The early investigators of cooling tower theory grappled with the problem presented by the
dual transfer of heat and mass. The Merkel theory overcomes this by combining the two into
a single process based on enthalpy potential. Dr. Frederick Merkel was on the faculty of the
Technical College of Dresden in Germany. He died untimely after publishing his cooling
tower paper. The theory had attracted little attention outside of Germany until it was
discovered in German literature by H.B. Nottage in 1938.
Cooling tower research had been conducted for a number of years at University of California
at Berkley under the direction of Professor L.K.M. Boelter. Nottage, a graduate student, was
assigned a cooling tower project which he began by making a search of the literature. He
found a number of references to Merkel, looked up the paper and was immediately struck by
its importance. It was brought to the attention of Mason and London who were also working
under Boelter and explains how they were able to use the Merkel theory in their paper.
Dr. Merkel developed a cooling tower theory for the mass (evaporation of a small portion of
water) and sensible heat transfer between the air and water in a counter flow cooling tower.
The theory considers the flow of mass and energy from the bulk water to an interface, and
then from the interface to the surrounding air mass. The flow crosses these two boundaries,
each offering resistance resulting in gradients in temperature, enthalpy, and humidity ratio.
For the details for the derivation of Merkel theory, refer to Cooling Tower Performance
edited by Donald Baker and the brief derivation is introduced here. Merkel demonstrated that
the total heat transfer is directly proportional to the difference between the enthalpy of
saturated air at the water temperature and the enthalpy of air at the point of contact with
water.