BIZOMAT 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. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Number of hoods
- What it is
- How many hoods or extraction points draw air through the main duct at the same time, which sets the total airflow.
- Units and format
- Enter a number in hoods: hoods. The form accepts 1 hoods to 1000 hoods. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 hoods as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Total airflow, duct velocity and pressure loss all rise with it.
- Get it right
- Count the hoods that can be open together in practice, not every hood connected to the system.
What this means: Airflow per hood
- What it is
- The volume of air each hood must draw to capture the dust or fume at its source, which the tool multiplies by the number of hoods.
- Units and format
- Enter a number in m³/h: cubic metres per hour. The form accepts 0.000001 m³/h to 10000000 m³/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 m³/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Duct velocity rises in proportion and the duct loss roughly with its square.
- Get it right
- Enter cubic metres per hour per hood: multiply cubic metres per second by 3600.
What this means: Main duct diameter
- What it is
- The inside diameter of the main duct carrying the total airflow from the hoods to the fan.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 5000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 250 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A smaller duct raises the velocity and the pressure loss steeply; a larger one may let dust settle.
- Get it right
- Use the inside diameter of a round duct; for a rectangular duct this tool does not apply directly.
What this means: Duct length to the fan
- What it is
- The length of duct the air travels from the furthest hood to the fan, used for the friction loss along the duct.
- Units and format
- Enter a number in m: metres. The form accepts 0.000001 m to 10000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The straight-duct friction loss rises in proportion to the length.
- Get it right
- Measure along the duct route in metres, including vertical runs, not the straight-line distance.
What this means: Duct roughness
- What it is
- The absolute roughness of the inside duct surface, the average height of the surface bumps that cause friction.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.15 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A rougher duct raises the friction factor and the duct loss.
- Get it right
- Enter it in millimetres from duct material data; it is a height, not a relative roughness ratio.
What this means: Fittings loss coefficient (total)
- What it is
- The total of the loss coefficients for every bend, junction, transition and the filter connection along the duct route, each counted in velocity heads.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 1000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The fittings loss rises in proportion to this total.
- Get it right
- Add the coefficients for all fittings on the route from the duct fittings maker’s data; do not include the hood entry here.
What this means: Air density
- What it is
- The mass per cubic metre of the air being moved, which depends on its temperature, altitude and moisture.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 5 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.2 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The duct pressure loss rises in proportion to the density.
- Get it right
- Take it for the actual air temperature and site altitude; hot or high-altitude air is less dense.
What this means: Air kinematic viscosity
- What it is
- The kinematic viscosity of the air in the duct, used for the Reynolds number and the friction factor.
- Units and format
- Enter a number in cSt: centistokes (kinematic viscosity, 1 cSt = 1 mm²/s). The form accepts 0.000001 cSt to 1000 cSt. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 cSt as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter centistokes: 1 cSt = 1 mm²/s = 0.000001 m²/s.
Limits
What this means: Least transport velocity for the dust
- What it is
- The lowest air speed in the duct that keeps the dust or particles carried along without settling, for the type of contaminant extracted.
- Units and format
- Enter a number in m/s: metres per second. The form accepts 0.000001 m/s to 100 m/s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 m/s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line for velocity; the duct velocity does not change.
- Get it right
- Take it from your ventilation design guidance for the specific dust; heavy or sticky dusts need more.
What this means: Hood entry loss
- What it is
- The pressure lost as air enters the hood itself, added to the duct loss to give the total pressure the fan must provide.
- Units and format
- Enter a number in Pa: pascals. The form accepts 0 Pa to 100000 Pa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 250 Pa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to the fan pressure needed.
- Get it right
- Take it from the hood maker’s data or ventilation design guidance for the hood shape.
What this means: Fan pressure available at this flow
- What it is
- The pressure the fan delivers at the total airflow, read from its curve, after the filter or collector loss is taken off.
- Units and format
- Enter a number in Pa: pascals. The form accepts 0.000001 Pa to 1000000 Pa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 Pa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass or fail line for the fan; the pressure needed does not change.
- Get it right
- Read the fan curve at the calculated total airflow and subtract the loss of a loaded filter.
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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