Ventilation Ducts - Roughness & Surface Coefficients

Surface coefficients to calculate friction and major pressure loss in ventilation ducts - concrete, galvanized steel, corroded steel and more

For turbulent flow the friction coefficient depends on the Reynolds Number and the roughness of the duct or pipe wall. Roughness for different materials can be determined by experiments.

Absolute roughness - k - for some common materials below:

SurfaceAbsolute Roughness Coefficient
- k -
(10-3 m)
(feet)
Copper, Lead, Brass, Aluminum (new) 0.001 - 0.002 (3.28 - 6.56) 10-6
PVC and Plastic Pipes 0.0015 - 0.007 (0.49 - 2.30) 10-5
Stainless steel 0.0015 0.49 10-4
Steel commercial pipe 0.045 - 0.09 (1.48 - 2.95) 10-4
Stretched steel 0.015 4.95 10-5
Weld steel 0.045 1.48 10-4
Galvanized steel 0.15 4.92 10-4
Rusted steel (corrosion) 0.15 - 4 (4.92 - 131) 10-4
New cast iron 0.25 - 0.8 (8.2 - 26.2) 10-4
Worn cast iron 0.8 - 1.5 (2.62 - 4.92) 10-3
Rusty cast iron 1.5 - 2.5 (4.92 - 8.2) 10-3
Sheet or asphalted cast iron 0.01 - 0.015 (3.28 - 4.92) 10-5
Smoothed cement 0.3 0.98 10-3
Ordinary concrete 0.3 - 1 (0.98 - 3.28) 10-3
Coarse concrete 0.3 - 5 (0.98 - 16.4) 10-3
Well planed wood 0.18 - 0.9 (5.9 - 29.5) 10-4
Ordinary wood 5 16.4 10-3
• 1 m = 3.28 feet

Relative Roughness

Relative roughness - the ratio between absolute roughness an pipe or duct diameter - is important when calculating pressure loss in ducts or pipes with the Colebrook Equation. Relative roughness can be expressed as

r = k / dh                                            (1)

where

r = relative roughness

k = roughness of duct, pipe or tube surface (m, ft)

dh = hydraulic diameter (m, ft)

Related Topics

• Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more

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