Roughness & Surface Coefficients
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:
| Surface | Absolute Roughness Coefficient - k - | |
|---|---|---|
| ( 10-3 m) | (feet) | |
| Drawn Copper, Lead, Brass, Aluminum (new) and the like | 0.001 - 0.002 | (3.28 - 6.56) 10-6 |
| PVC, PE and other smooth Plastic Pipes | 0.0015 - 0.007 | (0.49 - 2.30) 10 -5 |
| Stainless steel, bead blasted | 0.001 - 0.006 | (0.00328 - 0.0197) 10-3 |
| Stainless steel, turned | 0.0004 - 0.006 | (0.00131 - 0.0197) 10-3 |
| Stainless steel, electron-polished | 0.0001 - 0.0008 | (0.000328 - 0.00262) 10-3 |
| Commercial steel or wrought iron | 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 |
| Wood stove | 0.18 - 0.9 | |
| 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
- Galvanized Steel - standard and commonly most used material in comfort air conditioning systems
- Aluminum - is widely used in clean room applications. Preferred for moisture laden air, special exhaust systems and ornamental duct systems
- Stainless Steel - is used in duct systems for kitchen exhaust, moisture laden air, and fume exhaust
- Carbon Steel (Black Iron) - is widely used in applications involving flues, stacks, hoods, other high temperature and special coating requirements for industrial use
- Copper - is mainly used for certain chemical exhaust and visual ductwork
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 / d h (1)
where
r = relative roughness
k = roughness of duct, pipe or tube surface (m, ft)
d h = hydraulic diameter (m, ft)
Typical Duct Materials and Their Use
- Galvanized Steel - most common material used in fabricated duct work for most comfort air conditioning systems.
- Aluminum - widely used in clean room applications, moisture laden air, exhaust systems and ornamental duct systems.
- Stainless Steel - used for kitchen exhaust, moisture laden air and fume exhausts.
- Carbon Steel (Black Iron) - used for flues, stacks, hoods, high temperature industrial systems.
- Copper - used for certain chemical exhaust systems and ornamental duct work.
- Fibreglass Reinforced Plastic (FRP) - used mainly for chemical exhausts, scrubbers and underground systems. Resistant to corrosion, self insulated, excellent sound attenuation and high quality sealing.
- Polyvinyl Chloride (PVC) - used in chemical exhaust, fumes and underground duct systems. Resistant to corrosion, light weight, easy to modify.
- Fabric (textile ducts) - used for even air distribution.
- Flex Duct (inner liner supported by a helix wire coil) - used for connections.
Related Topics
• Ventilation Systems
Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
Related Documents
Colebrook Equation
Friction loss coefficients in pipes, tubes and ducts.
Energy Equation - Pressure Loss vs. Head Loss
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Manning's Roughness Coefficients
Manning's roughness coefficients for some common materials.
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Velocity Classification of Ventilation Ducts
Recommended air velocities in ventilation ducts