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Displacement ventilation - a forgotten concept with huge potential

Have you ever wondered why some indoor environments cause discomfort and draughts despite being a large and open space, a classroom for instance? As experts in the industry of heating, ventilation and air conditioning (HVAC) the above is an obvious example of when the selection of ventilation system has been unsuccessful. But, what should be chosen to promote well-being and productivity in large indoor environments? An almost forgotten ventilation principle from the 1970s might be the key. Let me, Welat Han, Product Manager and Business Development, tell you more.

As we spend increasingly more time indoors, the need for an effective ventilation system that can provide fresh air and ensure a healthy indoor climate grows. A system should supply the indoor environment with fresh outdoor air and also remove polluted air and other harmful substances in an efficient manner. There are different ways to so, and also ensure an optimal indoor climate for the people inside.

An almost forgotten ventilation principle is displacement ventilation. This principle is designed to supply air at low velocities at floor level and for the used air to be drawn out at ceiling level. The supply air is under-tempered, usually 3 to 4 degrees Celsius cooler than the room temperature, which causes it to spread along the floor surface before it starts to rise upwards with the heat sources in the room. The air movement is driven by differences in air density - warm air, which has lower density than cold air, rises to the ceiling. This creates two zones in the room, an upper zone with warm, polluted air and a lower zone of "clean" air.

Figure 1. Supply air, floor - Extract air, ceiling.

Displacement ventilation over time

In the 1970s, during the energy crisis, displacement ventilation emerged as a cost-effective solution for how to manage ventilation in buildings. It quickly became a preferred choice across various applications, among which kindergartens were noteworthy examples. However, this approach led to unintended consequences, including frequent colds among children who spent time close to the floor where the cold air was distributed. In an attempt to mitigate this issue, drawings and other means were placed on the air diffusers to block the cold supply air. So, instead of solving the problem, this makeshift solution deteriorated the indoor climate and contributed to a negative perception of displacement ventilation. As a result, the use declined throughout the 2000s.

Despite the challenges in the past, displacement ventilation can be highly effective when properly implemented, as it offers air exchange efficiency from 50 to 100%. Unlike mixing ventilation, which distributes air from the ceiling level, this method does not require to ventilate the entire room. It focuses only on the occupied zone or areas with high pollution levels.

How to benefit from displacement ventilation today

Displacement ventilation is particularly beneficial in large spaces with high ceilings, such as industrial facilities, stores, classrooms, and sports halls. Higher ceilings provide a zone to store polluted air above the occupied space. To fully benefit from this ventilation principle, a ceiling height of at least three meters is required, however optimal conditions are achieved with ceiling heights exceeding four meters.

Further, the position of the air supply diffusers plays a key role when to ensure effective and uniform air distribution throughout the room. Displacement air diffusers, which are often installed in the occupancy zone, may face challenges related to the layout of the room, the arrangement of furniture and other interior factors. Having that said, it is crucial to avoid that these air diffusers are placed too close to people, especially persons with sedentary jobs, as the air flow can cause discomfort. Displacement diffusers are usually equipped with a number of circular rotatable air nozzles which enable the diffusion pattern to be adjustable. As the layout or function of indoor spaces may change over time, the adjustability definitely becomes a distinct advantage.

Displacement air diffusers are available in numerous models to suit the function, geometry and design of a room, as well as to match the selected type of building construction and HVAC solution. The many different models provide a flexibility to blend ventilation with the design of the space, particularly valuable in places with limited space. Even though the variety of displacement air diffusers on the market is wide-ranging, there are examples of when something extra is needed to suit the demands of the client. See Malmö Live in Sweden for instance. From Swegon, we successfully integrated 1 400 custom-built air supply diffusers into the bottom of the seats in large concert hall. That way, we made the ventilation occupy as little space as possible and to also disappear from the sight of the visitor.

Figure 3. Seating with integrated diffusers in the legs (sketch by Swegon).

With all the above in mind, displacement diffusers are remarkably flexible in their “standard” make as semi-circular or as “fake pillars”, but the ventilation principle also allows for customisation to meet specific needs.

To summarise, displacement ventilation is a highly effective method for ventilating spaces with:

        • Large air flows
        • Large floor space
        • Heat sources in the room
        • High ceilings
        • Limited cooling capacities

The displacement ventilation is characterised by:

        • The supply air being introduced at floor level
        • No significant mixing of the room air in the supply air
        • The supply air being supplied at low velocity
        • The supply air being below room temperature
        • The extract air being evacuated at ceiling level

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