Gas Cooling Operations

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. 

This correction becomes important in gas cooling operations. Unfortunately, we cannot evaluate Equation 6.19 in a straightforward manner. 

For the gas cooling case in which Le = 1, we must redefine our basic equations for humidification and dehumidification developed back in Chapter 5. 

In the above equations we have redefined our temperatures so that T refers to the hot gas and t to the cold water. 

To evaluate Ntu, we must simultaneously integrate all three equations. This means a trial-and-error approach to the solution. 

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There is a special case sometimes encountered with gas coolers, which leads to a simplified calculation procedure. This is the case in which hot gas contacts a nonvolatile cooling liquid. In such a situation the mass transfer can be negligible, and for some systems the latent heat of vaporization may be so small that the diffusion heat load can be neglected. It is possible then to neglect mass transfer altogether in this type of gas cooling operation. 

Tray towers These are very effective but are not commonly used in humidification, dehumidification, or gas-cooling operations for reasons of cost and relatively high pressure drop, except under special circumstances. Design considerations are available [1,4]. 

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