Blog

How Do VAV Dampers Work?

January 20, 2020

HOW DO VAV TERMINAL UNITS WORK?

All-air systems are systems where temperature control of the environment is achieved solely by supplying conditioned air from a central air conditioning system. In all-air system applications, there are several ways to meet the heating and/or cooling load in a space.

By changing the air supplied to the space:

Changing the airflow rate while keeping the temperature constant,
Changing the temperature while keeping the airflow rate constant,
Changing both the temperature and the airflow rate.

In cases 1 and 3, VAV should be used in the system, and in case 2, CAV should be used.

So, What is a VAV Unit?

VAV = Variable Air Volume

VAV units are devices that adjust the conditioned airflow rate supplied to a room, independently of pressure, based on temperature data received from the room thermostat. In other words, these units provide ambient temperature control by changing the amount of heated/cooled air supplied to a space, according to the signal received from the thermostat or the building automation system (BMS).

In some cases, they are used not for temperature control, but to maintain a constant positive/negative pressure value set for the room.

Two operating modes can be set:

Operation in the 0…10V range (0V minimum flow, 10V maximum flow)
Operation in the 2…10V range (0-2V complete shut-off, 2V minimum flow, 10V maximum flow)

Where are VAV Dampers used?

They are used in living spaces where separate temperature control is desired due to different heat requirements, for example, in open-plan offices.
They are used in areas where pressure control is required, such as cleanrooms, hospital rooms, and special process manufacturing facilities.
Since the pressure values created by HEPA filters vary due to their loading rates, they are used before HEPA filter boxes to continuously provide the desired airflow independently of pressure.
They are used in fresh air supply systems to maintain the desired indoor air quality based on the carbon dioxide level of the environment.
They are used in constant volume systems that can be controlled by automation.

What criteria should be considered when selecting a VAV Damper?

In VAV damper selection, the airflow rate is determined by the cooling load in the most critical condition. This is your maximum airflow (Vmax). On the other hand, the minimum fresh air quantity to be determined for the environment it serves (depending on the number of people) is your minimum airflow (Vmin).
When selecting a VAV unit based on these values, it is recommended that the cross-sectional velocity at maximum airflow should be between 5 m/s and 8 m/s.
- If you select very low velocities, the VAV damper will generally operate inefficiently at its lowest position, and the noise problem may even increase.
- If you select very high velocities, in addition to the noise problem, you will have no limit to increase the airflow on the device if more airflow is required in the future.
The “device pressure difference” you determine during selection is the difference between the pressure before the VAV and the pressure after the VAV. Generally, it is taken as 150 – 200 Pa. The higher this difference, the more noise the device will produce to pass the same airflow. For better sound values, a low-pressure duct system should be designed.
After suitable cross-sections are selected according to the airflow, noise values should be checked. Sound pressure values are determined by applying system attenuation according to VDI 2081 to the sound power values. Based on these values, the procedures under the “Acoustic Performance” heading are applied.
The communication protocol of the actuator on the VAV damper (MF, MPbus, Modbus, Lonworks, Bacnet) must be determined in advance, and the VAV control unit must be selected accordingly.
For selection checks on your project, mutual consultation with the manufacturer (STEKON A.Ş.) should be ensured.

What makes the BEST and MOST ACCURATE VAV Damper?

We can examine this under three headings.

a. Measurement Accuracy

A good VAV unit's error margin between the set airflow and the actual airflow should not exceed 5%. The reason for this is that errors in airflow measurement require higher fan power due to the need to provide more airflow. For example, according to the fan law, a 40% error rate in airflow measurement can lead to a 170% fan power loss. In addition, the extra heating and cooling costs of the additional air supplied should be considered.

The design of the VAV unit's measurement probe is the most important factor in measurement accuracy. This probe must both read pressure differences as high as possible and have a measurement system minimally affected by air turbulence.

b. Acoustic Performance

Before delving into acoustics for VAV dampers, it is necessary to address the differences between Sound Power and Sound Pressure, which are stated in catalogs and apply to all devices.

Sound Power (Lw) is the sound energy emitted by a sound source.

Sound Pressure (Lp) is the audible form of the sound spectrum produced by a specific source, its sound power, heard at a certain distance and after a certain attenuation time in an environment where the source's location is precisely defined. In other words, sound pressure is the audible form of the sound power value.

There are two different sound paths in a VAV terminal unit.

1.Sound emitted from the casing is noise generated by the pressure created by the damper in the device and propagates from the casing to the suspended ceiling. If it is thought to exceed a critical value, acoustic insulation on the device should be considered.

2.Airborne sound is noise generated in the device that travels with the conveyed air and propagates into the environment through the diffuser/grille openings where the air exits. If it is thought to exceed a critical value, a silencer option after the device should be considered.

c. Airtightness

Since VAV dampers are also used in critical environments, their airtightness in the fully closed position must be very good to prevent the passage of infection or other critical solvent-containing air to other areas.

Furthermore, if airtightness is insufficient, leaks occurring in the fully closed position create unnecessary fan and heating/cooling costs.

Casing and damper blade airtightness values are determined according to EN 1751. These values should be obtained from the VAV manufacturer and checked.