Danfoss FC 102 Design Manual page 28

Introduction to VLT® HVAC D...
2.7.15 Constant Air Volume
2 2
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 utilising 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 a frequency converter, significant energy savings can be obtained while maintaining decent control of the building.
Temperature sensors or CO
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
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 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 the Danfoss HVAC dedicated frequency converter can be utilised 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 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 frequency converter will maintain enough supply air to satisfy the air quality
sensor. The frequency converter is capable of monitoring and comparing 2 feedback signals to control the return fan by
maintaining a fixed differential air flow between the supply and return ducts as well.
D1
D3
Illustration 2.18 The VLT Solution
26
Design Guide
sensors can be used as feedback signals to frequency converters. Whether controlling
2
Cooling coil
Heating coil
D2
Danfoss A/S © Rev. 06/2014 All rights reserved.
Temperature
signal
Frequency
converter
Filter
Supply fan
Pressure
signal
Frequency
converter
Return fan
sensor detects lower levels and
2
Temperature
transmitter
Pressure
transmitter
MG11BC02