Figure 311

Overhead system for vacuum column—small amount of inerts

For a large amount of inerts, it is certainly feasible to control pressure by throttling the vent flow; this procedure is recommended for columns with a vapor product (see next section) where condensate temperature is not controlled.

In designing controls for vacuum columns, the engineer should keep in mind that these columns have a narrow range of operation. The range in column pressure drop between flooding and tray instability for a perforated tray column in vacuum service may be no more than 10-15 percent.

One last consideration should be noted here—that of dynamics. We have previously indicated that pressure control, if used, should usually be of the "averaging" type, which provides slow, gradual correction. This fits in well

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