Page 96 - HVAC Pump Handbook

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Basics of Pump Design

Basics of Pump Design 93

where is the specific weight of the liquid in pounds per cubic foot. For
example, from Table 2.3, water at 180°F has a specific weight of 60.57
3
lb/ft . Thus H, for 180°F water, is 144/60.57, or 2.38 ft/lb of pressure.
The words pump duty are often used when describing a pump’s per-
formance. Pump duty is usually defined as a certain flow in gallons
per minute at a specific head in feet. This is the point at which the
pump has been selected for a particular application. Usually, this
point is selected as closely as possible to the best efficiency of a pump
impeller, which, for Fig. 4.1a, is 86 percent.
Other common terms that are encountered in the pump industry are:

1. Carry out or run-out. This means that the pump is operating at the
far right of its curve, where the efficiency is poor.
2. Shutoff head. This is the head produced by a pump when it is run-
ning at the “no flow,” or zero capacity, point.
3. Churn. A pump is said to be in churn when it is operating at shut-
off head or no flow.

4.2 Centrifugal Pump Impeller Design
Centrifugal pumps, as their name implies, depend on centrifugal force
to produce flow through the pumps. When an impeller rotates, the
water in the impeller rotates as well, so there are two forces acting on
that water, namely centrifugal and rotational. The impeller of the
pump rotates in a body to utilize these forces which are imposed on
the water in the impeller. These forces are aided by the shaping of the
impeller to produce the correct passages in the impeller that will use
this centrifugal force as well as the rotating velocity of the water in
the impeller. Internal vanes are designed into the impeller that direct
the flow through and out of the impeller. The relatively high efficien-
cy of HVAC pumps, up to 90 percent, is due to the fact that the pump
designer can design these vanes for maximum efficiency without con-
cern for dirty or stringy material and without consideration for
corrosive or erosive liquids. Figure 4.2a illustrates a cross section of a
centrifugal pump impeller with vanes that direct the water through
the impeller.
This figure provides the basic vector diagram for a centrifugal
pump. It describes the forces imparted on a molecule of water leaving
the impeller. W is the centrifugal force and V is the rotational force
2 2
developed by the rotating energy of the impeller. These two forces pro-
duce the pump head and flow. The net force acting on the water is the
vector diagram for these two forces. The vector diagram that the pump

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