Vortia 3DDuct pressure drop calculator
Calculate the pressure drop of a straight run with the method of the design handbooks (Darcy-Weisbach with Colebrook) and add that of its fittings if you know their ζ coefficient.
How it is calculated
Δp = f · (L / D) · ρ·v² / 2 + Σζ · ρ·v² / 2f: friction factor (Colebrook-White) · L and D in m · v in m/s · ρ in kg/m³1 / √f = −2 · log₁₀( ε / (3.7·D) + 2.51 / (Re·√f) ) · Re = v·D / νε: absolute roughness · ν: kinematic viscosity of air (≈ 1.51·10⁻⁵ m²/s at 20 °C)Air density and viscosity are calculated at the temperature you enter (ideal gas at sea level and Sutherland’s law). In a rectangular duct, friction is calculated with the hydraulic Ø Dh = 2·a·b / (a + b) and the actual velocity Q / (a·b), as ASHRAE does.
Reference values
| Type of installation | Typical pressure drop per metre |
|---|---|
| Low-pressure HVAC and ventilation | 0.8–1.2 Pa/m |
| Industrial | up to 2–3 Pa/m |
With the maximum velocity for the use. Roughness: galvanised sheet steel 0.09 mm; fully extended flexible duct 0.91 mm (ASHRAE Fundamentals, ch. 21).
Example
1,000 m³/h through 10 m of Ø250 galvanised sheet steel at 20 °C: v = 5.66 m/s, Re ≈ 94,000, f = 0.0199 and 1.54 Pa/m, i.e. 15.4 Pa of friction. With two bends and a damper (Σζ ≈ 1.0), 19.3 Pa is added: 34.6 Pa for the run.
Bear in mind
- The friction factor of this run is not valid for another: it changes with velocity, diameter and roughness.
- The ζ of a fitting depends on its geometry (bend radius, angle, airflow ratio at a tee). Look it up in ASHRAE, Idelchik or the manufacturer’s catalogue.
- A compressed or curved flexible duct loses considerably more than when fully extended.