Heat Exchangers

Heat is a form of energy which we need to exchange for usual and unusual reasons. And, that is why heat exchangers have been created. Heat exchangers are devices that facilitate heat transfer between two or more bodies. Heat exchanger is very different from condensers and boilers. Heat exchangers are just devices which facilitate heat transfer and this heat transfer can be done to condensate or to boil a fluid. So, in short the purpose of a heat exchanger is to transfer heat from a hot body to a cold body but it doesn't mean that it is a condenser or it's cooling something. 

High Vapor Velocity Effect on Condensation Film Theory

According to Nusselt's assumption on condensation film theory, the vapor will be stagnant at the interface and there will be no frictional drag between the condensate and the vapor. However, it is not valid when the vapor velocity is large compared to the condensate velocity. Vapor causes the film thickness to increase. It happens when the vapor flows upward. The film thickness decreases when the vapor flows downward. Thus the heat transfer co-efficient increases.
Carpenter and Colburn correlated the data for condensation with high vapor velocity and got the following equation,

here,
hm = heat transfer co-efficient
Gm= mean mass velocity of vapor
f= frictional co-efficient