Steam supplies for distillation columns are commonly arranged as shown in Figure 4.10. From a high-pressure header, steam is let down to a lower pressure through a "reducing station." This may feature a transmitter, controller, and valve, but is often a self-contained, self-actuated pressure regulator. In some cases there is also a desuperheater. This last item is sometimes installed because of a common misunderstanding about the effect of superheat on reboilers; it simply reduces slightly the amount of condensate8 and usually does not, as sometimes believed, reduce reboiler heat-transfer capacity.
The reduced-pressure header may serve one load or many. For a design with only two or three loads, there is often a severe interaction between the loads and the pressure reducer. Suppose, for example, that steam flow to one reboiler is increased. This drops pressure in the reduced-pressure header. The pressure controller opens the upstream valve to restore pressure, but in the meantime the lower pressure may have caused another reboiler supply valve to open. If all of the controllers are tuned tightly, there may ensue "fighting" among the controls and substantial swings in header pressure and steam flows to the various reboilers.
For a header with many loads, header pressure is really constant at only one point; at the far end of a distillation train, header pressure may be much lower. Particularly for a small number of loads, it may be more logical to eliminate the pressure reduction. A system such as shown in Figure 4.11 may
B = 2" FOR DISPLACER TRANSMITTERS = 6" FOR A P TRANSMITTERS
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