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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
VARIABLE SPEED PUMPS
From the pump affinity laws (Table 3), pump power decreases
by the cube of the decreased speed, and flow decreases linearly
with speed; so at 80 percent flow, the power (kW) is down to
nearly 50 percent (80 percent cubed). Since many systems have
sharply reduced flow requirements at medium or low loads,
pump speed control can provide economical operation for most
of the heating (or cooling) season. Figure 59 shows typical
performance at reduced speeds. The shaded area of Figure 60
shows the wide range of pressures and flows available from a
variable speed pump. Variable speed pumps are usually
controlled from a differential pressure sensor with either fixed
or load reset setpoints.
Control objectives of variable speed pumping systems in
networked digital control systems, is to keep the most demanding
load control valve full open by varying the pump speed.
195
1750 RPM
180
165
REGION OF BEST
EFFICIANCY
150
BEST
135
EFFICIANCY
AREA
120
105
90
1150 RPM
75
60
850 RPM
45
30
550 RPM
15
0
0
6.25
12.5
Fig. 59. Pump Performance and
Efficiency at Various Speeds.
PUMP HEAD-CAPACITY
CURVES
18.75
25
31.25
FLOW (L/s)
C4094
300
240
180
120
60
0
0
6.25
12.5
Fig. 60. Typical Variable Speed
Pump Performance Range.
Table 4 and Figure 61 show the Figure 58 system with all the
control valves remaining full open and the load controlled by
varying the pump speed. This is the ideal system wherein the
loads of all AHUs vary in unison and the pump speed is
controlled to satisfy the valve with the greatest demand. This
is usually accomplished via differential pressure control,
automatically reset.
240
210
FULL OPEN
CONTROL VALVE DROP
180
150
120
1050 RPM
PUMP CURVE
90
700 RPM
60
PUMP CURVE
30
0
0
5
10
SYSTEM FLOW IN L/s
Fig. 61. Ideal Variable Speed Pump Control.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
342
TYPICAL RANGE OF PERFORMANCE
FOR VARIABLE SPEED PUMP
25
31.25
43.75
18.75
37.5
FLOW (L/s)
1750 RPM
PUMP CURVE
1400 RPM
PUMP CURVE
SYSTEM CURVE FOR STATIC
ELEMENTS OF SYSTEM (CHILLER,
PIPING, FITTINGS, BALANCING
COCKS, COILS, STRAINERS, ETC.)
15
20
25
C4095
OPERATING
POINT
M15281
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