Method: Vary ejector driving steam pressure. Not normally
Process: Vacuum systems. Must always have vapor product.
Advantages: Minimizes total steam demand, hot well water generation, and condenser cooling requirements. Minimum piping changes. Small control valve required.
Disadvantages: Often only a very small control range available.
Application: Added to vacuum ejector systems after construction. Should never be selected as the preferred method to add to a system in pre-construction design.
Operation: Lower pressure supply steam reduces the mass rate of steam to the ejector. This reduces the pressure ratio across the ejector and the ejector suction pressure rises.
Warnings: Ejectors have limited motive steam mass and volume operability ranges. Sometimes this method works, other times it does not. Improper ejector operation can result in rapid and unstable shifts in vacuum pressure. The author has seen sudden swings from 10 torr (10 mmHg) vacuum to 40 torr (40 mmHg) vacuum and back resulting from minute changes in motive steam pressure. In other systems, motive steam pressure could be varied by as much as 10 to 20 percent and reasonable control was achieved.
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