The Merkel method, developed in the 1920s, relies on several critical assumptions to reduce the solution to a simple manual iteration.
These assumptions are:
The resistance for heat transfer in the water film is negligible,
The effect of water loss by evaporation on energy balance or air process state is neglected,
The specific heat of air-stream mixture at constant pressure is same as that of the dry air, and
The ratio of hconv/hdiff (Lewis factor) for humid air is unity.
Merkel combined equations for heat and water vapor transfer into a single equation similar as