Blow the air in through a filter and let it out through a matching one. The cabinet then sits 5-10 Pa above ambient, and every leak carries dusty air out instead of in. That is the whole trick behind a positive-pressure filtered enclosure on a dusty CNC router or machining cell. A cabinet losing 115 W needs about 20 CFM [34 m³/h] of net airflow at a 10 °C rise, which lands on a 50 CFM matched set once you allow for filter back-pressure. Know the trade before you cut the hole: while the filters pass air, the assembly is a ventilated IP54 enclosure, and a washdown-rated 4X box gives that rating up.
What blowing in does that pulling out cannot
A fan that pushes filtered air into the box raises the pressure inside. Air then leaves through the exhaust filter, along the door seam and past every gland, and all of it flows outward. Dust in a machining cell cannot swim against that current, so the leaks you cannot seal stop mattering.
Turn the fan round and the box runs under vacuum. Now every gap inhales, and shop air loaded with chips, coolant mist and fine carbide dust heads straight for the drives and supplies. A pull-out fan's filter cleans only the stream reaching the fan itself; the box takes whatever arrives through the seams. That asymmetry is why filter-fan kits ship as an intake fan with a passive exhaust filter on the opposite wall. The which side the fan belongs on question gets its own walkthrough, and the wider strategy sits in keeping a control cabinet clean in a dusty machining cell.
The pressure band to hold, and how to read it
Design for 5-10 Pa above ambient while the fan runs, which reads as 0.02-0.04 inH2O on a low-range manometer. That band is what a matched filter-fan set holds on a gasketed cabinet: enough to keep every leak flowing outward, not enough to lift the door gasket off its seat. The gauge pressure inside is simply what the outgoing air pays to cross the exhaust filter, so the reading tracks airflow and filter condition:
- Sagging from 5 Pa toward 2-3 Pa: the intake media is loading and the fan is starving. Clean or change the intake filter.
- Climbing past 10 Pa: the exhaust media is blocking. Service that side.
- Sitting at zero: the fan has stopped, a door is open, or an open cutout is short-circuiting the box.
Match the exhaust element to the intake kit and the band lands where it belongs. A much larger exhaust drops it toward 2-3 Pa; a much smaller one pushes it past 10 Pa. These are pascals, thousandths of a psi, so an ordinary HVAC manometer covers the job. Reading the gauge takes five minutes, and watching enclosure air filters for clogging before heat builds turns it into early warning rather than a post-mortem.
Size the set from the heat, not from the box
Air carries roughly 0.57 W of heat per CFM for each °C of rise, based on air at 1.2 kg/m³ and a specific heat of 1.0 kJ/kg·K. That gives the standard enclosure-fan sizing formula: CFM = 1.76 × watts ÷ °C of rise, the same figure as 3.17 × watts ÷ °F of rise. Drives and most industrial electronics are rated for 40 °C (104 °F) ambient unless the datasheet allows more. A shop that touches 30 °C in summer therefore leaves you a 10 °C rise to design around. The airflow calculation for fans near ambient temperature works the same math step by step.
Worked example, a bench-top CNC router cabinet. The 2.2 kW spindle VFD loses about 3% of its rating at load, call it 70 W. The 24 V DC 10 A supply, 240 W at its terminals, loses about 40 W running at 85% efficiency. Controller and relays add 5 W. Total heat: 115 W. Net airflow: 1.76 × 115 ÷ 10 = 20.2 CFM [34 m³/h]. Through filter media a fan gives up a third to a half of its free-air flow, and dust loading cuts deeper over the months, so double the net figure and round up. The order lands on a 50 CFM matched set: one intake fan-filter plus a same-size passive exhaust filter. On this cabinet the set holds the 5-10 Pa band and the interior to 40 °C on a 30 °C day, and the 2× margin keeps it there as the media loads.
Scaled up, the same math is where the bigger matched sets live. Sets from 80 to 200 CFM carry cabinets losing roughly 230-570 W at a 10 °C rise:
| Cabinet heat loss | Net airflow at 10 °C rise | Matched set with the 2× margin |
|---|---|---|
| 50 W | 9 CFM [15 m³/h] | 50 CFM |
| 115 W | 20 CFM [34 m³/h] | 50 CFM |
| 250 W | 44 CFM [75 m³/h] | 100 CFM |
| 500 W | 88 CFM [150 m³/h] | 200 CFM |
Above that range a fan cannot close the gap, and the filter fan selection guide for panel cooling shows where fans stop working and what replaces them.
What the filters cost in rating
Every hole with a filter in it drops the assembly to the weakest part. Filter-fan kits carry the kit maker's rating, typically IP54. Under IEC 60529, digit 5 means dust-protected, not dust-tight, because air has to flow and some fines pass the media by design; digit 4 means splashing water, with no jets and no hose. A ventilated cabinet therefore builds to what NEMA 250 calls a Type 12 assembly: indoor use, protection against circulating dust, falling dirt and dripping non-corrosive liquids. That is the right rating for a dry, dusty shop, and the wrong one wherever a hose comes out.
With the fan off and the door shut, a gasketed box still resists drifting dust, but the assembly keeps its ventilated figure. NEC 110.28 says it plainly for US installations: an IP code does not stand in for an enclosure Type rating, so a washdown specification gets met by sealing, not by filters. Ratings in this piece are manufacturer ratings — a box from us is rated NEMA 4X / IP66 by the manufacturer; not UL listed. The NEMA enclosure types guide lays out the full type list with its IEC pairings.
Where the air gets out when the fan stops
The passive exhaust filter is the backstop for the unpowered hours. With no pressure in the box, daily temperature swings breathe air through whatever opening exists, and a filtered exhaust lets that breathing happen without carrying chips in. Filter the intake for the same reason; an unfiltered intake is a dust funnel the moment air moves.
Seal the fixed leaks so the flow goes where the design wants it:
- Cable entries: fit a gland matched to each cable's outside diameter and plug every unused cutout. An open field-drilled hole vents the pressurization where it stands, and the cutouts, glands and window fittings guide covers the hardware.
- Door: keep the gasket intact and the latch compressing it evenly. A 2 mm slit under a 500 mm door edge is 10 cm² of leak, which still flows outward; a door standing open erases the band instantly.
- Thermostat: a fan that only runs above its setpoint leaves the box at zero pressure the rest of the day. In fine, aggressive dust, run the fan continuously. On a mild floor, thermostat control for an enclosure fan above setpoint around 30-35 °C buys long filter life and holds pressure through the working shift.
Service the filters with the power off and verify both fans have stopped before reaching in; blades and live terminals share the same small space. Keep a spare media set on the shelf, because the intake side does the work and loads first.
Two fans move the same air as one
A second fan in the airstream, one blowing in and one pulling out, looks like double the airflow. It is not: fans in series move the same air, their flows do not add, and the pull side cancels the overpressure the design depends on. Staged ventilation fans in ducts come to the same figure, and what fans in series actually deliver shows the arithmetic from a real build. When the sizing math calls for more net flow, go one set size up. A 250 W cabinet with 44 CFM of net demand takes a single 100 CFM set, not a pair of 50s.
The decision by environment, and what to order
- Dry, dusty shop — CNC router, machining cell, woodworking: build ventilated. Matched filter-fan set sized from the table, passive exhaust filter as the at-rest backstop, manometer check at commissioning, spare media on the shelf. This is the answer that keeps dust off the drives for the least hardware.
- Washdown, rain or hose-down anywhere in the specification: keep the enclosure sealed at NEMA 4X / IP66 and leave the filters out. Cool it closed: wall conduction with internal circulation, an air-to-air heat exchanger, or an AC unit. A sealed enclosure with internal fans traces where that heat goes when nothing flows through the walls.
- Conductive fines — graphite, metal powder, carbon fiber: filtered air can still carry them onto live parts. Seal the box here too and cool it closed-loop.
- Flammable vapor or gas in the air: filtered fans are the wrong tool. What the job needs instead is a purged and pressurized system to NFPA 496, with a pressure controller, purge timing and alarms, built on equipment certified for that atmosphere; it is a different purchase, and we do not build explosion-rated boxes. The positive pressure requirement walkthrough covers what it involves.
Order the cabinet with the fan and exhaust cutouts factory-cut to the kit template and the gasket lands left clean. Our custom stainless boxes take your cutout layout and are delivered in 10 business days, counted from your approval of the drawing for custom work. If the box itself is still an open question, the enclosure selector narrows the field by environment, listing and size, then hands off a shortlist or a quote request.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 60 × 36 × 16 in | 304 stainless | 60 x 36 x 16 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How much positive pressure should a dust enclosure hold?
Design for 5-10 Pa above ambient (0.02-0.04 inH2O) while the fan runs; that is enough to make every leak flow outward without lifting the door gasket off its seat. Read it with a low-range manometer at the exhaust filter. A reading sagging toward 2-3 Pa points at a loaded intake filter; a climbing one points at the exhaust media blocking. Formal purged systems for flammable atmospheres are a different design and hold their own set pressure under NFPA 496.
Should the enclosure fan blow in or pull out in dust?
Blow in, with the intake fan low on one wall and the passive exhaust filter high on the opposite one. Pushing filtered air in raises cabinet pressure, so every seam leaks outward. A pull-out fan puts the box under vacuum and draws dust in through those seams, and its filter only cleans the stream reaching the fan. The low-in, high-out placement also sweeps warm air off the top of the cabinet.
Can I keep a NEMA 4X rating with filter fans installed?
No. While air flows, the assembly takes the filter kit's rating, and a ventilated box cannot be dust-tight or jet-resistant at the same time. A washdown 4X/IP66 specification and filtered fans are alternatives, so pick by the water in the room: hose-down means a sealed build with closed-loop cooling, dry dust means the ventilated design. The kit makers rate their own filters; the enclosure maker's 4X figure stops describing the assembly.
What airflow does a filter fan need for a 115 W cabinet?
About 20 CFM net at a 10 °C allowed rise: 1.76 × 115 ÷ 10 = 20.2 CFM, using air's 0.57 W carried per CFM per °C. Order about twice that for filter back-pressure and later dust loading, so a 50 CFM matched set: intake fan-filter plus same-size exhaust filter. A 500 W cabinet needs 88 CFM net and a 200 CFM set at the same rise.
Does a positive pressure enclosure also keep water out?
Splashing only. The kit's IP code ends in 4, which IEC 60529 tests as splashing water from any direction, so splashes, drips and drizzle are handled and a washdown hose is not. Pressurized water walks the same path dust would like to, which is why hose-down specs and filtered fans stay separate decisions.
