Variable air volume

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Variable air volume (VAV) is a technique for controlling the capacity of a heating, ventilating, and/or air-conditioning (HVAC) system. The simplest VAV system incorporates one supply duct that, when in cooling mode, distributes approximately 55 degree F supply air. Because the supply air temperature, in this simplest of VAV systems, is constant, the air flow rate must vary to meet the rising and falling heat gains or losses within the thermal zone served.

The air flow rate control is provided via two techniques; for single zone systems the blower's flow rate is varied. For a single VAV air handler that serves multiple thermal zones, the flow rate to each zone must be varied as well.

A VAV terminal unit[1], often called a VAV box, is the zone-level flow control device. It is basically a quality, calibrated air damper with an automatic actuator. The VAV terminal unit is connected to either a local or a central control system. Historically, pneumatic control was commonplace, but electronic direct digital control systems are popular especially for mid-to-large size applications. Hybrid control, for example having pneumatic actuators with digital data collection, is popular as well.

Control of the system's fan capacity is critical in VAV systems. Without proper and rapid flow rate control, the system's ductwork, or its sealing, can easily be damaged by overpressurization. However, the fan capacity control is the primary advantage of VAV systems, especially with modern electronic variable speed drives, because the energy consumed by fans is a substantial part of the total cooling energy requirements of a building.

While invented earlier, Tempmaster Corporation of Kansas City, Missouri is credited with perfecting early VAV technology. The VAV flow control loop (actuator, flow control loop, and differential pressure sensor) is the heart of the technology and several product innovations were introduced by Kreuter Manufacturing Corp., now KMC Controls [1].

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[edit] References

  1. ^ Systems and Equipment volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2004
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