Exhaust Outlets - Capturing Air Velocities

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

v_c = q / (4 π x²)

    = v_o A / (12 x²)

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

where

q = air volume flow (m³/s, ft³/s)

A = duct area (m², ft²)

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 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

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

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)

Related Mobile Apps from The Engineering ToolBox

• Capture Air Velocity App

- 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 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.