GE AF-600 FP Design Manual page 17

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Illustration 2.2: When using a frequency converter to reduce fan
capacity to 60% - more than 50% energy savings may be ob-
tained in typical applications.
2.7.3 Example of Energy Savings
As can be seen from the figure (the laws of proportionality), the flow is controlled by changing the RPM. By reducing the speed only 20% from the rated speed,
the flow is also reduced by 20%. This is because the flow is directly proportional to the RPM. The consumption of electricity, however, is reduced by 50%.
If the system in question only needs to be able to supply a flow that corresponds to 100% a few days in a year, while the average is below 80% of the rated flow
for the remainder of the year, the amount of energy saved is even more than 50%.
The laws of proportionality
The figure below describes the dependence of flow, pressure and power consumption on RPM.
Q = Flow
Q = Rated flow
1
Q = Reduced flow
2
H = Pressure
H = Rated pressure
1
H = Reduced pressure
2
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P = Power
P = Rated power
1
P = Reduced power
2
n = Speed regulation
n = Rated speed
1
n = Reduced speed
2
AF-600 FP Design Guide