General Design Considerations and Packing Coefficients

Approximately 80% of the transfer of heat is by evaporation and the remaining 20% by temperature gradient. Practically, the cold water temperature approaches the wet-bulb temperature of the air and the magnitude 

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. 

Earlier we described the area bound by A-B-C-D and noted it to be proportional to the reciprocal of the value of the integral for Ntu. This value defines the characteristics of the tower, and it is clear that the lower the area, the higher the value KaV/L, i.e., high contact area (a) and high contact volume (V) therefore, high investment cost. The reduction of the investment cost and, therefore, the increase of area A-B-C-D, can be achieved by increasing 

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