Air Duct Calculator

Overview

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Source URL preserved from the original page: http://www.iklimnet.com/data/duct.html

This page covers the calculation of friction loss in air ducts, a core task in HVAC system design. The Darcy-Weisbach equation, combined with empirical friction factor correlations, provides the standard method for estimating pressure drop per unit length in straight ductwork. These results drive fan selection, duct sizing, and energy cost analysis.

Key Formulas

Darcy-Weisbach Equation

$\Delta P_f = f \cdot \frac{L}{D_h} \cdot \frac{\rho v^2}{2}$

Pressure drop due to friction along a straight duct section.

Swamee-Jain Approximation (Friction Factor)

$f = \frac{0.25}{\left[ \log_{10}\left(\frac{\varepsilon}{3.7 D_h} + \frac{5.74}{Re^{0.9}}\right) \right]^2}$

Valid for turbulent flow ($Re > 4000$) in commercial ductwork.

Reynolds Number

$Re = \frac{\rho v D_h}{\mu}$

Determines flow regime: laminar ($Re < 2300$), transitional, or turbulent ($Re > 4000$). Nearly all HVAC duct systems operate in the turbulent range.

Hydraulic Diameter

$D_h = \frac{4A}{P}$

For non-circular ducts, where $A$ is cross-sectional area and $P$ is the wetted perimeter. For a round duct, $D_h$ equals the duct diameter.

Velocity Pressure

$P_v = \frac{1}{2}\rho v^2$

Useful for relating velocity to pressure and for sizing fittings.

Variables

Duct Material Roughness

Standard Air Properties

Calculator: Duct Friction Loss

Unit Converter

The source page included a Unit Converter section. This converter preserves the common duct-design conversions for airflow, duct size, pressure loss, length, and velocity.

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

Engineering Notes

• Turbulent flow assumption: The Swamee-Jain formula used above is valid for $Re > 4000$. Almost all HVAC duct systems fall in this range. For $Re < 2300$, use $f = 64/Re$ (laminar flow).
• Rectangular ducts: Convert to equivalent round duct using $D_h = 4A/P$, or use the equal-friction method tables from ASHRAE/SMACNA.
• Fittings and accessories: The calculator covers straight duct friction only. Add local loss coefficients ($K$-factors) for elbows, tees, transitions, dampers, and grilles. Typical practice applies an equivalent length or loss coefficient method.
• Air density corrections: At elevations above sea level or elevated temperatures, air density drops. Use the density table above or calculate as $\rho = P/(R \cdot T)$ with $R = 287\ \text{J/(kg·K)}$.
• Flexible duct penalty: Corrugated flexible ducts have roughness 10–20× that of sheet metal. Manufacturers often recommend adding 50% to the friction loss computed from nominal diameter.
• Standard design velocity: Residential supply mains: 4–6 m/s. Commercial supply: 6–10 m/s. Exhaust: 8–13 m/s. Exceeding these values leads to noise and excessive energy use.
• Equal-friction method: A common design approach sizes all duct sections for a uniform pressure drop per unit length (typically 0.8–1.2 Pa/m for low-pressure systems).

References

• Original Source — Engineering ToolBox
• Original external calculator reference — http://www.iklimnet.com/data/duct.html
• Original source reference: http://www.iklimnet.com/data/duct.html
• ASHRAE Handbook — Fundamentals, Chapter 21: Duct Design
• SMACNA HVAC Duct Construction Standards