times its diameter, the average heat transfer coefficient for laminar film condensation will be the same whether the tube is positioned horizontally or vertically. For L = 10 D , the heat transfer coefficient and thus the heat transfer rate will be higher in the horizontal position since L > 2.77 D in that case. 10-40C The condensation heat transfer coefficient for the tubes will be the highest for the case of horizontal side by side (case b) since (1) for long tubes, the horizontal position gives the highest heat transfer coefficients, and (2) for tubes in a vertical tier, the average thickness of the liquid film at the lower tubes is much larger as a result of condensate falling on top of them from the tubes directly above, and thus the average heat transfer coefficient at the lower tubes in such arrangements is smaller.
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