Condensation heat transfer of R-410a and R-22 in U-tubes

This study investigates the condensation heat transfer experiments for two pure refrigerants, R-410A and R-22, in the U-bend of smooth and micro-fin tubes at low flow rates. Experiments were performed at saturation temperatures of 30°C and 40°C. The mass flux varied from 50 to 200 kg/m²s, and the inlet quality varied from 0.2 to 0.9. The test results showed that heat transfer coefficient increased as mass flux and quality increased. Condensation heat transfer coefficient of R-22 was higher than R-410A in both smooth and micro-fin tubes. The local heat transfer coefficient of R-22 and R-410A in the bend section was, in average, 1.21 and 1.12 times larger than those in the upper straight section, respectively. The maximum local heat transfer coefficient occurs in the middle of the bend. A condensation heat transfer correlation of straight section is proposed. They predict the data of straight section in smooth and micro-fin tubes with r.m.s. 9.68% and 5.49% respectively.