Single Duct Variable Air Volume Terminal Practical Knowledge Sharing Part 1

In the variable air volume air conditioning system, the single duct variable air volume terminal device (VAV-BOX) is one of the most common variable air volume terminal devices. Its main function is to automatically adjust the supply air volume according to the temperature requirements of the room, so that the indoor temperature is kept within the set range to ensure a constant and comfortable space environment.

How does it achieve precise air supply? What are its specific structures and common accessories? Let's continue.

Pitot tube VAV-BOX air volume measurement principle

The airflow sensor at the primary air inlet of VAV-BOX has two types of pressure measuring holes:

The first type of measuring holes faces the direction of the airflow and measure the total pressure of the airflow.

The second type of measuring holes, facing away from the airflow, measures the static pressure of the airflow.

The difference between total pressure and static pressure, i.e. dynamic pressure, can be measured by the differential pressure sensor on the controller.

According to the formula: dynamic pressure = ( ρ : atmospheric density (constant), v: velocity), the inlet airflow velocity can be obtained. The inlet cross-sectional area is constant. According to the formula: air volume = airflow velocity x area, the air volume passing through the VAV-BOX can be calculated.

Measuring airflow velocity range

The airflow velocity measurement range of the Pitot tube VAV is usually: 1.5m/s~15m/s

Round inlet and square outlet

Most VAV-BOXs commonly used for temperature control have a circular inlet and square outlet structure. The benefits of this structure are as follows:

1. The small-sized circular inlet can measure and control a smaller air volume, making the controlled air volume range larger.

2. The square plenum box connected behind can convert the dynamic pressure at the VAV-BOX inlet into static pressure, reducing the resistance loss at the terminal

3. The square static pressure box can be filled with internal insulation to play the role of heat preservation and noise reduction

Option for lining insulation

There are four common forms of lining insulation (RS Standard)1. BGT black tissue faced fiberglass (standard)-----The inner surface has an anti-corrosion BGT non-woven fabric adhesive surface, which does not fall off at an airflow velocity of 20m/s, Class A fire protection, and complies with NFPA-90A and UL-181 specifications

2. Aluminum foil faced fiberglass-----optional according to project requirements

3. Adding perforated metal sheet on the surface of BGT faced fiberglass - optional according to project requirements, an extra layer of protection to prevent the inner surface insulation wool from falling off and further enhance the soundproofing effect

4. Adding metal sheet on the surface of BGT faced fiberglass--- the inner surface of VAV-BOX has no gaps, is easy to clean, and does not breed bacteria. It is suitable for use in hospitals and clean rooms.

BGT black tissue faced fiberglass   BGT fiberglass surface with sheet metal diagram (外壳体：External sheet metal, 玻璃纤维隔热层：Fiberglass insulation, 内壳体：Internal sheet metal)

VAV-BOX Power Supply

The VAV-BOX control box is usually equipped with a 220V to 24V transformer(50/60Hz) as standard,

The internal controller/actuator power supply requirement is usually 24V

Common accessories and applications

1. ON/OFF type electric heater

ON/OFF type electric heater can achieve 3 stages of electric heating control

Split electric heater

PTC electric heater

2. Hot water coil, equipped with ON/OFF type valve/modulating type valve

According to the requirements, VAV-BOX can control the ON/OFF of the hot water on/off valve or adjust the damper position of the modulating water valve.

VAV-BOX connected to hot water coil

3. Multi-Discharge Attenuated Plenum

When one VAV-BOX needs to be connected to multiple air diffusers, a multi-discharge attenuated plenum can be selected, and the multiple outlets can be equipped with manual volume control damper to adjust the air volume of each air outlet.

VAV-BOX with integrated multi-outlet silencer plenum box

VAV-BOX communication method

At present, the mainstream communication methods of VAV-BOX controller are: BACnet-MSTP and Modbus-RTU