Design of forced-circulation reboilers

The normal practice in the design of forced-convection reboilers is to calculate the heat-transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporisation in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapour-liquid separation vessel. 

If a significant amount of vaporisation does occur, the heat-transfer coefficient can be evaluated using correlations for convective boiling, such as Chen s method. 

Conventional shell and tube exchanger designs are used, with one shell pass and two tube passes, when the process fluid is on the shell side and one shell and one tube pass when it is in the tubes. High tube velocities are used to reduce fouling, 3-9 m/s. 

Because the circulation rate is set by the designer, forced-circulation reboilers can be designed with more certainty than natural circulation units. 

The critical flux in forced-convection boiling is difficult to predict. Kern (1950) recommends that for commercial reboiler designs the heat flux should not exceed 63,000 W/m2 (20,000 Btu/ft2h) for organics and 95,000 W/m2 (30,000 Btu/ft2h) for water and dilute aqueous solutions. These values are now generally considered to be too pessimistic. 

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