Surface Roughness Ventilation Ducts
Reference data and engineering information about surface roughness ventilation ducts for hvac systems applications.
Overview
Engineering reference data for Surface Roughness Ventilation Ducts in HVAC systems.
Key Formulas
Sensible Heat
Heat causing temperature change.
Latent Heat
Heat causing moisture change.
COP (Cooling)
Coefficient of performance.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Heat transfer | W | |
| Mass flow rate | kg/s | |
| Specific heat of air | J/(kg·K) | |
| Temperature difference | K |
Surface Roughness Coefficients
Surface Material | Absolute Roughness k (10⁻³ m) | Absolute Roughness k (feet) |
|---|---|---|
| Drawn Copper, Lead, Brass, Aluminum (new) | 0.001 - 0.002 | (3.28 - 6.56) × 10⁻⁶ |
| PVC, PE & 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, electropolished | 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 |
Source: engineeringtoolbox.com
Relative Roughness
Relative roughness is the ratio between absolute roughness and the hydraulic diameter of the pipe or duct. It is a critical parameter when calculating pressure loss using the Colebrook Equation for turbulent flow.
Where:
- = relative roughness (dimensionless)
- = absolute roughness of the duct surface (m, ft)
- = hydraulic diameter (m, ft)
Duct Material Selection Guide
| Material | Typical Applications |
|---|---|
| Galvanized Steel | Most common material for comfort air conditioning systems; standard fabricated ductwork |
| Aluminum | Clean room applications, moisture-laden air, special exhaust systems, ornamental duct systems |
| Stainless Steel | Kitchen exhaust, moisture-laden air, fume exhaust systems |
| Carbon Steel (Black Iron) | Flues, stacks, hoods, high-temperature industrial systems, special coating requirements |
| Copper | Chemical exhaust systems, visual/architectural ductwork |
| Fibreglass Reinforced Plastic (FRP) | Chemical exhausts, scrubbers, underground systems — corrosion resistant, self-insulating, excellent sound attenuation |
| PVC | Chemical exhaust, fumes, underground duct systems — corrosion resistant, lightweight, easy to modify |
| Fabric (Textile) | Applications requiring even air distribution |
| Flex Duct | Equipment connections (inner liner supported by helix wire coil) |
For turbulent flow in ducts, the friction coefficient depends on both the Reynolds Number and the surface roughness. As surfaces degrade over time (e.g., rust formation on steel), roughness values can increase significantly — rusted steel can reach roughness values up to 100× greater than new commercial steel.