Honeywell AUTOMATIC CONTROL User Manual

ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
344
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.
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.
Fig. 59. Pump Performance and
Efficiency at Various Speeds.
50
0 100 200 300 400 500
0
5
10
15
20
25
30
35
40
45
55
60
65
TOTAL HEAD IN FEET
FLOW, GPM
C1056
1750 RPM
REGION OF BEST
EFFICIANCY
BEST
EFFICIANCY
AREA
1150 RPM
850 RPM
550 RPM
PUMP HEAD-CAPACITY
CURVES
FLOW IN GALLONS PER MINUTE
TYPICAL RANGE OF PERFORMANCE
FOR VARIABLE SPEED PUMP
TOTAL HEAD IN FEET
100
80
60
40
20
0
0
100
200
300
400
500
600
700
C1052
Fig. 61. Ideal Variable Speed Pump Control.
80
70
60
50
40
30
20
10
0
SYSTEM CURVE FOR STATIC
ELEMENTS OF SYSTEM (CHILLER,
PIPING, FITTINGS, BALANCING
COCKS, COILS, STRAINERS, ETC.)
OPERATING
POINT
1750 RPM
PUMP CURVE
0
100 200 300 400 500
1400 RPM
PUMP CURVE
1050 RPM
PUMP CURVE
700 RPM
PUMP CURVE
FULL OPEN
CONTROL VALVE DROP
M15057
HEAD IN FEET
SYSTEM FLOW IN GPM

Honeywell AUTOMATIC CONTROL Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL
Category	Controller
Language	English