DUCT CLASSIFICATION
Ducts are classified in terms of velocity and pressure.
Velocity Classification
Ducts are classified into 3 basic categories:
1. Low Velocity Systems: They are characterized by air velocities up to
2000 fpm.
2. Medium Velocity Systems: They are characterized by air velocities in the
range of 2,000 to 2,500 fpm.
3. High Velocity Systems: They are characterized by air velocities greater
than 2,500 fpm.
The low velocity system is used in most air conditioning installations because it is
quieter, has lower friction losses, lower fan power, and lower air leakage.
High duct velocities result in lower initial costs but require increased fan static
pressures; therefore, resulting in increased operating costs. Often these need
additional noise attenuation (use of noise silencers) and are not suitable for comfort
applications.
Generally, high-velocity systems are applicable to large multi-story buildings, primarily
because the advantage of savings in duct shafts and floor-to-floor heights is more
substantial. Small two- and three-story buildings are normally low velocity. A velocity
of 1,000 to 1,500 fpm for main ducts and a velocity of 700 to 1,000 fpm for the branch
take offs are recommended.
Pressure Classification
Duct systems are also divided into three pressure classifications, matching the way
supply fans are classified.
1. Low Pressure: The term low-pressure applies to systems with fan static
pressures less than 3 inches WC. Generally, duct velocities are less than
1,500 fpm.
2. Medium Pressure: The term medium pressure applies to systems with fan
static pressures between 3 to 6 inches WC. Generally, duct velocities are
less than or equal to 2,500 fpm.
3. High Pressure: The term high pressure applies to systems with fan static
pressures between 6 to 10 inches WC. Usually the static pressure is limited
to a maximum of 7 inches WC, and duct velocities are limited to 4,000 fpm.
Systems requiring pressures more than 7 inches WC are normally
unwarranted and could result in very high operating costs.
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