Thermowell installation under vertical condenser against fouling and corrosion, cooling water exit temperature should usually be limited to a maximum of 50-60°C (122—140°F), and sometimes lower. Increasingly, override controls are used to provide this protection.
Another alternative is to use exit-cooling-water temperature control, discussed in Section 3.4.
Finally, it is increasingly common to provide no condensate temperature control, but to run with full cooling at all times. This saves a control valve. Further, the quality of cooling water is sometimes so poor that a minimum velocity must be maintained in the exchanger to minimize fouling. For accurate control of internal reflux, an internal reflux computer is required (discussed in Section 11.1). As pointed out by Bolles,1 however, it is necessary to limit subcooling in some columns to avoid foaming on the top tray.
An additional problem with condensate temperature control, via cooling-water manipulation, relates to column safety. In an instance with which one of the authors is painfully familiar, an atmospheric column with such a control system was running at a very low feed rate. Condensate temperature became too low, so the controller closed the cooling-water valve located in the exit line from a vertical condenser. The water in the shell began to boil, the valve could not pass the required volume of steam, the cooling-water pump stalled, and product vapor issued in great quantities from the vent. Fortunately, an alert operator shut the column down before any damage occurred.
As a consequence, unaided condensate temperature control is not recommended. There should be an override from cooling-water exit temperature. Further, to minimize the hazard of winter freezeup, a limiter should be provided to prevent complete valve closure (see Chapter 9).
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