What straight-duct friction represents
Straight-duct friction is the pressure loss associated with air moving along a duct’s internal surface over a stated length. It is often expressed as a pressure loss per unit length, but it is not the total pressure loss of a real system.
Inputs that change the result
| Input | Why it matters |
|---|---|
| Airflow and duct area | They establish velocity, which strongly affects loss. |
| Length and hydraulic diameter | They determine the geometry over which the loss occurs. |
| Air density and viscosity | Air properties change the pressure calculation and friction behavior. |
| Surface and friction factor | Material, roughness, and the selected method affect the result. |
| Fittings and transitions | They add local losses beyond straight-run friction. |
Preliminary calculation form
This Darcy-Weisbach form requires compatible units and a stated friction factor. Use it only as transparently as its assumptions allow. It does not substitute fitting-loss coefficients, manufacturer data, or a complete fan/system analysis.
Example of the scope boundary
A straight-run calculation can estimate the loss of one duct section. A final fan check also needs elbows, takeoffs, transitions, dampers, terminals, filters, coils, heat-recovery devices, and the fan curve. Adding a blanket safety factor is not a replacement for identifying those components.
FAQ
Is friction loss the same as total static pressure?
No. Total system resistance also includes fittings, terminals, filters, coils, dampers, and other components.
Can I use one friction factor for every duct system?
No. State the material, air properties, geometry, and selected method. A friction factor is an input to a model, not a universal default.