RB3-1 Enhanced Boiling Surfaces as Single Tubes and Tube Bundles

S. Yilmaz and J. W. Palen

Two enhanced tube surfaces (Gewa-T and Thermoexcel-E) with re-entrant nucleation sites were tested by HTRI both as single tubes and in tube bundles as internal reboilers. Single enhanced and plain tubes were tested with pure p-xylene and isopropyl alcohol in pool boiling at atmospheric pressure (103 kPa) using steam as the heating medium. Both enhanced tubes performed about the same with p-xylene, except at lower temperature differences where Thermoexcel-E gave a slightly better performance. Tests with isopropyl alcohol indicated somewhat better performance for Thermoexcel-E than Gewa-T. Both tubes gave several times higher heat fluxes than the plain tube at the same temperature differences.

Two enhanced bundles and a plain tube bundle were tested as steam-heated internal reboilers in the HTRI Boiling and condensation Research Plant. p-Xylene was boiled at atmospheric pressure (103 kPa) and at 5 psia (35 kPa) with all three bundles. Additional data were obtained with pure Freon 11 and a mixture of n-pentane/p-xylene for the Thermoexcel-E bundle only. An unintentional fouling of the Thermoexcel-E bundle due to a water leak into the Freon 11 test fluid resulted in an unplanned by interesting fouling study of an enhanced boiling surface.

For p-xylene at two pressures tested, about the same amount of improvement in performance was observed for Gewa-T and Thermoexcel-E bundles when compared to plain tube bundle data. The amount of improvement in boiling heat flux (based on measured outside tube diameter) is less for the bundles than for the single tubes tested. Although HTRI has demonstrated that plain tube bundle performance is higher than that of a single plain tube, the opposite is true for the enhanced tubes. This could be explained by a nucleate boiling suppression effect on enhanced surface caused by bubble-induced natural circulation. Enhancement is still appreciable for the bundles, but is not as great as would be expected from single tube data. Further study of their phenomenon will be required before issue of the detailed report on this subject.