Method: Vary ejector driving steam pressure. Not normally
recommended.
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.
Variants:
Configuration notes:
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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