Duct Velocity
Calculate velocities in circular and rectangular ducts - imperial and SI-units - online calculator.
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 (df / 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)
df = 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 ).
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Air Duct Velocity - SI Units
Air velocity in a duct can be calculated in SI units
vm = qm / Am
= qm / (π (dm / 2)2)
= 4 qm / (π dm2)
= qm / (am bm ) (2)
where
vm = air velocity (m/s)
qm = air flow (m3 /s)
Am = area of duct (m2)
dm = diameter of duct (m)
am = width of duct (m)
bm = 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
vm = (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 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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Related Topics
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Ventilation Systems
Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
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