Method: Vary heat flux. Varies cooling medium rate.
Process: Net vapor rate zero.
Disadvantages: Coolant return temperature may be high.
Application: No longer commonly called for in the design of new plants. Often added to existing units when original control schemes prove inadequate. Often used in older plants. Commonly, a 'manual' form of this control scheme is used. Block valves on the cooling water are pinched to reduce cooling water flow on a seasonal basis or at reduced rates. Then a more conventional pressure control scheme handles day-to-day control to set point.
Configuration notes: Cooling water fouls more rapidly with increasing temperature and decreasing velocity. A minimum rate and/or return temperature over-ride with a secondary pressure control system may be required.
Operation: Varying coolant rate causes LMTD to change. This modifies total heat flux.
Warnings: Low velocity and high temperature cooling water increases probability and severity of condenser fouling.
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