Condensation of a Pure Vapor

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Chapter: Pharmaceutical Engineering: Heat Transfer

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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