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CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
MATCHING PUMPS TO WATER
DISTRIBUTION SYSTEMS
System Curves
The pump curves and affinity laws are used to select a pump
or pumps for a particular application. The first step is to establish
a system pressure curve. This is calculated from design flow
and pressure loss tables for all the piping, coils, control valves,
and other components of the system.
Plotting A System Curve
An example is shown in Figure 56. The design point is 195 kPa
at a 32.2 L/s flow. A system curve (a simple square root curve)
can be plotted once the flow and pressure loss are known at any
particular point, since:
flow 2
p 2
2
=
flow 1
p 1
Where:
flow
= flow at p
in kPa
1
1
flow
= flow at p
in kPa
2
2
Plot the points (Fig. 56) for flows of 12.5, 25, and 37.5 L/s:
2
( )
12.5
Point A:
x 195 kPa = 0.1507 x 195 kPa = 29.4 kPa
32.2
2
( )
25
Point B:
x 195 kPa = 0.6028 x 195 kPa = 117.5 kPa
32.2
2
( )
37.5
Point C:
x 195 kPa = 1.356 x 195 kPa = 264.5 kPa
32.2
The system curve assumes all balancing valves are set for
design conditions, that all controls valves are fully open, and
that flow through all loads is proportional. The system curve is
always the same, even if loading is not proportional, at no load
and full (100 percent) load. If the loads are not proportional
(such as, some loads off or some valves throttling), the curve
rises above that shown for values between full and no load.
The system curve in Figure 56 is used in Figure 57 to select the
single speed pump. Using this system curve to determine the
switching setpoint for dual parallel pumps when the load flow
is not proportional can result in damaging pump cycling.
300
270
240
210
180
150
120
90
60
A
30
0
0
6.25
12.5
Fig. 56. System Curve for Pump Application.
Combining System And Pump Curves
The design system flow is 32.2 L/s. Piping, control valve,
and equipment losses are calculated at 195 kPa. An impeller
size and a motor power are selected by imposing the system
curve on the pump curve (Fig. 57). The designer has the option
of selecting a pump with a 240 mm impeller (32.2 L/s at
233 kPa) or a 220 mm impeller (30.5 at180 kPa). The smaller
impeller requires a 7500 watt motor and the larger impeller
requires a 9000 watt motor. Selection of the 240 mm impeller
requires system balancing valves to reduce the system pressure
differentials to those matching the design flow of 32.2 L/s.
When selecting a pump, it is important to remember that:
— With constant speed pumps (and two-way AHU control
valves), flow rides the pump curve. The system curve is
plotted assuming that the control valves are full open,
which in any system, only occurs at the full load. As
control valves throttle and loads are turned off the system
becomes non-proportional and the system curve rises
between no load and design.
— With variable speed pumps, the system control objective
is to have the pump curve ride the system curve by keeping
at least one control valve open and reducing the pump speed
to reduce flow with the diminishing system drop.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
340
C
FLOW AT FULL LOAD
B
18.25
25
31.35
37.5
CAPACITY IN L/s
C4092
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