In this case th should be 20 minutes or more. In addition, the level controller should be cascaded to a steam-flow controller with a linear flow transmitter (or orifice AP transmitter, and square-root extractor). Further, for a thermosyphon reboiler, one should make the volume AB x AHT at least ten times the volume inside the reboiler tubes. For this design:
Ab A Ht
Ab A Ht
(wa)m = input span of steam flow transmitter, pounds per minute; that is, maximum steam flow ka = steam latent heat of condensation, pcu/lb Xp = process fluid latent heat of condensation, pcu/lb pp = process fluid density, lbm/ft3 Note that the term in brackets
Ppkp is the process vapor flow corresponding to (wa)m. A typical control system arrangement for level control via steam-flow control is shown in Figure 4.14.
Much difficulty has been encountered when trying to control base level by adjusting steam flow to a thermosyphon reboiler. It therefore is recommended that all such applications be subjected to a dynamic analysis such as presented in Chapter 16.
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