2. Introduction
• The air which is processed in air handling unit
is applied to the room which are to be air
conditioned.
• This requires an air distribution system.
• Components:
– Supply air duct
– Supply air outlet
– Return air outlet
– Return air duct
3. Classification
• High velocity duct
– Velocities higher than 750 m/min are considered high
velocities.
– The higher velocities cause greater noise and large pressure
drop.
• Low velocity duct
– Most of the air conditioning systems use low velocities due
to noise level considerations.
4. Shape of Duct
• The circular shape is most compact shape which
requires less material and has least frictional pressure
drop. It is difficult to construct.
• The square shape is also a compact shape and not
economical to maintain throughout the length of duct.
• The rectangular shape is the most common shape for
low velocity ducts because it is easy to construct at
site.
5. Lower aspect ratio
• Lower aspect ratio: ratio of larger to smaller
dimension of cross-section.
– Less material due to lesser perimeter.
– Less material due to higher gauge of sheet.
– Lesser cost of installation.
– Lesser insulation cost.
– Lesser running cost due to lesser pressure drop.
6. Pressure Losses
• Loss of pressure due to friction between moving
particles of fluid and the interior surface of duct
and it occurs throughout the duct length.
• Dynamic loss of pressure occurs due to change in
cross section of the duct and also due to change in
direction of the duct.
7. Velocity Reduction Method
• The velocity method is one of the simplest ways of
designing the duct system for both supply and return
air. However, the application of this method requires
selection of suitable velocities in different duct runs,
which requires experience.
8. Equal Friction Loss Method
• This method is based on the principle that friction rate
per unit length in the entire duct system would be
maintained constant and the duct sizes would be
calculated for different discharges by keeping friction
rate constant.
• This method is generally suitable when the ducts are
not too long, and it can be used for both supply and
return ducts. However, similar to velocity method, the
equal friction method also requires partial closure of
dampers in all but the index run, which may generate
noise
9. Static Pressure Recovery Method
• This method is commonly used for high velocity
systems with long duct runs, especially in large
systems. In this method the static pressure is
maintained same before each terminal or branch.
