Variation in tower size factor with approach

The number of transfer units or tower characteristics is based on the overall heat and mass transfer expression given by Equation 5.33. 

Curve A-B has been discussed already. It represents the saturation curve. The points A, B are fixed by the outlet, t2, and inlet, tlt water temperatures. 

Line C-D, the operating line, gives the actual enthalpy, H, of air plotted against water temperatures. The wet-bulb temperature at any point on C-D is found by projecting the point horizontally to the water operating line, then vertically to the abscissa of the diagram. 

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Figure 5.11 Variation in tower size factor with approach.

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