Preparing airflow intelligence
Calibrating industrial-grade components and rendering the next section.
Understanding the Darcy-Weisbach equation and how to accurately calculate friction losses in industrial duct systems.
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.
