FT-11 · Industrial ventilation

HVAC air duct sizing

Compares circular and rectangular ducts, computes Darcy friction and the equivalent diameter, and issues preliminary guidance on velocity, noise and insulation.

Geometry and air condition

Duct typeAir flow rate · Diameter · Width a · Alto b · Rugosidad ε · Air density · Dynamic viscosity · Target velocity ·

Air velocity

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Friction loss

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Equivalent diameter

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Hydraulic diameter

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Reynolds / f Darcy

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Diameter at target velocity

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Acoustic and insulation guidance

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Velocity and section check

Friction and equivalence

\[\frac{\Delta p}{L}=f\frac{\rho V^2}{2D_h}\]

\[D_e=1.30\frac{(ab)^{0.625}}{(a+b)^{0.25}}\]

\[D_h=\frac{4A}{P}\]

D_e is the equivalent circular diameter by carrying capacity; D_h is used in the Reynolds number and in Darcy–Weisbach.

Air duct sizing calculator

Duct sizing balances cross section, velocity, pressure loss, space and noise. A smaller duct lowers the initial cost but increases friction, fan power and noise generation.

Indicative screening velocities

Velocity	Lectura preliminar	Action
≤3 m/s	Baja	Suitable for noise-sensitive areas
3–5 m/s	Moderada	Common range in branches
5–8 m/s	Alta	Check acoustics and pressure
>8 m/s	Very high	Redimensionar o justificar

Friction in Pa/m and inWG/100ft

The tool computes Darcy–Weisbach and converts the gradient automatically. Fittings must be added as equivalent lengths or K coefficients in a full balance.

Preguntas frecuentes

Are the equivalent and hydraulic diameters the same?

No. The equivalent diameter compares carrying capacity; the hydraulic diameter is used in Reynolds number and friction.

Does the NC guidance replace an acoustic calculation?

No. It is a velocity-based screen; sound power, attenuation and enclosure analysis are still required.

How do I select insulation?

It depends on temperature, condensation, fire, environment, noise and the local code; the result shown is indicative only.