Study on Reducing R1336mzz(Z) Refrigerant Charge via Heat Transfer Analysis in a Falling Film Evaporator for Industrial High-Temperature Heat Pumps

This study examines the reduction of refrigerant charge for R1336mzz(Z) through heat transfer analysis in a falling film evaporator designed for industrial high-temperature heat pumps. A lab-scale test facility was established to evaluate two types of heat transfer tubes—bare tubes and E-tubes—under film Reynolds numbers ranging from 100 to 550 and heat fluxes between 5 and 25 kW/m². The results indicate that the falling film heat transfer coefficient for the bare tube increased with heat flux, demonstrating a strong dependence on surface nucleation boiling dynamics. In contrast, the E-tube showed nearly constant values regardless of heat flux, suggesting that the surface enhancement resulted in fully developed nucleation boiling. To further explore the potential for minimizing refrigerant charge, a hybrid evaporator configuration combining flooded and falling film operation was compared to a conventional flooded evaporator using E-tube conditions. The hybrid system achieved a 48% reduction in refrigerant charge per unit of evaporation capacity while maintaining comparable evaporation performance. These findings underscore the benefits of using falling film evaporators with advanced tube designs, which provide improved thermal performance and a significant reduction in the use of costly, low-GWP refrigerants like R1336mzz(Z). This work lays the groundwork for the practical design of high-temperature heat pumps aimed at industrial steam production.