Inputs
Equal-Friction & Velocity MethodsResults
Quick Guidance
- Main supply trunks: ~700–900 fpm typical; branches lower.
- Common friction rate target: 0.08 in. w.g. per 100 ft.
- Include equivalent length for elbows, tees, and transitions.
Example Data
Click a row to load it into the form.
| CFM | Velocity (fpm) | Friction (in/100ft) | Len (ft) | Eq Len (ft) | Roughness |
|---|---|---|---|---|---|
| 600 | 900 | 0.08 | 75 | 40 | Galv steel |
| 1200 | 800 | 0.08 | 120 | 60 | Lined metal |
| 300 | 700 | 0.10 | 50 | 30 | Flex duct |
Formulas Used
Continuity (round duct): \( V = \dfrac{Q}{A} = \dfrac{4Q}{\pi D^2} \), where Q is flow and V is velocity.
Velocity-based diameter: \( D = \sqrt{\dfrac{4Q}{\pi V}} \).
Darcy–Weisbach friction: \( \frac{\Delta P}{L} = f \dfrac{\rho V^2}{2D} \). We use the Haaland approximation for friction factor:
\( \dfrac{1}{\sqrt{f}} = -1.8 \log_{10}\!\left[\left(\dfrac{\varepsilon/D}{3.7}\right)^{1.11} + \dfrac{6.9}{Re}\right] \), where \( Re = \dfrac{VD}{\nu} \).
Air properties: Density from barometric relations with lapse rate; viscosity by Sutherland’s law; \( \nu = \mu/\rho \).
Rectangular ducts: Area matched to round sizing with aspect ratio W:H. Hydraulic diameter \( D_h = \dfrac{2ab}{a+b} \) is used for friction/velocity checks.
How to Use
- Enter required airflow in CFM, plus your target velocity or friction.
- Pick the method. Use “Compare” to see both approaches side-by-side.
- Set straight length and equivalent fittings length to estimate total drop.
- Choose material roughness. Flex duct is rougher and raises losses.
- For rectangular, set aspect ratio and constraints, then compute.
- Use the graphs to inspect velocity and friction trends with diameter.
- Export a summary to CSV for records or generate a quick PDF.
Velocity vs Diameter
Friction Rate vs Diameter
Velocity & Friction vs Diameter (Dual‑Axis)
Always verify with local codes, manufacturer data, and project standards.