Accurate pressure drop calculation is the foundation of any fan selection exercise. Underestimating system resistance leads to undersized fans and inadequate airflow; overestimating leads to oversized fans operating inefficiently.
The Darcy-Weisbach equation is the standard method for calculating friction pressure drop in straight duct sections: ΔP = f × (L/D) × (ρV²/2), where f is the Darcy friction factor, L is duct length, D is hydraulic diameter, ρ is air density, and V is velocity.
The friction factor f depends on the Reynolds number and duct roughness. For turbulent flow in galvanized steel ducts (roughness ε = 0.15 mm), the Colebrook-White equation or Moody chart provides accurate f values.
Fitting losses are expressed as equivalent lengths or loss coefficients (K-factors). A 90° round elbow has K ≈ 0.3–0.9 depending on the radius ratio. Transitions, tees, and dampers each have tabulated K-factors in ASHRAE Fundamentals.
The Aerotech Pressure Drop Calculator automates these calculations, allowing engineers to build a complete duct system model with multiple sections, fittings, and equipment items to generate an accurate system curve.