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The principle of displacement ventilation involves air supply and distribution in a room by upwards displacement, i.e. as direct as possible through-flow in the occupied zone in order to achieve high ventilation efficiency. In addition, air distribution by displacement generally makes it possible to supply a larger quantity of air than for conventional mixing ventilation, which requires concentrated supply at high velocity.
The air flow pattern differs greatly from that caused by conventional mixing supply jets. Air is supplied at low velocity to the occupied zone, often near the floor (see figure in full pdf). The new air is slightly cooler than the air in the room, and thus has a strong tendency to fall and spread out over the floor in a uniformly thin layer (approximately 20 cm), due to gravity, without mixing significantly with the room air above. This process leads to a continual upwards uniform displacement of air in the room, akin to filling a bathtub with water.Air in occupied zone becomes both heated and polluted by occupants etc., and rises upwards due to natural convection.
The air in the occupied zone is thus generally fresher than for mixing ventilation. Air is extracted from the room at ceiling level.
In addition, for localized pollutant sources that generate heat, such as humans, the released pollutants rise rapidly to above the occupied zone, due to buoyancy forces (an upwards flowing natural convection plume). This local upwards flow also brings up a steady stream of fresh air from the floor up to the breathing zone of occupants. The air in the breathing zone is thus slightly fresher than elsewhere in the room at the same height.
The supplied air flow rate and its cooling capacity are limited by the size of the air supply areas, and on the magnitude of the air flow rate that is technically/economically justifiable. The cooling capacity is also limited by how cold the supply temperature can be without causing local discomfort (cold draught along floor).