Danfoss VLT FC 102 Series Design Manual page 25

Introduction to VLT® HVAC D...
2.7.3 Example of Energy Savings
As shown in the figure (the laws of proportionality), the
flow is controlled by changing the RPM. By reducing the
rated speed by only 20%, 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 corresponding to 100% a few days each 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%.
The laws of proportionality
Figure 2.10 describes the dependence of flow, pressure and
power consumption on RPM.
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 2.5 Abbreviations Used in Equation
100%
80%
Flow ~n
50%
25%
12,5%
50%
Figure 2.10 The 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
MG11BC22
Design Guide
= Rated power
= Reduced power
= Rated speed
= Reduced speed
Pressure ~n 2
Power ~n 3
n
80% 100%
Danfoss A/S © Rev. 06/2014 All rights reserved.
2.7.4 Comparison of Energy Savings
The Danfoss adjustable frequency drive solution offers
major savings compared with traditional energy saving
solutions. This is because the adjustable frequency drive is
able to control fan speed according to thermal load on the
system and the fact that the adjustable frequency drive
has a built-in facility that enables the adjustable frequency
drive to function as a Building Management System, BMS.
Figure 2.12 shows typical energy savings obtainable with
three well-known solutions when fan volume is reduced to,
e.g., 60%.
Figure 2.12 shows more than 50% energy savings can be
achieved in typical applications.
Discharge
damper
Less energy savings
IGV
Costlier installation
Figure 2.11 The Three Common Energy Saving Systems
2
Maximum energy savings
23
2