Displacement ventilation uses stratification to its advantage. Cool air is introduced at low level, and warm air is allowed to stratify at the ceiling.
For this reason, displacement ventilation is usually employed in spaces above 3m high.
This creates a stratified flow where the cooled supply air flows out into the room under the warmer room air. The flow in the room is based on natural air movements where the air is driven by a difference in density and by convection flows from heat-releasing activities and processes.
Convection flows at heat sources generate a vertical air flow in the room, thereby creating a clean zone on the bottom and a polluted zone on top.
A high level of heat activity from heat sources generates bigger convection flows, resulting in the air rising more strongly and greater entrainment of the air around the source. Textile ducting can create displacement ventilation even when fitted at high level because there is no mixing of the flows.
In general, displacement ventilation systems operate at both low velocity and very low pressure. This delivers an immediate additional benefit of low operational noise combined with a minimal draught within the occupied area.
In the temperature gradient associated with displacement ventilation, the ceiling-level air temperature is higher than that of the occupied zone. The first effect of this is that the supply temperature can be higher than that of traditional mixing systems.
This higher air temperature means longer spells of free-cooling, reducing energy use through the lifetime of the system. A higher return air temperature also renders the system ideal for use with an air handling unit (AHU). There is a consequent dramatic reduction in the heating energy needed to meet room conditions after the energy recovery device.
In some cases, particularly if energy recovery is via a thermal wheel, the heating energy requirement can be removed altogether.
There are also bonus benefits: the higher supply temperature enables free cooling to be available for much of the year.