Preferred overhead system for atmospheric column
5. The controller should have auto overrides (see Chapter 9), or perhaps adaptive gain and reset, to compensate for changes in condenser dynamics as condensate rate changes. An override from coofing-water exit temperature is also normally needed.
6. As an alternative to Item 5, one may use a recirculating coolant system ("tempered" coolant) with condensate temperature control of makeup coolant. This keeps the condenser dynamics constant and eliminates the problem of retuning the controller as the load changes (see Figure 3.9).
7. Another, completely different approach is to run the column at a slight pressure, say 3-5 psig, or under vacuum. Then the condenser cooling water may be manipulated by the pressure controller while subcooling is controlled by manipulating the vent (see Figure 3.10). This is discussed more fully in the next section. There is, however, a limitation to this technique: for protection
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