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Displacement ventilation is a method for directing cool air flow in a space through the movement of air from floor to ceiling. Air introduced at floor level is carried upward with warm air from heat sources within the room and is discharged at the ceiling. Air can enter the occupied space at a lower velocity and warmer temperature than mixed-flow systems, offering energy savings, noise reduction, and fewer drafts. Displacement ventilation can be used with outside air or cooled air. Sufficient ceiling height is necessary for proper function of this type of ventilation, however.
A gentle flow of air moves into a room at the floor level through vents or diffusers and covers the floor area. The heat created by people, lights, and machinery in the occupied room draws the cooler air up from the floor, a movement known as thermal stratification. The warmed air mixture continues upward toward the ceiling, where it is discharged through vents at ceiling height. Airborne contaminants, such as smoke, tend to move upward with the air flow and are also exhausted through the ceiling vents.
A mixed-flow ventilation system injects air high up in the room at a higher velocity, where it mixes with the room air to adjust the temperature. It generally requires lower temperature air to achieve a comfort level, and the higher speed of the air entering the space can increase noise levels. In contrast, the displacement ventilation system is generally moving air at a lower speed, thus producing fewer drafts and less noise. The heated air is carried upward away from the occupants, so the inlet temperature does not have to be as cold as that required by a mixed-flow system.
In moderate climates, displacement ventilation can be supplied with outside air, requiring no energy expenditure for modifying the temperature, only for air movement. The temperature in a room cooled with displacement ventilation is lowest near the floor and hotter near the ceiling, as opposed to the room cooled with a mixed-flow system where the temperature is more consistent throughout. This distinction allows for the use of fresh air for a longer portion of the year with a displacement system, a process known as free cooling. In hotter or more humid climates or seasons where air conditioning is necessary, cooled air is supplied instead of fresh air.
There are some limitations with displacement ventilation. It typically works best with a ceiling height of at least 10 feet (3 meters). Spaces with lower ceilings provide insufficient space for the thermal stratification to function well and may be more comfortable with a mixed-flow system. Heating is usually not combined with the vents for a displacement system because the buoyancy of the heated air tends to disrupt the thermal stratification. The accelerated flow of the heated air toward the ceiling may not provide a balanced temperature in the room.