AIR DISTRIBUTION (Additional information. Also see Chapter 18)

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AIR DISTRIBUTION (Additional information. Also
see Chapter 18)

• General
• The proper delivery of air for heating, cooling,
  or ventilation is a crucial part of the ducted
  air distribution system.
• When conditioned air is supplied to a space by
  ductwork, the duct has to terminate in an outlet
  device capable of introducing the air into the
  space in an air pattern that maintains the
  desired air temperature, humidity, and motion.
• The resulting conditions will be unsatisfactory
  if the air is not properly introduced.

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AIR DISTRIBUTION

• Throw (See section 18.0)
• The horizontal distance that an air stream
  travels from an outlet before it slows to a
  comfortable velocity (usually 50 to 100 fpm) is
  called its throw.
• The throw should be long enough to give the air
  stream time to mix sufficiently with room
  temperature so that its temperature and velocity
  reach comfortable levels before the air drops
  down into the occupied zone. Otherwise,
  uncomfortable drafts will result.
• If the air stream is warmer than the room air, it
  will tend not to drop at all.

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AIR DISTRIBUTION

• 3.00 Types of outlets
• Ceiling diffusers
• Ceiling outlets that spread air out in a
  horizontal discharge pattern,creating a plane of
  conditioned air that blankets the ceiling.
• As the supply air mixes with the entrained room
  air close to the ceiling, its velocity and
  temperature gradually decrease, until it finally
  drops to the occupied zone where it comes in
  contact with the occupants of the room.
• Square, round, and rectangular shapes.
• A ceiling diffuser, when controlled by dampers,
  may be termed as a register.

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AIR DISTRIBUTION

• Registers (see section 18.0)
• A grille with damper directly behind the louvered
  face to regulate the volume of airflow.
• Apart from ceiling, registers are also used for
  wall and floor outlets.
• Perforated ceiling panels
• Used when a large proportion of the ceiling may
  be used as an outlet.
• Perforated ceiling panels provide a
  well-distributed air supply straight down.

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AIR DISTRIBUTION

• Slotted diffusers
• Linear in shape, slotted diffusers are elongated
  outlets consisting of single or multiple slots
  usually installed in long continuous lengths.
• Available in two basic flow patterns
  perpendicular and parallel.
• The perpendicular type discharges air within 30º
  of perpendicular to the face of the diffuser it
  creates long, narrow bands of conditioned air
  flowing into the room, and is best suited for
  perimeter heating applications.
• The parallel flow type discharges air parallel to
  the surface of diffuser and is suited for cooling
  applications.

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AIR DISTRIBUTION

• Suitability of outlets
• No single type of outlets is suitable for both
  heating and cooling.
• Heating outlets
• The heating outlet location that most effectively
  provides comfort is near the floor at outside
  walls, especially below windows.
• It provides vertical stream of air that flows
  upward to blanket the cold surfaces and
  counteracts perimeter downdrafts of cold air.
• Adding heat at the area of highest heat loss
  prevents uneven air temperatures and an uneven
  radiant temperature distribution.

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AIR DISTRIBUTION

• Cooling outlets
• Proper cooling requires larger quantities of air
  than is generally needed for heating.
• This air must be carefully and evenly diffused.
• The best method is to deliver air through
  multiple outlets strategically located in the
  ceiling.
• When a compromise is required for year-round
  operation of a single system, the optimum
  location is determined by the predominant
  application.
• Ideally, a ducted air system with ceiling
  registers/ diffusers provides cooling and
  ventilation while a separate perimeter system,
  such as fin-tube radiation, is used for heating.
• When this is not possible, it is important that
  delivery device be carefully selected and
  located.

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AIR DISTRIBUTION

• Return air (see section 18.3)
• Whenever air is supplied to a room, provisions
  must be made either to return the room air to the
  AHU or to exhaust it to the outside.
• This requires the installation of return air
  inlets.
• They should not be located close to the supply
  outlet and, if possible, not at the same height.
• This will result in short circuiting which means
  that supply air will bypass portions of the space
  without conditioning it.
• Return air inlets may be connected to a duct.

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AIR DISTRIBUTION

• Return air inlets for cooling systems can be
  located in ceilings or high above on walls,
  provided they do not cause short circuiting.
• Return air inlets for heating systems should
  preferably be located near the floor to
  discourage stratification of warm air near the
  ceiling, and across from the supply outlets to
  ensure heating throughout the room.
• Return inlets actually on the floor are
  undesirable because they readily collect dirt,
  which clogs the filter, and imposes an undue
  strain on the AHU.

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AIR DISTRIBUTION

• Face velocity
• This is the velocity of air in fpm at some point
  in the distribution system based on overall area
  through which air is passing. e.g. face velocity
  of a register with a face area of 18 in. x 6 in.,
  supplying a volume of 375 CFM of air, would be
  375/(18x6)/144 500 fpm.