Danfoss VLT Series Design Manual page 26

2. Introduction to the VLT HVAC Drive
2.6.12. Variable Air Volume
VAV or Variable Air Volume systems, are used to control both the ventilation and temperature to
satisfy the requirements of a building. Central VAV systems are considered to be the most energy
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efficient method to air condition buildings. By designing central systems instead of distributed
systems, greater efficiency can be obtained.
This greater efficiency is a result of utilizing larger fans and larger chillers, which have much higher
efficiency rates than small motors and distributed air-cooled chillers. Savings are also a result of
decreased maintenance requirements.
2.6.13. The VLT solution
While dampers and IGVs work to maintain a constant pressure in the ductwork, an adjustable
frequency drive solution saves much more energy and reduces the complexity of the installation.
Instead of creating an artificial pressure drop or causing a decrease in fan efficiency, the adjustable
frequency drive decreases the speed of the fan to provide the flow and pressure required by the
system.
Centrifugal devices such as fans behave according to the centrifugal laws. This means the fans
decrease the pressure and flow they produce as their speed is reduced. Their power consumption
is thereby significantly reduced.
The return fan is frequently controlled to maintain a fixed difference in airflow between the supply
and return. The advanced PID controller of the VLT HVAC Drive can be used to eliminate the need
for additional controllers.
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Cooling coil
Heating coil
Filter
D1
D2
D3
MG.11.B4.22 - VLT
Pressure
signal
VAV boxes
Supply fan
3
Pressure
transmitter
Flow
Return fan
Flow
3
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VLT ® HVAC Drive Design Guide
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