KEVOS® MAKEOS · Engineering
Extraction Duct Check (Total Airflow, Duct Velocity, Friction Loss, Fan Pressure)
For a dust or fume extraction system with several hoods on one duct, is the duct velocity high enough to carry the dust, what is the pressure loss, and can the fan supply it?
Check of a local exhaust ventilation duct: the total airflow from the hoods, the duct velocity against the least transport velocity for the dust, the Reynolds number, friction factor and pressure loss in the duct and fittings at the full flow (as the pipe friction tool with air properties), and the fan pressure needed with the hood entry loss, against the fan pressure available.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Number of hoods | hoods | Number of hoods or extraction points open at once. |
| Airflow per hood | m³/h | Airflow each hood needs to capture the contaminant. |
| Main duct diameter | mm | Diameter of the main duct. |
| Duct length to the fan | m | Length of duct from the furthest hood to the fan. |
| Duct roughness | mm | Roughness of the duct (galvanised spiral duct about 0.15 mm). |
| Fittings loss coefficient (total) | Sum of loss coefficients of bends, junctions and the filter connection. | |
| Air density | kg/m³ | Air density. |
| Air kinematic viscosity | cSt | Kinematic viscosity of air (about 15 cSt at 20 °C). |
| Limits | ||
| Least transport velocity for the dust | m/s | Least velocity to keep the dust airborne (from your ventilation guidance; heavy dusts need 20 m/s or more). |
| Hood entry loss | Pa | Entry loss of the hood itself. |
| Fan pressure available at this flow | Pa | Fan pressure from its curve at the total airflow, less the filter loss. |
Method and formulas
- Total airflow = hoods × airflow per hood; velocity = flow ÷ duct area.
- Duct loss = (f L ÷ d + ΣK) ρ v² ÷ 2 with f from Colebrook (as the pipe friction tool with air properties).
- Fan pressure needed = duct loss + hood entry loss.
- Example: 3,200 m³/h at 18.108 m/s; 959.06 Pa in the duct and 1,209.1 Pa in all.
Limitations
- Full flow taken over the whole duct length; branches carry less and need their own checks.
- Clean duct; dust build-up and a loaded filter raise the loss.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size the main duct for a workshop extraction system
- Check a fan before adding a hood
- Check dust will not settle in a duct
Who it is for
- Ventilation engineers
- Safety advisers
- Plant engineers
What you get
- Total airflow and duct velocity
- Duct pressure loss
- Fan pressure needed
Interpreting the result
- Duct velocity must stay above the transport velocity everywhere, so a duct that is too large is as bad as one that is too small.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3024–3027. Topic reference; no source tables or recommended settings are reproduced.
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