ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
345
Table 4. Variable Speed Pump Relationships
Figure 64 shows the system curve adjusted for the 10 ft of
static head and the actual operating points of Pump A (selected
in Figure 63) and Pump B. Notice Pump A supplies only 175
gpm at 32 ft of head.
PUMPS APPLIED TO OPEN SYSTEMS
In cooling towers (Fig. 62) and other open systems, static
head must be considered when establishing system curves and
selecting a pump. Notice that the 90 ft of vertical pipe (static
discharge head) is partially offset by the 80 ft of vertical pipe
(static suction head) in the suction line. When a system curve
is drawn for such a system, static head of the tower must be
added to the system curve. The system is designed to operate
at 200 gpm against 30 ft of head for piping and valves. The
system curve in Figure 63 is drawn through zero head (ignoring
the static head of 10 ft) which leads to choosing Pump A.
Fig. 62. Typical Cooling Tower Application.
Fig. 63. System Curve for Open
Circuit without Static Head.
CONDENSER
200GPM
COOLING
TOWER
STATIC
DISCHARGE
HEAD
10' TOTAL
STATIC
HEAD
STATIC
SUCTION
HEAD
80'
90'
C2413
FLOW IN GALLONS PER MINUTE
TOTAL HEAD IN FEET
60
50
40
30
20
10
0
50
100
150
200
250
300
350
C1057
PUMP A
PERFORMANCE CURVE
SYSTEM
CURVE
INDICATED
OPERATING
POINT
Fig. 64. System Curve for Open
Circuit with Static Head.
MULTIPLE PUMPS
Multiple pumps are used when light load conditions could
overload a single pump. These conditions normally occur when
two-way control valves are used in the control system. Two-
way control valves sharply reduce flow when they begin to
close. Figure 65 shows that in a single-pump system, over
pressure can result at low flow. At one-third flow, the pump
head has increased, the source and piping drop is reduced to
one-ninth of the design drop, and the control valve drop has
increased greatly. Bypass, variable speed, or throttling valve
pressure relief should be used with a single pump. Where the
heat exchanger (such as a chiller) requires a high minimum
flow rate, a single pump is used, and diversity is not used, three-
way load control valves should generally be used.
60
50
40
30
20
10
0
50
100
150
200
250
300
350
FLOW IN GALLONS PER MINUTE
TOTAL HEAD IN FEET
10
C1058
ACTUAL
OPERATING
POINT PUMP B
PUM
P B
PERFORMANCE
CURVE
PUMP A
PERFORMANCE
CURVE
SYSTEM
CURVE
ACTUAL
OPERATING
POINT PUMP A
Load (percent)
Condition 100 80 60 40
Flow (gpm) 500 0.80 x 500 = 400 0.60 x 500 = 300 0.40 x 500 = 200
Speed (rpm) 1750 0.80 x 1750 = 1400 0.60 x 1750 = 1050 0.40 x 1750 = 700
Total Head (ft) 70
70 x
()
1400
1750
2
= 44.8 70 x
()
1050
1750
2
= 25.2 70 x
()
700
1750
2
= 11.2
Fully Open Valve Drop (ft) 10
10 x 0.80
2
= 6.4 10 x 0.60
2
= 3.6 10 x 0.40
2
= 1.6
Brake Horsepower 13.6
13.6 x
()
1400
1750
3
= 6.96 13.6 x
()
1050
1750
3
= 2.94 13.6 x
()
700
1750
3
= 0.87
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