Exhaust Outlets  Capturing Air Velocities
Capturing air velocity in front of an exhaust outlet  online exhaust outlet velocity calculator.
The capturing air velocity in a distance x from an exhaust outlet can be calculated as
v_{c} = q / (4 π x^{2})
= v_{o} A / (12 x^{2})
= v_{o} π d^{2} / (48 x^{2}) (1)
where
q = air volume flow (m^{3}/s, ft^{3}/s)
A = duct area (m^{2}, ft^{2})
v_{c} = capture air velocity in distance x from exhaust outlet (m/s, ft/s)
v_{o} = 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
v_{c} = v_{o }A / (A + 10 x^{2})
= q / (A + 10 x^{2}) (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
v_{c} = (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
Related Mobile Apps from The Engineering ToolBox
 free apps for offline use on mobile devices
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
v_{c} = 1.33 v_{o }A / (A + 10 x^{2})
= 1.33 q / (A + 10 x^{2}) (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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