Duct reference
Duct friction rate reference
Use this page to identify the inputs and scope of a straight-duct pressure-loss check before applying a detailed duct-design method.
Definition and units
Friction rate commonly means straight-duct pressure loss per unit length, such as Pa/m or in. w.g. per 100 ft. Always state both the pressure unit and the length basis. It is not total external static pressure.
Inputs that must be stated
| Input | Why it must be recorded |
|---|---|
| Airflow and duct size | They establish velocity and the flow regime. |
| Shape or hydraulic diameter | Round and rectangular ducts require geometry-aware treatment. |
| Length basis | Prevents confusion between total loss and loss per unit length. |
| Air properties | Density and viscosity change pressure and friction behavior. |
| Surface and friction factor | The selected method and assumptions must be reproducible. |
| Fittings | They require separate local-loss or equivalent-length treatment. |
Formula
Friction rate = ΔP ÷ L
Use compatible units for density, velocity, length, and diameter. The formula applies to a modeled straight section; do not add values from incompatible units or use a generic friction factor without stating its basis.
How to use the reference
- Determine the assigned airflow and actual duct geometry.
- Calculate velocity and choose a documented friction method.
- Calculate straight-run loss and state the length basis.
- Add fittings, terminals, filters, coils, and equipment pressure drops separately for a system check.
- Compare the total resistance with the fan’s actual performance point.
Limit
This page intentionally does not publish a universal friction-rate target. Appropriate criteria depend on system type, route, sound, fan pressure, energy, and project requirements.