The principle of passive thermal displacement ventilation is that cooled air is supplied with low momentum in the lower part of the room. The cold air displaces the contaminated air from the
occupied zone upwards in the room. Buoyancy forces (temperature differences) control the air movement in the room as the free convection around heat sources (persons, machines, lightning
etc.) creates vertical air movements in the room. In the same way a cold window or a cold wall will result in a downward convective flow.
A very important characteristic of displacement ventilation is the formation of horizontal layers of air. The warm air stays under the ceiling, and the cold air stays in the occupied zone.
The air caneasily move in horizontal direction in the “temperature equilibrium zone” but relatively high forces are needed to move the air vertically across the different layers. The convective flows from heat
sources thereby work as an engine of the displacement ventilation. The exhaust openings are often placed at the ceiling to draw out hot and contaminated air.
The convection flow rates relative to the ventilation flow rates determine the height of the boundary between the two zones. The sum of the warm convection flow rates to the upper zone minus the
downward directed flows from cold surfaces to the lower zone is equal to the ventilation flow rate in the room. An increased ventilation flow rate thus moves the boundary upwards and a decreased
flow rate moves the boundary downwards.
As mentioned the passive displacement ventilation creates a stratified flow, and thereby there will be a vertical temperature gradient (°C/m) in the room. The vertical temperature distribution has to
be given attention, as a too high difference in temperatures will cause a potential risk of discomfort (the feeling of draught).
The temperature gradient is strongly influenced by the elevation of the heat sources in the room. In rooms where the heat sources are located at a high level, the temperature gradient will be low in
the lower part of the room and high in the upper part. In rooms where the heat sources are located in the lower part of the room, the temperature gradient is larger in the lower part, and the
temperature is more constant in the upper part.
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