The water cooling process is one of the simultaneous heat and mass-transfer in which sensible
heat is transferred as a result of the difference of temperature between water and air, and a small
proportion 'of the water is evaporated as result of the differential of vapor pressure between the
water surface and the general air steam. The dual nature of the exchange process does not lend
itself to an accurate treatment, but a fortunate relationship between the sensible heat and mass
transfer coefficients allows the two transfers to be combined in one simple transfer equation with
enthalpy as driving force and the mass transfer coefficient as overall coefficient for the combined
transfer process. This total -heat method was originally suggested by Merkel in 1926[1] and has
since been elaborated by several authors, notably Lichtenstein in 1943[2] in the design of
mechanical draught cooling towers, also Woods and Betts in 1950 [3, 4] and Chilton in 1950 [5] in
the design of natural draught cooling towers. Development of the combined equation for the dual
transfer can be found in many papers, in particular one published by Carey and Williamson 1950
[6]. The equation of transfer over the full depth of the packing for a counter current flow tower