Comparison of refrigerant charge requirements in an optimized fin and tube evaporator versus plate heat exchangers

Air-to-water heat pump systems with natural refrigerants require a minimum refrigerant charge. Two systems with direct evaporation in a fin-and-tube evaporator and a brazed plate evaporator with an additional brine-to-air heat exchanger will be compared. Experimental data obtained for evaporation of propane (R290) in small sized smooth tube and an internally enhanced tube is used for an optimal design of a fin-and-tube evaporator with a heat output of 5 kW. Two separately developed simulation codes were used to calculate the required refrigerant charge and the efficiency of both systems. The results indicate the feasibility and limitations of a fin-and-tube heat exchanger in a system with direct evaporation in charge reduced air-to-water heat pump systems.