2.7.13 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 efficient method to air condition buildings. By designing central systems instead of distributed systems, a
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 seen from the decreased maintenance requirements.
2.7.14 The VLT Solution
While dampers and IGVs work to maintain a constant pressure in the ductwork, a frequency converter 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 frequency converter 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
frequency converter can be used to eliminate the need for additional controllers.
Pressure
signal
VAV boxes
Flow
Flow
Cooling coil
Heating coil
D1
D2
D3
Filter
Pressure
transmitter
Supply fan
Return fan
T
3
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2 Introduction to VLT HVAC Drive
VLT
®
HVAC Drive Design Guide
26
MG.11.B9.02 - VLT
®
is a registered Danfoss trademark
2