Danfoss VLT AQUA Drive FC 202 Design Manual page 17

Introduction
2.8.3 Example of Energy Savings
As can be seen in Illustration 2.3, the flow is controlled by
changing the RPM. Reducing the speed only 20% from the
rated speed reduces the flow by 20%. This reduction
occurs because the flow is directly proportional to the
RPM. The consumption of electricity, however, is 50%
lower.
If the system in question must supply a flow that
corresponds to 100% only 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%.
Q = Flow P = Power
Q = Rated flow P
1 1
Q = Reduced flow P
2 2
H = Pressure n = Speed regulation
H = Rated pressure n
1 1
H = Reduced pressure n
2 2
Table 2.4 Laws of Proportionality
100%
80%
Flow ~n
50%
25%
12,5%
50%
Illustration 2.3 Dependence of Flow, Pressure and Power
consumption on RPM
Q 1 n 1
Flow : =
Q 2 n 2
H 1 n 1 2
Pressure : =
H 2 n 2
P 1 n 1
3
Power :
=
P 2 n 2
MG20Z202
Design Guide
= Rated power
= Reduced power
= Rated speed
= Reduced speed
Pressure ~n 2
3
Power ~n
n
80% 100%
Danfoss A/S © Rev. 06/2014 All rights reserved.
2.8.4 Example with Varying Flow Over One
Year
Illustration 2.4 is calculated based on pump characteristics
obtained from a pump datasheet.
The result obtained shows energy savings in excess of 50%
at the given flow distribution over a year. The pay back
period depends on the price per kWh and the price of the
frequency converter. In this example, it is less than a year
when compared with valves and constant speed.
Energy savings
P =P
shaft shaft output
[h] t
2000
1500
1000
500
100 200
Illustration 2.4 Flow Distribution over 1 Year
Q
300 400
[m 3 /h]
15
2 2