Specifications

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 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

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 CO

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 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.

Frequency

converter

Frequency

converter

Pressure

signal

Cooling coil

Heating coil

Filter

Pressure

transmitter

Supply fan

Return fan

Temperature

signal

Temperature

transmitter

130BB451.10

Illustration 2.18 The VLT Solution

Introduction to VLT® HVAC D... Design Guide