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Water enters the eye of
the impeller and is thrown out by centrifugal
force. As water leaves the eye of the impeller a
low pressure area is created causing more liquid to flow
toward the inlet because of atmospheric pressure and
centrifugal force. Velocity is developed as the
liquid flows through the impeller while it is turning at
high speeds on the shaft. The liquid velocity is
collected by the diffuser or volute and converted to
pressure by specially designed passageways that direct
the flow to discharge into the piping system; or, on to
another impeller stage for further increasing of
pressure.
The head or pressure that
a pump will develop is in direct relation to the
impeller diameter, the number of impellers, the eye or
inlet opening size, and how much velocity is developed
from the speed of the shaft rotation. Capacity is
determined by the exit width of the impeller. All
of these factors affect the horsepower size of the motor
to be used; the more water to be pumped or pressure to
be developed, the more energy is needed.
A centrifugal pump is not
positive acting. As the depth to water increases,
it pumps less and less water. Also, when it pumps
against increasing pressure it pumps less water.
For these reasons it is important to select a
centrifugal pump that is designed to do a particular
pumping job. For higher pressures or greater
lifts, two or more impellers are commonly used; or, a
jet ejector is added to assist the impellers in raising
the pressure. |
