The paper describes a theoretical investigation for the steady-state counter flow wet cooling tower with modified definitions for both the number of transfer units and the tower thermal effectiveness. The modified number of transfer units is dependent on both air and water heat capacity. The effectiveness is defined by the tower cooling range and the approach to equilibrium. A new expression relating the tower effectiveness to the modified number of transfer units and the capacity rate ratio has been developed. The model considered the resistance to heat transfer in the water film, the nonunity of the Lewis number, and the curvature of the saturated air enthalpy curve. A procedure for implementing the model in designing or rating cooling towers has been outlined and demonstrated through illustrative examples. The model compares very satisfactorily with other methods such as Logarithmic Mean Enthalpy Difference (LMED) and conventional effectiveness—NTU. Within the ranges used, the obtained results showed that substantial errors varied from +4.289 to −2.536 percent can occur in calculating the cooled water outlet temperature, and errors from +42.847 to −16.667 percent can occur in estimating the tower thermal characteristics.
Issue Section:
Applications of Heat Transfer
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