Coupled ϵ-NTU Method to Design and Evaluate the Performance of Energy Exchangers With Coupled Heat and Mass Transfer

Abstract The classical ϵ-number of transfer units (NTU) method is widely used to design and evaluate the performance of heat and mass energy exchangers. In energy exchangers, where the heat and mass transfer are coupled, i.e., the magnitude of heat transfer impacts the magnitude of mass transfer and vice-versa, the classical ϵ-NTU method fails to capture the outlet fluid conditions of the energy exchanger accurately. It cannot be used for designing/evaluating the performance of energy exchangers where heat and mass transfer are coupled. The coupled ϵ-NTU model uses modified heat and mass capacity ratios to capture the effects of coupled heat and mass transfer. The use of the coupled ϵ-NTU model to design and evaluate the performance of energy exchangers is illustrated, specifically on a liquid-air-membrane energy exchanger (LAMEE), but the model can be extended to other coupled energy exchangers. The coupled ϵ-NTU model is validated using a numerical model of a LAMEE in counterflow and crossflow configurations. The validation is completed for over 14,500 test points representing a wide range of operating conditions. The average error in estimating sensible and moisture transfer effectiveness using the coupled ϵ-NTU method is less than ±1.5% for both configurations, compared to the numerical model illustrating the robustness of the coupled ϵ-NTU model. Of the 14,500 tested points, the error in estimating sensible or moisture transfer effectiveness is greater than 4% for less than 5% of the test points.