Evaporator development for adsorption heat transformation devices - influencing factors on non-stationary evaporation with tube-fin heat exchangers at sub-atmospheric pressure

Evaporators for adsorption heat pumps, chillers and storage devices mostly use water as refrigerant and work at sub-atmospheric pressures. However, there are hardly any applicable performance correlations or sizing guidelines for these rather unusual operating conditions. Within this study geometric and process-related impacts on non-stationary evaporation performance of copper tube-fin heat exchangers are investigated to start filling that gap. Cyclic condensation/evaporation measurements were performed in a thin film evaporation mode and in partially flooded operation with changing refrigerant filling level to cover different applications. For thin film evaporation a thermal resistance model was developed to quantify resistance contributions and identify performance-limiting factors. The presented measurement results reveal that evaporation performance in thin film operation is crucially governed by fluid side heat transfer (UA raise by 146% within tested Reynolds numbers) and wetting conditions while fin sheet thickness plays a marginal role (8% increase of UA). In partially flooded operation performance strongly depends on filling level. Apart from some refinement potential of the thin film model, the simulations reflect evaporation dynamics and effects of influencing factors fairly well which indicates that the employed model approach could be a suitable tool for an effective optimization of evaporator geometry and process parameters.