An overview of displacement ventilation

Displacement ventilation is essentially a buoyancy driven "displacement" process. "Fresh" ventilation air is introduced at low velocity and at low-level into the occupied zone at a temperature slightly cooler (usually around 19°C) than the design room air temperature. This air spreads out across the floor forming a reservoir of cool fresh air (see Figure 1, displacement ventilation). Any source of heat will generate a buoyant thermal plume rising upwards and this will entrain surrounding air which is replaced with air drawn from the fresh pool. In this way fresh cool air continually replaces the warmed and contaminated air which rises to upper levels of the room from where it is extracted. This causes a vertical temperature gradient to develop (typically 5-6°C between supply and extract), resulting in higher temperatures at ceiling level than with standard mixing ventilation systems.

Figure 1 Typical displacement ventilation room layout

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Why is there need for improvement?

Although displacement ventilation systems have some cooling capability, their prime function is to provide ventilation. The vertical temperature gradient characteristics of these systems may impose some risk of cold discomfort for the legs and feet, and heat discomfort at the head .

The cooling capacity of displacement ventilation alone may therefore be insufficient in offices with normal internal heat loads and solar gains. There is broad concensus within the industry taht the maximum cooling performance of displacement ventilation alone is around 20 W/m² for buildings with mainly sedentary type occupancy (such as offices). In buildings with higher occupant activity levels (for example industrial buildings), lower supply air temperatures and higher velocities can usually be tolerated without causing thermal discomfort, allowing a higher displacement ventilation cooling performance to be achieved.