Equivalent Enthalpy Model and Log-Mean Enthalpy Difference Method for Heat-Mass Transfer in Cooling Towers

Abstract

Abstract In industrial processes involving both heat and mass transfer, the enthalpy or total energy difference, not the temperature difference, is a more comprehensive driving-force potential for heat exchanger systems, such as a cooling tower. However, the common practice of current enthalpy methods based on Merkel integral requires numerical integration with implied linear distributions of air enthalpy and water temperature. If the cooling range is relatively small, the outcome of the integration may carry little error. But for a large cooling range and a substantially increased temperature, the error involved may be significant. Like log-mean temperature difference, a log-mean enthalpy difference method may require only the inlet and outlet conditions without prior knowledge of the enthalpy distributions of the fluids within the heat exchanger. The known Berman's enthalpy difference method requires correction factors, which was derived based on the linearization of the interfacial enthalpy and the enthalpy difference between the interfacial moist-air enthalpy and the bulk air enthalpy without underlying interpretation to demonstrate that the method could be broadly used. In this paper, an equivalent enthalpy model is proposed, and the log-mean enthalpy difference is derived without any constraints for broad uses. The derived method is compared with the results of Merkel's method from literature with excellent agreement. Thus, the work in this paper would open the possibility for broad uses of the log-mean enthalpy difference method for many industrial processes involving mass transfer.