As shown by Figure 4.3, a flooded thermosyphon reboiler operates by throttling condensate rather than steam flow. The basic principle is that of varying the heat-transfer surface. The chief advantage is that for a given steam flow in pounds per hour, a condensate valve is much smaller than a vapor supply valve. At a high heat load such that shell-side liquid level is low, condensate subcooling is small and there is some cavitation in or flashing across the condensate valve. Fortunately new methods developed by the ISA (see Chapter 11) permit much more accurate prediction of flashing and cavitation, and many valve manufacturers now can provide anticavitation trim. Care must be taken to avoid excessive reboiler AT, which may cause either film boiling or choked flow. A steam-supply pressure regulator is often used to protect against this.
Control of the condensate valve is sometimes achieved by a steam-flow measurement, sometimes by a condensate-flow measurement (more accurate and cheaper), and sometimes by another process variable. Response is more sluggish than when steam flow is throttled. No trap is required. The theory and design equations are presented in Chapter 15. For troubleshooting, performance tests, startup, and, in some cases, overrides, a liquid-level transmitter should be installed on the reboiler shell side.
A special version of a flooded reboiler designed for low-boiling materials (requiring low-temperature steam) is shown in Figure 4.4. Steam to the reboiler is throttled in a conventional fashion—usually flow controlled or flow-ratio controlled—but there is neither a trap nor a condensate pot. Instead condensate is removed through a loop seal whose top is vented to atmosphere. The height
of the loop is typically 5-10 feet. This arrangement has the equation:
Hl = loop-seal or standpipe height, feet Hs = condensate level in the shell, feet Ps = shell pressure, lbf/ft2 absolute p = condensate density, lbm/ft3
Now the absolute value of Ps does not change very much so boilup rate is mosdy controlled by variation in exposed tube area.
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