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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 m 2 )
= q m / (a m b m ) (2)
v m = air velocity (m/s)
q m = air flow (m 3 /s)
A m = area of duct (m 2 )
d m = diameter of duct (m)
a m = width of duct (m)
b m = width of duct (m)
The velocity in a rectangular 0.5 m x 0.5 m duct with air flow 1 m 3 /s can be calculated as
v m = (1 m 3 /s) / ((0.5 m) (0.5 m))
= 4 (m/s)
Air velocity can be calculated with the calculator below. Add air volume - q - and diameter - d - (or length a and b ).
The chart below can be used to estimate velocities in air ducts.
The default values are for air flow 400 cfm (680 m 3 /h) , duct size 8 in (200 mm) and velocity 1150 fpm (5.8 m/s) .
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