BX-5 Heat Transfer and Pressure Drop in Horizontal Thermosiphon Reboilers: Plain-Tube Bundles

L. W. Swanson and J. W. Palen

Using horizontal thermosiphon reboiler data, HTRI developed a generalized flow boiling method for predicting heat transfer and pressure drop in TEMA-type shell-and-tube horizontal thermosiphon reboilers. These units, with different shellside geometries, simulate flow boiling on baffled tube bundles. A wide range of fluid conditions, from vacuum to near the critical pressure, were covered for pure hydrocarbons and mixtures, water, and water-glycol mixtures.

Wet-wall mechanisms for nucleate boiling, convective boiling, natural convection, and thin film evaporation were superposed to determine the boiling heat transfer coefficient. Depending on flow regime and critical heat flux, two major dry-wall regimes, film boiling and mist flow, were included in the model. Recent methods for boiling heat transfer in multicomponent hydrocarbon mixtures were also reviewed and extended to predict wide boiling range hydrocarbon mixtures. The Martinelli-type separated flow model was used for pressure drop, and specific constants for various bundle and piping geometries were developed from the data.

The combined model, used to predict heat transfer coefficients in the wet-and dry-wall regimes for a wide range of data, produced a mean error ratio of 0.982 with a standard deviation of 0.143. The model also predicted the static head for pure components and mixtures and yielded a mean error ratio of 0.989 with a standard deviation of 0.177. In general, the nucleate boiling suppression factor, the boiling range, and the mass transfer coefficient have a significant impact on mixture flow boiling phenomena; ignoring these effects produces erroneous results.

The model was also used to determine the performance of typical thermosiphon reboilers. The results showed that

  • The heat transfer coefficient decreased as the boiling range increased.
  • The maximum heat flux decreased as the number of tubes in the bundles increased.
  • The static head decreased as the tube pitch increased.