ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
339
Figure 55 illustrates a pump fitted with a 165 mm impeller,
operating at 130 kPa, and delivering 4.2 L/s of water.
Water power = 4.2 L/s x 130 kPa
= 546 W
The curves show that the motor output is 850 watts.
Efficiency = (water power)/(motor power) x 100 = (546/850)
x 100 = 64 percent. This agrees with the efficiency curves
shown in Figure 55.
Pump Affinity Laws
Pump affinity laws (Table 3) show how pump flow, pressure,
and motor power vary as impeller diameter or speed change.
These laws help when adjusting an installed pump to changes
in the system served. For example, if a pump with a 210 mm
impeller delivers 240 kPa, with a 190 mm impeller it would
deliver 196 kPa. It is calculated as follows:
Table 3. Pump Affinity Laws.
Fig. 55. Pump Curve for 1750 RPM Operation.
New pressure = Old pressure x
()
New Diameter
Old Diameter
2
= 240
()
190
210
2
= 196 kPa
C4091
CAPACITY (L/s)
45%
55%
60%
68%
68%
65%
60%
55%
45%
2000W
1500W
1000W
750W
500W
400W
180mm DIA
165mm
DIA
140mm DIA
150mm DIA
125mm DIA
0
60
90
120
150
180
0 0.6 1.2 1.8 2.4
3.0
3.6 4.2 4.8 5.4 6.0 6.6 7.2 7.8 8.4 9.0 9.6 10.2 10.8 11.4
TOTAL PRESSURE (kPa)
SYSTEM CURVE OPERATING POINT
65%
Impeller
Diameter Speed
Specific
Gravity
(SG)
To
Correct
for
Multiply
by
Flow
()
New Speed
Old Speed
Constant Variable Constant Pressure
()
New Speed
Old Speed
2
Power
or kW
()
New Speed
Old Speed
3
Flow
()
New Diameter
Old Diameter
Variable Constant Constant Pressure
()
New Diameter
Old Diameter
2
Power
or kW
()
New Diameter
Old Diameter
3
Constant Constant Variable Power
or kW
New SG
Old SG
Source: ASHRAE Handbook--1996 System and Equipment
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