Page 156 - HVAC Pump Handbook

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HVAC Pump Performance

HVAC Pump Performance 153

Three pump performance
90
Best efficiency area
50%
60%
80 75% 1750 RPM
85%
70
88%
System and pump head—ft 50 5100 GPM
60

at 60 ft
40

30

20

10

0
0 1000 2000 3000 4000 5000
System flow—GPM

Figure 6.12 Three-pump performance under variable speed.

friction loss in the pump fittings around each pump. The flow in an
individual pump is not the same as the total flow for a water system.
This will be explained further in Chap. 9, Section 9.3.3, “Pumping
System Losses.”

6.4 Affinity Laws of Pumps
Centrifugal pumps are variable-torque machines, like centrifugal
fans, and they obey the same laws as do centrifugal fans. The term
variable torque indicates that the horsepower required to turn the
pump or fan does not vary directly with their speed. It varies with the
cube of the speed. Constant-torque pumps, like most positive-
displacement pumps, are those in which the horsepower does vary
directly with the speed.
The laws that define centrifugal pump performance are called the
affinity laws. They dictate the changes in pump performance with
variations in speed, flow, head, and impeller diameter. The basic affin-
ity laws are as follows.

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