Input Parameters
Results
Recommended Size
--
Velocity
-- FPM
Friction Rate
-- in. w.g./100ft
Area
-- ft²
Velocity Pressure
-- in. w.g.
Equivalent Diameter
-- in.
Hydraulic Diameter
-- in.
Reynolds Number
--
Friction Factor (f)
--
Shape Comparison
Compare all duct shapes for current airflow
| Shape | Size | Velocity (FPM) | Friction (in. w.g./100ft) | Area (ft²) |
|---|
Calculation Methodology
Based on ASHRAE Fundamentals Handbook
Darcy-Weisbach Equation
dP = f x (L/D) x (rho x V^2 / 2)
Where: f = friction factor, L = length, D = diameter, V = velocity
Velocity Pressure
Pv = (V / 4005)^2
For standard air at 70F (0.075 lb/ft3)
Reynolds Number
Re = V x D / nu
Where nu = 0.000157 ft2/s (air at 70F)
Swamee-Jain Friction Factor
f = 0.25 / [log10(e/3.7D + 5.74/Re^0.9)]^2
Explicit approximation of Colebrook equation for turbulent flow
Equivalent Diameter (Rectangular)
De = 1.30 x (a x b)^0.625 / (a + b)^0.25
Round duct with equal friction loss at same airflow
Equivalent Diameter (Oval)
De = 1.55 x A^0.625 / P^0.25
Where A = cross-sectional area, P = perimeter
Reference: Roughness Values
| Material | Absolute Roughness (ft) |
|---|---|
| Galvanized Steel | 0.0003 |
| Stainless Steel | 0.00015 |
| Aluminum | 0.0002 |
| Fiberglass Lined | 0.003 |
| Flexible Duct | 0.012 |
| PVC | 0.0001 |
| Fabric | 0.005 |