Danfoss VLT Refrigeration Drive FC 103 Design Manual page 17

Product Overview
2.1.3 Example of Energy Savings
As shown in Illustration 2.2, the flow is controlled by
changing the pump speed, measured in RPM. By reducing
the speed only 20% from the rated speed, the flow is also
reduced by 20%. The flow is directly proportional to the
speed. The consumption of electricity is reduced by up to
50%.
If the system only has 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 energy
savings are even greater than 50%.
Illustration 2.2 describes the dependence of flow, pressure,
and power consumption on pump speed in RPM for
centrifugal pumps.
100%
80%
Flow ~n
50%
25%
12,5%
50%
Illustration 2.2 Affinity Laws for Centrifugal Pumps
Q n
1 1
Flow : =
Q n
2 2
2
H n
1 1
Pressure : =
H n
2 2
3
P n
1 1
Power : =
P n
2 2
Assuming an equal efficiency in the speed range.
Q=Flow P=Power
Q =Flow 1 P
1 1
Q =Reduced flow P
2 2
H=Pressure n=Speed regulation
H =Pressure 1 n
1 1
H =Reduced pressure n
2 2
Table 2.1 Affinity Laws
MG16G202
Design Guide
Pressure ~n 2
3
Power ~n
n
80% 100%
=Power 1
=Reduced power
=Speed 1
=Reduced speed
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2.1.4 Example with Varying Flow over 1
Year
This example is calculated based on pump characteristics
obtained from a pump datasheet, shown in Illustration 2.4.
The result obtained shows energy savings in excess of 50%
at the given flow distribution over a year,
see Illustration 2.3. The payback period depends on the
price of electricity and the price of the frequency
converter. In this example, payback is less than a year,
when compared with valves and constant speed.
[h] t
2000
1500
1000
500
100 200
t [h] Duration of flow. See also Table 2.2.
Flowrate
Q [m 3 /h]
Illustration 2.3 Flow Distribution over 1 Year (Duration versus
Flowrate)
Q
300 400
[m 3 /h]
15
2 2