Vertical-thermosyphon reboilers or calandrias are commonly designed for a mass circulation ratio of about 5-20 parts of liquid to one part of vapor. This means that only about 5-25 percent by weight of reboiler effluent is vapor; on a volumetric basis, however, the numbers are almost reversed, t The tubes are filled with froth, and pure liquid exists in the tubes only for a few inches above the bottom tube sheet.
For most applications, if shell-side steam pressure, tube-side pressure, and composition are fixed, heat-transfer flux will be a maximum when column-base liquid level relative to the bottom tube sheet is about one third of the distance between the two tube sheets2—see Figure 4.1 A.
If liquid level goes below this point, heat transfer falls off rapidly. As liquid level goes above this point, heat transfer declines only slightly; at a liquid elevation corresponding to the top tube sheet, flux is perhaps 10-15 percent less than maximum. In view of this, it is a conservative practice to maintain column-base liquid level below the top tube sheet and above an elevation corresponding to the midpoint between the two tube sheets. As pointed out by Smith,3 the above may be influenced by column pressure.
For many applications the practical implementation of the preceding is as illustrated by Figure 4.IB. A level nozzle spacing of 44 inches is suggested for AP transmitters and 48 inches for displacer transmitters. The top nozzle center should be at least 1 inch below the lip of the vapor-return nozzle, or 1 inch below the elevation of the bottom lip of the internal vapor downspout (if used). The bottom nozzle should be centered 6 inches above the KRL (knuckle radius line) if a AP transmitter is used, and 2 inches above the KRL if a displacer instrument is used. For either type of transmitter, the following calibration procedure is recommended.
—Zero level should be 6 inches above the KRL.
—The transmitter output span should be 3—15 psig for pneumatics, or 4— 20 mA for electronics, for 44 - 6 = 38 inches of process fluid. This allows 6 inches below the top level nozzle to accommodate changes in liquid-specific gravity.
* Particularly in the petroleum industry, horizontal-thermosyphon reboilers are sometimes encountered.
t A literature review of thermosyphon reboilers is given by McKee.4
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