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Air Volume Regulators CAV & VAV- overview

Volume regulators

Nowadays almost every new designed building has a mechanical ventilation system. This solution has many benefits like air temperature regulation, better quality of the air supplied to the premises or possibility to recover energy from exhausted air in winter (heat recovery system). It directly improves comfort for building occupants and decrease operating costs of the building comparing to building with natural ventilation.
Heat recovery systems are more and more popular also because of changes introduced in Construction Law Legislation and other directives concerning buildings energy consumption. Requirements stated by those legislations can be met on two ways:

  • by attaining lower heat transfer coefficients than required;
  • by attaining lower value of building primary energy consumption than the value of reference building.

Sometimes it is impossible to classify the building as an energy efficient and reach correct ratios without taking the air heat recovery system into account.
Many factors can influence operation of mechanical ventilation system. One of them is correct regulation of air flows in specific branches of duct system. Main role of ventilation system is to supply designed amount of fresh air to the premises and extract the same amount of used air out of the building. To achieve it, regulation dampers and air flow regulators are used. Depending on ventilation system: with variable air volume (VAV) or constant air volume (CAV), the air flow regulator construction differs. The most common solutions are described in the next part of the article.

What is an airflow regulator & how it works?

The role of airflow regulators is to maintain desired air flow rate in particular branches of ducting system regardless pressure variation inside the ducts. It makes the process of system design much easier but noise generation and operating costs should be considered that is why low initial pressure set up is crucial. Air flow regulation can take place both in supply and extract air duct. Few basic types of air flow regulators can be distinguished:

  • constant air volume regulator with a fixed set point;
  • air flow regulator with two selectable set points (V ̇min, V ̇max,);
  • constant air volume regulator with an adjustable set point;
  • variable air volume regulator VAV.

Air flow regulator controls the air volume by means of rotating damper blade, rubber membrane, perforated metal sheet surface change or other elements throttling air flow in the duct. Air flow regulator performance is normally guaranteed in a specific range of airspeed and pressure. Usually, limit values for air speed is 2-15 m/s and for pressure difference is 30-1000 Pa, but it can vary depending on regulator design.
Such air speed values can generate noise in the ventilation system, thus installation of acoustic dampers or air diffusers with plenum has to be considered when using air flow regulators. Installation drawing of the equipment connection is presented on figure bellow.

Installation of air flow regulator in the ductwork

Installation of airflow regulator in the ductwork.

Automatic Air Volume Regulators without External Power Supply

They are also known as self-regulating. The most popular on the market is circular version of these regulators but few manufacturers offers also rectangular one. Air flow rate is regulated by rotating damper blade or other elements that throttle air flow and are connected with spring to counteract closing torque. Closing torque depends on air speed through the damper. Required air flow rate is set in specific range, depending on air flow regulator size, by adjusting spring tension.
Such regulator design has several disadvantages in comparison with actuator controlled air flow regulator. Because of little tension force, it is impossible to use these dampers as an air tight shut-off dampers, which could completely close the air flow. Another feature is that, there is no possibility to have feedback signal regarding damper position or actual air flow rate.
On the other hand it is the great advantage that, these regulators do not need external power supply. Also the lack of actuator, reduce the price of self-regulating dampers comparing to motorized air flow regulators. Regulator operating accuracy ranges in 5-10% depending on its design and operating point of specific regulator. [1] 
Photos below present two types of CAV volume regulators: CFR-PC-ABS airflow regulator dedicated to installing inside the circular air duct and RACAV airflow regulator.

CFR-PC-ABS constant ar volume regulatorRACAV Constant Airflow Regulator

CFR-PC-ABS Constant Air Volume Regulator  without external power supply (left photo);
RACAV Constant Air Volume Regulator (right photo)

Motorized Air Flow Regulators with External Power Supply

In this kind of solutions air flow rate measurement is made, which is transformed into control signal to actuator adjusting position of damper blade. At the moment the most popular measuring method is a solution based on differential pressure measurement by means of measuring cross. There are two methods of measurement: 

  • using static pressure sensor - directly inside measuring box, air flow rate is proportional to pressure difference. There is no direct air flow through the sensor and possibility to get the sensor dirty is limited. The disadvantage of the solution is measurement accuracy which depends on the sensor location and the high price of the device;
  • using dynamic pressure sensor - part of the air flow is directed through the sensor and the measuring tubes are connected with bypass tube. Air flow rate is proportional to air speed. The advantage of the solution is high measurement accuracy and the low price of the device. However, applicability is limited excluding dusty and aggressive air flows.

 

RAVAV air volume regulator with belimo attenuatorRAVAV air volume regulator with gruner attenuator

RAVAV Air Volume Regulator VAV type available with Belimo and Gruner attenuators

RAVAV-Q Air Volume Regulator - SquareRAVAV-Q-I Air Volume Regulator - Square - Insulated

RAVAV-Q Square Volume Air Regulator and RAVAV-Q-I -insulated version

Some manufacturers use also orifice plate and static pressure sensor for air flow measurement. In this case the total pressure drop on the device is higher and the fan available pressure has to be considered, if it will be integrated with already operating ventilation system.

Sensor output signal is processed into linear signal by the sensor itself or by the regulator and can be a base value for further calculations. As an example it can be used to check if the desired air flow is reached. Air flow regulator can maintain the air flow rate set point value if it operates as the constant air volume regulator, or set point value could be adjusted by control signal from external sensors like temperature, CO2 etc.. - it works more like variable air volume regulator.

Application of the actuator with suitable torque determines the possibility to design airtight dampers (max leakage up to 10 m3/h at the pressure difference of 100Pa).

References: Recknagel, H. i in.. Handbook for heating, air conditioning and domestic hot water, Wroclaw, OMNI SCALA, 2008 (s. 1260-1264)Act of July 7th, 1994 r. Construction Law Legislation (with further changes)

 
Air flow regulators catalogue
Air flow regulators for ventilation installations
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