Pump Affinity Laws - Honeywell AUTOMATIC CONTROL Engineering Manual

Figure 55 illustrates a pump fitted with a 6-1/2 inch impeller,
operating at 45 ft of head, and delivering 65 gpm of water.
Water hp = 65 gpm x 45 ft x 1.0/3960
= 0.74 hp
The curves show that the motor output is 1.17 horsepower.
Efficiency = (water horsepower)/(motor horsepower) x 100 =
(0.74/1.17) x 100 = 63 percent. This agrees with the efficiency
curves shown in Figure 55.

Pump Affinity Laws

Pump affinity laws (Table 3) show how pump flow, head,
and brake horsepower 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 an
8-inch impeller delivers 80 ft of head, with a 7.21-inch impeller
it would deliver 65 ft of head. It is calculated as follows:
New Diameter

New head = Old head x

Old Diameter
45%
60
7"
50
6 1/2"
6"
40
5 1/2"
30
5"
20
0
0 10 20 30
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
2 2
7.21
  
= 80 = 64.98 ft
  
8
55%
60%
SYSTEM CURVE
65%
1/2HP
40 60 70 80
50
CAPACITY IN GALLONS PER MINUTE
Fig. 55. Pump Curve for 1750 RPM Operation.
Table 3. Pump Affinity Laws.
Impeller
Diameter Speed
Constant Variable
Variable Constant
Constant Constant
Source: ASHRAE Handbook--1996 System and Equipment
OPERATING POINT
68%
68%
65%
3/4HP 1HP
90 100 110 120 130
341 ENGINEERING MANUAL OF AUTOMATIC CONTROL
Specific To
Gravity Correct Multiply
(SG) for
(
Flow
New Speed
Old Speed
Constant Head
(
New Speed
Old Speed
Bhp
(
New Speed
or kW
Old Speed
(
Flow
New Diameter
Old Diameter
Constant Head
(
New Diameter
Old Diameter
Bhp (
New Diameter
or kW
Old Diameter
New SG
Variable Bhp
or kW Old SG
60%
55%
45%
2HP
1 1/2HP
140 150 160 170 180
by
)
2
)
3
)
)
2
)
3
)
3HP
190
C1051