Specifications

2.7.15 Constant Air Volume
CAV, or constant air volume systems are central ventilation
systems used to supply large common zones with the
minimum amounts of fresh tempered air. They preceded
VAV systems and are found in older multi-zoned
commercial buildings as well. These systems preheat fresh
air with air handling units (AHUs) that have heating coils.
Many are also used for air conditioning buildings and have
a cooling coil. Fan coil units are often used to help with
the heating and cooling requirements in the individual
zones.
2.7.16 The VLT Solution
With a frequency converter, significant energy savings can
be obtained while maintaining decent control of the
building. Temperature sensors or CO
2
sensors can be used
as feedback signals to frequency converters. Whether
controlling temperature, air quality, or both, a CAV system
can be controlled to operate based on actual building
conditions. As the number of people in the controlled area
decreases, the need for fresh air decreases. The CO
2
sensor
detects lower levels and decreases the supply fan speed.
The return fan modulates to maintain a static pressure
setpoint or fixed difference between the supply and return
air flows.
Temperature control needs vary based on outside
temperature and number of people in the controlled zone.
As the temperature decreases below the setpoint, the
supply fan can decrease its speed. The return fan
modulates to maintain a static pressure setpoint.
Decreasing the air flow, reduces the energy used to heat
or cool the fresh air, resulting in further savings.
Several features of the Danfoss HVAC dedicated frequency
converter can be used to improve the performance of a
CAV system. One concern of controlling a ventilation
system is poor air quality. The programmable minimum
frequency can be set to maintain a minimum amount of
supply air regardless of the feedback or reference signal.
The frequency converter also includes a 3-zone, 3 setpoint
PID controller which allows monitoring both temperature
and air quality. Even if the temperature requirement is
satisfied, the frequency converter maintains enough supply
air to satisfy the air quality sensor. The controller can
monitor and compare two feedback signals to control the
return fan by maintaining a fixed differential air flow
between the supply and return ducts.
Frequency
converter
Frequency
converter
Pressure
signal
Cooling coil
Heating coil
D1
D2
D3
Filter
Pressure
transmitter
Supply fan
Return fan
Temperature
signal
Temperature
transmitter
130BB451.10
Illustration 2.12 Frequency Converter used in a CAV System
Introduction VLT® HVAC Drive FC 102 Design Guide
MG16C102 - Rev. 2013-08-20 25
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