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Exhaust Outlets - Capturing Air Velocities

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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.

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

Larger Outlets

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Exhaust Outlets in Walls or with Flanges

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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Related Topics

Ventilation Systems

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

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Duct Velocity

Calculate velocities in circular and rectangular ducts - imperial and SI-units - online calculator.

Evacuation of Air - minimum Capture Velocities to avoid Pollution products transferred to the Room

Capture velocities to avoid pollution products from galvanic baths, spray painting boxes and more contaminating the surrounding room and environment.

Exhaust Hoods

Sizing of exhaust hoods - air volume flow and capture velocities - online exhaust hood calculator.

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