BX-7 Heat Transfer and Pressure Drop in Horizontal Thermosiphon Reboilers: Enhanced Tube Bundles

L. W. Swanson

This report describes shellside heat transfer and pressure drop phenomena in horizontal thermosiphon reboilers with enhanced-tube bundles. HTRI used its horizontal thermosiphon reboiler enhanced-tube data to develop and verify expressions for the low-finned tube fin efficiency factor and the Turbo-B tube surface factor.

A fin efficiency factor for smooth annular fins was applied to the comprehensive shellside model developed in Reports BX-5 and BX-6. The model was compared with the experimental data and yielded excellent results. The mean ratio of the calculated-to-measured shellside heat transfer coefficients was 1.00 with a standard deviation of 0.166. The mean ratio of the calculated-to-measured thermosiphon reboiler static heads was 0.976 with a standard deviation of 0.191.

From Turbo-B nucleate pool boiling data provided by Wolverine Tube, Inc., HTRI developed nucleate boiling surface factor correlations for n-pentane and p-xylene. Because mixture data were not available, a nucleate boiling surface factor for mixtures was derived in terms of pure-component surface factors and plain-tube heat transfer coefficients. These surface factor relationships were applied to the flow boiling model in BX-5 and BX-6, and the model was compared with experimental data. HTRI ascertained the mixture penalty for p-xylene/n-pentane under flow boiling conditions to be less than that predicted by the theoretical pool boiling mixture surface factor. The results also showed that additional nucleate boiling suppression occurred for large bubbles with departure diameters greater than the thermal layer thickness. HTRI developed a flow boiling surface factor correction to reconcile these discrepancies. After this correction was applied to the flow boiling model, the mean ratio of calculated-to-measured heat transfer coefficients was 0.981 with a standard deviation of 0.172.