Variable Speed Pumps - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS

VARIABLE SPEED PUMPS

From the pump affinity laws (Table 3), pump horsepower
decreases by the cube of the decreased speed, and flow decreases
linearly with speed; so at 80 percent flow, the horsepower 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 heads 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.
65
1750 RPM
60
55
REGION OF BEST
EFFICIANCY
50
BEST
45
EFFICIANCY
AREA
40
35
30
25
1150 RPM
20
850 RPM
15
10
550 RPM
5
0
0 100 200
Fig. 59. Pump Performance and
Efficiency at Various Speeds.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
PUMP HEAD-CAPACITY
CURVES
300 400 500
FLOW, GPM
C1056
100
80
60
40
20
0
0 100 200
FLOW IN GALLONS PER MINUTE
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.
80
70
FULL OPEN
CONTROL VALVE DROP
60
50
1050 RPM
40
PUMP CURVE
30
700 RPM
PUMP CURVE
20
10
0
0 100 200
SYSTEM FLOW IN GPM
Fig. 61. Ideal Variable Speed Pump Control.
344
TYPICAL RANGE OF PERFORMANCE
FOR VARIABLE SPEED PUMP
400 500 700
300 600
OPERATING
1750 RPM
POINT
PUMP CURVE
1400 RPM
PUMP CURVE
SYSTEM CURVE FOR STATIC
ELEMENTS OF SYSTEM (CHILLER,
PIPING, FITTINGS, BALANCING
COCKS, COILS, STRAINERS, ETC.)
300 400 500
C1052
M15057