Exhaust airflow and duct sizing

Researched from published specifications and material data · Standard formula, convention inputs

Quick answer

A laser enclosure of 0.2 cubic metres exchanged every 6 seconds needs about 71 CFM, which suits a 4 inch duct at roughly 815 feet per minute. Keep duct runs short and rigid wherever possible.

Airflow equals velocity times area, which is standard. The target transport velocity and the penalties for flexible duct and bends are conventions drawn from industrial ventilation practice.

Exhaust airflow and duct sizing Standard formula, convention inputs

Enclosure width (mm)

Enclosure depth (mm)

Enclosure height (mm)

Full air change every (s) Laser smoke needs a fast exchange. Six to ten seconds is the usual target.

Duct size

Duct run (ft)

Number of bends

Duct type

Extraction required 31 CFM

Enclosure volume0.087 m3, 3.1 ft3

Duct velocity354 fpm

Target velocity1000 to 2000 fpm

Measured run plus bends20 ft equivalent

Effective run for fan sizing50 ft equivalent

Below about 1000 feet per minute, soot and resinous particulate settles out inside the duct instead of being carried outside, and over time the run gums up and catches smells. A smaller duct raises the velocity.

Flexible duct costs far more pressure than smooth rigid duct for the same length, which is why the effective run below is much longer than the measured one. Use rigid pipe for the straight sections and keep flex for the last short connection.

Formula: CFM = volume_ft3 x (60 / exchange_seconds) ; velocity_fpm = CFM / duct_area_ft2

Fans, ducting and filtration

Cenipar
Cenipar Dryer Vent Hose-4 Inch Ducting Vent Hose Flexible Aluminum Foil Non-Insulated (4 Inch 8 Feet) with 2 Clamps

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AC Infinity
AC Infinity Flexible 4-Inch Aluminum Ducting, 8-Feet Long

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VIVOSUN
VIVOSUN 4 Inch Flex Aluminum Ducting, 8 Ft Air Duct with 2 Clamps, Silver

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VIVOSUN
VIVOSUN P4 4" Inline Fan &Speed Control 130 CFM Exhaust Fan for Home

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VIVOSUN
VIVOSUN D4 4" AC Motor Inline Fan, 195 CFM for Ventilation

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Voltset
Voltset 4 Inch Duct Fan 280 CFM Inline Ventilation Exhaust Fan

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Safety

Common questions

How much CFM does a laser cutter need?

It depends on the enclosure volume rather than the machine wattage, because what you are doing is replacing the air in the cabinet fast enough that smoke never builds up. A typical desktop enclosure of around 0.1 to 0.2 cubic metres wants roughly 40 to 90 CFM for a six second exchange. Larger cabinets and faster exchanges scale it up proportionally.

Is a 4 inch duct big enough?

For almost every desktop machine, yes. Four inch duct carries up to roughly 175 CFM while staying inside the sensible velocity band, which covers the great majority of desktop lasers and printer enclosures. Move to 6 inch when the airflow is high enough that a 4 inch run becomes noisy.

Why does the calculator penalise flexible duct so heavily?

The corrugations inside flexible duct create turbulence along the entire length, so it costs several times the pressure of smooth rigid pipe for the same run. A fan rated for 200 CFM in free air can deliver well under half that through a long flexible run with bends. Short straight rigid runs are worth real money in delivered airflow.

Can I just use a carbon filter instead of venting outside?

It is a fallback, not an equivalent. A filter captures odour and particulate while the carbon has capacity, but it does not remove all gases, it gives no warning when it is spent, and its capacity falls quickly with heavy cutting. Vent outside whenever a window or wall penetration is possible, and treat filtration as the option for when it genuinely is not.