Danfoss VLT Series Design Manual page 23

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VLT HVAC Drive Design Guide
2.7.14. 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 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.
2.7.15. The VLT solution
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 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 CO2 sensor detects lower levels and de-
creases 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 require-
ments 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 frequency converter, the VLT HVAC Drive 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 min-
imum amount of supply air regardless of the feedback or reference signal. The frequency converter
also includes a 3-zone, 3 setpoint PID controller which allows monitoring both temperature and
air quality. Even if the temperature requirement 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.
Cooling coil
Heating coil
D1
D2
D3
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MG.11.B2.02 - VLT is a registered Danfoss trademark
2. Introduction to VLT HVAC Drive
Temperature
signal
Filter
Supply fan
Pressure
signal
Return fan
2
Temperature
transmitter
Pressure
transmitter
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