In the age of energy-saving buildings and innovative solutions, new developments are made for mechanical ventilation systems as well. One of such solutions is the variable air volume control (VAV) which has gained popularity in the recent years even on the Polish market. The major advantages of this solution is the lower amount on energy spent on pretreating the intake air to meet the required parameters (heaters or coolers).
Compared to constant air volume control solutions (CAV), it is possible to supply exactly the required amount of air to selected zones/facilities. The parameters influencing the amount of supplied air may be heat gains (temperature), or for example CO2 concentration.
The main components of the VAV installations are regulator, also referred to as terminals. These units mostly act as adjusters – they adapt the air flow in the ventilation duct depending on the received signal.
The operating principle relies on exact air flow speed measurement. The measurement is carried out on the accumulating element, for example, a flange or a measuring profile.
Alnor Ventilation Systems has introduced VAV regulators into its product offer. The RAVAV regulator is dedicated to forced ventilation systems and is an important component for installations with variable air flow.
The regulator unit consists of three basic components:
Chart no. 1 shows the relation between the air flow speed and pressure drop.
The regulator casing is made of Z 275 galvanized sheet. Optionally, it is possible to use the 1.4301 stainless steel to build the casing. Casing sides with factory-installed EPDM rubber gaskets ensure tight connection to ventilation ducts. Inside the casing, the adjusting damper is also insulated. The servomotor is installed on the control unit casing so as to allow for thermal insulation of the system.
The measuring system based on aluminium impact pressure tubes and pressure relieve stub pipes ensures reliable measuring of the air flow.
The servomotor (a), a control unit (b) and sensor (c) are integrated into one device. The servomotor adjusts the damper position, the control unit compares the current air flow rate with the set value and the sensor turns the differential pressure value into the electric signal.
Characteristics of the RAVAV terminal:
At the design stage and during subsequent installation of VAV regulators, it is recommended to ensure that the longest possible straight duct section are placed before an after the regulator. This ensures the very high accuracy of air flow measurement. It is possible to supply the regulator with a flow straightener with normalized dimensions, allowing to balance the disrupted air flow. Observe the proper air flow direction through the regulator, as indicated on the product label.
Available RAVAV regulator diameters are shown in table 1. Table 2 shows airflow ranges, dependant on the average minimum (2 [m/s]) and maximum (12 [m/s]) airspeed.
Depending on the selected regulator control solution, the following configurations are possible:
a. constant setting (VAV) – air flow adjusted depending on a constant control signal 0...10 or 2…10 V,
b. forced setting (CAB) – air flow adjusted depending on a forced setting:
c. digital control via MP bus (e.g. DDC regulator, adjusting fan operation, systems: LonWorks, EIB-Konnex, BACnet, MODBUS RTU).
It is also possible to deliver special automated solutions such as: short adjustment time, highly polluted environments, rooms with a specific requirement of under and overpressure.
As a standard, the RAVAV control units are supplied with LMV-D3-MP (NMV-D3-MP) servomotors, according to the specification below:
VAV – variable flow operation (constant setting)*:
*diagram based on the catalog card VAV-Compact-D3_pl / BELIMO Siłowniki S.A.
CAV – step adjustment (forced setting)*:
*diagram based on the catalogue card VAV-Compact-D3_pl / BELIMO Siłowniki S.A.
To summarize, the main characteristics of the RAVAV terminal boxes are: