Page 207 - Centrifugal Pumps 2E

High Speed Pumps 185

Figure 11-3. Partial emission efficiency comparison.

Concentric bowl pump peak efficiency occurs at about N s = 800, and
declines in efficiency at higher specific speeds as shown. This character-
istic of RE. pumps exists in general because with high specific speeds the
inlet and discharge passages enlarge toward overlap causing declining
head coefficients and efficiency reduction. This characteristic establishes
a boundary region delineating the suitability of P.E. and RE. pump types.
Although very low specific speed design must inherently entail effi-
ciency sacrifice, such design can have overall attraction. Efficiency is
not the sole consideration in pump selection, and can be overridden by
factors such as simplicity, low initial cost, and quick, easy maintenance.
These alternative considerations tend toward dominance at modest power
levels and for intermittent or low-usage services.

Suction Specific Speed

Another dimensionless parameter highly important in pump design is
known as suction specific speed involving a parametric group nearly
identical to the pump specific speed expression. This subject is discussed
in detail in Chapters 2 and 8.