What is displacement ventilation?
In systems using the concept of displacement
ventilation, the air is supplied at a lower temperature in relation
to the room air. The air is usually supplied at floor level,
directly in the occupied zone.
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.