HVAC & Ventilation
Duct Sizing (Equal Friction)
Size rectangular or circular HVAC ducts based on air flow rate and maximum allowable velocity or pressure drop per metre.
Units:
Recommended velocities: main ducts 5–9 m/s, branch ducts 3–6 m/s, diffusers 2–4 m/s. Higher velocities increase noise and pressure drop significantly (proportional to v²).
A = Q / v | D = √(4A/π) | ΔP/m = λ/D × (ρv²/2)
Input
Air flow Q iVolume flow rate through the duct in m³/h.
m³/h
Design velocity v iMaximum air velocity in the duct. Main duct: 5–9 m/s. Branch: 3–6 m/s.
m/s
Duct type iRound ducts are more efficient. Rectangular used where space is limited.
Wall roughness ε iSheet metal (galvanised): 0.09 mm. Flexible duct: 1.5–3.0 mm. Concrete: 1.0–3.0 mm.
mm
Result
Required cross-section A [m²]
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Round duct diameter D [mm]
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Nearest standard size [mm]
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Pressure drop ΔP/m [Pa/m]
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Actual velocity (std. duct) [m/s]
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