Page 140 - HVAC Pump Handbook

Rishel_CH06.qxd 20/4/06 6:06 PM Page 137

HVAC Pump Performance

HVAC Pump Performance 137

6.2.1 Constant-speed head-flow curves
Most HVAC pump head-flow curves provided by the pump manufactur-
ers are constant-speed curves for a particular speed such as 875, 1150,
or 1750 rev/min. These curves are derived from dynamometer tests in
the pump manufacturer’s plant. The newer, high efficiency motors have
higher, full speeds such as 885, 1180, and 1775 rev/min. Pump manu-
facturers are now furnishing flow-head curves at such speeds.
Head-flow curves for pumps can have several shapes depending on
the specific speed of the pump. Figure 6.1a is for a pump of moderate
specific speed, 1560, and is called a continuously rising characteristic
curve because the head increases continuously to the shutoff, or no-flow,
condition. High-specific-speed pumps such as mixed-flow or propeller
pumps can have a looped head-flow curve, as shown in Fig. 6.2a.
Normally, manufacturers of such pumps do not provide a complete
head-flow curve in their catalogs. Only the right-hand portion of the
curve where the pump is to be operated is provided, as shown in Fig. 6.2b.
These pumps require some care in their application due to the fact
that they can operate at several flows at the same head; this is
shown in Fig. 6.2a, where the pump at 34 ft of head can operate at
800, 1700, and 2700 gal/min. The pump can shift from one flow to anoth-
er, causing shock and vibration in the pump itself and in the water sys-
tem. Only experienced pump application engineers should handle the
selection, installation, and operation of these large pumps.
The head-flow curve describes the net head available for useful
pumping after all internal losses of the pump are deducted from the
theoretical head that could be produced by an impeller of a particu-
lar diameter and operating at a certain speed. The efficiency curves
superimposed on the head-flow curve demonstrate how efficient the
pump is when operated at a specific point on its head-flow curve.
Some pump manufacturers provide efficiency curves similar to those
in Fig. 6.1. Others provide a separate efficiency curve from zero effi-
ciency at zero flow to maximum flow, as shown in Fig. 6.3. This is a
rising curve to the point of maximum efficiency at the optimal flow.
This maximum efficiency point on such a curve is called the best effi-
ciency point. Great emphasis will be placed on the importance of this
best efficiency point in Chap. 10. Some pump manufacturers, partic-
ularly of small pumps, do not provide efficiency curves; instead, they
provide a performance curve for a certain impeller diameter and
with a specific size of electric motor. Such pump curves should not be
used for selection of HVAC pumps. If the manufacturer cannot pro-
vide an efficiency curve for a pump, another manufacturer should be
consulted.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.