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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
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
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,
Properties The properties of water at the saturation temperature of 100°C are
103 J/kg and
0.60 kg/m3. The properties of liquid water
at the film temperature of Tf
(Table A-9) 1 atm
Ts = 80°C 2m Condensate FIGURE 10–30
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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.
- Spring '10