Handbook of Turbomachinery Second Edition Revised - Ventech!

operates at a relatively low volumetric flow rate (particularly in comparison
to the head rise imparted). Conversely, the ‘‘low’’ Ns pump operates at a
relatively high head rise, in comparison to the flow rate being processed by
the pump. Lastly, the ‘‘low’’ Ns pump operates at a relatively low operating
speed. Although we have not quantitatively defined what ‘‘low’’ means in
each of the above references, it should be thought of as ‘‘low’’ in comparison
to the hydrodynamic optimum under the conditions in question.
As with many other generalized design parameters, specificity in the
interpretation of Ns is quite important. For example, in a single-stage
centrifugal pump, a particular reference to Ns may refer to either the
impeller alone, or to the entire pump, and the actual Ns of each of these will
be different. Likewise, a very high head multistage pump may have a low Ns
for the entire pump (Fig. 15), while having individual stages with quite
favorable
N s-values. When N s data are being generated, studied, or exchanged, written
reference in regard to the point of reference of these data should be
considered mandatory.
We shall see in our upcoming detailed discussions that typical liquid
rocket engine systems are relatively ‘‘low’’ Ns environments. That is, liquid
rocket systems tend to demand relatively low volume flow rates of
propellants at relatively high pump head rises. This is true of even large
Figure 15 Elements of a typical centrifugal pump. For details see Figs. 15a
through 15l.
Copyright © 2003 Marcel Dekker, Inc.