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HVAC Ducts - Air Velocities

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Flow velocity in air ducts should be kept within certain limits to avoid noise and unacceptable friction loss and energy consumption. Low velocity design is very important for the energy efficiency of the air distribution system. Doubling the duct diameter reduces the friction loss by factor 32.

Low and Medium Pressure Loss Ducts

  • Maximum friction rate 0.1 - 0.2 inches W.G./100 ft
  • Velocity 1500 - 2000 ft/min (8 - 10 m/s)
Air Flow RateMaximum Velocity
(m3/h)(CFM)(m/s)(ft/min)
< 300 < 175 2.5 490
< 1000 < 590 3 590
< 2000 < 1200 4 785
< 4000 < 2350 5 980
< 10000 < 5900 6 1180
> 10000 > 5900 7 1380

High Pressure Loss Ducts

  • Maximum friction rate less than 0.4 inches W.G./100 ft
  • Velocity 2000 - 3500 ft/min (10 - 18 m/s)

Shafts

Air Flow RateMaximum Velocity
(m3/h)(CFM)(m/s)(ft/min)
< 5000 < 2950 12 2350
< 10000 < 5900 15 2950
< 17000 < 10000 17 3350
< 25000 < 14700 20 3940
< 40000 < 23500 22 4300
< 70000 < 41000 25 4900
< 100000 < 59000 30 5800

It is common to keep main duct velocity above 20 m/s (3940 ft/min).

Corridors

Air Flow RateMaximum Velocity
(m3/h)(CFM)(m/s)(ft/min)
< 5000 < 2950 10 2000
< 10000 < 5900 12 2350
< 17000 < 10000 15 2950
< 25000 < 14700 17 3350
< 40000 < 23500 20 3940

User Areas

  • Offices, receptions, lounges and similar
Air Flow RateMaximum Velocity
(m3/h)(CFM)(m/s)(ft/min)
< 5000 < 2950 10 2000
< 10000 < 5900 12 2350
< 17000 < 10000 14 2750
< 25000 < 14700 16 3150

Typical Air Velocity Guidelines

Low Velocity Duct Systems

  • typical 400 - 2000 ft/min (2 - 10 m/s)
ApplicationsRecommended max. Velocity
(ft/min)

Load Calculator!
Main DuctsBranch Ducts
SupplyReturnSupplyReturn
Apartments 1000 800 600 600
Banks 2000 1500 1600 1200
Hospital rooms 1500 1300 1200 1000
Apartments, Hotel rooms 1500 1300 1200 1000
Industrial 3000 1800 2200 1500
Libraries 2000 1500 1600 1200
Meeting rooms, cafeterias 2000 1500 1600 1200
Offices 2000 1500 1600 1200
Residences 1000 800 600 600
Restaurants, stores 2000 1500 1600 1200
Auditoriums, churches, cinemas, theaters 1300 1100 1000 800

Medium Velocity Duct Systems

  • typical 2000 - 2500 ft/min (10 - 12.5 m/s)

High Velocity Duct Systems

  • typical 2500 - 3000 ft/min (12.5 - 15 m/s)

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Related Topics

• Ventilation

Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

Related Documents

Natural Draft - Air Flow Volume and Velocity

Air flow - volume and velocity - due to stack or flue effect caused by indoor hot and outdoor cold temperature difference.

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