Sizing of air diffusion equipment Once the type of diffusion to be adopted has been chosen, the designer must gather a series of additional data essential for the correct sizing:
- The final floor plan of the rooms, preferably complete with the furniture layout, to identify the position of workstations and other sources of localized thermal load.
- A cross-section drawing of the spaces that clearly shows the usable heights.
- The values of the supply and return airflow rate for each individual room.
- The specifications on the maximum noise limits.
After selecting the suitable diffuser models for the application, the next step is to use the “quick selection” tables provided by the manufacturer. These tables, for each diffuser size, summarize the key performance data such as the range of operating flow rates, the corresponding pressure drop, and the generated noise level.
The preliminary sizing phase allows the designer to:
- check the required pressure head for the air handling unit (AHU);
- perform an initial check of the sound level;
- estimate the number of diffusers to be installed, their model, and the preliminary costs.
This is followed by the detailed sizing phase, during which the designer focuses on:
- checking the final average velocity at critical points, comparing it with the design data (for this, manufacturers provide specific diagrams based on room dimensions and diffuser position);
- checking the pressure drop of the diffuser and its accessory components (e.g., damper, sensor, deflector);
- performing a final check of the sound level.
The following paragraphs illustrate the preliminary and detailed sizing process applied to swirl diffusers.
These diffusers, equipped with fixed or adjustable deflectors, are particularly suitable for air distribution systems by mixing. Their operation is based on dividing the airflow into multiple radial jets, which generate a helical (vortex) motion. This air motion produces a rapid reduction in the jet’s velocity, a quick decrease in the temperature differential (Δt), and a very high inductive effect. Thanks to these properties, they can be used in a wide range of flow rates and with thermal differentials up to ±10 K.
These diffusers can ensure effective and optimal air diffusion even with high airflow rates, corresponding to 15-20 air changes per hour. They are also particularly suitable for variable air volume (VAV) systems, as they maintain an acceptable air distribution even when the flow rate is reduced to 30-40% of the nominal value, avoiding problems of jet drop.
The most common installation for these diffusers is flush-mounted to the ceiling, with a horizontal air throw. This configuration makes the most of the Coanda effect and is ideal for rooms with heights between 2.7 and 4 meters.
In certain circumstances, a ceiling installation with a “free jet” can be planned. This can be achieved in two ways: by connecting the diffuser’s neck directly to a branch from the main duct, or, alternatively, by using a specific plenum that is left exposed.
Let’s consider a practical example: the requirement is to supply 2000 m³/h of air into a room, using swirl diffusers installed flush with the ceiling.
To perform the preliminary sizing, we will refer to the data in the manufacturer’s quick selection table.
The detailed sizing is used by the designer to verify that the actual performance of the diffusers corresponds to the design data. Specifically, the checks to be performed are:
- The check of the final average velocity between diffusers (vH1).
- The check of the final average velocity at the wall (vP).
- The check of the inductive effect, using the ΔtX / Δtm ratio.
- The check of the pressure drop (Δp) and the sound power level (LWA).
Based on the design data and assuming a standard installation for ceiling diffusers, the following parameters are defined:
- 4 diffusers of nominal size 600 are planned, arranged in 2 rows that are 3 m apart (X) and with a spacing of 3 m (Y) between diffusers in each row.
- The flow rate for each individual diffuser will be 500 m³/h (Q).
- The minimum distance from the wall will be 1.6 m (L).
- The installation height from the floor will be 3 m (H), and consequently, the value of H1 (reference height for velocity) will be H1 = 3 m – 1.8 m = 1.2 m.
- The temperature differential between supply air and ambient air will be -10°C (Δtm).
To calculate the wall velocity (vP), proceed as follows:
- Calculate the reference throw (P), given by the sum of the distance from the wall (L) and the reference height (H1): P = L (1.6 m) + H1 (1.2 m) = 2.8 m.
- On the manufacturer’s selection diagram, starting from a throw of 2.8 m, move horizontally until intersecting the curve corresponding to the flow rate of 500 m³/h.
- From this intersection point, proceed vertically until meeting the line that represents the distance between diffusers (3 m).
- By reading the corresponding value on the appropriate axis, the wall velocity is obtained: vP = 0.24 m/s.
It is essential to remember that this velocity is measured very close to the wall (at 75 mm). The actual velocity at the edge of the conventionally occupied zone (for example, at 300 mm from the wall) will be lower. Applying typical reduction factors, the value drops to approximately 0.12 m/s.
The calculation of the final average velocity between diffusers (vH1) is performed by following these steps on the selection diagram:
- Start from the value of the distance between diffusers, Y = 3 m, and move horizontally until intersecting the airflow rate curve, equal to 500 m³/h.
- From the intersection point, proceed vertically until reaching the horizontal line that corresponds to the reference height H1 = 1.2 m.
- Finally, from this last point, follow the inclined guideline for the final velocities to read the result. A value of vH1 = 0.15 m/s is thus obtained.
Experimental tests on this type of diffuser have highlighted a specific characteristic: the residual air velocity between two diffusers (vH1) reaches its minimum value when the spacing is approximately 1 meter. As the distance increases, the velocity peaks at about 1.8 m, and then decreases again to low values with a spacing greater than 2.4 meters. This feature can be exploited to manage concentrated thermal loads.
To determine the inductive effect of the diffuser at a point in the occupied zone, diagrams providing the ΔtX / Δtm ratio are used. Starting from the throw P = 2.8 m, the corresponding value for the ratio is found on the diagram, which in this case is 0.07. Consequently, the calculation is:
ΔtX = 0.07 x Δtm = 0.07 x (-10°C) = -0.7°C
This means that at the edge of the occupied zone, the temperature difference of the jet relative to the room is only -0.7°C, demonstrating excellent mixing.
Finally, using the “Pressure Drop – Sound Power” diagram and our flow rate Q = 500 m³/h, the following values are obtained:
- The pressure drop (Δp) is approximately 15 Pa (with the damper fully open).
- The sound power level (LWA) generated by the diffuser is approximately 33 dB(A) (also with the damper open).
