4. 5 A model with latent heat transfer
In heat transfer under normal conditions, normal air is wet and is defined as the mixed
ideal gases of dry air and water vapor. It is not difficult to observe this by putting a glass
of cooled beer on a desk in summer time. The air around the glass is cooled and water
vapor in the air condenses at the surface of the glass. This means that the air temperature
at the surface of the glass is cooled lower than the dew-point temperature of the air at that
temperature. This is shown well on the psychrometric chart (Fig. 4.9).