Effectiveness–NTU relation for packed bed liquid desiccant–air contact systems with a double film model for heat and mass transfer

One-dimensional models were usually utilized to describe the coupled heat and mass transfer processes in packed bed liquid desiccant–air contact systems. In this paper, a double film model was utilized for both parallel and countercurrent flow configurations. The model considered the effects of non-unity values of Lewis factor, unequal effective heat and mass transfer areas, liquid phase heat and mass transfer resistances, changes in solution mass flow rate and concentration. Within the relatively narrow range of operating conditions usually encountered in a specified application, a linear approximation was made to find out the dependence of equilibrium humidity ratio on solution temperature and concentration. Constant approximations of some properties and coefficients were further made to render the coupled equations linear. The original differential equations were rearranged and an analytical solution was developed for a set of newly defined parameters. Analytical expressions for the tower efficiency and other effectiveness values were further developed based on the analytical solution. Comparisons were made between analytical results and numerical integration of the original differential equations and the agreement was found to be quite satisfactory.