Fully developed circular-pipe model
Round-Duct Straight-Run Pressure Loss
Calculate distributed wall-friction loss for a straight constant-diameter circular duct with supplied fluid density, viscosity and absolute roughness. Transitional flow and unmodelled fittings are not guessed.
Straight-run result
An NREL/US DOE HVAC modelling presentation documents Darcy-Weisbach with laminar and turbulent friction-factor treatment. NASA defines Re=ρvL/μ as the inertial-to-viscous similarity parameter.
A NASA pipe-flow reference places transition usually between Re 2300 and 4000 depending on disturbances. No single regime equation is asserted in that interval, so this calculator reports it as out of scope. The DOE fluid-flow handbook notes viscosity is a fluid property affected by temperature.
Removed claims: the former page hard-coded ρ=1.2 kg/m³ and μ=1.81×10⁻⁵ Pa·s without conditions, called a Haaland explicit approximation “Colebrook”, and applied it immediately above Re 2300. Unsourced roughness/velocity/equivalent-length presets and rectangular-duct shortcut were removed.
