Danfoss VLT AQUA Drive FC 202 Design Manual page 18

Product Overview
2.1.3 Example of Energy Savings
2 2
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%. This is because the flow is directly
proportional to the speed. The consumption of electricity,
however, is reduced by up to almost 50%.
If the system only needs 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 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%
Illustration 2.2 Affinity Laws for Centrifugal Pumps
Q 1 n 1
Flow : =
Q 2 n 2
H 1
Pressure :
=
H 2
P 1 n 1
Power :
=
P 2 n 2
Assuming an equal efficiency in the speed range.
Q=Flow
Q =Flow 1
1
Q =Reduced flow
2
H=Pressure
H =Pressure 1
1
H =Reduced pressure
2
Table 2.1 Affinity Laws
16
Pressure ~n
3
Power ~n
50% 80%
n 1 2
n 2
3
P=Power
P =Power 1
1
P =Reduced power
2
n=Speed regulation
n =Speed 1
1
n =Reduced speed
2
Danfoss A/S © 09/2014 All rights reserved.
Design Guide
2.1.4 Valve Control versus Speed Control of
Centrifugal Pumps
Valve control
As the demand for process requirements in water systems
varies, the flow has to be adjusted accordingly. Frequently
used methods for flow 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
flow 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 with resultant
pump damage.
Throttling the flow 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 car speed. Illustration 2.3 shows that
throttling makes the system curve turn from point (2) on
the pump curve to a point with significantly reduced
efficiency (1).
100% speed
2
n
100%
Pump curve
1
2
3
Illustration 2.3 Flow Reduction by Valve Control (Throttling)
1
3
Flow
Operating
point
operating point
1
3
Flow
Operating point using a throttle valve
Natural operating point
Operating point using speed control
2
Natural
2
H
p
H
s
MG20N602