Page 157 - HVAC Pump Handbook

Rishel_CH06.qxd 20/4/06 6:06 PM Page 154

HVAC Pump Performance

154 HVAC Pumps and Their Performance

For a fixed-diameter impeller:

1. The pump capacity varies directly with the speed.

Q S
1
1
(6.1)
Q S
2 2
2. The pump head varies as the square of the speed.
h S 2
1
1
(6.2)
h S 2
2 2
3. The pump brake horsepower required varies as the cube of the
speed:

bhp S 3
1
1
(6.3)
bhp S 3
2 2
Figure 6.13a describes these three laws graphically. These are the
basic laws that must be understood in any attempt to comprehend
centrifugal pump performance. It must be remembered that these
laws pertain to the pump itself. They do not describe pump perfor-
mance when the pump is connected to a system of piping containing
both constant and variable head.
Figure 6.13b describes the variation in flow, head, and power for a
variable-speed pump operating with a constant head of 20 ft; data for
these curves are provided in Table 6.2 later in this chapter. This con-
stant head can be static head on a cooling tower or constant differen-
tial pressure on a secondary chilled or hot water system. It is obvious
that there is no comparison between the curves in Fig. 6.13a and b. At
50 percent speed and 885 rev/min, the flow is 36 percent, not 50 per-
cent, the head is 30 percent, and the brake horsepower is 10 percent,
not 12.5 percent. This demonstrates the need to evaluate pump per-
formance with the actual system conditions and not depend on the
classic affinity laws of Fig. 6.13a.
This is emphasized at this point because it may appear from Fig. 6.1a
that pump impellers can be changed just through observation of the
basic affinity laws of Fig. 6.13a. Pump impeller diameter should not be
changed without a thorough evaluation of the system head curve or
area.
Changing the pump impeller diameter changes pump performance,
as does pump speed.

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