Condensation of refrigerants CFC11 and CFC113 in the annulus of a double-tube coil with an enhanced inner tube Original Research Article

Local heat transfer and pressure drop measurements were made during condensation of pure refrigerants CFC11 and CFC113 in the annulus of a double-tube coil consisting of three U-bends and four straight lengths. The inner tube is a 19.1-mm O.D. corrugated tube with wire fins soldered onto the outer surface, and the inner diameter of the outer duct is 25.0 mm. The mass velocity of the test fluids ranged from 100 to 235 kg/(m2 s). The local heat transfer coefficient decreased along the tube length, with the U-bends showing higher coefficients than the straight lengths. The amplitude of the coefficient variation decreased as the condensation proceeded and was less remarkable at a lower vapor velocity. The experimental data were compared with the previous results for the straight annuli obtained using the same inner tube as the present study. Empirical equations for the local heat transfer coefficient and frictional pressure gradient were developed for both the U-bends and the straight lengths.