Ductwork Friction Loss

Overview

Engineering reference data for Ductwork Friction Loss in fluid mechanics.

Key Formulas

Reynolds Number

$Re = \frac{\rho v D}{\mu}$

Ratio of inertial to viscous forces — determines flow regime.

Bernoulli's Equation

$P + \frac{1}{2}\rho v^2 + \rho g h = \text{const}$

Conservation of energy for steady, inviscid, incompressible flow.

Continuity Equation

$A_1 v_1 = A_2 v_2$

Conservation of mass for incompressible flow.

Darcy-Weisbach

$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$

Pressure drop due to friction in a pipe.

Variables

Example Calculation

Friction loss in a 60-inch diameter duct with an airflow of 100,000 CFM can be estimated to approximately 0.4 inches of water gauge (w.g.) per 100 feet of duct. Under these conditions, the air velocity is approximately 5,000 feet per minute (fpm).

Unit Conversions

The following conversions are frequently used in ductwork calculations:

• Pressure:
  1 inch of water = 248.8 N/m² (Pa) = 0.0361 lb/in² (psi) = 25.4 kg/m² = 0.0739 inches of mercury
• Volumetric Flow Rate:
  1 ft³/min (CFM) = 1.7 m³/h = 0.47 L/s
• Velocity:
  1 ft/min = 5.08×10⁻³ m/s
• Length:
  1 inch = 25.4 mm = 2.54 cm = 0.0254 m = 0.08333 ft

References

• Original Source