What Is the Friction Factor?
The Darcy friction factor (also called Darcy-Weisbach friction factor) is a dimensionless parameter used to calculate pressure loss due to friction in pipe flow. It appears in the Darcy-Weisbach equation, which is the standard method for calculating frictional head loss in pipes and ducts across all flow regimes.
The friction factor depends on the Reynolds number (characterizing flow regime) and the relative roughness of the pipe wall. For laminar flow (Re less than 2300), it is simply 64/Re. For turbulent flow, it must be found iteratively using the Colebrook-White equation or estimated with explicit approximations.
Colebrook-White Equation
This implicit equation requires iterative solution. For laminar flow: f = 64/Re. The Fanning friction factor is one-quarter of the Darcy factor.
Pipe Roughness Values
| Material | Roughness (mm) |
|---|---|
| Drawn tubing (glass, brass) | 0.0015 |
| Commercial steel | 0.045 |
| Cast iron | 0.26 |
| Galvanized iron | 0.15 |
| Concrete | 0.3-3.0 |
| PVC pipe | 0.0015 |
Frequently Asked Questions
What is the difference between Darcy and Fanning friction factor?
The Darcy friction factor is four times the Fanning friction factor. The Darcy factor is more common in civil and mechanical engineering, while the Fanning factor appears in chemical engineering texts. Always check which convention is being used to avoid errors by a factor of four.
When does flow become turbulent?
Flow transitions from laminar to turbulent around Reynolds number 2300-4000. Below 2300 is definitively laminar, above 4000 is fully turbulent. The transition zone between these values is unpredictable and flow may be intermittently turbulent.
Does pipe age affect friction factor?
Yes significantly. Pipe aging increases surface roughness due to corrosion, scaling, and biological fouling. Old cast iron pipes can have roughness values 5-10 times higher than new ones, dramatically increasing friction losses and pumping costs. Regular maintenance and pipe replacement are essential for efficient systems.