Lewis number correlation for the air-water system

from Equation 6.22. Hence, for a gas-cooling problem the evaluation of process conditions depends entirely on the value of the outlet air humidity. If, however, we assume a humidity for the exit air for a specified discharge temperature, we can complete the heat balance and determine a value for the L G ratio. 

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. 

Convection alone can be estimated from mass transfer data in packed towers through the relationship developed in Chapter 5. 

If there is no mass transfer occurring, then the enthalpy of the gas is directly proportional to the difference in temperatures between the gas and cooling medium. We know that 

This equation can be rearranged to give the number of transfer units (see Kern [5] for details)  

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Figure 6.16 Lewis number correlation for the air-water system [4].

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