Flow rate regulation can be achieved using simple control dampers. However, for applications requiring greater precision, V.A.V. (Variable Air Volume) terminal units are used, which are electronic flow rate controllers.
Downstream of the flow rate controller (VAV unit), it is necessary to install specialized air terminals capable of handling highly variable air flow rates without creating discomfort for the occupants.
When the air flow rate varies within a range of 60-70% to 100% of the nominal value, standard diffusers and grilles can be used. In this case, sizing is typically performed for a flow rate of around 80% of the maximum.
The choice of diffusers becomes much more critical, however, when the flow rate variation is wider, for example, between 40% and 100% of the nominal value, requiring terminals specifically designed for such conditions.
Diffusers for Variable Air Flow
As is well known, the residual velocity of an air jet () at a given distance () from the supply point is directly proportional to the flow rate.
Consequently, if a diffuser is selected to achieve a velocity of m/s at 100% flow rate, it is likely that when the flow rate is reduced to 50%, the velocity will drop to about 0.1 m/s. This value is often insufficient to ensure proper mixing between the supply air and the room air.
Conversely, sizing the diffuser to have a velocity of m/s at 50% flow rate would risk reaching a velocity of m/s at 100% flow rate, an unacceptable value that would create annoying drafts in the occupied zone.
It follows that the use of standard diffusers in variable volume systems is technically advisable only when the flow variation is limited, typically within a range of 70% to 100% of the nominal flow rate.
When flow rate variations are larger, for example, in a range between 30% and 100%, the use of specific diffusers is recommended: swirl diffusers for ceiling installations, or micro-nozzle diffusers for wall installations.
Swirl diffusers (Fig. 2) are composed of a series of fixed or adjustable blades, while micro-nozzle diffusers (Fig. 3) are equipped with one or more rows of small nozzles, each of which can be aimed individually.
The division of the airflow into multiple small jets generates a swirling (or helical) motion. This phenomenon produces three important advantages: a very rapid decrease in the jet’s velocity, a quick reduction of the temperature differential (Δt), and an extremely high induction effect.
Thanks to these specific properties, such diffusers can operate over a wide range of flow rates and handle temperature differences between the supply air and the ambient air of up to ±10 K.
These terminals are suitable for ensuring effective and optimal air diffusion even with high air flow rates, equivalent to 15-20 air changes per hour. They are also particularly effective in variable air volume (VAV) systems, as they ensure an acceptable air distribution even at very low flow rates (down to 30-40% of the nominal value), without encountering problems with jet drop.
Both swirl diffusers and micro-nozzle plates can be installed in two different configurations: for an airflow that utilizes the Coandă effect or for a free-field airflow (free jet).
In even rarer cases, the required flow rate variation is more extreme, reaching a range between 10% and 100% of the nominal flow rate.
It is therefore clear that to handle flow rate variations below the 30% threshold, it is essential to use devices in which the free area for air passage varies in proportion to the flow rate itself.
