A. Kettle, internal, and horizontal thermosiphon reboilers

Kettle, internal, and horizontal thermosiphon reboilers are similar in that heat is transferred to a vaporizing two-phase mixture flowing across a tube bundle. Nucleate and convective boiling mechanisms both take part in the boiling process. For kettle reboilers, convective circulation takes place in an enlarged shell with mostly vapor going overhead. For the thermosiphon, a two-phase mixture is discharged from the reboiler and convective circulation involves the external piping and liquid reservoir. Nevertheless, the heat transfer processes are essentially the same in both types, so thermal design is covered in the same section. The following elements are important.

(a) Single-tube nucleate boiling

The nucleate boiling mechanism has considerable influence on the behavior of kettle and horizontal thermosiphon reboilers, so a correlation for nucleate boiling on single tubes is a necessary (but not sufficient) component of the design calculations For a more detailed discussion of nucleate boiling theory, refer to Section 2.7.2.

The nucleate boiling heat transfer coefficient can be described by either of the two following forms, both of which may be convenient in different types of calculations: