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

Condition
Flow (gpm)
Speed (rpm)
Total Head (ft)
Fully Open Valve Drop (ft)
Brake Horsepower

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.
10' TOTAL
STATIC
COOLING
HEAD
TOWER
STATIC
SUCTION
HEAD
80'
200GPM
Fig. 62. Typical Cooling Tower Application.
60
PUMP A
PERFORMANCE CURVE
50
40
30
20
SYSTEM
CURVE
10
0
150
50
100
FLOW IN GALLONS PER MINUTE
Fig. 63. System Curve for Open
Circuit without Static Head.
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
Table 4. Variable Speed Pump Relationships
100 80
500 0.80 x 500 = 400
1750 0.80 x 1750 = 1400
70
( )
1400
70 x
1750
10 10 x 0.80 2 = 6.4
13.6
( )
1400
13.6 x
1750
STATIC
DISCHARGE
HEAD
90'
CONDENSER
C2413
INDICATED
OPERATING
POINT
200 250 300
350
C1057
Load (percent)
60
0.60 x 500 = 300
0.60 x 1750 = 1050
2 2
( )
1050
= 44.8 70 x
1750
10 x 0.60 2 = 3.6
3
( )
1050
= 6.96 13.6 x
1750
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.
PUMP A
60
PERFORMANCE
CURVE
50
40
30
SYSTEM
CURVE
20
10
10
0
100
50
FLOW IN GALLONS PER MINUTE
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.
345 ENGINEERING MANUAL OF AUTOMATIC CONTROL
40
0.40 x 500 = 200
0.40 x 1750 = 700
2
( )
700
= 25.2 70 x
1750
10 x 0.40 2 = 1.6
3 3
( )
700
= 2.94 13.6 x
1750
ACTUAL
OPERATING
POINT PUMP B
ACTUAL
OPERATING
POINT PUMP A
150 200 250 300 350
C1058
= 11.2
= 0.87