e m = Ce/Cm* 100% where:

ce = Concentration of pollution in the exhaust air
cm = Mean concentration of pollution in the entire room

Ventilation efficiency is a frequently used parameter to evaluate a system’s ability to provide good atmospheric comfort. e m depends on the geometry of the room, location of the valve and distribution of pollution in the room.
With ventilation by displacement, it is possible to achieve ventilation efficiency exceeding 100%, while the goal with ventilation by diffusion is to achieve e m of about 100%.

Air-change efficiency Air-change efficiency is a measure of how fast the air in a room is replaced. It depends on the distribution of air in the room, i.e. location and size of valves, room geometry, location of heat sources, etc.

.e a = Tn/2Tm *100% where:

T n = volume of fresh air supplied/room volume
tTm = average "age" of air in the room

With ventilation by displacement, it is possible to achieve e a of between 50% and 100%, while full ventilation by diffusion yields e a of approximately 50%.

There are two types of air distribution systems. The traditional type is mixing ventilation. It's main purpose is to efficiently mix supply air with room air. The contaminants in the room are diluted by supply air and then extracted through the exhaust grill (Fig.1). Displacement ventilation (Fig. 2) does not mix the impurities, it displaces them into the upper uninhabited zone from where it is extracted. With a displacement ventilation system, the supply air temperature is slightly lower than room air and supplied at low velocity directly into occupied zone . The impurities and heat loads in the space are carried by convective flows created by heat sources into the upper part of the room. Air in the upper zone, having a higher temperature, does not re-circulate back into the occupied zone. That is why the temperature and concentration of impurities at the exhaust outlet is higher than in the occupied zone.

Ventilation by Diffusion

Ventilation by displacement