Air Volume Variables ( VAV ) are types of heating, ventilation and/or AC (HVAC) systems. Unlike a constant air volume system (CAV), which supplies constant airflow at variable temperatures, the VAV system varies airflow at constant temperature. The advantages of a VAV system versus a constant volume system include more precise temperature control, reduced compressor wear, lower energy consumption by system fans, less fan noise, and additional passive dehumidification.
Video Variable air volume
Sistem VAV sederhana (VAV)
The simplest VAV system incorporates a single supply line which, when in cooling mode, distributes supply air at a constant temperature of about 55 ° F (13 ° C). Because the supply air temperature is constant, the airflow rate must vary to meet the rising and falling heat losses in the thermal zone presented.
Even a simple VAV system has several advantages over the CAV system. One of them is a more precise temperature control. To meet the cooling load of the chamber, the CAV unit operates the fan and compressor at full capacity until the temperature falls to the specified limit, and then the compressor dies. This on/off cycling causes the temperature to fluctuate above and below the temperature setpoint. In a one zone VAV unit, fan speeds vary depending on the actual space temperature and temperature setpoint, whereas the compressor modulates the refrigerant flow to maintain a constant supply air temperature. The result is a more precise room temperature control.
Other advantages are energy saving and reduced wear. The VAV fan control, especially with the modern variable speed electronic speed, reduces the energy consumed by fans, which can be an important part of the total energy requirements of building cooling. The compressor modulation control also reduces wear and provides further energy savings.
The final advantage is the increase in dehumidification. Because the VAV airflow is reduced under partial load conditions, air is exposed to the cooling coil for a longer time. More moisture condenses on the coil, moistening the air. Thus, despite the constant volume and one zone VAV unit maintaining the same room temperature, the VAV unit provides more passive dehumidification and more comfortable space conditions.
Maps Variable air volume
Multi-zone VAV system
Air blower flow rate varies. For a single VAV air handler that serves multiple hot zones, the flow rate to each zone should be varied as well.
VAV terminal unit , often called the VAV box, is a zone-level flow control device. This is basically an air damper that is calibrated with an automatic actuator. The VAV terminal unit is connected to a local or central control system. Historically, pneumatic control is common, but digital direct control systems are very popular especially for medium to large size applications. Hybrid controls, for example, have pneumatic actuators with a collection of digital data, are also popular.
Common commercial applications are shown in the diagram. This VAV system consists of a VAV box, requires a working channel, and four air terminals.
Fan control
System fan capacity control is very important in VAV systems. Without proper and rapid flow rate control, the system air duct, or its sealing, can be easily damaged by excess pressure. In the cooling mode of operation, since the temperature in the room is satisfied, the VAV box closes to restrict the flow of cold air into space. As the temperature rises in space, the open box to bring the temperature back down. The fan maintains a constant static pressure in the drain regardless of the VAV box position. Therefore, when the boxes are closed, the fan slows down or limits the amount of air entering the supply line. When the box is open, the fan accelerates and allows more airflow into the channel, keeping the static pressure constant.
One challenge for VAV systems is providing adequate temperature control for multiple zones with different environmental conditions, such as offices in the glass perimeter of buildings vs. interior offices in the hallways. The dual channel system provides cool air in one channel and warm air in the second duct to provide the appropriate mixed air supply temperature for each zone. The additional channels, however, are complicated and expensive. Reheating air from a single channel, using an electric heater or hot water, is often a more cost-effective solution.
References
Source of the article : Wikipedia
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