The vapor now must diffuse through the noncondensable

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Unformatted text preview: ith a noncondensable gas condenses, only the noncondensable gas remains in the vicinity of the surface (Fig. 10–29). This gas layer acts as a barrier between the vapor and the surface, and makes it difficult for the vapor to reach the surface. The vapor now must diffuse through the noncondensable gas first before reaching the surface, and this reduces the effectiveness of the condensation process. Experimental studies show that heat transfer in the presence of a noncondensable gas strongly depends on the nature of the vapor flow and the flow velocity. As you would expect, a high flow velocity is more likely to remove the stagnant noncondensable gas from the vicinity of the surface, and thus improve heat transfer. EXAMPLE 10–4 Vapor + Noncondensable gas Cold surface Condensate Noncondensable gas Vapor FIGURE 10–29 The presence of a noncondensable gas in a vapor prevents the vapor molecules from reaching the cold surface easily, and thus impedes condensation heat transfer. Condensation of Steam on a Vertical Plate Saturated steam at atmospheric pressure condenses on a 2-m-high and 3-mwide vertical plate that is maintained at 80°C by circulating cooling water through the other side (Fig. 10–30). Determine (a) the rate of heat transfer by condensation to the plate and (b) the rate at which the condensate drips off the plate at the bottom. SOLUTION Saturated steam at 1 atm condenses on a vertical plate. The rates of heat transfer and condensation are to be determined. Assumptions 1 Steady operating conditions exist. 2 The plate is isothermal. 3 The condensate flow is wavy-laminar over the entire plate (will be verified). 4 The density of vapor is much smaller than the density of liquid, l. Properties The properties of water at the saturation temperature of 100°C are hfg 2257 103 J/kg and 0.60 kg/m3. The properties of liquid water at the film temperature of Tf (Tsat Ts)/2 (100 80)/2 90°C are (Table A-9) 1 atm 3m Ts = 80°C 2m Condensate FIGURE 10–30 Schematic...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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