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Unformatted text preview: efore, the actual heat transfer will be larger. Rohsenow showed in 1956 that the cooling of the liquid below the saturation temperature can be accounted for by replacing hfg by the
modified latent heat of vaporization h* , defined as
fg
h*
fg hfg 0.68Cpl (Tsat Ts) (109a) where Cpl is the specific heat of the liquid at the average film temperature.
We can have a similar argument for vapor that enters the condenser as
superheated vapor at a temperature T instead of as saturated vapor. In this
case the vapor must be cooled first to Tsat before it can condense, and this heat
must be transferred to the wall as well. The amount of heat released as a unit
mass of superheated vapor at a temperature T is cooled to Tsat is simply
Tsat), where Cp is the specific heat of the vapor at the average temCp (T
Tsat)/2. The modified latent heat of vaporization in this case
perature of (T
becomes
h*
fg hfg 0.68Cpl (Tsat Ts) Cp (T Tsat) (109b) With these considerations, the rate of heat transfer can be expressed as
·
Q conden hAs(Tsat Ts) mh*
fg (1010) where As is the heat transfer area (the surface area on which condensation oc·
curs). Solving for m from the equation above and substituting it into Eq. 10–8
gives yet another relation for the Reynolds number,
Re Re = 0
Laminar
(wavefree)
Re ≅ 30 ·
4Q conden
p l h*
fg 4As h(Tsat Ts)
p l h*
fg (1011) This relation is convenient to use to determine the Reynolds number when the
condensation heat transfer coefficient or the rate of heat transfer is known.
The temperature of the liquid film varies from Tsat on the liquid–vapor interface to Ts at the wall surface. Therefore, the properties of the liquid should
be evaluated at the film temperature Tf (Tsat Ts)/2, which is approximately
the average temperature of the liquid. The hfg, however, should be evaluated
at Tsat since it is not affected by the subcooling of the liquid. Flow Regimes
Laminar
(wavy)
Re ≅ 1800
Turbulent FIGURE 10–23
Flow regimes during film
condensation on a vertical plate. The Reynolds number f...
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 Spring '10
 Ghaz

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