Design of an air diffusion system
The design of an air diffusion system begins with the preliminary definition of the strategy to be used (mixing or displacement) and with the selection of the supply and return terminals. The latter are responsible for introducing the supply air and extracting the return air, in such a way that:
- the air distribution occurs in a controlled and predictable manner, also thanks to the use of specific diagrams provided by the manufacturer;
- dead zones or undesirable stratification effects are avoided;
- within the occupied zone, excessively high air velocities, which could generate an unpleasant sensation of draft, do not occur.
The selection and sizing of air diffusion devices, despite appearances, are not simple operations and are influenced by numerous factors, including:
- the magnitude and location of internal loads;
- the presence and position of any pollutant sources;
- the position of the occupants;
- the airflow rate to be supplied;
- the temperature difference between the supplied air and the average temperature of the room;
- the furniture layout.
Of fundamental importance, both in the design and implementation phases, is the availability of technical documentation that allows for the identification of the operating values for each device. Such data, obtained from laboratory tests, must be as close as possible to real conditions so that they can be confirmed during system commissioning.
Fundamental parameters for selecting an air diffusion system
The essential parameters for the selection of air diffusion devices, as well as for their correct sizing and positioning, are illustrated below.
- Qn – Nominal airflow rate: This is the volumetric flow rate that must be introduced into the environment, measured in or . By dividing this value by the number of terminals, the specific airflow rate for each individual diffuser is obtained.
- ΔTm – Maximum temperature difference: This indicates the maximum difference, expressed in or , between the temperature of the supplied air and the average temperature in the occupied zone.
- Vmedia – Average air velocity: This is the point velocity within the occupied zone, expressed in and averaged over time to disregard fluctuations due to turbulence. Industry standards establish, for each application, the maximum acceptable limit for this velocity. Practical experience identifies the critical points—those with the presumably highest velocity—at a height of from the floor, at from the walls, and/or on the centerline between two diffusers. Verifying the velocity at these points, using manufacturers’ tables, allows for the system to be sized so that the regulatory limits are respected throughout the occupied zone.
- Occupied volume. The “conventionally occupied volume” is defined as the space where people are stationed. It generally corresponds to the volume of the room up to a height of from the floor and at a distance of from the walls and the system terminals.
The objective of any air supply strategy is to achieve and maintain the desired comfort conditions precisely within this volume.
This result can be achieved by supplying treated air in various ways, which involve installing the terminals inside or outside the control zone.
In the case of supply from outside the control zone, the air is supplied from above (from the ceiling or high on a wall), and the system is configured as a mixing system. When the supply is internal or very close to the control zone, the air is supplied from below (from a low wall position or the floor), and the system is either a displacement or a localized mixing type. The latter is defined as mixing that occurs entirely within the controlled zone, without drawing air from outside this volume. If the zone to be controlled extends to the entire room, induced flow systems can be added as an option to the mixing systems.
Referring to Figure 1, which relates to a system with ceiling diffusers, it is possible to identify the points of greatest interest for assessing air velocity. These are specifically those near the wall and those on the centerline between two diffusers.
- Vp – Average air velocity at the wall: This is the average air velocity, measured in , detected at a height of from the floor and at a distance from the wall in the range of to . Laboratory tests show that if the distance of the measurement point from the wall is doubled, the value of the Vp velocity is reduced by about . For example, a Vp velocity of measured at from the wall will drop to a value between and at the entrance to the occupied zone (at from the wall). This demonstrates how wall velocities that may seem high translate into perfectly acceptable values at the boundary of the occupied zone. In installations where the diffusers are not positioned symmetrically, the reference Vp velocity is the one generated by the diffuser closest to the walls.
- VH1 – Average air velocity between two diffusers: This parameter, expressed in , is the value that manufacturers provide for the measurement point located on the median line (centerline) between two adjacent diffusers, at a height of from the floor. Its value is influenced by the airflow rate, the distance between the diffusers, and the room height.
I – Induction ratio: This value represents the ratio between the supply airflow rate and the ambient room airflow rate that is entrained by the jet. If a manufacturer states a value for the induction ratio, such data is only significant if the exact measurement point is also specified (for example, at the end of the throw).
Induction, also defined as the volume of air set in motion per unit volume, is a dimensionless parameter. Since no instruments exist that can measure it directly, the stated induction value for a diffuser has no practical significance, as it is not verifiable. For this reason, the effectiveness of induction is evaluated indirectly by checking the degree of thermal uniformity achieved in the room through temperature measurements.
The ΔtL / Δt0 Ratio
This ratio is defined by the following terms:
- ΔtL: Represents the difference between the temperature of the air jet at a specific point (at a distance L from the diffuser) and the average room temperature.
- Δt0: Represents the difference between the temperature of the air jet measured at the diffuser outlet and the average room temperature.
This ratio is a key indicator of the level of temperature distribution. A low value for the ratio signals high temperature uniformity in the room, which directly translates into better thermal comfort conditions.
To give a practical example, let’s assume that at a given point the following values are recorded: ΔtL / Δt0 = 0.05 and Δt0 = -10 K.
Consequently, the temperature difference at point L will be: ΔtL = 0.05 x (-10 K) = -0.5 K
This result means that in the center of the jet, at distance L, the temperature differs by only 0.5 K from the room average. Such a low value indicates that excellent mixing has been achieved, thus ensuring high conditions of thermal comfort.
Aeff – Effective air passage area: This parameter, expressed in , indicates the actual area of the flow passage. Its calculation is not simple for two main reasons:
- The complex geometries of diffusers.
- Fluid dynamic effects that make the geometrically measurable area unsuitable for calculating the ratio between airflow rate and velocity.
For this reason, manufacturers directly provide the effective area value for each terminal model for which this data is relevant during the sizing phase.
The effective area is calculated with the following formula:
Aeff = Ageom x K where:
- Ageom is the geometric area.
- K is the contraction factor of the fluid streams, for which typical values are:
- Supply: 0.7 – 0.9
- Return: 0.5 – 0.7
These K values are obtained experimentally through aeraulic laboratory tests. Although the technical documentation for products usually provides the Aeff value directly, if this data is missing, the coefficients indicated above can be used to obtain an estimate.
Throw: “Throw” is defined as the conventional distance, measured in meters, between a diffusion terminal and a point in the room where a specific air velocity is to be verified.
Note: Since the definition of this quantity can vary significantly among different manufacturers, it is essential to always consult the specific technical documentation for each product.
In simpler terms, the throw corresponds to the sum of the horizontal and vertical paths traveled by the air, from the outlet point to the point where its residual velocity is measured.
Coanda Effect: Also known as the “ceiling effect,” this is the phenomenon whereby an air jet supplied horizontally from an opening on a vertical wall (located less than 0.3 m from the ceiling) tends to deviate upwards and adhere to the ceiling itself. This occurs due to a low-pressure zone that forms between the jet and the wall-ceiling corner.
This effect can be advantageously utilized to optimize air diffusion, provided that certain parameters are met:
- The inclination of the vanes must form an angle no greater than 40° with the ceiling.
- The air outlet velocity from the terminal must be greater than 2 m/s.
Modern technical literature recommends, for an effective wall throw, maintaining a distance between the ceiling and the upper edge of the diffuser of less than or equal to 0.2 m.
If this distance exceeds 0.3 m, the Coanda effect does not occur, and the air jet behaves as a “free jet.” In this case, the flow does not adhere to the ceiling but deviates upwards (if the air is warmer than the ambient air) or downwards (if it is colder), a phenomenon known as “jet drop.”
Pressure Drop: This data is a fundamental parameter for calculating the overall pressure drop of the entire ductwork system. Generally, each manufacturer provides the designer with tables or diagrams that indicate the terminal’s pressure drop as a function of the airflow rate. The values for accessories, such as balancing dampers and the plenum, can then be added to these figures.
Sound Level: Air diffusion terminals have a dual acoustic function: on one hand, they generate noise, and on the other, they help to attenuate noise coming from the rest of the system. It is therefore essential to know the sound level produced by the device alone, technically defined as “self-generated noise.”
Since self-generated noise is a sound power, the correct way to express it is as a “sound power level,” using dB, dB(A), or NR as the unit of measurement.
Sometimes, manufacturers express this value in terms of sound pressure. This data, however, does not represent the absolute value of the source, as it already includes a standard attenuation of 4 dB, corresponding to a room with 10 m² of absorption units.
