Page 99 - HVAC Pump Handbook

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Basics of Pump Design

96 HVAC Pumps and Their Performance

Outside of impeller Outside of impeller
polished not polished
Only hubs
machined

Vanes sharply
defined Vanes poorly
defined
Smooth interior
of impeller
Rough interior
of impeller

a. Quality finish. b. Poor finish.
Figure 4.4 Comparison of pump impellers.

port of the impeller? How smooth are the internal surfaces of the
pump body, namely, the volute, or the bowl.
Figure 4.4 illustrates these points and describes the difference
between a pump impeller with quality finish and one with poor finish.
It is obvious which impeller will develop the highest pump efficiency.
Likewise, the quality of the volute or bowl will determine the pump’s
efficiency. A critical point in a volute pump’s construction is at the
tongue or cut water where water continues to the discharge connection
or is recirculated inside the pump; Figure 4.5 describes this. A poorly
defined or cast tongue will increase turbulence and loss of energy. A
well defined and cast tongue will improve pump performance. One
ongoing effort that must be made in pump manufacturing is mainte-
nance of the patterns for the pump castings.

4.3 General Performance of a Centrifugal Pump
Figure 4.1a provides general performance of a centrifugal pump, as
well as typical hydraulic efficiencies for that pump. It should be noted
that this efficiency drops to zero at zero flow and then increases to a
point called the best efficiency point for the pump. As flow increases

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