# 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

air volume flow (m^{3}/s, ft^{3}/s)

distance from outlet (m, ft)

#### Larger Outlets

air volume flow (m^{3}/s, ft^{3}/s)

distance from outlet (m, ft)

area (m^{2}, ft^{2})

### 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. *