Dual Fan Cooling Box: How Airflow and Heat Load Really Work

• 1
  What a dual fan cooling box actually does
• 2
  Series or parallel: the one rule that decides your layout
• 3
  Estimating the heat load before you print
• 4
  Material and wall choices for a printed enclosure
• 5
  Where a printed dual fan box stops working
• 6
  Ducting, louvers and the recirculation trap
• 7
  Parallel vs series: pick by system impedance
• 8
  Printed enclosure materials compared

A dual fan cooling box is a printed enclosure that holds two fans and channels their output through a defined path. The fans do not cool anything by themselves. They move air, and the moving air carries heat away from a surface. If the air short-circuits back to the intake, you get noise and almost no cooling.

The open-source version of this box is popular because the geometry is simple. A rectangular shell, two fan cutouts, a louver or grille, and a lid. Anyone with a 260 × 260 mm printer bed can produce it. The hard part is not printing it. The hard part is predicting whether the finished box will remove the watts you need removed.

Air is a weak coolant by volume. It carries roughly 1.2 kg per cubic meter at room temperature, and its specific heat is about 1,005 J/kg·K. That means a small temperature rise in the exhaust is the only lever you have. To dump 100 W, you need either a lot of air or a large temperature delta.

So the design question becomes concrete. How many cubic feet per minute can the fans push through this restriction, and how much temperature rise will the exhaust show? Everything else, the color, the louver shape, the screw bosses, follows from those two numbers.

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  Fans move airHeat removal happens at the heat source surface, not inside the box.
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  Restriction sets flowGrilles, filters and bends cut the free-air rating, often by half.
• 3
  Exhaust temperature is your gaugeA 10 °C rise over ambient is a useful first-order check.

Two identical fans can be stacked in series or mounted side by side in parallel. The choice is not cosmetic, and it is the most common mistake in printed cooling boxes. Series doubles the static pressure at the same flow. Parallel doubles the flow at the same pressure.

If your box is open, with a short path and a large exit, pressure is not the limiting factor. Parallel wins. You get roughly twice the volumetric flow, so roughly twice the heat removal for the same temperature rise.

If your box pulls through a filter, a narrow duct, or a dense louver stack, the system curve is steep. Series wins. Each fan adds pressure, which pushes more air through a restriction that would otherwise choke a single fan.

Two fans in series with a wide-open inlet are a waste. You pay for a second fan, add noise, and gain almost nothing. Two fans in parallel behind a clogged filter are also a waste, because the restriction limits both of them. Match the layout to the impedance, not to the picture in your head.

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  Open path, low impedanceRun the fans in parallel.
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  Filter, duct or tight louversStack the fans in series.
• 3
  Mixed caseAdd pressure first, then add flow if the exhaust is still hot.

Start with the watts. A small server board may dissipate 25 W to 60 W. A bench power supply under load can reach 150 W. A 3D printer mainboard with stepper drivers is usually under 20 W. Write the number down before you open the slicer.

Then apply the airflow equation. Heat removed equals mass flow times specific heat times temperature rise. In practical units, roughly 1 CFM of air removes about 1.8 W for every 1 °C of temperature rise. So 30 CFM with a 10 °C rise handles about 540 W. That sounds generous until you subtract real losses.

Real losses are large. A printed grille with 40 percent open area can cut flow by a third. A 90-degree bend costs more. A dust filter can halve the flow within a week of use. Derate the free-air fan rating by 40 to 60 percent for a typical printed box.

Now compare. A 60 W load with 15 CFM of real flow needs only a 2.2 °C rise. That is easy. A 300 W load with the same 15 CFM needs an 11 °C rise, which pushes exhaust temperature well above what most electronics tolerate in a closed room.

If the numbers say you need a bigger fan than the box can hold, the box is the wrong answer. Move to a metal chassis with a larger fan, or split the load across two enclosures.

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  Write the watts firstNo airflow estimate is meaningful without a heat load number.
• 2
  Derate by 40-60 percentPrinted grilles, bends and filters all cost flow.
• 3
  Check the riseMore than 15 °C over ambient usually means undersized airflow.

The enclosure itself sits in the exhaust stream, so it sees warm air, not the heat source temperature. PLA softens near 60 °C. PETG holds to about 75 °C. ABS and ASA hold to about 95 °C. PC and PA are higher still.

For most electronics cooling, PETG is the sensible default. It prints easily, tolerates a warm exhaust, and resists the small knocks a bench enclosure takes. Use ABS or ASA if the exhaust regularly exceeds 60 °C.

Wall thickness matters less than stiffness. A 2.4 mm wall with three perimeters at a 0.4 mm nozzle is usually enough. Add ribs or a slight curvature to the large flat panels. Flat panels on a 200 mm span will drum and radiate noise.

Fan mounting is where prints fail. Fan screws bite into plastic threads that strip after two or three removals. Model through-holes and use brass heat-set inserts, or design a clamp ring with M4 screws and nuts.

Vibration isolation is worth the effort. A thin TPU gasket between the fan and the printed face cuts structure-borne noise noticeably. Hard-mounting a 3,000 RPM fan to a thin printed wall turns the whole box into a speaker.

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  PETG defaultGood to about 75 °C and easy to print.
• 2
  ABS or ASA for hot exhaustUse when exhaust regularly passes 60 °C.
• 3
  Heat-set insertsPrinted threads strip after a few fan swaps.

Printed cooling boxes work well for low to moderate heat loads, clean air, and a stationary bench setting. A 20 W to 80 W board in a room at 25 °C is a comfortable fit. So is a small power supply that only runs intermittently.

They work less well when the ambient is already hot. If the room sits at 40 °C, an exhaust at 52 °C gives you only 12 °C of headroom, and the components inside are running near their limits. Air cooling cannot beat ambient, no matter how many fans you add.

They fail when the air is dirty. Filters clog, and a clogged filter turns a parallel setup into a stalled one. If the environment has oil mist, metal dust or fibers, plan a cleanable metal filter and inspect it on a schedule.

They also fail when the part needs tight temperature control. A printed box with two fans gives you air movement, not a setpoint. If you need the surface held to ±2 °C, you need a controller, a sensor and a fan that accepts PWM or voltage control.

Finally, consider service access. A box that needs eight screws removed to reach the filter will not get serviced. Design a hinged or sliding lid, or at least a tool-less filter tray.

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  Air cooling cannot beat ambientA 40 °C room caps what any fan box can do.
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  Dirty air kills parallel layoutsOne clogged filter stalls both fans.
• 3
  No setpointAir movement is not temperature control.

The single biggest performance loss in a printed cooling box is recirculation. Warm exhaust leaves the box, curls around, and gets pulled back into the intake. The fans run, the noise is there, and the intake air is already warm.

Separate the intake and exhaust by at least the fan diameter, and ideally more. If the box must exhaust and intake on the same face, add a divider plate that extends outward at least 30 mm past the face.

Louvers look tidy but cost flow. A stack of angled slats with 40 percent open area behaves like a partial blockage. Straight-through honeycomb or a simple wire grille with 70 percent open area is better if you care about CFM more than appearance.

Duct length adds resistance roughly with the square of velocity. Keep runs short and keep the cross-section larger than the fan hub area. A duct that necks down to 60 percent of fan area will choke the flow and raise noise.

If you need directed cooling, put the nozzle at the heat source, not at the box exit. Blowing a narrow jet at a hot component is far more effective than blowing into the general volume.

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  Separate intake and exhaustAt least one fan diameter apart.
• 2
  Prefer open grilles70 percent open area beats decorative louvers.
• 3
  Aim the jetDirect air at the hot part, not at the room.