NH₃ condensation in a plate heat exchanger: Flow pattern based models of heat transfer and frictional pressure drop

This paper develops predicting models for NH₃ condensation in plate heat exchangers based on the experiments of flow patterns, heat transfer coefficients and frictional pressure drop previously reported by the authors. The aim is to provide design methods of compact plate condensers used in NH₃ systems, which are not available in open literature. The experimental data are firstly compared with selected correlations, showing a poor agreement. A heat transfer model is developed based on flow patterns, which represents the transition from convective condensation to gravity-controlled condensation. The physical interpretation of the two-phase multiplier approach and the deviation from Nusselt's theory are discussed. A transition criterion of condensation mechanisms is proposed based on the wetting characteristics. Since the flow patterns indicate separated flow, the Lockhart and Martinelli model is selected and is modified to predict the frictional pressure drop. The model is the sum of the liquid pressure drop, vapor pressure drop and interface pressure drop. The contributions of vapor pressure drop and interface pressure drop are discussed and quantified. The proposed heat transfer and frictional pressure drop models show good predictive performances. NH₃ flow has large two-phase slip because of the large density ratio. Plate heat exchangers have corrugated channels and tend to break up the liquid film. The models identify the distinct flow characteristics based on flow patterns.