Page 105 - HVAC Pump Handbook

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Basics of Pump Design

102 HVAC Pumps and Their Performance

4.9.1 Volute-type centrifugal pumps
The pump impeller produces the pump head, but it must be housed in
a body that collects the water and delivers it to the system piping. In
volute-type pumps, the body collects the water from the impeller and
moves it around to the pump discharge connection (Fig. 4.2b, c). This
body is called the volute. Unlike the axial-flow pump, normally, there
are no collecting vanes in the body or volute to aid the flow. The cost
of adding these volute vanes or diffusers would be prohibitive in volute-
type pumps.
Most small HVAC centrifugal pumps are volute pumps, since this
type of pump lends itself to many different configurations that will be
described in Chap. 5. Larger volute pumps may be equipped with
what is called a double volute (Fig. 4.2c). This pump has a second volute
cast into its casing. Radial thrust on a pump shaft can be reduced by
means of the double-volute construction. The second passage in the
volute should not be confused with the diffusers of axial-flow pumps.
There is very little need for double-volute pumps in HVAC work, par-
ticularly with variable-speed pumps, in which the radial thrust is very
low under most operating conditions. Radial thrust will be described
in detail in Chap. 6.

4.9.2 Axial-flow centrifugal pumps
Axial flow pumps are arranged to pass water through a body that is
called a bowl. This bowl has suction and discharge ports and contains
diffuser vanes that aid the flow of water through the bowl. Because of
the vanes, these pumps are often called diffuser pumps. Water flows
out uniformly 360 degrees around the impeller; Fig. 4.7 illustrates
this flow. The pump body or bowl vanes are similar to the impeller
vanes and they aid the diffusion of water out of the impeller into the
bowl and then into the discharge pipe or another bowl.

4.10 Open or Closed Impellers
There are several types of impellers available for clear service (clean
water): closed, semiopen, and open. Most of the impellers of pumps in
the HVAC industry are of the closed type. Closed-type impellers have a
shroud and utilize case wear rings to impede bypassing. Other impellers
are open type without shrouds or case wear rings. Usually, they are
found in larger axial-flow pumps. For most HVAC applications, the
closed impeller with case wear rings provides the highest efficiency;
this efficiency is sustained longer due to the ability of the pump designer
to develop the smallest possible clearance between the impeller and the

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