Constant Air Volume; The Vlt Solution - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

Product Overview

3.1.14 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
3 3
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.

3.1.15 The VLT Solution

With a frequency converter, significant energy savings can
be obtained while maintaining decent control of the
building. Temperature sensors or CO
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
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
airflows.
D1
D3
Illustration 3.12 Constant Air Volume
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®
VLT HVAC Basic Drive FC 101
sensors can be used
2
2
Cooling coil
Heating coil
D2
Danfoss A/S © 04/2016 All rights reserved.
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 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 frequency
converter 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 frequency converter also includes a PI controller, which
allows monitoring both temperature and air quality. Even if
the temperature requirement is fulfilled, the frequency
converter maintains enough supply air to satisfy the air
quality sensor. The controller is capable of monitoring and
comparing 2 feedback signals to control the return fan by
maintaining a fixed differential airflow between the supply
and return ducts as well.
sensor
Temperature
signal
Frequency
converter
Filter
Supply fan
Pressure
signal
Frequency
converter
Return fan
Temperature
transmitter
Pressure
transmitter
MG18C702