Reboiler design forced circulation

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

CHANTRY, W. A. and Church, D. M. (1958) Chem. Eng. Prog. 54 (Oct.) 64. Design of high velocity forced circulation reboilers for fouling service. 

Forced circulation reboilers should be designed for velocities of 10-15 ft/sec (3-5 m/sec).53... 

Most often, forced circulation is used with fired reboilers. If flow is lost to such a reboiler, furnace tube damage is likely to result. Hopefully, this is less likely to occur with a forced-circulation reboiler. Also, the higher pressure drop of a furnace may force the designer to use a pump. Sometimes, we also see a forced-circulation reboiler system, if the reboiler heat is to be recovered from a number of dispersed heat sources that are far away from the tower, and hence a lot of pressure drop has to be overcome. 

Example 8.12 summarizes the results of such calculations made on the basis of data in Heat Exchanger Design Handbook (1983). Procedures for the design of kettle, thermosiphon and forced circulation reboilers also are outlined by Polley (in Chisholm, 1980, Chap. 3). 

It has been recommended to design a forced-circulation reboiler for a high pressure drop (134). In some cases (68), a restriction is placed in the vapor line downstream of the reboiler and sized to prevent vaporization in the reboiler. This restriction is often placed at the column inlet nozzle (68), but this may generate an undesirable high-velocity jet at the column inlet (see Sec. 4.1, guideline 5). Further, a restriction downstream of the reboiler may interfere with the action of a control valve located in the liquid line to the reboiler. The author is also familiar with a case where such a restriction experienced erosion at an intolerable rate. 

Forced-Recirculation Reboilers In forced-recirculation reboilers, a pump is used to ensure circulation of the liquid past the heattransfer surface. Force-recirculation reboilers may be designed so that boiling occurs inside vertical tubes, inside horizontal tubes, or on the shell side. For forced boiling inside vertical tubes. Fair s method (loc. cit.) may be employed, making only the minor modification that the recirculation rate is fixed and does not need to be balanced against the pressure available in the downcomer. Excess pressure required to circulate the two-phase fluid through the tubes and back into the column is supphed by the pump, which must develop a positive pressure increase in the liquid. 

Circulation rate through the reboiler is fixed by the driving force and the resistance to flow. In a fixed piping system, it is a function of the liquid level in the reboiler sump in the case of natural circulation, and of pump design and operation in the case of forced circulation. 

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