Explain the Darcy-Weisbach equation and how to calculate friction factor.

The Darcy-Weisbach equation calculates pressure drop due to friction in pipes: h_f = f × (L/D) × (V²/2g), where f is Darcy friction factor, L is pipe length, D is diameter, and V is velocity. For laminar flow (Re < 2300): f = 64/Re. For turbulent flow, use Colebrook-White equation: 1/√f = -2log₁₀(ε/3.7D + 2.51/Re√f) or Moody diagram. Relative roughness (ε/D) depends on pipe material: commercial steel (0.046mm), PVC (0.0015mm), cast iron (0.26mm). This equation is fundamental for pipe sizing and pump head calculations.