Heat transfer water cooling theory

The cooling tower cools hot water tvith cool air by countercurrent (or cross-current) fiow of the tw o fluids past each other in a tower filled with packing. This involves both mass and heat transfer. The water surface that exists on the tower packing is covered with an air film assumed to be saturated at the water temperature. The heat is transferred between this film and the main body of air by diffusion and convection. Detailed presentations of the development of cooling tower theory are given in References 39 and 46. 

Our discussion up to now has concerned the cooling of hot process waters exclusively. However, we insisted back in Chapter 1 that a cooling tower is nothing more than a device that transfers heat from one mass to another. Therefore, gas coolers are governed by the same theory of operation and design principles as are water cooling towers. 

First, applications will be considered where a condensible vapor is present in an inert, noncondensible gas. An example of this situation is cooling a chlorine cell gas with water to condense the majority of the gas stream water vapor content. The simplified design method described here is based on a large bank of practical experience with the particular system discussed. Thus, an empirical adjustment of the heat transfer coefficients provides a suitable design basis to overcome the lack of precise theory. 

The principal drawback of the process is the possibility of producing poorly dried gas for a brief period when a hot, regenerated bed is first put on stream for dehydration service. However, operating experience of more than a year with a commercial unit did not show evidence of such occurrences (possibly due to instrumentation limitations). In theory, the heat transfer rone moves through the bed several hundred times faster than the mass transfer zone. As a result, except for a brief period at the beginning of the cycle, the heat transfer zone moves rapidly ahead of the mass transfer zone, providing an adequate supply of cool adsorbent to pick up water contained in the feed gas. 

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