HEAT EXCHANGER APPLICATIONS
DOE-HDBK-1018/1-93
Heat Exchangers
To prevent the condensate level from rising to the lower tubes of the condenser, a hotwell level
control system may be employed. Varying the flow of the condensate pumps is one method used
to accomplish hotwell level control. A level sensing network controls the condensate pump
speed or pump discharge flow control valve position. Another method employs an overflow
system that spills water from the hotwell when a high level is reached.
Condenser vacuum should be maintained as close to 29 inches Hg as practical. This allows
maximum expansion of the steam, and therefore, the maximum work. If the condenser were
perfectly air-tight (no air or noncondensable gasses present in the exhaust steam), it would be
necessary only to condense the steam and remove the condensate to create and maintain a
vacuum. The sudden reduction in steam volume, as it condenses, would maintain the vacuum.
Pumping the water from the condenser as fast as it is formed would maintain the vacuum. It
is, however, impossible to prevent the entrance of air and other noncondensable gasses into the
condenser. In addition, some method must exist to initially cause a vacuum to exist in the
condenser. This necessitates the use of an air ejector or vacuum pump to establish and help
maintain condenser vacuum.
Air ejectors are essentially jet pumps or eductors, as illustrated in Figure 10. In operation, the
jet pump has two types of fluids. They are the high pressure fluid that flows through the nozzle,
and the fluid being pumped which flows around the nozzle into the throat of the diffuser. The
high velocity fluid enters the diffuser where its molecules strike other molecules. These
molecules are in turn carried along with the high velocity fluid out of the diffuser creating a low
pressure area around the mouth of the nozzle. This process is called entrainment. The low
pressure area will draw more fluid from around the nozzle into the throat of the diffuser. As the
fluid moves down the diffuser, the increasing area converts the velocity back to pressure. Use
of steam at a pressure between 200 psi and 300 psi as the high pressure fluid enables a single-
stage air ejector to draw a vacuum of about 26 inches Hg.
Figure 10 Jet Pump
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