Duct Friction Loss and Static Pressure
Darcy-Weisbach with Swamee-Jain Colebrook; round or rectangular; fitting losses from a C_o library; total external static.
Formula and source
Hydraulic diameter D_h = 4A/P (round D_h = D; rectangular Huebscher D_eq = 1.30 × (W×H)^0.625 / (W+H)^0.250). 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).
ASHRAE Handbook Fundamentals chapter 21-2022 (Ventilation for Acceptable Indoor Air Quality). (referenced by name; principles in published engineering texts).
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