Thermosyphon Reboiler Piping

For thermosyphon reboilers, the hydraulic aspects are as important as the heat transfer aspects. The design of thermosyphon reboiler piping is too broad a subject for this handbook. Some good articles on the subject can be found in References 2-14. Reference 3 is particularly good for horizontal thermosyphon reboilers. Table 1 has typical vertical thermosyphon design standards. 

Kern, Robert, Thermosyphon Reboiler Piping Simplified, Hydrocarbon Processing, December 1968. 

Figure 7-72. Typical flow in thermosyphon reboiler piping.

The upward flow of gas and Hquid in a pipe is subject to an interesting and potentially important instabiHty. As gas flow increases, Hquid holdup decreases and frictional losses rise. At low gas velocity the decrease in Hquid holdup and gravity head more than compensates for the increase in frictional losses. Thus an increase in gas velocity is accompanied by a decrease in pressure drop along the pipe, a potentially unstable situation if the flows of gas and Hquid are sensitive to the pressure drop in the pipe. Such a situation can arise in a thermosyphon reboiler, which depends on the difference in density between the Hquid and a Hquid—vapor mixture to produce circulation. The instabiHty is manifested as cycHc surging of the Hquid flow entering the boiler and of the vapor flow leaving it. 

When the diagram is laid out, it is only necessary to show the relative elevation of the process connections to the equipment where they affect the process operation for example, the net positive suction head (NPSH) of pumps, barometric legs, siphons, and the operation of thermosyphon reboilers. Full details of pipe layout are usually shown in a different drawing, known as a piping isometric drawing. See Figure 5.19 for an example. 

Exchangers. Thermosyphon reboilers and condensers use short pipe runs. Short reboiler and overhead lines are essential for both economy and reliable operation. 

Shell-and-tube heat exchangers can be broken down into (1) pipe-coil (2) double-pipe (3) fixed-head, single-pass (4) fixed-head, multipass (5) floating-head, multipass (U-tube) (6) kettle reboiler (7) thermosyphon reboiler and (8) shell nomenclature. These devices can be mounted vertically or horizontally. 

Used for small duties, clean process, and only infrequent cleaning required. Vaporization is usually less than 30%, but less than 15% if the fractionator pressure is below 50psig. The viscosity of the reboiler feed should be less than 0.5 cp. Put a butterfly valve in the reboiler inlet piping. This type is used in nearly 100% of chemical plant thermosyphon applications (70% of petrochemical). 

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