Displacement ventilation is achieved by supplying low-velocity air at floor level. This supply air must have a temperature lower than the ambient temperature in order to displace the warmer air upwards without significant mixing. This “pool” of cool air feeds the base of the upward convective plumes that naturally form around heat sources, such as people and electronic equipment.
This upward motion continues to a certain level, known as the stratification level, above which the air stabilizes. This creates two distinct zones: a lower, stratified zone where the temperature gradually increases with height, and an upper, mixed zone where the temperature is higher and nearly constant. It is important to note that the concentration of contaminants generated within the space follows a profile very similar to that of the temperature, accumulating to a greater extent in the upper zone.
Given that both the temperature and the concentration of contaminants increase with height, it follows that the air return must be located above the stratification level to be effective.
From a design perspective, for a displacement system to function correctly, two fundamental conditions must be met:
- The air velocity leaving the diffuser must be around 0.25 m/s. It must not exceed 0.30 m/s for commercial applications and 0.4 – 0.5 m/s for industrial ones; otherwise, it would induce turbulence that would compromise the displacement effect.
- The supply air temperature must always be lower than the ambient temperature. Generally, the design uses a differential of 4 – 5°C for commercial applications and 6 – 7°C for industrial ones (relative to the desired temperature at a height of 1.8 m).
The ability of the supply air to spread across the floor depends on its velocity and temperature. It is possible to use a lower temperature differential by increasing the velocity, although this might limit the displacement effect to only the area near the diffuser. In some cases, a localized effect can also be achieved with neutral temperature air.
It is crucial to point out that displacement ventilation works exclusively for cooling. It cannot be used for heating, as the supplied hot air, being lighter, would immediately rise towards the ceiling just a few centimeters from the diffuser, without conditioning the occupied zone.
In case of specific heating needs, it is recommended to supplement displacement diffusion with a radiant system, as this does not cause an air mixing effect.
Typical applications for displacement diffusers
The height of the stratification level (referenced in Figure 1) varies depending on the activity in the room: it is considered to be about 1.5 m from the floor in offices with seated people, and it rises to about 1.8 m in commercial or industrial environments where people are mostly standing. For this reason, displacement diffusion is particularly advantageous in rooms with high ceilings, as it allows for conditioning only the occupied zone (up to 1.5-1.8 m), without the need to treat the entire volume of the room.
Regarding operating temperatures, in commercial environments, the air is supplied at about 21°C. In contexts with higher thermal loads due to more intense activities (such as shopping centers or recreational areas), the supply temperature can be lowered to 18°C. A further advantage occurs during the intermediate seasons when, if external conditions are favorable, these systems can operate in free cooling mode, using only outside air for cooling.
Displacement Diffusers
The typical displacement diffuser has a vertical design with a cylindrical, semi-cylindrical, corner, or rectangular shape. Depending on its type, it can be installed on the floor, either in the center of a room, against a wall, or in a corner. The air supply to the diffuser is provided by a vertical circular duct, which can be connected from above or below.
Displacement diffusers are designed to have a uniform air velocity profile across the entire supply face. This face is composed of a perforated outer sheet and an internal equalizer located immediately behind it. The equalizer can be a honeycomb panel made of galvanized steel or a series of low-efficiency filter-media cones, which prevent the airflow from concentrating on the side opposite the inlet. A common characteristic of displacement diffusers is their low pressure drop.
To correctly size this specific category of diffusers, manufacturers provide special diagrams. These diagrams indicate the extent of the proximity zone for a given temperature difference between the supply air and the average ambient temperature.
In the example considered, the proximity zone (indicated as a0.2 and b0.2) is calculated considering a maximum value for the residual jet velocity of = 0.2 m/s and a temperature differential of Δt = -3°C, which are typical parameters for a “commercial” application. For different values of and/or Δt, each manufacturer provides specific correction factors.
In a room cooled by a displacement diffuser, the air temperature increases from the floor to the ceiling; this means the occupied volume always remains the coolest zone in the space. This is different from what happens in mixing systems where, theoretically, the high induction effect of the diffusers causes a complete mixing of the supply air with the indoor air, with the goal of achieving (ideally) the same temperature at every point.
Figure 6 graphically illustrates the results of a fluid dynamics simulation that shows the progressive heating of the supply air. It can be observed how the fresh air, introduced at 22°C (blue color), comes into contact with heat sources, progressively increasing its temperature to about 30°C (red color) near the ceiling. The air then rises through the room until it reaches the return grilles positioned high up.
As Figure 7 clearly shows, the only areas where air movement is registered (with velocities always less than or equal to 0.3 m/s) are the proximity zone of the displacement diffuser, the areas near heat sources (people, window fixtures), and the zone surrounding the ceiling returns.
