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 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.
2.6.15 The VLT Solution
With an adjustable frequency 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 adjustable frequency 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 air flows.
With temperature control (especially used in air conditioning systems), as the outside temperature varies and the number of
people in the controlled zone changes, different cooling requirements arise. 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, energy used to heat or cool the fresh air is also reduced, adding further savings.
Several features of the Danfoss dedicated adjustable frequency 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 adjustable frequency
drive also includes one PI controller, which allows monitoring both temperature and air quality. Even if the temperature
requirement is satisfied, the adjustable frequency drive maintains 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.
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
Figure 2.12 Constant Air Volume
Product Overview Design Guide
MG18C522 Danfoss A/S © Rev. 2014-01-14 All rights reserved. 19
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