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Exhaust Outlets - Capturing Air Velocities

Capturing air velocity in front of an exhaust outlet - online exhaust outlet velocity calculator.

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Ventilation exhaust outlet - air velocity

The capturing air velocity in a distance x from an exhaust outlet can be calculated as

vc = q / (4 π x2)

    = vo A / (12 x2)

    = vo π d2 / (48 x2)                                    (1)


q = air volume flow (m3/s, ft3/s)

A = duct area (m2, ft2)

vc = capture air velocity in distance x from exhaust outlet (m/s, ft/s)

vo = air velocity in the exhaust outlet opening (m/s, ft/s)

d = diameter of exhaust outlet (m, ft)

x = distance from exhaust outlet (m, ft)

Note! According to line one of the equation (1) - the capture velocity is not dependent on the size of duct. This equation can only be used for exhaust outlets with relatively small diameters.

Ventilation - small exhaust outlet - capturing air velocities

For larger exhaust outlets equation (1) should be modified to

vc = vo A / (A + 10 x2)

    = q / (A + 10 x2)                                      (2)

Example - Capturing air velocity for a  smaller exhaust outlet

Capturing air velocity in distance 250 mm from a 250 mm duct with internal air velocity of 3 m/s can be calculated as

vc = (3 m/s) π (0.250 m)2 / (48 (0.250 m)2)

        = 0.2 (m/s)

Note! The air velocity in distance one diameter from the duct outlet - is less than 10% of the duct air velocity.

Exhaust Outlet Velocity Calculators

The velocity in distance x from an exhaust outlet can be calculated with the calculators below.

Smaller Outlets

air volume flow (m3/s, ft3/s)

distance from outlet (m, ft)

Larger Outlets

air volume flow (m3/s, ft3/s)

distance from outlet (m, ft)

area (m2, ft2)

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Exhaust Outlets in Walls or with Flanges

Ventilation - wall mounted exhaust hood

If the exhaust outlet is in a wall or with a flange the evacuation efficiency is improved and can be expressed as

vc = 1.33 vo A / (A + 10 x2)

    = 1.33 q / (A + 10 x2)                           (3)

The calculator above can be used if volume is multiplied with 1.33. Use of exhaust hoods improves the evacuation efficiency further.

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  • Engineering ToolBox, (2005). Exhaust Outlets - Capturing Air Velocities. [online] Available at: [Accessed Day Mo. Year].

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