Dimensionamento delle tubazioni di aspirazione delle polveri: velocità, diametro e disposizione

Size a dust extraction pipe from airflow and transport velocity, not fan power. Divide each pickup point’s airflow by the minimum velocity that keeps your dust airborne, convert that area to a diameter, then round down to a standard size. Size the main from the farthest station, enlarging it only where branches join.

Why Velocity Decides Whether Dust Settles

Air speed in a duct is airflow divided by the duct’s cross-sectional area. That is the whole relationship. A 315 mm pipe carrying 5,600 m³/h moves air at about 20 m/s. The same pipe carrying 1,500 m³/h moves it at about 5 m/s, and dust drops out onto the floor of the pipe.

Fan power enters the picture only through airflow. A larger fan raises velocity if it actually pushes more air through the same pipe. It cannot rescue a duct that is too wide for the airflow that really flows through it, and that situation is common: the main was sized for every station running, but on most shifts half the blast gates are closed.

Compared with the common shortcut of sizing ducts to match the collector inlet, velocity-based sizing usually produces smaller branches and a main that steps up in diameter along its length. That shape is the visible sign of a network designed for transport rather than convenience.

Settled dust is not only a performance problem. It reduces the effective pipe area, raises resistance, and in combustible dusts creates fuel inside the duct. NFPA 660, Standard for Combustible Dusts and Particulate Solids (2025 Edition) addresses combustible-dust hazards in dust collection systems, and OSHA provides combustible-dust guidance for U.S. general industry.

Typical Transport Velocity Starting Points

Higher is not automatically better. Pressure loss rises roughly with the square of velocity, so running at 28 m/s where 20 m/s would do costs fan energy every hour the system runs, adds noise, and wears elbows faster with abrasive dust. The goal is to stay above the minimum with a sensible margin, not to maximize speed.

How to Size a Dust Extraction Pipe in 8 Steps

The worked example uses a woodworking line with four stations. Machine A is farthest from the collector, D is nearest.

Step 1: Record the airflow each pickup point needs

Take the required airflow from the machine manufacturer’s data or from a hood calculation. Do not use the collector’s rated airflow divided by the number of machines.

Done when: every pickup point has a documented airflow figure and its source.

Step 2: Choose design velocities for branches and main

From the table above, this woodworking example uses 20 m/s in branches and 18 m/s in the main.

Done when: you have one branch velocity and one main velocity, and you have recorded why.

Step 3: Calculate each branch diameter

Use:

• Area (m²) = Airflow (m³/h) ÷ (3,600 × velocity in m/s)
• Diameter (mm) = 1,000 × √(4 × Area ÷ π)

For station A: 1,500 ÷ (3,600 × 20) = 0.0208 m², which gives 163 mm.

Done when: each branch has a calculated diameter.

Step 4: Round down to a standard size, then recheck velocity

If 163 mm is not a stock size, choose the next size down, not up. A 160 mm pipe carrying 1,500 m³/h runs at about 20.7 m/s. Rounding up to 180 mm drops it to about 16.4 m/s, below the target.

Done when: every selected size runs at or above the design velocity.

Step 5: Size the main from the farthest station outward

Start at A and add each branch’s airflow where it joins. Enlarge the main only at a junction, and never step it back down toward the collector.

If 224 mm is not in your supplier’s range, the next size down (200 mm) runs at about 23.9 m/s. That works, at a higher pressure cost on that section.

Done when: the main increases in diameter at each junction and every section stays above 18 m/s.

Step 6: Lay out the fittings

Most avoidable pressure loss sits in fittings, not straight pipe.

• Elbows: use long-radius elbows. A centerline radius of at least 1.5 times the diameter is a common minimum, and 2 times is better where space allows.
• Branch entries: join branches to the main with an angled lateral (typically 30° or 45°) rather than a 90° tee, so the incoming air merges with the flow instead of colliding with it.
• Two branches never enter at the same point on opposite sides of the main.
• Flexible hose: keep it to the short final connection at the machine. Corrugated hose has far higher resistance per meter than smooth pipe.
• Valves: place a manual or automatic blast gate on each branch, close to the main, so closed branches leave little dead pipe for dust to sit in.
• Cleanout access: add inspection ports on long horizontal runs and before major direction changes.

Done when: a layout drawing shows every fitting type, its angle and its location.

Step 7: Calculate system pressure loss and select the fan

Add the losses for the longest or highest-resistance path: straight pipe, each fitting converted to an equivalent length or loss coefficient, the hood entry, the filter at its expected dirty condition, and any spark arrestor or silencer. Select the fan where its curve meets that total pressure at the required airflow.

Done when: the fan’s operating point is plotted at the design airflow and the dirty-filter pressure, not at clean-filter conditions.

Step 8: Measure after commissioning

Measure velocity in each branch with a pitot tube or a suitable anemometer, and record static pressure at the collector inlet. These readings are your baseline. A gradual drop in branch velocity later points to filter loading, a leaking connection or buildup in the pipe.

Done when: a commissioning sheet lists measured velocity per branch and a baseline static pressure.

What Happens When You Add or Close Workstations?

This is where networks that worked on commissioning day start to fail.

Adding a station. Suppose station E (1,500 m³/h) joins near the collector two years later. The last main section now needs to carry 7,100 m³/h through the same 315 mm pipe, about 25.3 m/s. Pressure loss on that section climbs, and unless the fan has spare capacity, total system airflow falls short of the new demand. The shortfall does not spread evenly. The far branches, with the longest path and the highest resistance, lose the most airflow, so station A drifts below transport velocity first. The symptom reported from the floor is “weak suction at the end of the line,” while the collector itself looks fine.

Closing stations. With a fixed-speed fan and only station A running, the 315 mm section near the collector carries 1,500 m³/h at roughly 5 m/s. Dust from A travels well through its own branch, then settles in the oversized main.

Four ways to plan for this:

• Design for realistic simultaneity. Size around the stations that actually run together, and document that assumption on the drawings.
• Control the fan. A variable-speed drive controlled to a static pressure setpoint lets the fan respond as gates open and close. Multi-fan collectors achieve a similar effect by running only as many fans as current demand requires.
• Add a controlled air bleed. A bleed-in valve near the far end of the main lets in ambient air to hold main velocity when few stations are open. It costs fan energy, so it is a fix for a known low-demand mode, not a default.
• Recalculate before adding branches. Any new pickup point means rerunning steps 3 to 7 for the affected path. Do it before the tee is cut into the main.

Common Mistakes in Dust Extraction Pipe Sizing

• Matching branch size to the machine’s outlet. A machine with a 200 mm port that only needs 1,500 m³/h should get a reducer to a 160 mm branch, not 200 mm pipe all the way.
• Rounding up “for safety.” A bigger pipe lowers velocity. The safe direction is usually down.
• Using HVAC fittings. Short-radius ventilation elbows and thin-wall fittings are not built for abrasive dust at transport velocity.
• Ignoring static electricity. Plastic pipe and ungrounded metal sections can build charge with dry dust. Use conductive, grounded ductwork where combustible dust is present, and follow the applicable combustible-dust standard.
• Treating sticky or fibrous dust like dry powder. Moist, oily or fibrous material can build up even above typical transport velocities. These cases need application-specific design and more frequent inspection.

Senserui Dust Collection Components

Senserui supplies the components used to build the branch-and-main network described above, including:

• Tee connectors for joining branches to the main
• Manual valves for branches that are opened and closed by hand
• Automatic valves for branches that open when a machine starts, which pairs with variable-speed or multi-fan control
• Explosion vents for combustible-dust protection on the system
• Filter cartridges and related ducting accessories

Parameters vary by component and size, and no single specification range applies to the whole category. Diameters, materials and valve actuation options should be confirmed for your project during system design. Pricing for ducting and componenti is available on richiesta based on system configuration and project requirements, since it depends on network length, diameters, fitting count and safety equipment.

The Senserui Engineering Team supports airflow calculation, duct-network design and safety planning as part of project assessment.

FAQ

Q: What diameter should a dust extraction pipe be?

A: There is no single correct diameter. It depends on the airflow at that point in the network and the transport velocity your dust needs. Calculate area as airflow divided by velocity, convert it to a diameter, then round down to a standard size.

Q: Will a more powerful fan fix poor suction at the far end of the line?

A: Sometimes, but often not. If the cause is undersized fan capacity after added stations, a larger fan helps. If the cause is an oversized main, leaks or settled dust, more fan power raises energy use without restoring velocity where it is needed.

Q: Is round pipe better than rectangular duct for dust extraction?

A: For conveying dust, round pipe is generally preferred. It holds velocity more evenly across its section, leaves no corners for dust to collect, and is easier to seal.

Q: How often should dust extraction pipes be inspected?

A: Base it on your dust and your measurements. Compare branch velocity and static pressure against the commissioning baseline, and inspect sections where readings change. Combustible-dust facilities should follow the inspection requirements in the standard that applies to them, such as NFPA 660.

Q: Can I add a new machine by cutting a tee into the existing main?

A: Only after recalculating. A new branch changes airflow, pressure and velocity across the network, and the far stations usually feel it first.