Chips and oil mist ride straight into an air-to-air heat exchanger unless you stop them upstream, so fit a washable media filter on a remote intake plenum. Size the filter for twice the fan's rated airflow, set the plenum 18 in [450 mm] off the floor, and duct it to the exchanger's ambient intake without touching the cabinet wall. The exchanger keeps its sealed loop sealed, the enclosure holds the rating it left the shop with, and the core stays clean for years instead of months. The worked example below uses a 130 CFM [221 m³/h] ambient fan, the size most small cabinet exchangers move.
Why does an enclosure heat exchanger need filtered air?
Fit a washable media filter on a plenum ahead of the exchanger's ambient intake, sized for at least twice the fan's rated airflow and set 18 in [450 mm] off the floor, so chips and oil mist load the filter instead of the core.
An air-to-air exchanger pulls room air through a finned or plate core and blows it straight back out; the cabinet side is a separate, closed loop that recirculates the air inside the box. Plate-type units for control cabinets commonly run from a 48 V DC supply, and their fans move roughly 65-250 CFM [110-425 m³/h] on the ambient side. Whatever the room holds, that side breathes it in: steel fines from a mill, wood dust from a saw, coolant mist from a machining cell. The load packs the fins, chokes the fan and quietly cuts cooling capacity.
A filter turns that problem into a sink rinse. Dirt loads a media pad you can wash in a minute, not a core that takes an afternoon to pull. When a core is already choked, the heat exchanger cleaning walkthrough shows what the teardown takes. When you are still choosing between an exchanger, a filter fan and an air conditioner, the cooling comparison lays out where each one wins.
Does filtering the intake cost the cabinet its NEMA 4X rating?
The rating survives when the filter sits on the ambient side of the core: an air-to-air exchanger keeps the cabinet-side airstream sealed inside its own loop, so the wall stays unbroken and the box holds the NEMA 4X / IP66 rating the manufacturer gave it.
The core separates two airstreams: fins carry the heat across, and the air itself never crosses. The filter and the plenum live entirely on the room side of that wall, so the sealed envelope of the enclosure stays untouched. Our stainless boxes are rated NEMA 4X / IP66 by the manufacturer; not UL listed. The exchanger is the same story: its Type 4X marking is the manufacturer's rating for hose-directed water plus corrosion resistance, the combination NEMA 250 defines as Type 4X. The NEMA 4 vs 4X vs IP66 guide explains what those hose tests look like, and Type 4X heat exchanger requirements for drive panels covers the exchanger side of the same rule.
The plenum adds furniture around an opening the exchanger already has, so it adds no hole to the cabinet. An assembly carries the rating of its lowest-rated opening, and here that opening is the exchanger's own ambient side, built for the job. Bolt the plenum to the machine frame and the wall of the box you paid for keeps zero holes.
How big should the filter and plenum be?
Size the filter for at least twice the exchanger fan's rated CFM, which holds face velocity under about 250 fpm [1.3 m/s]; for a 130 CFM [221 m³/h] fan, a common 16 x 20 x 1 in washable furnace filter covers it four times over.
Worked example, 130 CFM ambient fan:
- Minimum rated airflow: 2 x 130 CFM = 260 CFM [442 m³/h].
- Minimum open media: 130 CFM / 250 fpm [1.3 m/s] ceiling = 0.52 ft² = 75 in² [484 cm²].
- Candidate filter: a 16 x 20 x 1 in washable MERV 8 furnace filter; 16 x 20 = 320 in² of face = 2.22 ft².
- Face velocity in service: 130 CFM / 2.22 ft² = 59 fpm [0.30 m/s], about a quarter of the ceiling.
- Clean pressure drop: residential 1-in MERV 8 filter data shows about 0.2 in wg [50 Pa] at 300 fpm [1.5 m/s]; at 59 fpm the pad reads roughly a tenth of that, 0.02-0.05 in wg [5-12 Pa].
The pad passes with four times the velocity margin and a drop the fan cannot feel. Where your media's own datasheet differs, the datasheet wins.
| Ambient fan on the exchanger | Minimum open media at 250 fpm | Washable filter that covers it |
|---|---|---|
| 65 CFM [110 m³/h] | 38 in² | 12 x 16 in (192 in²) |
| 130 CFM [221 m³/h] | 75 in² | 16 x 20 in (320 in²) |
| 200 CFM [340 m³/h] | 115 in² | 16 x 25 in (400 in²) |
| 250 CFM [425 m³/h] | 144 in² | 20 x 25 in (500 in²) |
Stay with washable MERV 8-class media. Chips, fines and mist droplets are coarse particles, so a higher-MERV pad catches little extra here while loading faster. The air filter selection for dusty environments article carries the full media trade-offs.
How far off the floor should the filtered intake sit?
Set the filter face 18 in [450 mm] off the floor at minimum, high enough that sweeping chips and washdown splash never reach it, and go to 24 in [600 mm] where carts, pallet trucks or a flood-prone slab share the room.
The height is about the two things that live at floor level. Sweepings bounce: a broom flicks steel fines a foot or more off the slab, and washdown spray rebounds into the bottom 12 in [300 mm] of anything standing at floor level. At 18 in [450 mm] the filter face clears both, and a squeegee or broom passes underneath. On the 130 CFM example the plenum hangs on the machine frame at exactly that height.
Two things move the number upward. Cart or pallet traffic wants 24 in [600 mm], so a bumper never finds the intake. A slab that takes standing water wants the opening faced downward, with the drain carrying water past the filter rather than into it. Keep the opening on the side away from the cut too: a mill throws chips and coolant meters sideways, and the plenum nearest the spindle breathes that throw.
How do you build and mount the filtered intake plenum?
Build a shallow sheet-metal or polycarbonate plenum that seats the filter on two ledges, mount it to the machine frame or wall brackets at 18 in [450 mm] or higher, and duct it to the exchanger's ambient intake with the shortest run you can route.
- Read the nameplate. Take the ambient-side CFM and the intake opening dimensions off the exchanger's datasheet.
- Pick the pad. Choose the filter from the table above and buy two: one in service, one dry spare.
- Lay out the shell. Make the interior 1 in [25 mm] larger than the filter all round and 3-4 in [75-100 mm] deep. That depth lets flow spread over the whole face instead of piping through the centre, the same plenum logic behind sizing a plenum for uniform airflow.
- Seat and retain the media. Form two ledges the pad rests on and fit a retainer bar or screw-kept strap, so a loose pad cannot fall out in service.
- Cut the intake low and aim it away. Put the opening in the underside or lower face, pointed away from the machine's chip throw, and fit a down-facing hood outdoors.
- Mount on the frame. Bolt the plenum to the machine frame or wall brackets at the 18 in [450 mm] height from the section above, and leave the cabinet wall untouched.
- Duct it short. Run the shortest transition you can to the exchanger's ambient intake, clamp every joint, and tape seams with weatherproof foil tape in a washdown cell.
- Drain and label. Drill a 6 mm [1/4 in] drain at the plenum's lowest corner, below the filter, and mark the shell with the clean interval and the twice-clean-drop trigger.
The shell is a good small job for a custom box. A shallow custom stainless enclosure with the cutouts and fixing holes you mark arrives cut your way, and the 3D enclosure builder lays out the size and the holes before you order. Build the ledges and the retainer from offcut angle; they carry nothing but a pad that weighs a few hundred grams.
Which checks prove the filter and the seal are working?
Prove the job with three numbers: plenum airflow within 10 % of the fan's nameplate CFM, a clean-filter pressure drop under 0.1 in wg [25 Pa], and the hottest spot inside the cabinet at or under 40 C [104 F], the ambient most drives tolerate.
- Airflow. A vane anemometer traverse at the plenum intake should read nameplate CFM ±10 %; on the example that is 117-143 CFM [199-243 m³/h]. The replacement fan airflow check shows the traverse method.
- Pressure drop. A manometer across the filter reads 0.02-0.05 in wg [5-12 Pa] clean on the example pad. Write the number on the shell; it becomes the trigger in the servicing schedule.
- Temperature. Run the machine at full load for an hour and read the air near the hottest component. At or under 40 C [104 F] the cooling is keeping up; most drives and electronics carry that ambient rating unless the datasheet says more.
- Water. In a washdown cell, hose the cabinet top and the plenum the way you wash the floor. The interior stays dry and the plenum drain runs clear.
- Dust, two weeks on. Open the door after a fortnight of cutting: a film inside means air is bypassing the seat, so add soft foam tape to both ledges and re-clamp the retainer.
Which mistakes cost you the rating or choke the airflow?
The two expensive mistakes are piercing the cabinet wall to mount the plenum, which unseals the enclosure, and fitting media too small for the fan, which starves the exchanger; the 2x sizing rule and a bracket that stays off the box prevent both.
- A filter fan instead. A wall-mounted filter fan opens the box, and the assembly then carries the fan unit's own rating rather than the exchanger's sealed-loop arrangement. The filter fan selection guide explains when that trade makes sense.
- Media too small. Double the designed face velocity and the pressure drop climbs steeply, the fan gives back airflow, and the cabinet runs hot. The choking arithmetic in the filter intake without choking the fan walkthrough applies word for word to this plenum.
- Bypass at the seat. Air takes the gap round an unseated pad and carries the dirt with it. Foam tape on both ledges and a firm retainer close it.
- Intake in the chip throw. Keep the opening on the side away from the cut and hold the 18 in [450 mm] height.
- A core already loaded. Wash the core before you trust the new filter. A clean pad upstream of a choked core just hides capacity you no longer have.
- A wet pad back in service. Dry the rinsed pad fully or run the dry spare. A soggy pad adds drop and can shed fibers into the core.
When should you clean or replace the filter media?
Rinse a washable filter monthly in a chip-and-mist shop and quarterly in dry dust, and pull it early whenever a gauge or a temperature trend shows the pressure drop at twice its clean reading, about 0.1 in wg [25 Pa] on the 16 x 20 in example.
Washable media comes back with a low-pressure rinse and a full dry. Keep the spare in rotation so the panel never runs with the pad out. Where the room runs dry dust only, stretch to a quarterly rinse and let your gauge call the early pulls; the filter clogging monitoring walkthrough shows how to trend the drop before heat builds. Retire the pad when a rinse stops restoring the clean reading, or when the media tears or hardens.
The decision in one line: monthly rinse where mist lives, quarterly where dust is dry, and the twice-clean-drop reading overrides both.
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
Can a filter pad go straight on the exchanger intake without a plenum?
Yes, when the intake has a flat flange and the pad still keeps face velocity under about 250 fpm [1.3 m/s]. A 130 CFM [221 m³/h] fan needs at least 75 in² of open media, so a 12 x 16 in pad clears it with margin. Hold the pad on a frame with foam round the edge; a pad taped bare over the opening sags and lets air bypass.
What pressure drop says the filter is loaded?
Write the clean reading on the shell first: a 16 x 20 in washable pad at 59 fpm [0.30 m/s] reads about 0.05 in wg [12 Pa] clean. Clean or swap the media when the gauge shows twice that, about 0.1 in wg [25 Pa]. Past it, the fan gives back airflow on its curve and the cabinet temperature climbs before any dirt is visible.
Will a filter recover a core that is already clogged?
No, a filter only stops new dirt. Wash or replace the core first, then prove the ambient fan moves its nameplate airflow again; on the 130 CFM [221 m³/h] example that means 117-143 CFM at the plenum intake. If the washed core still strangles the flow, the capacity printed on the nameplate is no longer the capacity you have.
Does the plenum itself need a drain hole?
Yes outdoors, and anywhere hose water can reach it. Drill a 6 mm [1/4 in] hole at the plenum's lowest corner, below the filter, so water that blows in leaves instead of soaking the media. The plenum sits outside the cabinet's sealed envelope, so a drain there costs the enclosure's rating nothing.
