If a mixture of condensable and noncondensable gases is cooled below its dew point at a surface, the former condenses, leaving the adjacent layers richer in the latter, thus creating an added thermal resistance.
CONDENSATION OF MIXED VAPORS
If
a mixture of condensable and noncondensable gases is cooled below its dew point
at a surface, the former condenses, leaving the adjacent layers richer in the
latter, thus creating an added thermal resistance. The condensable fraction
must diffuse through this layer to reach the film of condensate, and heat
transfer coefficients are normally very much lower than the corresponding value
for the pure vapor. For example, the presence of 0.5% of air has been found to
reduce the heat transfer by condensation of steam by as much as 50%.
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