Conference Dates

July 1-6, 2007

First Page



Two causes of under-performance within the tubes of vertical thermo-siphon re-boilers (VTR’s) are considered: premature dry-out within the unit and chemical reaction fouling at the tube wall. In many cases dryout is itself a cause of fouling. Both of these causes depend on re-boiler geometry. Some geometry performs satisfactorily. Other geometry performs poorly. There are two basic forms of flow boiling: nucleate flow boiling and convective flow boiling. In nucleate flow boiling part of the vaporisation results from the formation of bubbles of vapour on the heated surface. Unfortunately, a thin liquid film (the micro-layer) formed beneath these bubbles can fully evaporate and dissolved solids can deposit on the heated surface. Under convective flow boiling conditions the vapour shear generated by the two-phase flow results in the formation of a thin liquid film on the tube wall. This film moves at high velocity and provides very effective heat transfer. The result is a marked reduction in wall temperature and suppression of the nucleate boiling process. It is clearly desirable to move from a nucleate boiling mechanism to a convective boiling situation over as short a tube length as possible. The length of tube that is subject to nucleate boiling is again a function of re-boiler geometry. Wall shear is important in suppression of chemical reaction type fouling. Shear is function of recirculation rate and vapour mass quality. Again these factors depend on reboiler geometry. In this paper the authors describe how the better geometry can be identified.