V A V bo x es
Fl o w
Fl o w
P r essur e
tr ansmitt er
R etur n fan
3
3
Illustration 11: Variable Air Volume
3.1.2.2 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 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 VLT Solution
With a drive, 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 drives. 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 airflows.
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 setpoint, the
supply fan can decrease its speed. The return fan modulates to maintain a static pressure setpoint. By decreasing the air flow, ener-
gy used to heat or cool the fresh air is also reduced, adding further savings.
Several features of the Danfoss dedicated drive can be utilized 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 drive also includes a PI controller, which allows monitoring
both temperature and air quality. Even if the temperature requirement is fulfilled, the drive 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 main-
taining a fixed differential airflow between the supply and return ducts as well.
AJ363928382091en-000101 / 130R0983 | 21Danfoss A/S © 2021.04
Product Overview
VLT® Flow Drive FC 111
Design Guide
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