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Most condensation processes encountered in

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Unformatted text preview: al pipes at a rate of 32.8 g/s per meter length of the tubes. 10–6 I FILM CONDENSATION INSIDE HORIZONTAL TUBES So far we have discussed film condensation on the outer surfaces of tubes and other geometries, which is characterized by negligible vapor velocity and the unrestricted flow of the condensate. Most condensation processes encountered in refrigeration and air-conditioning applications, however, involve condensation on the inner surfaces of horizontal or vertical tubes. Heat transfer analysis of condensation inside tubes is complicated by the fact that it is strongly influenced by the vapor velocity and the rate of liquid accumulation on the walls of the tubes (Fig. 10–34). For low vapor velocities, Chato recommends this expression for condensation Liquid Vapor hinternal 0.555 g l( l l (Tsat ) k3 l Ts) hfg 3 C (T 8 pl sat 1/4 Ts) (10-34) for Tube Revapor D inlet 35,000 (10-35) FIGURE 10–34 Condensate flow in a horizontal tube with large vapor velocities. where the Reynolds number of the vapor is to be evaluated at the tube inlet conditions using the internal tube diameter as the characteristic length. Heat transfer coefficient correlations for higher vapor velocities are given by Rohsenow. 10–7 I DROPWISE CONDENSATION Dropwise condensation, characterized by countless droplets of varying diameters on the condensing surface instead of a continuous liquid film, is one of the most effective mechanisms of heat transfer, and extremely large heat transfer coefficients can be achieved with this mechanism (Fig. 10–35). In dropwise condensation, the small droplets that form at the nucleation sites on the surface grow as a result of continued condensation, coalesce into large droplets, and slide down when they reach a certain size, clearing the surface and exposing it to vapor. There is no liquid film in this case to resist heat transfer. As a result, with dropwise condensation, heat transfer coefficients can be achieved that are more than 10 times larger than those ass...
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