Air distribution: definitions
Definition: Air distribution means supplying air to and extracting air from a room in a comfortable way.
Supplying air to a room can have various reasons, cooling
and/or heating, humidification and/or dehumidification or
ventilation. The supplied air shall not cause any draughts in
the room and needs to flush the room completely.
The final goal is an optimum thermal comfort, a draught free
environment and an acceptable temperature. This thermal
comfort must apply to standards as recorded in the NEN-ISO
7730. The section of the room where these standards apply
for
is called the comfort zone.
In a closed off space, air will circulate under the influence of air streams. Circulating air masses are also called swirls. The way a room is flushed depends on the air supply position and the temperature difference between room and supply air. The area where room air mixes with supply air is called induction zone. When the air streams widens, velocity drops and the temperature becomes almost equal to the room temperature as a result of mixing. Then the air falls, expands on floor level and flows back opposite the supply direction, this is called return swirl. The travel distance of air streams mainly depends on room height and supply direction, unless a partition wall or other air stream prematurely turns the stream.
Comfort zone
The comfort zone is the area which starts 1,0 m from the façade, 0,5 m from the internal partition walls, up until a height of 2,0 m (DIN 1946 part 2).
InductionThe diffuser supplies air and extracts stagnant room air. This extraction is called induction. As a result the velocity of the supply air decreases and the velocity of the room air increases. The proportion between the jet mass flow and the mass flow from the supply air is called induction ratio. The velocity of the streaming air is related to the jet velocity.
Jet shapeThe total mass of air to a room is directly related to the available free space through which the air can be supplied. The shape of the free area results in various shapes of jets:
- Jet with a plane pattern; slot diffusers, rectangular diffusers and (long) side wall grills
- Jet with an axial pattern; jet diffusers and (short) side wall grills
- Jet with a radial pattern; round “louver face” diffuser and perforated face diffusers
- Jet with a tangential pattern; swirl diffusers
Influential parameters
In the course of supplying air various parameters have to be taken into account:
● Temperature,
Air stream with a different temperature than the room air temperature, and therefore a different specific mass, is influenced by gravity. Cold air is curved downwards and hot air is curved upwards.
● Walls and ceilings,
When an air stream is coming near a wall or ceiling, the stream will bend towards this wall or ceiling. This phenomenon is called coanda effect; it is caused by a lower pressure on the wall or ceiling side of the air stream related to the room side of the air stream, as a result the air stream is pressed against the wall or ceiling. The coanda effect is important for good air distribution. A second air stream behaves like a wall or ceiling, so air streams attract each other.
● Obstacles,
Obstacles, like for instance construction beams, close to the air supply diffuser can cause disturbance of the air stream. The coanda effect can be neutralized, which causes the air stream to drop too early in the occupied zone, which causes draught complaints.
Air distribution in four principles:
A) Air distribution with high induction systems (also called tangential mixing systems).
Characteristics:
- Induction factor up to 18;
- Starting velocity approx. 3 m/s
- A heat source has less influence on air streams compared to the other systems;
- Air supply temperature with cooling to this system like slot, sidewall, louver face, and perforated face and swirl diffusers.
Applications:
…. Air supplied from ceiling/side wall:
- Offices, schools, cafeterias, theaters, cinemas, hospitals.
…. Air supplied from floor:
- Theaters, vides.
B) Air distribution with diffuse mixing systems.
Characteristics:
- Induction factor up to 25;
- Starting velocity > 4 m/s;
- Deliberately blowing air on an object or through a zone;
- Air supply temperature with cooling 10ºC below and with heating 15 ºC above room temperature;
- Most suitable for this application are Jet-Flo diffusers
Applications:
…. Air supplied from side wall/airduct:
- High ceilings and large air volumes, f.e. Congress- and event centers, sports halls, exhibition centers and atriums.
C) Air distribution with low inducing systems
(also called displacement ventilation)
Characteristics:
- Induction factor 3 to 5;
- Starting velocity approx. 1 m/s;
- A heat source can have strong local influence on an air stream;
- Air supply temperature approx. 3ºC lower than the room temperature;
- Various diffusers are suitable like slot diffusers, sidewall grills, louver face diffusers, and perforated face and swirl diffusers.
Applications:
…. Air supplied from ceiling:
- Operating theaters, spray paint cabins, kitchens (when supplying from the ceiling this system is not suitable for heating)
…. Air supplied from sidewall:
- Industrial, and to a lower level also theaters, congress centers and offices.
…. Air supplied from floor:
- Computer rooms
D) Air distribution with non-inducing systems
(also called free displacement ventilation).
Non-inducing systems are through displacement systems. It appears that the room is part of the ductwork.
Characteristics:
- Starting velocity is the same as the room air velocity; between 0,05 and 0,4 m/s;
- The temperature increases or decreases in the direction of the air stream;
- A heat source can have strong local influence on an air stream;
- The air supply location (ceiling, side wall or floor) is mainly determined by the required flow direction of the pollution;
Applications:
…. Air supplied from the ceiling, side wall or floor:
- Clean rooms, operation theaters, spray paint cabins, shooting ranges.
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