Page 278 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 275

the points that should be brought out for selecting a pump for a partic-
ular duty.

10.2.1 Selecting constant-speed pumps
The pump head-flow curve shown in Fig. 10.1a is for an excellent
pump for a pump duty of 2500 gal/min and 120 ft of head. The peak
efficiency for this pump is 86 percent. For constant-speed operation,
this would be satisfactory for system flows between 2000 and 2700
gal/min. To operate this pump at lesser or greater flows would pro-
duce poorer efficiencies, and the radial thrust would increase so that
there could be increased wear within the pump. One pump company
urges that constant-speed pumps operate at no greater flow range
than 25 percent of the flow at the best efficiency point. Normally,
pumps for constant-speed operation would be selected just to the left
of the best efficiency point, or at 2200 to 2500 gal/min for the preced-
ing pump.
A dangerous point to operate a pump is at 1000 gal/min and 130 ft
of head, as shown in Fig. 10.1a. Often, inexperienced pump represen-
tatives who do not have a smaller pump at higher heads will operate
this pump improperly. This is an unacceptable selection because of
the poor efficiency and the high radial thrust existing at this point.
Additional wear may occur due to hydraulic imbalance within the
pump. When a pump shaft breaks, it may be an indication that the
pump is running too closely to the no-flow or shutoff head.
Sometimes, HVAC water systems are designed with more estimated
pump head than actually exists in the system. The result is that the
pump does not operate at the design point but at a point farther out
on its curve. This is shown in Fig. 10.1b. This pump was selected for a
capacity of 1000 gal/min at a head of 100 ft. When new, the system
had much less head requirements, and this resulted in the pump
being operated at 1200 gal/min at 80 ft of head.
Another example of this so-called carryout of a pump’s operation is
given in Fig. 10.2 for the model building of Chap. 9 when located at
the end of a campus chilled water loop. This figure uses the system
head area of Fig. 9.4a. If a constant-speed pump is selected at the
design condition of 300 gal/min at 90 ft of head for this broad system
head area, the pump can operate at capacities as high as 540 gal/min.
At this point, the pump would operate noisily, at a poor efficiency, and
with a high radial thrust.
In an attempt to avoid poor pump operation, and recognizing the
inability to compute system head accurately, the usual practice with
constant-speed pumps has been to add a pump head conservation factor

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