Application of VSD in VAV Air-Handling Units

This article describes the application of variable speed drive (vsd) in variable air volume (vav) air handling unit.

            Variable Air Volume (VAV) systems typically bring conditioned air from Primary air handling unit (PAU) and returned air from the air-conditioned space into Air Handling Units (AHU) where the air temperature and humidity can be adjusted. Fans blow air across filter, cooling coils and volume control dampers or inlet guide vanes into ductwork, which distributes the air throughout the zones served. The air passes into each zone from the ductwork through individual VAV terminal boxes. A temperature sensor located in each zone is connected to its VAV box and opens or closes the VAV box to maintain the defined temperature set point. As the zone becomes satisfied, the VAV box modulates to a close position. The pressure in the ductwork would then begin to rise as the openings in the VAV box close.

Traditionally, inlet guide vanes or discharged dampers are installed in the AHUs to prevent this over pressurization and save energy. These devices work by creating resistance and a pressure drop to the air entering the ductwork or reducing the efficiency of the fan. The more the VAV boxes in the system close, the more the dampers close to maintain static duct pressure. The dampers or inlet guide vanes for the fan are commonly controlled by a controller maintaining a fixed pressure in the supply ductwork downstream of the AHU.

While dampers and inlet guide vanes work to maintain a constant pressure in the ductwork of a VAV system, the utilization of VSD could save much more energy and reduce the complexity of the installation. Instead of creating an artificial pressure drop or causing a decrease in fan efficiency, the VSD decrease the speed of the fan to provide the flow and pressure required by the system. Typical configuration schematic is below.

There are two most important advantages to VAV systems. The fan capacity control, especially with modern electronic variable speed drives, reduces the energy consumed by fans which can be a substantial part of the total cooling energy requirements of a building. Dehumidification is greater with VAV systems than it is with constant volume system which modulates the discharge air temperature to attain part load cooling capacity.