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Velocity Classification of Ventilation Ducts

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The duct velocity in air condition and ventilation systems should not exceed certain limits to avoid unnecessary noise generation and pressure drop in the duct work.

The limits of velocities depends on the actual application. The background noise in an industrial building is significant higher than the noise in a public building and more duct generated noise can be accepted.

Commonly accepted duct velocities are indicated in the table below.

Recommended air velocities in ventilation ducts
ServiceRecommended Maximum Velocity
- v -
Public buildingsIndustrial plant
Air intake from outside 2.5 - 4.5 500 - 900 5 - 6 1000 - 1200
Heater connection to fan 3.5 - 4.5 700 - 900 5 - 7 1000 - 1400
Main supply ducts 5.0 - 8.0 1000 - 1500 6 - 12 1200 - 2400
Branch supply ducts 2.5 - 3.0 500 - 600 4.5 - 9 900 - 1800
Supply registers and grilles 1.2 - 2.3 250 - 450 1.5 - 2.5 350 - 500
Low level supply registers 0.8 - 1.2 150 - 250
Main extract ducts 4.5 - 8.0 900 - 1500 6 - 12 1200 - 2400
Branch extract ducts 2.5 - 3.0 500 - 600 4.5 - 9 900 - 1800

Ducts are commonly classified as

  • Low Velocity Systems - with air velocities up to 2000 fpm (10 m/s)
  • Medium Velocity Systems - with air velocities in the range 2000 to 2500 fpm (10 - 13 m/s)
  • High Velocity Systems - with air velocities greater than 2500 fpm (> 13 m/s)
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