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Basics of Pump Design

Basics of Pump Design 97

Poorly defined Accurately defined
or cast tongue
and cast tongue

Turbulence with Smooth flow with
energy loss little turbulence
a. Poor construction. b. Quality construction.

Figure 4.5 Comparison of pump constructions.

through the pump beyond this point, friction increases internally, and
the efficiency starts to fall again. The best efficiency point of a pump
will be emphasized in many parts of this book. Every water system
designer should know where the best efficiency point occurs for each
of the pumps involved in an HVAC water system.
The pump designer develops a basic configuration of a pump impeller
and then selects various pump impeller diameters and speeds to cre-
ate the vector diagrams that will produce the flow-head relationships
needed for the broad application of that pump to HVAC water services.
The capacity or flow range for a volute-type pump is determined by
the size of its discharge connection. Therefore, the pump designer
works with the vector diagrams and discharge connection areas to pro-
duce a number of pumps that will cover this broad range of flows and
heads. The result is a number of head-capacity curves, as shown in
Fig. 4.1b. This is often called a family of pump curves and provides a
quick guide to the water system designer as to which size of pump
may fit the needed flow and head for a particular application.

4.4 Sizing Centrifugal Pumps
As will be shown later, two types of centrifugal pumps are used in
HVAC service, namely, volute and axial-flow pumps. Volute pump size
is designated by the sizes of the suction and discharge connections, as
well as by the pump diameter. A typical sizing for such a pump would

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