Danfoss VLT FC 102 Series Design Manual page 31

Introduction to VLT® HVAC D...
2.7.15 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.
2.7.16 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
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
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 HVAC dedicated adjustable frequency drive can be utilized 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 adjustable
frequency drive also includes a 3-zone, 3-setpoint PID controller which allows monitoring of both temperature and air
quality. Even if the temperature requirement is satisfied, the adjustable frequency drive will maintain enough supply air to
satisfy the air quality sensor. The adjustable frequency drive 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.
Cooling coil
D1
D2
D3
Figure 2.18 The VLT Solution
MG11BC22
Design Guide
sensors can be used as feedback signals to adjustable frequency drives. Whether
2
Frequency
Heating coil
converter
Filter
Frequency
converter
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sensor detects lower
2
Temperature
signal
Supply fan
Pressure
signal
Return fan
2 2
Temperature
transmitter
Pressure
transmitter
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