Handbook of Turbomachinery Second Edition Revised - Ventech!

Figure 14 Approxmate values of pump stage efficiency versus stage-specific speed.
one or more candidate configurations will perform more or less equally well,
and the choice must then be made according to the skill and experience of
the designer.
The reader should note that, of the three variables that fix the N s
parameter, two of them (the pump flow rate Q, and the pump head rise H)
are specified by the engine system Power Balance Model, and thus are
outside the control of the pump designer. It should now be apparent that the
designer’s freedom in sizing of a pumping element is very limited, and in
many cases barely exists at all. The only input to the pump Ns parameter
over which the designer has control is the pump shaft speed [the ‘‘N’’ term in
Eq. (1)]. The designer must then use the shaft speed to appropriately set the
N s parameter for a given type of pumping element that is under
consideration. Sometimes, the designer may have the freedom to set the
pump speed to a value that is at or near the hydrodynamic optimum for a
given type of pump configuration. However, more often one or more other
items (available pump suction performance, rubbing speed of seals,
rotordynamic considerations, etc.) will conspire to limit pump speed such
that Ns is set to a value lower than desired.
Although the Ns parameter does not correlate to anything physical in
an ‘‘explicit’’ sense, it can give great insight into both the geometry and
performance of a pump (or pump stage) if thought of in a ‘‘relative’’ sense.
For example, consider a ‘‘low’’ specific speed pump stage. From the terms
that make up the Ns-term, we can say the following: a ‘‘low’’ Ns pump
Copyright © 2003 Marcel Dekker, Inc.