To provide back up to the current rather simple design rule deployed to avoid dryout, and thence coking, in fired reboilers, an extension of a phenomenological model has been developed. This has integrated elements for annular flow up to dryout and for dispersed flow for the post burnout region. The complication caused by the bends in the traditional serpentine geometry is dealt with by focusing on the effect identified as the most important: deposition of drops. A dryout map, plots of allowable combinations of mass fluxes/heat fluxes, and a wall temperature profile, have been produced. The model developed in the current work serves as a starting point for future refinement.