A sealed cabinet with fans inside and no intake is not a mistake. The fans do not push heat out; they stir the air so heat moves from the hot parts to the walls. The walls then conduct that heat to the outside air. Whether the arrangement works is arithmetic: wall area times the temperature rise you can tolerate, versus the watts the panel makes. This is how it adds up, and how to check a cabinet that already exists.
Our enclosures are rated NEMA 4X / IP66 by the manufacturer; not UL listed, so the sealed-fan approach fits them directly: cut nothing, add fans inside, let the box do the heat rejection.
What the fan actually does
A fan moves air, not heat. In a sealed enclosure the air can only carry heat from one surface to another inside the box: from a drive's heatsink to the walls, from a power supply to the mounting panel, from the top of the cabinet to the bottom. Nothing leaves the enclosure except through the walls by conduction, then by natural convection and radiation on the outside.
That still helps enormously. In still air, a hot component builds a blanket of warm air around itself, and its junction temperature climbs far above the enclosure's average. A fan strips that blanket off. Component temperatures drop toward the mixed-air temperature even though the total heat leaving the box has not changed one watt.
So the internal fan does two jobs:
- It destratifies the cabinet, so the top is not 15 C warmer than the bottom.
- It scrubs the walls with warm air, raising the inside surface heat transfer so the walls run closer to the bulk air temperature.
What it cannot do is increase the heat the walls pass to the room. That depends on wall area, wall material, and the inside-to-outside temperature difference. For that side of the problem, the heat calculation guide walks the whole load estimate.
Where the heat goes, step by step
Follow one watt through a sealed cabinet:
- A component dissipates it into the nearby air, or into the mounting panel by conduction.
- The fan carries that warm air across the enclosure walls and the backplate.
- Heat conducts through the wall. Steel and stainless are decent conductors; a gasketed door loses little through its face because the gasket interrupts conduction, but a large door still counts.
- On the outside, natural convection and radiation carry it away. A wall-mounted box in still room air sheds roughly 5 to 10 W per square meter of surface per degree C of temperature rise, depending on orientation and finish.
For a quick engineering check, sealed steel enclosures in free air are commonly estimated at about 0.05 to 0.1 W per square foot of surface area per degree C of rise. Use the total exposed surface, not just the sides; a wall-mounted box loses its back face's share to the wall behind it.
Worked example: a 20 x 16 x 8 in enclosure has roughly 9.5 sq ft of exposed surface (counting the back at half). At 0.07 W/sq ft/C, that is about 0.7 W/C. With 100 W inside, the walls sit about 145 C above room. That fails. The same box with 15 W inside settles around 20 C above room, which usually passes. Sealed boxes live or die on watts per square foot.
If the numbers fail, you have three levers: a bigger enclosure, less heat, or an active closed-loop cooler. The enclosure cooling comparison covers when each one wins.
Why machine builders seal the cabinet and fan the inside
Machine tool and washdown cabinets are sealed for good reasons. Oil mist, coolant aerosol, chips and dust destroy electronics faster than heat does, and every intake vent is a filter that someone must service. Sealing the box and stirring the inside air gives clean electronics and zero filter maintenance, at the price of a higher internal temperature.
Many builders go one step further and fit a heat exchanger or an air conditioner: two air loops, one inside the sealed cabinet and one outside, exchanging heat through a wall without exchanging air. The fans you see may belong to such a loop, and the loop's outside half may be quiet enough to miss. Before assuming a cabinet has no cooling path, look for a core in a wall, a roof-mounted unit, or an external filter box. Closed-loop options are compared in closed-loop enclosure cooling where fumes are in the air.
Checking an existing sealed cabinet
You do not need the drawings to judge a cabinet. You need a thermometer, the nameplate watts, and a few hours of running.
- Record the room ambient near the cabinet, away from its exhaust.
- Record the air temperature inside, at the top and at the hot component's intake, after a few hours at real load.
- Subtract ambient from the internal reading. That is your temperature rise.
- Divide the heat load in watts by the rise to get the box's actual dissipation in W/C.
- Compare the component temperature against its rating at your worst-case summer ambient. If the drive is rated 40 C and the internal air sits 25 C above the room, anything above a 15 C room puts you over.
A rise that holds steady means the walls are rejecting exactly what the components make, just at a higher temperature. Whether that higher temperature is acceptable is a component-rating question, not a cooling question. The load estimate behind the measurement, and the same arithmetic in reverse, is the heat calculation guide starting from watts.
If the cabinet is small and the load is fixed, removing heat from a small sealed IP66 enclosure covers the options that fit tight boxes, including conduction to the mounting surface as described in using a sealed enclosure as a heat sink.
What the layout adds
Internal fans only work if air can actually reach the parts. A tightly packed cabinet with one fan at the door can leave dead zones where a component cooks in its own exhaust. Leave a path: components that run hot up high or near the fan sweep, wiring ducts not blocking the wall-to-air gap, and the hottest parts downwind of cooler ones where possible. Component layout inside a sealed enclosure to manage heat gives the placement rules; a well-laid-out box can cut component temperatures as much as the fan itself.
Fan choice matters less than fan placement. Any quiet industrial fan moves far more air than the heat requires; what you need is coverage, so aim the flow along the walls and across the drive heatsinks rather than at the door.
When to stop trusting the walls
Passive conduction through the walls fails in three common situations:
- High heat density. More than roughly 10 to 20 W per square foot of exposed surface needs help, depending on the allowed rise.
- High ambient. A cabinet bolted next to an oven or in full sun loses its temperature difference. Outdoors, add the solar load; how a sun shield cuts solar heat load on an enclosure quantifies it.
- Sealed room. A box in a small closed room eventually heats the room, which heats the box. The whole room becomes the heat sink.
In those cases the fix is a closed-loop air conditioner or heat exchanger sized for the load and the ambient, as covered in cabinet air conditioner sizing for an overheating enclosure. Do not cut vents into a sealed cabinet to add airflow; that trades a thermal problem for a contamination problem, and destroys the rating that justified the box.
Specifying a box that rejects heat passively
If you want the sealed-fans-only approach to work from day one, put these on the order:
| Item | What to specify |
|---|---|
| Size | Largest surface area the mounting allows; a shallow wide box beats a deep narrow one |
| Material | 304 or 316L stainless; both conduct heat similarly, chosen for corrosion, not cooling |
| Mounting | Standoff feet or a frame so air can move over the back, not flat against a wall |
| Finish | Bare or light stainless outside; dark paint raises solar gain outdoors |
| Interior | Removable mounting panel in contact with the walls, so backplate conduction reaches the shell |
The stainless enclosure size finder filters wall-mount boxes from 6 x 4 x 4 in to 60 x 36 x 16 in by outside size, so you can pick the surface area first. If the size you need is not standard, custom enclosures are built to your dimensions and delivered in 10 business days from drawing approval. For choosing between rating, size and cooling in one pass, the enclosure selector carries your environment and inside dimensions through to a shortlist.
The bottom line: fans inside a sealed enclosure are a heat-spreading device, not a heat-removal device. Heat leaves only through metal, and metal area times allowed temperature rise is the whole budget. Size the box for that budget and the internal fans will do their job; exceed it and no number of fans will fix it.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 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 |
Questions
Do internal fans cool a sealed enclosure?
They cool the components, not the enclosure. A fan inside a sealed box moves heat from hot parts to the walls and the air, which stops hot spots and drops component temperatures well below still-air values. The heat still leaves through the walls by conduction and outside natural convection, and only wall area and the allowed temperature rise set how much can leave. If the load exceeds that capacity, internal fans alone will not save the design.
How do I know if my sealed cabinet is dissipating enough heat?
Measure the air temperature inside and the ambient outside after the panel has run at full load for a few hours. The difference is the temperature rise. Divide the heat load in watts by that rise to get the enclosure's effective dissipation in watts per degree C. Then compare against the components: if a drive tops out at 40 C, the rise is 25 C, and a 35 C day is possible, the design fails. Measure at the hottest spot, not at the bottom of the box.
Why does a machine cabinet have fans but no intake vents?
Many industrial cabinets are sealed on purpose to keep oil mist, chips and dust off the electronics, and the internal fans are part of that design. The fans scrub heat off drives and power supplies and carry it to the walls, or to a sealed heat exchanger or air conditioner loop. No outside air passes through the electronics compartment, so the inside stays clean while heat leaves through metal. It looks wrong if you expect a filtered intake, but it is the standard approach in dirty plants.
When do internal fans stop being enough?
When the temperature rise you measure exceeds what the components allow at your worst-case ambient, or when the enclosure is small relative to its heat load. A small sealed box dissipates on the order of a few watts per degree C through its walls; a 500 W drive array will cook in one. At that point you need a closed-loop cooler sized for the load, or a bigger box, or a lower load. Fans still help by spreading heat to the whole wall area, but they cannot raise the wall's own heat transfer much.
Do internal fans cause condensation in a sealed enclosure?
Fans do not add moisture, and a sealed box admits little new air, so they do not cause condensation by themselves. They can reduce it slightly by eliminating cold spots where the air's dew point is reached. Condensation in a sealed enclosure comes mostly from day-night temperature cycling breathing air past gaskets, and the fix is a heater or desiccant, not removing the fans.
