Page 101 - HVAC Pump Handbook

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Basics of Pump Design

HVAC Pumps and Their Performance
98
98 HV A C Pumps and Their P erf ormance
be 6 in 4 in 8 in. This indicates that the pump has a 6-in diame-
ter suction, a 4-in discharge, and an 8-in nominal size impeller. The
3
impeller may have been trimmed to a specific diameter such as 7 ⁄ 4 in
to achieve the best efficiency at the specified flow and head. Many
pump companies would call this a 4-in pump, since they use the dis-
charge connection size to classify their pumps.
Axial-flow pump sizing is determined by the suction and discharge
connections of the body or bowl. The suction and discharge connec-
tions are almost always the same size. A 6-in suction and discharge
would indicate a 6-in pump or bowl.

4.5 Specific Speed of a Pump
The experienced pump designer uses a formula called specific speed N
s
to develop a pump impeller for a specific range of flows and heads,
knowing that a certain specific speed will produce the desirable ratio
between pump head and pump capacity. This formula (Eq. 4.2) is of great
importance to the designer but of little interest to the user of pumps.

S Q
N (4.2)
s 3/4
h
where S pump speed, in rev/min
Q pump flow, in gal/min
h pump head, in ft

For example, assume that a pump has a capacity of 1500 gal/min at
100 ft of head and is rotating at 1760 rev/min. Then,

1750 1500
Specific speed 2143
100 3/4
With the aid of this formula and a great amount of experience and
testing, the designer can develop the above-mentioned family of pump
curves.
Most HVAC pumps are of moderate specific speed in the range of
1000 to 5000; some large cooling tower pumps may have specific speeds
as high as 8000. High-specific-speed pumps, as the preceding formula
would indicate, are high capacity with low head, while low-specific-
speed pumps are high head with low capacity. Figure 4.6 describes the
variation of impeller types with specific speed. The high-head pumps
with low specific speed have narrow vanes indicating high head and
low flow. The high-specific-speed pumps have the shorter and wide
vanes for high delivery at low-pump heads.

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