US How the two coolers differ
An enclosure air conditioner is a closed-loop refrigeration cooler mounted on the side, door or roof of an electrical cabinet. It pumps heat from the air inside the enclosure to the air outside, and because it uses a compressor and refrigerant it can do so even when the room is hotter than the cabinet. An air-to-air heat exchanger does a similar job without refrigeration: it lets heat flow through a sealed core from the warm inside air to the cooler outside air.
Both are closed-loop coolers. The internal air circulates through the cooler and back into the cabinet, and the outside air passes through a separate path, so dust, oil mist and moisture stay out. That is what separates them from filtered fans, which blow room air straight through the enclosure.
The difference that matters is the direction heat is allowed to flow. Heat only moves on its own from hot to cold, so a heat exchanger can only work while the outside air is cooler than the inside. An air conditioner does work on the heat to move it uphill, and that costs power, maintenance and a way to deal with condensate.
How an air-to-air heat exchanger works
Two fans drive two air streams across a heat exchange element. The element is usually either a plate or convoluted core, a folded metal or polymer sheet with inside air on one face and outside air on the other, or a bank of finned heat pipes with a sealed working fluid that evaporates on the warm side and condenses on the cool side.
Capacity is quoted as a specific heat output in watts per kelvin (W/K) of temperature difference. A 30 W/K heat exchanger removes about 300 W when the inside is held 10 K above the room, and only about 150 W when the gap is 5 K. As the room approaches the target inside temperature, capacity falls towards zero, so a heat exchanger suits sites where the hottest air is still comfortably cooler than the equipment needs.
In return it draws only fan power, has no refrigerant circuit and no condensate, and needs little more than cleaning of the outside air path.
How an enclosure air conditioner works
An enclosure air conditioner uses the vapour-compression cycle found in any refrigerator. An evaporator coil in the internal air loop absorbs heat, a compressor raises the refrigerant pressure, and a condenser coil in the outside air loop rejects the heat, together with the compressor’s own power. Capacity is rated in watts at stated inside and outside temperatures, commonly L35/L35 (35 °C inside, 35 °C outside) and L35/L50 to the EN 14511 test standard, so check that two models are rated at the same point before comparing them.
Cooling air below its dew point condenses water out of it. Inside a sealed cabinet that should only happen briefly after the door has been opened, and the water is removed through a drain or an evaporator built into the cooler. Most air conditioners take a door switch so they stop while the cabinet is open, instead of drawing in humid room air. Refrigerants are also regulated: in the USA and EU, F-gas rules govern which refrigerants can be used and how systems are serviced.
Air conditioner vs heat exchanger compared
| Feature | Air-to-air heat exchanger | Enclosure air conditioner |
|---|---|---|
| Can cool below ambient | No | Yes |
| How capacity is rated | W/K of inside-to-outside difference | W at stated conditions, such as L35/L35 to EN 14511 |
| Power drawn | Fans only | Compressor and fans; several times the heat exchanger’s draw for the same load |
| Condensate | None | Drain or built-in evaporator needed |
| Refrigerant | None in plate cores; sealed fluid in heat pipe types | Refrigerant charge subject to F-gas rules |
| Maintenance | Clean the outside air path | Clean coils and filters; check drain and refrigerant circuit |
| Best fit | Dusty or oily sites with cooler ambient air | Hot rooms, outdoor cabinets in sun, high heat loads |
Many sites need both answers at different times of year, so some manufacturers offer hybrid coolers that run as a heat exchanger while the outside air is cool enough and only start the compressor when it is not.
Choosing between them
Start with two numbers: the hottest air the cabinet will ever stand in, and the highest temperature allowed inside it, set by the lowest-rated device in the enclosure. Then work through three checks.
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Temperature gap. If the hottest outside air is well below the inside limit, a heat exchanger can do the job. If the gap is small, zero or negative, only an air conditioner, an air-to-water heat exchanger, a thermoelectric cooler or a vortex cooler can.
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Heat load. Add up the losses inside the cabinet, plus solar gain outdoors, and size the cooler with margin. Our guide to electrical enclosure cooling and heat-load calculation works through the sums.
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Enclosure rating. The cooler must carry the same rating as the cabinet, such as NEMA 4X for washdown and corrosive sites, or it becomes the weak point in the seal.
Cutting the cooling load in the first place
The cheapest cooler is the one the cabinet does not need. Every watt that equipment loses as heat inside the enclosure has to be removed, and the inside limit is set by whichever device is most sensitive to temperature. Raising that limit can turn an air conditioner job into a heat exchanger job.
Computers are often where this can be done. Our wide temperature computers are designed for higher ambient temperatures than office hardware, and compact DIN rail PCs with low-power processors add relatively little heat to the cabinet. Outdoor cabinets need sun shields and light-coloured surfaces as well, which our guide to designing outdoor computer enclosures covers.
Quick answers on enclosure air conditioners
Enclosure air conditioners and heat exchangers: common questions
Can a heat exchanger cool below ambient temperature?
No. It can only bring the inside down towards the outside air temperature, so it needs a cooler room or a shaded site.
What does W/K mean on a heat exchanger?
Watts removed per kelvin of temperature difference. Multiply it by the gap between the inside limit and the hottest outside air to get the usable capacity.
Does an enclosure air conditioner need a drain?
It needs somewhere for condensate to go. Many models evaporate it on the hot condenser coil, so no external drain is needed in normal operation.
How do you size an enclosure air conditioner?
From the heat load at the hottest ambient. Add internal losses and solar gain, then pick a model whose rated capacity at the matching conditions covers it with margin.
Heat exchanger or filtered fan?
A heat exchanger, if the cabinet must stay sealed. Filtered fans move more heat for less money but let room air in, which limits the ingress rating.
We specify computers for sealed cabinets as standard, choosing the processor and temperature rating so the system suits the cooling the enclosure already has.