Experimental study on the Heat transfer performance of Enhanced tubes for falling film evaporation
In response to global carbon neutrality goals, improving energy efficiency in the industrial sector has become increasingly important, leading to the expansion of industrial heat pump applications utilizing waste heat. Development and application of low global warming potential (GWP) refrigerants to replace HFCs are accelerating. R1233zd(E) is suitable for industrial heat pump systems utilizing low-temperature waste heat due to its non-flammability, low toxicity, and relatively higher latent heat compared to other low-GWP refrigerants. Falling film evaporators spray refrigerant directly onto tube surfaces, offering the advantage of lower refrigerant charge compared to flooded evaporators. Falling film evaporators have primarily been used in small-capacity systems, and because refrigerant is directly sprayed to enhanced tube surfaces, accurate evaluation of the external heat transfer performance of enhanced tubes is necessary. This study experimentally investigates the overall heat transfer coefficient of enhanced evaporation tubes with different fins per inch (FPI). The Outer and overall heat transfer coefficients are measured over a falling film Reynolds number range of 50–300, with a refrigerant temperature of 36.5 °C. Overall heat transfer coefficient is 5 kW/(m²·K) for 45 FPI tube, 5.5 kW/(m²·K) for 48FPI tube, and 6 kW/(m²·K) for 52FPI tube at 0.9 m/s water velocity. At 2.0 m/s water velocity, the overall heat transfer coefficient for all enhanced tubes increases by more than 10% compared to the overall heat transfer coefficient at 0.9 m/s water velocity. The Outer heat transfer coefficient is 9 kW/(m²·K) for 45 FPI tube, 10 kW/(m²·K) for the 48 FPI tube, and 12 kW/(m²·K) for 52 FPI tube at 2.0 m/s water velocity.