Constant air volume

Constant Air Volume (CAV) is a type of heating, ventilating, and air-conditioning (HVAC) system. In a simple CAV system, the supply air flow rate is constant, but the supply air temperature is varied to meet the thermal loads of a space.[1]

Most CAV systems are small, and serve a single thermal zone. However, variations such as CAV with reheat, CAV multizone, and CAV primary-secondary systems can serve multiple zones and larger buildings.

In mid to large size buildings, new central CAV systems are somewhat rare. Due to fan energy savings potential, variable air volume systems are more common. However, in small buildings and residences, CAV systems are often the system of choice due to simplicity, low cost, and reliability. Such small CAV systems often have on/off control, rather than supply air temperature modulation, to vary their heating or cooling capacities.

There are two types of CAV systems that are commonly in use to modify the supply air temperature: the terminal reheat system and the mixed air system.

The terminal reheat system cools the air in the air handling unit down to the lowest possible needed temperature within its zone of spaces. This supplies a comfortable quality to the space, but wastes energy.

The mixed air system has two air streams, typically one for the coldest and one for the hottest needed air temperature in the zone. The two air streams are strategically combined to offset the space's load. The mixed air system option is not as proficient at controlling the humidity, yet it does do well at controlling the temperature. (reference: Heating/Piping/Air Conditioning, December 1993 p.53-57)

Variable air volumeVariable 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 °F (13 °C) 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.

There are two primary 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 modulate the discharge air temperature to attain part load cooling capacity.

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The air blower's flow rate is variable. 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. In the cooling mode of operation, as the temperature in the space is satisfied, a VAV box closes to limit the flow of cool air into the space. As the temperature increases in the space, the box opens to bring the temperature back down. The fan maintains a constant static pressure in the discharge duct regardless of the position of the VAV box. Therefore, as the boxes close, the fan slows down or restricts the amount of air going into the supply duct. As the boxes open, the fan speeds up and allows more air flow into the duct, maintaining a constant static pressure.

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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