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Ventilation by displacement
Ventilation by displacement is a ventilation form that has traditionally been applied for ventilation of industrial premises. It can also be used to advantage for many types of so-called comfort ventilation. If properly dimensioned, it can remove major heating effects and achieve a high degree of ventilation efficiency. Before this principle is described in further detail, the following terms should be defined:

Occupied zone
The occupied zone is that part of the room normally occupied by people. For offices, schools, etc., it is normal to calculate with a distance of 0.5 m from an outer wall with window, 0.2 m from other walls and 0.1 m - 1.8 m above floor level.

Near zone
This denotes the zone near a low-impulse valve where there is a floor draft.

Velocity v = 2.0 m/s is used as the draft limit for a low impulse valve in a comfort system. The goal is to achieve the smallest possible near zone for a low-impulse valve. With ventilation by displacement, the supply mechanism is located at a low level, and air is supplied directly to the occupied zone at a low velocity. Convection flows from potential heat sources give the air an upward motion and the hot/polluted air is removed via exhaust valves located at the ceiling.

Air is supplied to the occupied zone at a temperature of 1 - 5° C below the room temperature. It is important that the temperature is not too low below the room temperature since this will yield a so-called convection draft from the cold surface. Temperatures of more than about 2° C below the room temperature place special requirements on the air-supply fittings.

A typical distribution of temperature and concentration of impurities in a room with displacement ventilation is shown in Fig. 3. The main objective is to maintain the temperature and concentration in the occupied zone at design conditions, while allowing higher temperature and concentration levels to rise above the occupied zone. Instead of maintaining design conditions in the whole room, displacement ventilation system works where it is needed - in the occupied zone, thus saving energy required to air condition the space.


Properly designed systems for ventilation by displacement yield air of very good quality. However, the principle has obvious restrictions such as:

* The supply valves are large and require a lot of wall space
* The supply valves are often covered
* The near zone, where velocity exceeds 0.2 m/s, becomes too large
* The vertical temperature gradient becomes too large; should not exceed 2-3° C.


It should also be mentioned that several heat sources at various levels and with different temperatures complicate matters. Combined with movement in the room, this causes displacement of air from the upper layer to the lower zones.

Where to use Displacement Ventilation?

Displacement ventilation systems have been successfully used in Northern Europe during the last twenty years. Initially used in industrial applications, designs have been successfully used for the ventilation of offices and other commercial spaces, where in addition to energy conservation, indoor air quality and comfort are important considerations. Displacement ventilation is efficient to use in spaces with high heat loads where contaminants are carried together with the warm convective flows. Recent studies conducted at MIT show that the load can be as high as 38 Btu/ft2 (120 W/m2). Displacement ventilation is especially effective when designing air conditioning for spaces with high ceilings, over 9 ft. Displacement ventilation is mostly recommended in theaters, auditoriums, atriums, restaurants, commercial kitchens, gyms, stores and factories. It is not recommended to use displacement ventilation when contaminants are heavier than

Sample for ventilaiton by dispalacament.

Ventilation of auditoriums



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