Page 206 - Centrifugal Pumps 2E

184 Centrifugal Pumps: Design and Application

the diameter, while flow or throughput is proportional to the cross-sec-
tional area and so is proportional to the square of the diameter. Small
pipes thus experience relatively higher flow loss than do large pipes. In
fact, pipe friction data provide excellent corollary with the N s—^ data for
pumps in that pipe diameters commonly appear as parameters on a di-
mensionless plot of friction factor versus Reynolds Number.
With specific speed as well as head and flow expressions having been
defined for P.E. pumps, convenient expressions for impeller and throat
size may be derived

The concentric bowl pump has been unjustly criticized as having only
low efficiency potential, probably because this pump type is frequently
designed for very low specific speed where only low efficiency potential
exists. Barske states that efficiency was of secondary importance in his
development efforts, yet reports an efficiency island of 57% in the vicin-
ity of H = 1,000, Q = 40, N = 28,000 (N, = 1,000). This is seen to
be representative of good pump performance as indicated by the general
pump population data discussed in Chapter 2.
Because partial emission pumps range so widely in speed, it is sensible
to use impeller diameters for scale or size parameters on N S-T| maps,
rather than the flow parameters widely used for the higher specific speed
types. Direct comparison of P.E. and RE. efficiency potentials from
these data is a little elusive since these maps define explicitly only two of
the four parameters involved in the specific speed expression. But by
making the quite reasonable assumption that the low specific speed data
collected by Karassik derived from pumps at 3,600 RPM, direct compar-
ison can be made as shown in Figure 11-3. The dotted curves reflect the
Karassik data and the solid curves represent P.E. performance at 3,600
RPM. Distinct P.E. efficiency superiority is seen to exist at low specific
speeds and low to medium flow rates.