STEAM CONDENSATION ON HORIZONTAL INTEGRAL-FIN TUBES OF LOW THERMAL-CONDUCTIVITY

This work identifies preferred integral-fin geometries for steam conde nsation on low conductivity, integral-fin tubes (admiralty, copper-nic kel, and titanium). Although much work has been done to measure and pr edict condensation coefficients for refrigerants on high thermal condu ctivity copper tubes, very little has been done for the problem of pre sent interest. Because of the low tube thermal conductivity, and conde nsate retention, it is necessary to solve a conjugate problem with tub e side coolant flow. An adaptation of a model previously published by Adamek and Webb is used for the steam side, and the heat transfer to t he coolant, accounting for circumferential wall heat conduction is inc luded in the model. The model was validated by predicting 53 data poin ts for steam, R-11 and R-113. Ninety four percent of the data were pre dicted within +/- 15%. A parametric study was performed to determine t he effect of fin height, fin spacing, and fin shape on the condensing coefficient for steam condensing at 35 degrees C on the three tube mat erials. The results show that the enhancement level decreases as the t ube thermal conductivity decreases. The predicted enhancement level fo r admiralty, copper-nickel, and titanium (or stainless steel) increase s as the fin height is reduced from 1.0 mm to 0.5 mm. The preferred fi n geometry for titanium, copper-nickel, and admiralty tubes is a 0.5 m m fin height, 0.2 mm tip thickness, and 0.9 mm base thickness. A maxim um enhancement level is achieved at 512 fins/m (13 fins/m) for admiral ty, copper-nickel, and titanium, for 0.5 mm fin height. The economic o ptimum fins/in is expected to be less than 512 fins/m. This work has r esulted in the identification of preferred fin geometries for low ther mal conductivity materials, which are different from those commerciall y available.