Page 281 - HVAC Pump Handbook

Rishel_CH10.qxd 21/4/06 6:20 PM Page 278

Basics of Pump Application for HVAC Systems

278 The HVAC World

Following is an example of this pump selection procedure. Assume
pump duty at 700 gal/min at 70 ft of head. There is one selection at
1750 rev/min: a 6 6 9 pump with a 25-hp motor and efficiency of
72 percent at the design point (Fig. 10.3a). A desirable pump would be
the 6 4 9 pump, but it does not quite achieve the head capacity
required at 1750 rev/min (Fig. 10.3b). An alternate selection would be
the same 6 4 9 pump with a 20-hp motor operating at 1789
rev/min and at 87 percent efficiency at the design point (Fig. 10.3c).
The alternate selection was determined by computing the capacity
of the smaller pump at a higher speed using the pump affinity laws.
The pump speed was calculated at a point where the required condi-
tion of 700 gal/min at 70 ft was achieved by the smaller pump.
Following is the method of computing the desired speed. From the
affinity laws:
Q 2 Q 2
2
1

h h
1 2
where Q and h are the desired conditions of 700 gal/min and 70 ft.
1 1
Q and h are the equivalent points on the known pump curve.
2 2
By trial and error, the equivalent flow Q can be computed and is 685
2
gal/min (see Eq. 10.1 developed in Sec. 10.4 on the use of the affinity
laws for pumps). The required speed S of the pump, therefore, is
1
700
S 1750 1789 rev/min
1 685
Figure 10.3c describes the actual operating curve for the 6 4 9
pump operating at 1789 rev/min. It is obvious that a 20-hp motor and
variable-speed drive will be adequate because the maximum brake
horsepower at any point on this curve is 16.8 bhp. The motor and
drive manufacturers can certify that their equipment is acceptable for
operation at this speed and brake horsepower.
An examination of Figs. 10.3b and c reveals that the use of this tech-
nique has increased the pump efficiency from 72.5 to 87 percent and
has reduced the nonoverloading motor horsepower from 25 to 20 hp.
This demonstrates the added dimension that variable-speed drives can
provide in the selection of pumps. Engineers should not be concerned
about the operation of a pump near its maximum impeller diameter;
pump manufacturers will certify their pumps within the impeller
diameters listed in their catalogs. Other engineers are reluctant to
select pumps at maximum impeller diameter in anticipation of future
increases in pumping requirements. This procedure of using speeds as

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.