Last edited 05 Jul 2021

Displacement ventilation

Displacement ventilation (DV) first emerged as a ventilation strategy in the 1970’s. It can lower energy consumption and improve air quality by supplying ‘fresh’ air direct to the occupied zones in buildings.

In ‘conventional’ mixing ventilation systems, air is supplied to occupied spaces at a relatively high velocity at ceiling level. This supply air mixes within the internal air, and a proportion of the mixed air is then extracted. Mixing ventilation systems may also re-circulate some of the internal air with the supply air.

Displacement ventilation systems supply low-velocity ventilation air at low level, close to the floor, creating a pool of cool, fresh air in the zone where people are likely to be. This air then heats up when it comes into contact with building occupants, equipment, lighting and so on. As it becomes warmer, so it becomes more buoyant and rises through the space, creating a thermal plume that drives ‘stale’ air upwards, where it is extracted. So, the fresh supply air ‘displaces’ the stale internal air.

This avoids the potential for ‘short circuiting’ which can occur in conventional mixing systems, where supply air is extracted before it has properly ventilated the occupied space.

Displacement ventilation tends to operate with lower air velocity, lower fan speeds, and higher supply air temperature compared to mixing ventilation systems. This avoids draughts and large temperature differences within the occupied zone which might otherwise cause discomfort.

Air is supplied through special, low-velocity diffusers which must be relatively large in area to provide adequate ventilation at low speeds. Diffusers are typically wall mounted, corner mounted or freestanding. The lower fan speed associated with displacement ventilation tends to result in less noise.

Displacement ventilation can supply cool air, or can simply supply fresh ventilation air, sometimes with cooling provided by other sources, such as radiant chilled ceilings. It can be used to provide low level heating, but is not a particularly effective method of heating, as warm air will tend to rise rapidly, before it has properly heated the occupied space.

Displacement ventilation is particularly suited to high-occupancy, open-plan spaces with high ceilings that allow thermal stratification, such as theatres.

In small spaces, such as school classrooms, displacement ventilation may be relatively straightforward and might be designed by following accepted ‘rules of thumb’. However in more complex spaces such as theatres, computational fluid dynamics (CFD) analysis might be necessary to model air flows and help size equipment.

NB Underfloor air distribution (UFAD) is sometimes considered to be a form of displacement ventilation. This uses the underfloor plenum beneath a raised floor to provide air through floor diffusers directly to the occupied zone. However, in some UFAD systems, the available diffuser area for UFAD is smaller, and so the supply velocity higher and supplied through diffusers that create greater mixing of the internal air than is generally found in displacement ventilation.

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