Duct Friction Loss and Static Pressure
Darcy-Weisbach with Swamee-Jain Colebrook; round or rectangular; fitting losses from a C_o library.
Example
You enter
- Duct shape round
- Round D (in) 12
- Material galv_smooth
- Airflow (CFM) 1200
- Run length (ft) 60
- Fittings kind elbow_90_smooth_radius, count 4; kind filter_typical, count 1; kind diffuser_typical, count 2
You get
- Velocity 1528 fpm
- Friction per 100 ft 0.2665 in WC / 100 ft
- Straight-duct static 0.1599 in WC
- Hydraulic diameter (in) 12
Checked against published worked examples from: ASHRAE Fundamentals Darcy-Weisbach + ACCA Manual D fitting tables.
Details, formula, and sources
ScopeTotal external static.
Formula or tableHydraulic diameter D_h = 4A/P (round D_h = D), used with the duct's own velocity in the friction-factor and loss terms; the rectangular Huebscher equivalent round D_eq = 1.30 × (W×H)^0.625 / (W+H)^0.250 is reported for sizing and is not mixed into the loss. Velocity V_fpm = CFM / A. Velocity pressure VP = (V/4005)². Friction factor from Swamee-Jain explicit Colebrook: f = 0.25 / [log10(eps/(3.7 D) + 5.74/Re^0.9)]². Pressure loss dP = f × (L/D) × (rho_air × V²/(2g)) converted to in WC. Fitting losses dP_fit = Σ (C_o × VP).
Source and editionASHRAE Handbook, Fundamentals volume, Duct Design chapter (ch. 21), by name; principles in published engineering texts.
Free accessFree read-only access to ASHRAE standards at ashrae.org (Technical Resources, read-only standards).
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesshape, D_in, material, cfm, length_ft, fittings, velocity_fpm, friction_loss_per_100ft_in_wc, straight_loss_in_wc, hydraulic_diameter_in
- Air density / kinematic viscosity 0.075 lb/ft³ / 1.62×10⁻⁴ ft²/s at 70 °F sea levelASHRAE Fundamentals
- Roughness table data/hvac/duct-roughness.json (galv smooth 0.0003 ft → flex metal 0.012 ft)engineering-practice consensus
- Fitting C_o library data/hvac/duct-fittings.json keyed to fitting kindengineering-practice consensus