The
process to reduce the condensable vapor content from the mixture of condensable
and non-condensable gas mixture is called dehumidification.
Condensation is the change of vapor in a non-condensable gas to liquid.
The Dehumidification
of air can be effected by bringing it into contact with a cold surface, like
the contact of moist air with the surface of a glass containing cold
water. If the temperature of the surface is lower than the dew point of
the gas, condensation takes place and the temperature of the gas falls.
The temperature of the surface tends to rise because of the transfer of latent
and sensible heat from the air.
It would
be expected that the air would cool at constant humidity until the dew
point was reached, and that subsequent cooling would be accompanied by
condensation. It is found, in practice, that this occurs only when the air is
well mixed.
To illustrate the case, let us assume
that a glass is containing water at 25 oC. The surfaces of glass are
in thermal equilibrium with that of water. The glass is surrounded with air at
40 oC and relative humidity of 70 %. What is dew point of the mixture?
Will the condensation occur?
Pg = Psat40 = 7.375
kPa => Pv = φ Pg = 0.7×7.375 = 5.1625 kPa
Tdew is T when Pv = Pg = 5.1625 kPa
From the
tables;
Tdew = 33.5 oC
Yes,
condensation will occur because Twater/glass
< Tdew.
Normally
the temperature and humidity are reduced simultaneously throughout the
whole of the process. The air in contact with the glass surface is cooled below
its dew point, and condensation of vapor therefore occurs before the more
distant air has time to cool.
Where the
air stream, above the surface of cold water, is cooled by cold water of the
glass, then an interfacial film is always produced above the surface of
cold water that is due to decrease of temperature of the moist air. This temperature
decrease shifts the moist air mixture status from under saturation state
towards saturation state and this new status of moist air appears as the
interfacial film of condensable/non-condensable/partially cooled/partially
condensed mixture. The thin layer of humid air in direct contact with the cold
water is saturated air, the next layer is partially saturated.
The
distant humid air has to cross this interfacial film to reach the surface of
cold water and thus the condensation of water vapor in humid air by the surface
of the cold water is highly resisted.
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