SINOPSIS

Accurate design of air-cooled condensers using inclined flat tubes requires a reliable method for determining heat transfer and pressure drop in the downflow and upflow sections. This paper presents experimental study of downflow and upflow condensation in an inclined flat tube under vacuum. Experimental results and analysis confirm that the heat transfer mechanism underlying the condensation in an inclined flat tube with a large flatter cross section is the same as for downflow condensation on a vertical plate under gravity influence. The characteristics of two-phase pressure drop in an inclined flat tube are also similar for downflow condensation inside a circular tube. Upflow reflux condensation has a better heat transfer performance compared to downflow condensation due to the opposite vapor and condensate flow, thus enhancing heat transfer on the vapor–liquid interface. Using experimental data, the empirical constants in the Nusselt model and the Chisholm two-phase frictional multiplier in the Lockhart–Martinelli pressure drop model were modified. Comparisons show that the modified Nusselt correlation predicts heat transfer data within ± 15% and pressure drop predictions can be achieved within ± 30% for the majority of experimental data. This study provides an alternate approach for design and optimization of air-cooled condensers with inclined flat tubes.