Specifications

2.6.14 Constant Air Volume
CAV, or Constant Air Volume systems, are central
ventilation systems usually used to supply large common
zones with the minimum amounts of fresh tempered air.
They preceded VAV systems and are therefore found in
older multi-zoned commercial buildings as well. These
systems preheat amounts of fresh air utilising Air Handling
Units (AHUs) with a heating coil, and many are also used
to air condition buildings and have a cooling coil. Fan coil
units are frequently used to assist in the heating and
cooling requirements in the individual zones.
2.6.15
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 fans speed.
The return fan modulates to maintain a static pressure
setpoint or fixed difference between the supply and return
air flows.
With temperature control, especially used in air
conditioning systems, as the outside temperature varies as
well as the number of people in the controlled zone
changes, different cooling requirements exist. As the
temperature decreases below the set-point, the supply fan
can decrease its speed. The return fan modulates to
maintain a static pressure set-point. By decreasing the air
flow, energy used to heat or cool the fresh air is also
reduced, adding further savings.
Several features of the Danfoss HVAC dedicated frequency
converter can be utilised to improve the performance of
the 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 one PI 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 is capable of
monitoring and comparing 2 feedback signals to control
the return fan by maintaining a fixed differential air flow
between the supply and return ducts as well.
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 Constant Air Volume
Product Overview
VLT
®
HVAC Basic Drive FC 101 Design Guide
MG18C502 - Rev. 2013-09-06 19
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