Duct Velocity

Air Duct Velocity - Imperial Units

Air velocity in a ventilation duct can be calculated in imperial units as

v_i = q_i / A_i

= q_i / (π (d_f / 2)²))

= q_i / (π ((d_i / 12) / 2)²))

= (576 / π) ( q_i / d_i ²)

= 144 q_i / (a_i b_i )                                   (1)

where

v_i = air velocity (ft/min)

q_i = air flow (ft³/min)

A_i = area of duct (ft²)

d_f = diameter of duct (ft)

d_i = diameter of duct (in)

a_i = width of duct (in)

b_i = width of duct (in)

Example - Air Flow Velocity in a Duct

The velocity in a 12 inch duct with air flow 1000 cfm can be calculated as

v_i = (576 / π)  (1000 cfm) / (12 in)²)

= 1273 (ft/min)

Air Flow Velocity Calculator - Imperial Units

Air velocity can be calculated with the calculator below. Add air volume - q - and diameter - d - (or length a and b ).

Air volume - q_i - (cfm)

Diameter - d_i - (inches)

or alternatively

Length side - a_i - (inches)

Length side - b_i - (inches)

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• Air Duct Flow App
• Air Duct Velocity App
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Air Duct Velocity - SI Units

Air velocity in a duct can be calculated in SI units

v_m = q_m / A_m

= q_m / (π (d_m / 2)²)

= 4 q_m / (π d_m²)

= q_m / (a_m b_m)                                  (2)

where

v_m = air velocity (m/s)

q_m = air flow (m³/s)

A_m = area of duct (m²)

d_m = diameter of duct (m)

a_m = width of duct (m)

b_m = width of duct (m)

Example - Air Flow Velocity in a Duct

The velocity in a rectangular 0.5 m x 0.5 m duct with air flow 1 m³/s can be calculated as

v_m = (1 m³/s) / ((0.5 m) (0.5 m))

= 4 m/s

Air Flow Velocity Calculator - SI Units

Air velocity can be calculated with the calculator below. Add air volume - q - and diameter - d - (or length a and b ).

Air volume - q_m - (m³/s)

Diameter - d_m - (m)

or alternatively

Length side - a_m - (m)

Length side - b_m - (m)

Air Ducts - Velocity Diagram

The chart below can be used to estimate velocities in air ducts.

The default values are for air flow 400 cfm (680 m³/h) , duct size 8 in (200 mm) and velocity 1150 fpm (5.8 m/s) .

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Air Ducts Friction Loss

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