Short answer: put the pair in a sealed, one-piece stainless box rated NEMA 4X / IP66 by the manufacturer, sized around a 520 W heat load, and cool it with a closed-loop air-to-air heat exchanger — a 50 W/K unit holds 40 C inside at a 30 C ambient. Leave the fan-and-filter kit out: it drops the whole assembly to about IP54 and hands the ash a road in. If fines have beaten gaskets at your site before, add a regulated clean-air purge holding 0.1 in w.c. [25 Pa] behind a relief valve. Order the body custom, because a 20 in [508 mm] deep rack PC outgrows the 16 in [406 mm] deepest standard body, and custom work is delivered in 10 business days counted from your approval of the drawing.
What ash dust asks of the rating
Ash runs finer than most shop dust, and it turns mildly alkaline once it picks up moisture, so the box needs two things: a dust-tight seal and a material that shrugs off a damp deposit. On an IP66 box the first digit 6 means dust-tight and the second 6 means powerful jets — IEC 60529 tests that with a 12.5 mm nozzle delivering 100 L/min. NEMA 4X covers the same hose-directed water and adds corrosion resistance. Type 12 stops dry indoor dust and nothing else; the first time someone hoses that room, a Type 12 door seam leaks. Choose 4X for an ash room even indoors.
Two points shape the paperwork. These enclosure ratings are manufacturer ratings — rated NEMA 4X / IP66 by the manufacturer, not UL listed. And NEC 110.28 states that an IP code is not a substitute for an enclosure type rating, so the order should carry both marks. The NEMA 4 vs 4X vs IP66 guide and the control cabinet in a dusty machining cell article cover the same ground for readers weighing the rating.
304 or 316L
Keep 304 where the dust is inert and the room stays dry — packaging lines and plastic fines. Wood, biomass and coal ash carry sulfates and chlorides, and damp ash sits in door seams as an alkaline deposit, which is exactly the crevice attack 304 is weakest against. Order 316L whenever ash can get wet or the area gets hosed down; its molybdenum lifts pitting and crevice resistance. The 4X type itself is partly a material question: NEMA 250 / UL 50E give Type 4X an extra 200 hours of salt spray tested against a 304 stainless specimen. On a custom order, stepping from 304 to 316L is a small line next to the cost of replacing two rack PCs.
Count the heat before you pick a cooler
One number here gets mistaken for heat: 750 VA is UPS capacity, not dissipation. A line-interactive 750 VA unit at half load rejects about 60 W — 92-94 % conversion efficiency plus float charging — while an online unit at full load can reject 150 W. For the PCs, take the heat-dissipation line from the datasheet; 150-250 W is the typical spread for an industrial 19 in rack PC, so use 200 W when the sheet is not in front of you. The pair, on those numbers:
- Two rack PCs: 2 x 200 W = 400 W
- Two 750 VA UPS units, line-interactive: 2 x 60 W = 120 W
- Total to shed: 520 W
Hold the inside at 40 C, the safe ambient for drives and most electronics unless your datasheet allows more, and take 30 C as the plant air near the cabinet.
Size the box, then the exchanger
Stack the gear first. Two 4U PCs (7 in [178 mm] each) and two 2U UPS units (3.5 in [89 mm] each) come to 21 in [533 mm] of rack height. A rack PC is 19 in [483 mm] wide and about 20 in [508 mm] deep, and connectors plus cable bend want 2 in [50 mm] behind it — that rules out the standard stainless line, so the build goes through custom enclosures. Order 36 x 24 x 24 in [914 x 610 x 610 mm] (H x W x D) and you keep a foot of height for wiring and growth.
The walls do part of the cooling. A 36 x 24 x 24 in body has about 3.0 m² of surface, and stainless sheds heat at 3.7 W/m²·K, so the box itself is an 11 W/K path. With 40 C inside and 30 C outside, the working difference is 10 K:
- Walls carry 11 W/K x 10 K = 110 W
- Exchanger carries the rest: (520 - 110) / 10 = 41 W/K
- Order a 50 W/K unit and keep a 20 % margin
Ambient moves this hard. At 35 C the difference is only 5 K, the walls carry 55 W, and the exchanger needs (520 - 55) / 5 = 93 W/K — order 100 W/K, or step to a sealed air conditioner around 700 W [2400 BTU/h]. Every 5 C of extra ambient roughly halves the cooling a given exchanger delivers. The enclosure sizing guide runs the same arithmetic for other shapes.
Cool it closed-loop or the dust wins
An assembly carries the rating of its weakest part. A fan-filter package is rated about IP54, so your 4X box becomes an IP54 box the day the kit goes on, and the fan then feeds ash onto a mat that fine dust packs solid within weeks — monitoring enclosure air filters for clogging becomes a weekly chore. That trade is spelled out in putting a fan on a NEMA 4X enclosure and enclosure air filter selection for dusty environments.
| Cooling choice | Effect on the assembly rating | Use it when |
|---|---|---|
| Fan-filter kit | Drops to about IP54 | Clean, dry rooms only |
| Air-to-air heat exchanger | Keeps NEMA 4X / IP66 when the unit is 4X-rated | Ambient at or below 35 C, at least 5 K of difference |
| Sealed enclosure air conditioner | Keeps the rating, both circuits closed | Ambient 35 C and up, or heavy airborne load |
| Vortex cooler | Keeps the rating, runs on compressed air | Instrument air already reaches the cabinet |
An air-to-air heat exchanger keeps two loops apart: the inside loop recirculates past a core, the outside loop blows plant air through the other side, and the streams stay separate. The external side still breathes your room, so fit its washable pre-filter and wipe the core monthly. Order the unit itself rated NEMA 4X / IP66 so the door cutout keeps the assembly's rating, and have us cut that hole on the drawing rather than field-cutting it — a factory cutout in a one-piece body is why the box holds its rating. Where the comparison gets closer, enclosure cooling: fan vs air conditioner vs heat exchanger and closed-loop cooling where fumes are in the air cover the same decision next to chemical loads. A vortex cooler suits a cabinet already fed with instrument air, but meter its air draw against your supply first.
Purge at 0.1 in w.c. when sealing alone has failed
Sealing handles most ash rooms. Fine dust still beats gaskets sometimes: in one documented plant, chalk fines passed both HEPA-filtered vents and the door gaskets of IP67 boxes until the cabinets were retrofitted with a clean-air purge regulated to about 0.1 in w.c. [25 Pa] — just enough that every seam leaks outward. Start there. Set the regulator on a 1/4 in instrument-air line and read the pressure at the door with a manometer; 0.05 in w.c. [12 Pa] held at the latched door means ash has no inward path, and the flow is small enough to add no heat.
Three details make the purge safe and dry. Fit a relief valve rated NEMA 4X so a stuck regulator cannot turn the box into a pressure vessel. Feed it dried instrument air — a -20 C [-4 F] dew point is ordinary plant supply and keeps the purge from feeding condensation — through a coalescing filter to keep oil off the electronics. And keep the supply below 40 C, because warmer air adds to the heat load you just sized the exchanger for. Once it runs, verifying a positive-pressure enclosure gives the acceptance checks and a purge losing pressure the causes to walk first.
Layout, mounting and cable entry
Bring the cables in low: a bottom gland plate with glands rated IP66 or better, each sized to its cable OD, so water and falling dust run away from the entries rather than onto them. Set 19 in rack angles or a shelf pair for the stack and figure about 50 kg [110 lb] of gear plus the box, which points to a floor stand rather than a wall arm. Leave 3 ft [900 mm] clear in front so the door swings a full 90 degrees — NEC 110.26(A) asks for that working space in front of equipment serviced while energized, and you will pull a PC eventually, so lock out the feed before the door stays open. The cutouts and glands guide lists the standard hole sizes for common gland bodies.
What to put on the order
| Line | What to specify | This build |
|---|---|---|
| Body | One-piece 316L stainless, H x W x D | 36 x 24 x in [914 x 610 x 610 mm] |
| Rating | Manufacturer rating, not UL listed | NEMA 4X / IP66 |
| Door | Full gasket, 90-degree swing, key latch, exchanger cutout | 50 W/K air-to-air unit, 4X-rated |
| Purge | 1/4 in air inlet, regulator, relief valve | Set 0.1 in w.c. [25 Pa] |
| Cable entry | Bottom gland plate | IP66 glands sized to cable OD |
| Interior | Rack angles or shelves, DIN rail, wiring tray | 50 kg [110 lb] equipment load |
| Delivery | Counted from drawing approval | 10 business days |
Decide by the room. Dry, inert dust at 30 C: a 304 body and the 50 W/K exchanger. Ash that can dampen, or a hose anywhere in the plant: 316L, with the purge added where fines have beaten gaskets before. Ambient at 35 C or above: step up to the 100 W/K exchanger or the 700 W air conditioner. Send the drawing with both PC outlines and the exchanger bolt pattern; the enclosure selector and the stainless size finder are the starting point when a standard size will do.
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 |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How much heat do two 750 VA UPS units really add?
About 120 W for the pair if they are line-interactive units at half load: roughly 60 W each, from 92-94 % conversion efficiency plus the battery float current. An online (double-conversion) pair at full load can reject up to 300 W. Take the heat-dissipation line from each datasheet when you have it, and allow extra for recharge current during the few hours after an outage.
Is 0.1 in w.c. the right purge pressure against ash?
Treat 0.1 in w.c. [25 Pa] as the starting setpoint and the number that worked in the field: a documented chalk-dust retrofit held about that pressure and stopped ingress that HEPA-filtered vents and IP67 gaskets had let through. What matters is outward flow at every seam, so 0.05 in w.c. [12 Pa] read at the latched door is working. Fit a relief valve so a stuck regulator cannot pressurize the box.
Why is a fan-and-filter kit wrong for this cabinet?
Because the assembly takes the rating of its weakest part, and a fan-filter package is rated about IP54 — your 4X box becomes an IP54 box the day the kit is fitted. The fan also pulls a steady stream of ash onto the mat, which packs it within weeks and chokes the airflow the PCs need. Closed-loop cooling keeps plant air out of the equipment chamber entirely.
Does an air-to-air heat exchanger also clog with ash?
Its external loop does: it moves room air through one side of the core, so fit the washable pre-filter and wipe the core on a monthly walk-through. The internal loop is sealed and stays clean, which is the point — dust lands on a core you can brush, not on PC boards and connectors. In rooms where even that is too much, use a sealed air conditioner instead.
How deep does the cabinet need to be for 19 in rack PCs?
Figure 20 in [508 mm] for the PC body plus about 2 in [50 mm] for connectors and cable bend, so 22-23 in [559-584 mm] of inside depth. The deepest standard stainless body runs 16 in [406 mm], so two rack PCs is a custom order; specify 24 in [610 mm] outside depth and let the drawing carry the exact rack depth.
