Inline fan for a 3D printer: what size you need

Buy a 4-inch inline duct fan with a speed controller. One desktop printer needs about 9 to 23 cubic feet per minute (CFM), or 16 to 40 m³/h, to hold its enclosure at 40 °C (104 °F) in a 25 °C (77 °F) room. Common 4-inch inline fans are rated 165 to 226 CFM, so the fan runs slow and spends its spare pressure on a carbon filter and the duct.

What size inline fan do you need for a 3D printer?

A 4-inch inline duct fan. One desktop printer needs about 9 to 23 CFM (16 to 40 m³/h) of exhaust to hold an enclosure at 40 °C in a 25 °C room. Common 4-inch inline fans are rated 165 to 226 CFM, so one fan covers a single printer at a low speed setting, carbon filter and duct included.

How much air one printer needs

The exhaust has to carry out the printer’s heat. nVent Hoffman, an enclosure maker, sizes cooling fans with the formula below. The enclosure fan calculator runs the same math in metric:

CFM = 3.16 x watts / temperature rise (°F)

Bambu Lab P1S printing PLA: 105 W average
rise = 40 °C limit - 25 °C room = 15 K = 27 °F
3.16 x 105 / 27 = 12.3 CFM (20.9 m³/h)

Use the printer’s average draw while printing, not its peak rating. Bambu Lab publishes average figures for its printers:

Printer and filament	Average power	Airflow, 40 °C limit, 25 °C room
Bambu Lab A1 mini, PLA	80 W	9.4 CFM (16.0 m³/h)
Bambu Lab P1S, PLA	105 W	12.3 CFM (20.9 m³/h)
Bambu Lab P1S, ABS	140 W	16.4 CFM (27.9 m³/h)
Bambu Lab H2D, PLA	197 W	23.1 CFM (39.2 m³/h)
Bambu Lab A1, ABS	200 W	23.4 CFM (39.8 m³/h)

The 40 °C limit protects PLA: Bambu Lab’s X1E guide puts the softening point of most PLA at about 45 °C. The room matters as much as the limit. In a 30 °C (86 °F) room the allowed rise drops to 10 K (18 °F), and the P1S on PLA needs 18.4 CFM (31.3 m³/h). ABS goes the other way. For warp-prone ABS prints Bambu Lab suggests a 60 °C chamber, and with that limit the P1S needs only 7.0 CFM (11.9 m³/h).

Why the fan is rated so far above that

An inline fan’s CFM rating is free air, the flow with nothing in front of the fan. A filter and a duct resist the flow, and the fan delivers only where its pressure curve meets that resistance.

Oriental Motor, a fan maker, gives a rule for when the resistance is hard to pin down. Assume about half the rated flow, and “select a fan that can produce double the required air flow.” For the P1S on PLA that means a fan rated 25 CFM. The smallest 4-inch fan in the table below is rated 165 CFM.

The margin is static pressure: the push a fan keeps against resistance, given in pascals (Pa) or inches of water gauge (in. w.g.). Can-Filters, a carbon filter maker, says a filter at its maximum rated flow drops about 0.7 in. w.g. (174 Pa). Oriental Motor adds that a fan never reaches its maximum airflow and its maximum static pressure at the same time.

That pressure is the reason to buy a duct fan rather than a PC fan. ARCTIC rates its 120 mm P12 PWM case fan at 56.3 CFM (95.7 m³/h) free air and 2.20 mm of water (22 Pa) static pressure. AC Infinity rates its 4-inch CLOUDLINE S4 at 223 Pa, ten times as much. A PC fan’s whole 22 Pa has to push through the filter and the full duct run.

4-inch or 6-inch

A 6-inch duct has 2.25 times the area of a 4-inch one: 28.3 in² (182 cm²) against 12.6 in² (81 cm²). Fan ratings scale with the duct. AC Infinity’s 6-inch CLOUDLINE S6 is rated 425 CFM and 503 Pa at 70 W maximum draw. Its 4-inch S4 is rated 226 CFM and 223 Pa at 28 W.

For one printer a 6-inch fan is oversized even on its lowest setting. Can-Filters rates its 6-inch Max-Fan at 201 CFM on low, 16 times what the P1S needs. A 6-inch fan earns its place when several printers share one exhaust. This table assumes P1S printers on PLA (105 W each), a 40 °C limit, a 25 °C room and the doubled rating Oriental Motor suggests:

Printers on one exhaust	Heat	Airflow needed	Free-air rating to look for	Duct
1	105 W	12.3 CFM (20.9 m³/h)	25 CFM	4-inch
4	420 W	49.2 CFM (83.6 m³/h)	98 CFM	4-inch
8	840 W	98.3 CFM (167 m³/h)	197 CFM	4-inch rated 226 CFM, or 6-inch
12	1,260 W	147.5 CFM (251 m³/h)	295 CFM	6-inch

Published specs of 4-inch and 6-inch inline fans

All figures are the makers’ own. Airflow is free air, at zero static pressure, and noise is in A-weighted decibels (dBA).

Fan	Duct	Rated airflow	Static pressure	Power, max / average	Noise	Speed control
AC Infinity CLOUDLINE A4	4 in (102 mm)	165 CFM (280 m³/h)	324 Pa (1.30 in. w.g.)	35 W / 18 W	29 dBA	10 speeds
VIVOSUN AeroZesh G4	4 in (102 mm)	195 CFM (331 m³/h)	Not published	20 W	39 dBA	10 speeds
AC Infinity CLOUDLINE S4	4 in (102 mm)	226 CFM (384 m³/h)	223 Pa (0.90 in. w.g.)	28 W / 21 W	26 dBA	10 speeds
AC Infinity CLOUDLINE T4	4 in (102 mm)	226 CFM (384 m³/h)	223 Pa (0.90 in. w.g.)	28 W / 21 W	26 dBA	Temperature and humidity triggers
Can-Fan Max-Fan 6”	6 in (152 mm)	334 CFM (567 m³/h) high, 201 CFM (341 m³/h) low	Not published	75 W high, 53 W low	Not published	High, medium, low
AC Infinity CLOUDLINE S6	6 in (152 mm)	425 CFM (722 m³/h)	503 Pa (2.02 in. w.g.)	70 W / 38 W	30 dBA	10 speeds

How to set up an inline exhaust fan on a printer enclosure

Mount the fan as an exhaust at the top of the enclosure and duct it outdoors. The makers’ manuals fill in the details:

• Exhaust high, intake low. AC Infinity’s CLOUDLINE manual puts the exhaust fan at the highest corner and the intake, a second fan or just an opening, at the opposite bottom corner. Point the fan’s airflow arrow away from the enclosure.
• Pull air out rather than blow it in. An exhaust fan keeps the enclosure below room pressure, so gaps pull room air inward. Bambu Lab warns that an X1E filtration fan turned below its default speed in a hot chamber can lose that negative pressure. Some gases may then “escape through gaps in the chamber.”
• Put the filter on either side of the fan. Can-Filters says you can “pull through the filter or push air through the filter.” AC Infinity’s tent manual says either order works while air is pulled out, and its typical setup puts the fan and filter inside the tent.
• Vent outdoors. For schools, Washington State’s Department of Health prefers an enclosure that “captures emissions and exhausts them directly outside of the building.” It says HEPA (high-efficiency particulate air) and charcoal filters help but “are not sufficient under the code.”
• Keep the duct short and straight. The vent hose guide covers diameter, length and bends. Can-Filters puts the loss over 25 ft (7.6 m) of duct at about 3% for straight rigid duct and 7% for flex duct, plus 1% to 4% for every 90° bend.

Matching a carbon filter to the fan

Can-Filters’ advice is to match the fan to the filter. That means a 4-inch flange on a 4-inch fan, and a rated flow above what you will run. AC Infinity rates its 4-inch carbon filter, with a 38 mm carbon bed, at 210 CFM (357 m³/h). Can-Filters rates its 4-inch Can-Lite, with a 2-inch bed, at 250 CFM (425 m³/h).

The 0.7 in. w.g. drop applies at the filter’s maximum rating. A single printer runs a 4-inch filter at a tenth of that rating or less. Resistance falls faster than airflow, and Oriental Motor models it as the square of the flow. The 4-inch fans that publish a static pressure rate it at 223 to 324 Pa, above the filter’s full-flow 174 Pa.

Setting the speed

AC Infinity and VIVOSUN list only the maximum CFM for their 10-speed fans. Asked for the rest, AC Infinity’s manual points back to that listing. Set the speed by temperature instead:

1. Put a thermometer at the top of the enclosure, where the warm air collects.
2. Start a long print with the fan on its lowest setting.
3. Step the speed up until the reading holds under 40 °C (104 °F) for PLA.

A temperature-triggered controller automates those steps. AC Infinity’s T-series controller raises the fan one level each time the temperature climbs another threshold step. With a 90 °F (32 °C) trigger, 2 °F steps and level 6 as the top, the fan starts at level 1 at 90 °F and reaches level 6 at 100 °F (38 °C).

What inline fan does a 3D printer tent need?

A 4-inch inline fan on a speed controller. A 105 W printer in a 2x2x4 ft grow tent (16 ft³) needs 12 CFM to stay at 40 °C in a 25 °C room. Changing a 3x3x6 ft tent’s 54 ft³ of air every minute takes 54 CFM. A 4-inch fan rated 165 to 226 CFM covers both.

A grow tent used as a printer enclosure, a vent tent, sizes like any other box: by the printer’s heat or by air changes per hour. Both AC Infinity tents listed below have 4-inch duct ports.

CFM = volume (ft³) x air changes per hour / 60

3 x 3 x 6 ft tent = 54 ft³
one change a minute: 54 x 60 / 60 = 54 CFM

Tent, nominal	Volume	20 changes an hour	60 changes an hour	One P1S on PLA, by heat
2 x 2 x 4 ft	16 ft³ (0.45 m³)	5.3 CFM (9.0 m³/h)	16 CFM (27.2 m³/h)	12.3 CFM, 46 changes an hour
3 x 3 x 6 ft	54 ft³ (1.53 m³)	18 CFM (30.6 m³/h)	54 CFM (91.7 m³/h)	12.3 CFM, 14 changes an hour

The 20-an-hour column is Can-Filters’ grow-room minimum: the volume in ft³ divided by 3 gives the CFM, a full change every three minutes. No published guideline sets an air-change rate for a printer, so the heat column is the floor. It stays at 12.3 CFM in both sizes, because the printer’s watts don’t change. The largest figure, 54 CFM, is a third of the 165 CFM rating on the smallest fan in the table above.

Nominal sizes run a little large. AC Infinity sells its two tents as 24 x 24 x 48 in and 36 x 36 x 72 in, and lists these measured sizes:

Tent	Measured size	Volume	Duct ports
AC Infinity CLOUDLAB 422 (2 x 2 x 4 ft)	60 x 60 x 120 cm	15.3 ft³ (0.43 m³)	four 4-inch, three 6-inch
AC Infinity CLOUDLAB 733 (3 x 3 x 6 ft)	90 x 90 x 180 cm	51.5 ft³ (1.46 m³)	four 4-inch, two 6-inch, three 8-inch

Setting up the tent exhaust

Hang the fan and filter from the roof bars and exhaust from the top. AC Infinity’s tent manual says the extraction fan and the carbon filter both perform better at the highest point, since heat rises. Open one of the low vents or a passive intake screen for make-up air. Run the duct out a window and set the speed by temperature.

Check the tent fabric

Creality, a printer maker, says an enclosure “should be made of metal or, if not, a fireproof material.” AC Infinity’s CLOUDLAB pages list 2000D Oxford canvas with a Mylar lining and give no temperature or fire rating. Before a printer goes in, ask the tent maker for the fabric’s rating.