Principles of Direct-Contact Transfer

Shell and tube heat exchangers evolved because of the need to prevent contamination of the hot fluid by the cold fluid in many heat transfer operations. In the case of a water cooling tower, where one of the fluids is a gas and the other a liquid, an impervious surface or separation is not necessary because the gas and liquid are. readily separable after mixing and exchanging heat. 

Cooling towers fall under the general category of direct-contact equipment. In direct-contact equipment, fouling resistances are automatically eliminated because a surface is no longer available on which they can form. This is a unique feature because it allows direct-contact equipment to operate indefinitely without interruption in thermal performance. 

The principles of direct-contact heat transfer, treated in the discussions to follow, are presently almost exclusively applied to water cooling and humidification of air. It should be noted, however, that these very same principles can be applied to the cooling or heating of just about any insoluble gas or liquid. 

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