Evaporation and Condensation Heat Transfer and Pressure Drop in Horizontal, 12.7-mm Microfin Tubes With Refrigerant 22

Using R-22 as the working fluid, a series of tests was performed to determine the evaporation and condensation performance of three 12.7-mm o.d. tubes having many small, spiral inner fins. The tubes, referred to as microfin tubes, had a 11.7-mm maximum i.d., 60 or 70 fins with heights ranging from 0.15 to 0.30 mm, and spiral angles from 15 to 25 deg. A smooth tube was also tested to establish a basis of comparison. The test apparatus had a straight, horizontal test section with a length of 3.67 m and was heated or cooled by water circulated in a surrounding annulus. Nominal evaporation conditions were 0 to 5°C (0.5 to 0.6 MPa) with inlet and outlet qualities of 15 and 85 percent, respectively; condensation conditions were 39 to 42°C (1.5 to 1.6 MPa) with inlet and outlet qualities of 85 and 10 percent, respectively. Mass flux varied from 75 to 400 kg/m²·s. The average heat transfer coefficients in the microfin tubes, based on a nominal equivalent smooth tube area, were 1.6 to 2.2 times larger for evaporation and 1.5 to 2.0 times larger for condensation than those in the smooth tube. The pressure drop increased, but by a smaller factor than the heat transfer coefficient.