For film condensation, a theoretical analysis of the laminar flow of a liquid film down an inclined surface and the progressive increase in thickness due to condensation yields.
CONDENSATION OF A PURE VAPOR
For
film condensation, a theoretical analysis of the laminar flow of a liquid film
down an inclined surface and the progressive increase in thickness due to
condensation yields the following expression for the mean heat transfer
coeffi-cient, hm:
where
λ is the latent heat
of vaporization, and ρ,
k, and η are the density, the thermal
conductivity, and the viscosity of the liquid, respectively. ΔT is the dif-ference in temperature
between the surface and the vapor. Experimentally deter-mined coefficients
confirm the validity of equation (3.14). In practice, however, coefficients are
somewhat higher because of disturbance of the film arising from a number of
factors. As the condensation rate rises, the thickness of the condensate layer
increases and the film coefficient falls. However, a point may be reached in
long vertical tubes at which flow in the layer becomes turbulent. Under these
conditions, the coefficient again rises and equation (3.14) is not valid.
Coefficients may also be increased if high vapor velocities induce ripples in
the film.
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