Pump Affinity Laws - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

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:
New pressure = Old pressure x
45%
180
180mm DIA
150
165mm
DIA
150mm DIA
120
140mm DIA
90
125mm DIA
60
0
0 0.6 1.2 1.8
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
2
New Diameter 190
( ) ( )
= 240
Old Diameter 210
= 196 kPa
55%
60%
SYSTEM CURVE
65%
400W
2.4 3.0 3.6 4.2 4.8
Fig. 55. Pump Curve for 1750 RPM Operation.
Table 3. Pump Affinity Laws.
Impeller
Diameter Speed
Constant Variable
Variable Constant
2
Constant Constant
Source: ASHRAE Handbook--1996 System and Equipment
OPERATING POINT
68%
68%
65%
750W
500W
5.4 6.0 6.6 7.2 7.8
CAPACITY (L/s)
339
Specific To
Gravity Correct Multiply
(SG) for
(
Flow
New Speed
Old Speed
Constant Pressure
(
New Speed
Old Speed
Power
(
New Speed
or kW
Old Speed
(
Flow
New Diameter
Old Diameter
Constant Pressure
(
New Diameter
Old Diameter
Power (
New Diameter
or kW
Old Diameter
New SG
Variable Power
or kW Old SG
60%
55%
45%
1500W
1000W
8.4 9.0 9.6 10.2 10.8
by
)
2
)
3
)
)
2
)
3
)
2000W
11.4
C4091