Danfoss VLT AQUA FC 202 - 3.3.3 Terminal Locations - Enclosure Type D

n
100%
50%
25%
12,5%
50% 100%
80%
80%
e75ha208.10
P o w er ~n
3
P r essur e ~n
2
Fl
o
w ~n
Illustration 7: Anity Laws for Centrifugal Pumps
Flow: 
Q1
Q2
 = 
n1
n2
Pressure: 
H1
H2
 = 
n1
n2
2
Power: 
P1
P2
 = 
n1
n2
3
Assuming an equal eciency in the speed range.
Anity Laws
Q=Flow
P=Power
Q
1
=Flow 1
P
1
=Power 1
Q
2
=Reduced ow (or increased ow)
P
2
=Reduced power (or increased power)
H=Pressure
n=Speed regulation
H
1
=Pressure 1
n
1
=Speed 1
H
2
=Reduced pressure (or increased pressure)
n
2
=Reduced speed (or reduced speed)
4.3.2 Valve Control versus Speed Control of Centrifugal Pumps
Valve control
As the demand for process requirements in water systems varies, the ow to be adjusted accordingly. Frequently used methods for
ow adaptation are throttling or recycling using valves. A recycle valve that is opened too wide can cause the pump to run at the
end of the pump curve with a high ow rate at a low pump head. These conditions do not only cause a waste of energy due to the
high speed of the pump, but can also lead to pump cavitation resulting in pump damage.
Throttling the ow with a valve adds a pressure drop across the valve (HP-HS). This can be compared with accelerating and pulling
the brake at the same time in an attempt to reduce the car speed. As shown in Illustration 8, throttling makes the system curve turn
from point (2) on the pump curve to a point with signicantly reduced eciency (1).
AJ300847815559en-000101 / 130R0337 | 23Danfoss A/S © 2020.09
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