Performance evaluation and optimal configuration analysis of a CO ₂/NH₃ cascade refrigeration system with falling film evaporator-condenser

This study presents a CO₂/NH₃ cascade refrigeration system, in which a falling film evaporator-condenser is used as the cascade heat exchanger. The thermodynamic analysis results of the proposed system show an improvement in the coefficient of performance (COP) due to the smaller temperature difference provided by this type of cascade heat exchanger. Furthermore, an effectiveness-NTU method based model is developed by considering the constraint of the total thermal conductance. The developed model is subsequently used to examine the influences of the main parameters on the system configurations under the maximum COP condition. Results obtained reveal that, when the overall system COP is maximized, thermal conductance allocation ratios are dominated mainly by the temperature differences of the three heat exchangers and the effectiveness factors of the condenser and the evaporator. This study could contribute to the further development and the optimal design of CO ₂/NH₃ cascade refrigeration systems.