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 therefore are found in older
multi-zoned commercial buildings as well. These systems preheat amounts of fresh air utilizing 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.
The new standard
With a frequency converter, significant energy savings
can be obtained while maintaining decent control of
the building. Temperature sensors or CO2 sensors
can be used as feedback signals to frequency con-
verters. 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 CO2 sensor detects lower
levels and decreases the supply fans speed. The re-
turn fan modulates to maintain a static pressure set-
point or fixed difference between the supply and return
air flows.
With temperature control, especially used in air condi-
tioning 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 Danfoss HVAC dedicated frequen-
cy converter, the VLT 6000 HVAC can be utilized to
improve the performance of your 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 re-
gardless of the feedback or reference signal. The fre-
quency converter also includes a two zone, 2 setpoint
PID controller which allows monitoring both tempera-
ture and air quality. Even if the temperature require-
ment is satisfied, the drive will maintain enough supply
air to satisfy the air quality sensor. The controller is
capable of monitoring and comparing two feedback
signals to control the return fan by maintaining a fixed
differential air flow between the supply and return
ducts as well.
Pressure
signal
Cooling coil
Heating coil
D1
D2
D3
Filter
Pressure
transmitter
Supply fan
Return fan
Temperature
signal
Temperature
transmitter
VLT
®
6000 HVAC Series
MG.61.B5.02 - VLT
®
is a registered Danfoss trademark 19
Introduction to HVAC
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