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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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