Page 118 - HVAC Pump Handbook

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Physical Description of HVAC Pumps

Physical Description of HVAC Pumps 115

5.3.2 Double-suction pumps
The workhorse of the HVAC field is the double-suction pump. It is by
far the preference of most designers when the flow in a water system
exceeds 1000 gal/min. The principal reasons for this popularity are its
relatively high efficiency and its ability to be opened, inspected, and
serviced without disturbing either the pump rotor, motor, or the con-
necting piping.
The latter feature is available if the pump casing is split parallel to
the pump rotor or shaft. With this construction, called axially split,
the top half of the casing can be removed, affording internal inspection
of the impeller, case rings, and internal surfaces of the casing.
Usually, there is no need for realignment of the pump and motor after it
has been inspected, since the rotating element has not been disturbed.
The double-suction pump can be furnished in a casing that is split
vertically or perpendicularly to the pump shaft. This construction
requires pulling the rotating element out of the pump end to inspect
the impeller interior; the pump should be realigned with its motor
after being inspected and reassembled. This pump is almost always
mounted horizontally. Contrary to claims, this pump does not save
equipment room space in most cases because of the distance required
at the pump end for removal of the rotating element.
The double-suction pump (Fig. 5.7) that has a casing split parallel
to the pump shaft can be furnished mounted horizontally or vertically.
This is why the popular name horizontal split case should not be
used. The double-suction pump is always flexibly coupled to the elec-
tric motor, never close coupled. The vertically mounted double-suction
pump uses a vertical, solid-shaft motor.

5.3.3 Axial-flow pumps
Axial-flow pumps are furnished in a number of different configura-
tions. They are classified by impeller design and by their general con-
struction and arrangement. An axial-flow pump consists of four basic
subassemblies: (1) the bowl assembly that houses the impellers, (2)
the column assembly that consists of pump shafting and piping to
transfer the water from the bowl assembly to the discharge head, (3)
the discharge head that receives water from the column assembly and
delivers it to the system piping, and (4) the motor or other type of dri-
ver that powers the pump.
The three principal impeller classifications are turbine, mixed flow,
and propeller.

1. Turbine. Turbine pumps are designed with lower specific speeds and
are the type usually found in HVAC applications. Most of these pumps
are equipped with enclosed impellers, although some manufacturers

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