Room air distribution can be defined into two distinct categories:
Mixing and Displacement systems
In mixing systems, primary air is mixed into the room (or secondary) air from the ceiling, wall or the floor by forcing the air with med to high velocity into the room.
In displacement systems, primary air is mixed into the room at low speed in most cases directly into the occupied zone. In some specific applications such as hygienic the air is also introduced at low speed from the ceiling. In applications of commercial or industrial, displacement and mixing are mixed into one solution to accommodate a specific zone needs to isolate the ventilation/ or cooling in one specific product or zone.
In mixing systems, air is introduced into the zone or room at med to high speeds to achieve the entrainment ration necessary to have a good mixture between the primary and secondary air. This ratio helps the change of temperature from supply to room air at a short time. In principal the swirl and nozzle diffusers are known for their high entrainment, and simple double deflection grilles and linear diffusers are known for lower entrainment ratio.
Another advantage of high entrainment ratio diffuser is to bring higher comfort for the occupants in the occupied zone. However, the higher mixing ratio between supply and room air, the higher the rate of mixture between the polluted air and fresh air. As people walk in and out of the offices, they carry one level of pollution known as dust into the occupied zone through our clothing, shoes and so forth. So, there is a compromise in rate comfort against the rate of pollution mixed into the fresh air. Another disadvantage of mixing diffusers where are typically installed on walls and ceilings, this means the volume of conditioned air from the point where the diffuser is installed to the floor, this in most cases is ceiling or near to the ceiling. The larger the volume of conditioned air, the larger the dimension of system, (air handling unit, chiller, duct system and air diffusion as the m3/h is designed for the total volume of the needed conditioned air. (Note: as the occupied zone is no more than 1.8m from the floor and 0.5 from perimeter walls, the volume of the conditioned air needed is a percentage of the total room volume). This will impact the energy and installation costs of the total building. It should also be taken into consideration where the mixing air systems are introducing primary air into the room or an occupied zone, the higher the risk of higher sound levels into the room and higher-pressure losses of the system.
In a displacement system, air is introduced directly into the occupied zone in a low to very low speed with very low temperature difference between supply and room air. Typically, the displacement diffusers are constructed by having various preformation on the front face of the diffuser. This will typically build an over pressure in the diffuser by reducing the free area and have a linear front face velocity. Such construction and linear velocity will result in low entrainment ratio between supply air and room air. Advantage of such system is conditioning the occupied zone directly and extracting the air through placing the inlet grille or diffuser in the or near the ceiling. This system will reduce or limit the total volume of condition air to the occupied zone, this reduces system installation and energy cost. Equally by introducing the primary air at low to very low speeds, the sound levels will remain low. Disadvantage of such system is the complexity of the calculation, and installation. An ideal condition for installation of displacement system is large halls such as shopping centers, airports, factories or industrial halls.