Danfoss VLT FC 102 Series Design Manual page 30

Introduction to VLT® HVAC D...
2.7.13 Variable Air Volume
2 2
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 efficient method to air condition buildings. By
designing central systems instead of distributed systems, greater efficiency can be obtained.
The efficiency comes from utilizing larger fans and larger chillers which have much higher efficiencies than small motors
and distributed air-cooled chillers. Savings are also a result of decreased maintenance requirements.
2.7.14 The VLT Solution
While dampers and IGVs work to maintain a constant pressure in the ductwork, a 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 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 HVAC can be used to eliminate the need for additional controllers.
D1
D3
Figure 2.17 The VLT Solution
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Design Guide
Cooling coil
Heating coil
D2
Danfoss A/S © Rev. 06/2014 All rights reserved.
Pressure
signal
Frequency
converter
Filter
Supply fan
3
Frequency
converter
Return fan
3
VAV boxes
Pressure
transmitter
Flow
Flow
T
MG11BC22