BK1-7 Circulation Boiling Model for Analysis of Kettle and Internal Reboiler Performance

J. W. Palen and C. C. Yang

The Circulation Boiling Model is a method to estimate performance of a kettle type reboiler by predicting the natural convection circulation rate within the tube bundle. For this presentation a one-dimensional analysis is used. The advantages of this method over the previous empirically based method (described in HTRI Report BK1-1) are

  1. more accurate prediction at low temperature differences,
  2. estimation of local vapor fractions for dryout protection, and
  3. improved ability to account for the effects of enhanced surfaces and finned tubes.

The model was tested and tuned against the HTRI kettle reboiler data bank including a very wide range of operating pressures, fluids, and bundle geometries. Boiling curves are shown from the data and as they were predicted by the method. A parametric evaluation is made illustrating the effects of critical bundle geometries on the maximum vapor fraction in the bundle.

The previous kettle reboiler method developed in HTRI Report BK1-1 was based mainly on the nucleate boiling mechanism and adjusts for the higher performance of tube bundles as compared to single tubes by a bundle correction factor. This relatively simple empiricism produced a reasonable way of accounting for the fact that bundle boiling coefficients are enhanced by vigorous internal circulation.

It is recognized that additional data with local wall temperatures will be required before really adequate understanding of boiling in bundles is achieved. The proposed new method represents a great improvement in the treatment of bundle boiling calculations. It is expected to be a good basic framework for present estimations and for future improvement as additional HTRI boiling data are obtained. However, the method presented in HTRI Report BK1-1 is still considered a valid alternative for design estimations.