Duct reference
Duct friction rate reference
Use a friction-rate calculation to document one modeled straight duct section. It is not a total-system static-pressure calculation or a universal duct-design target.
Definition and units
Friction rate is straight-duct pressure loss per unit length, for example Pa/m or in. w.g. per 100 ft. Always show both the pressure unit and the length basis. Total external static pressure also includes fittings, filters, coils, dampers, grilles, terminals, and other components.
Inputs that must be stated
| Input | Why it matters |
|---|---|
| Assigned airflow and actual duct size | They establish velocity and the flow regime. |
| Shape or hydraulic diameter | Round and rectangular ducts need geometry-aware treatment. |
| Length basis | It prevents confusion between a total section loss and a loss per unit length. |
| Air properties | Density and viscosity affect pressure and friction behavior. |
| Surface and friction factor | The selected model and assumptions must be reproducible. |
| Fittings and components | They need separate local-loss or equivalent-length treatment. |
Modeled straight-section formula
Friction rate = ΔP ÷ L
Use compatible units for density, velocity, length, and diameter. In this expression, f is the Darcy friction factor, L is straight length, D is the applicable diameter, ρ is density, and V is velocity. Do not combine values from incompatible unit systems or apply an unstated generic factor.
How to use this reference
- Confirm the airflow and actual inside geometry for the duct section.
- Calculate the corresponding velocity and document the chosen friction method.
- Calculate straight-run loss and state the length basis.
- Add fitting, terminal, filter, coil, and equipment losses separately for a complete path.
- Compare the full resistance with the fan’s published operating point and project requirements.
Limits
This reference intentionally does not publish one recommended friction rate. Appropriate criteria depend on system type, route, acoustic goals, fan pressure, energy objectives, materials, and project requirements. A straight-section result cannot prove a fan will deliver the required airflow.
Related tools and guidance
- Duct Friction Loss Calculator and Duct Velocity Calculator
- Duct Friction Loss Explained and How Duct Velocity Affects HVAC Performance
- Duct Size Calculator — use only after the airflow is identified.