NH3/H2O based systems are promising for thermal energy storage and thermal energy conversion. These systems are used for absorption energy storage and Kalina cycles. This paper investigates the condensation of high-concentration NH3/H2O in vertically downward plate heat exchangers. A combined method is proposed by discussing the applicability of equilibrium and non-equilibrium models. Both models are necessary for zeotropic mixtures with large temperature glide. The non-equilibrium model applies where the temperature glide is non-linear or the vapor is in non-equilibrium with the liquid. The equilibrium model becomes applicable with decreasing vapor qualities. Heat transfer correlations are proposed according to the equilibrium model, which interpret convective condensation and gravity-controlled condensation. The additional heat transfer resistance is calculated considering mass transfer. The non-equilibrium model is further developed quantifying the heat and mass transfer of vapor and neglecting the mass transfer resistance of the liquid. The non-equilibrium model transforms into the equilibrium model as the concentration gradient of vapor approaches zero. Additionally, a frictional pressure drop model for separated flow conditions is proposed and quantifies the two-phase shear force.
