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The air stream is further heated in the contact body (cooling fill) until a heat-exchange balance between the air flow and the circulating cooling water is reached. The decrease of the temperature of the cooling water and the increase of the air’s heat content in the cooling tower is a gradual process.

The operation of an (evaporative) cooling tower is based on the principle of combined heat and mass transfer. This is a transfer of sensible (dry) heat between circulating cooling water and passing air by convection and the transfer of latent (wet) heat by evaporation of the cooling water.Air with condition A and a matching wet bulb temperature Tnb1 passes a wet surfacewith the temperature Tw2. The condition of the saturated air in the border between air and water (just above the wet surface) equals the point Tw2,lying on the saturation line in the Mollier diagram.

When air and water pass one another in counter flow, the colder air comes into contact with the warm cooling water and the air absorbs more heat. For this reason the condition of the air moves to the right and follows the curve A-B. In the point B, with a matching wet bulb temperature Tnb2, is a curve directed to the point Tw1. The air flow absorbs more and more water and will eventually be saturated. The force behind the total enthalpy increase of the air is the enthalpy difference
between the air in the border layer air-water and the passing air. In short, in the cooling tower the cooling water is cooled from temperature Tw1 to Tw2. The drawn-in air with condition Tnb1 is heated and humidified to a condition Tnb2, where the air is almost saturated. A rule of thumb is that the wet bulb temperature of the air leaving the cooling tower almost equals the average of the cooling water’s in and outlet temperatures. This depends on the cooling tower’s efficiency.