Constant Air Volume; The Vlt Solution - Danfoss VLT HVAC Drive FC 102 Design Manual

Introduction

2.7.15 Constant Air Volume

CAV, or constant air volume systems are central ventilation systems used to supply large common zones with the minimum
amounts of fresh tempered air. They preceded VAV systems and are found in older multi-zoned commercial buildings as
well. These systems preheat fresh air with air handling units (AHUs) that have heating coils. Many are also used for air
conditioning buildings and have a cooling coil. Fan coil units are often used to help with 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
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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 fan speed. The return fan modulates to maintain a static pressure setpoint or fixed difference between
the supply and return air flows.
Temperature control needs vary based on outside temperature and number of people in the controlled zone. As the
temperature decreases below the setpoint, the supply fan can decrease its speed. The return fan modulates to maintain a
static pressure setpoint. Decreasing the air flow, reduces the energy used to heat or cool the fresh air, resulting in further
savings.
Several features of the Danfoss HVAC dedicated frequency converter can be used 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 maintains enough supply air to satisfy the air quality sensor.
The controller can monitor and compare two feedback signals to control the return fan by maintaining a fixed differential
air flow between the supply and return ducts.
Cooling coil
D1
D2
D3
Illustration 2.11 Frequency Converter used in a CAV System
MG16C202
Design Guide
Frequency
Heating coil
converter
Filter
Frequency
converter
Danfoss A/S © Rev. 05/2014 All rights reserved.
sensor detects lower levels and
2
Temperature
signal
Supply fan
Pressure
signal
Return fan
2 2
Temperature
transmitter
Pressure
transmitter
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