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.

High concentration NH₃/H₂O is suitable for Kalina cycles used for the recovery of low grade heat. Plate heat exchangers (PHEs) are compact and reduce the charge of working fluid. This paper investigates the condensation of NH₃/H₂O with NH₃ mass concentrations of 80%-96%. The vapor and liquid concentrations are close to equilibrium state, which are different from normal absorbers. The apparent heat transfer coefficients (HTCs) and frictional pressure drop are presented, covering the mass fluxes of 32–86 kgm⁻²s⁻¹, the averaged vapor qualities of 0.08–0.65 and the saturated pressure of 610 to 780 kPa. Larger mass fluxes noticeably increase the apparent HTCs and frictional pressure drop. At the mass concentrations of 96%, 91% and 88%, higher vapor qualities increase the apparent HTCs for large mass fluxes. The apparent HTCs decrease slightly with vapor qualities for 80% mass concentration. The experimental results are compared with those of pure NH₃. The flow patterns of high concentration NH₃/H₂O are considered as full film flow and partial film flow, which are the same as for NH₃. The mass transfer resistance deteriorates the heat transfer especially for partial film flow, which happens at small liquid mass fluxes. The mass transfer resistance has negligible influences on frictional pressure drop.