Pumps Applied To Open Systems; Multiple Pumps - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

Condition
Flow (L/s)
Speed (rpm)
Total Pressure (kPa)
Fully Open Valve Drop (kPa)
Motor Power (kW)

PUMPS APPLIED TO OPEN SYSTEMS

In cooling towers (Fig. 62) and other open systems, static
pressure must be considered when establishing system curves
and selecting a pump. Notice that the 27m of vertical pipe
(270 kPa static discharge pressure) is partially offset by the
24m of vertical pipe (240 kPa static suction pressure) in the
suction line. When a system curve is drawn for such a system,
static pressure of the tower must be added to the system curve.
The system is designed to operate at 12 L/s against 90 kPa for
piping and valves. The system curve in Figure 63 is drawn
through zero pressure (ignoring the 3m or 30 kPa static
pressure) which leads to choosing Pump A.
3m
TOTAL
STATIC
COOLING
PRESSURE
TOWER
(30 kPa)
24m
STATIC
SUCTION
PRESSURE
(240 kPa)
12 L/s
Fig. 62. Typical Cooling Tower Application.
180
PUMP A
PERFORMANCE CURVE
150
120
90
60
SYSTEM
CURVE
30
0
3 6 9
FLOW IN L/s
Fig. 63. System Curve for Open
Circuit without Static Pressure.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
Table 4. Variable Speed Pump relationships
100 80
31.5 0.80 x 31.5 = 25.2
1750 0.80 x 1750 = 1400
210 2
( )
1400
210 x
1750
30 30 x 0.80 2 = 19.4
10.1 3
( )
1400
10.1 x
1750
27m
STATIC
DISCHARGE
PRESSURE
(270 kPa)
CONDENSER
C4096
INDICATED
OPERATING
POINT
15
12 18 21
C4097
Load (percent)
60
0.60 x 31.5 = 18.9
0.60 x 1750 = 1050
2
( )
1050
= 134.4 210 x
1750
30 x 0.60 2 = 10.8
( )
1400
= 5.17 10.1 x
1750
Figure 64 shows the system curve adjusted for the 30 kPa of
static pressure and the actual operating points of Pump A
(selected in Figure 63) and Pump B. Notice Pump A supplies
only 10.5 L/s at 125 kPa.
180
PUMP A
PERFORMANCE
CURVE
150
120
90
SYSTEM
CURVE
60
30
30
0
3 6
Fig. 64. System Curve for Open
Circuit with Static Pressure.

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
pressure 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.
343
40
0.40 x 31.5 = 12.6
0.40 x 1750 = 700
2
( )
700
= 75.6 210 x
1750
30 x 0.40 2 = 4.8
3 3
( )
1400
= 2.18 10.1 x
1750
ACTUAL
OPERATING
POINT PUMP B
ACTUAL
OPERATING
POINT PUMP A
9
12 15 18 21
FLOW IN L/s
C4098
= 33.6
= 0.65