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Panel & Electrical·7 min read·

Sizing an enclosure for the heat inside it

Adding up the load, what an enclosure dissipates on its own, and the decision between a filter fan, a heat exchanger and a cooler.

Short answer

Add up the heat dissipated by everything inside, usually a few percent of throughput for drives and power supplies, and compare it with what the enclosure dissipates naturally, roughly 5.5 watts per square metre of free surface per degree of temperature difference. If the internal temperature would exceed the most sensitive component's limit at the site's maximum ambient, you need cooling, and which type depends on whether outside air is clean enough to bring in.

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Panel cooling is decided by looking at the panel and forming an opinion, on more projects than anybody would like to admit. The calculation is not difficult and it takes twenty minutes.

Step one: add up the heat

Every component dissipates. The figures come from datasheets, and the approximations are close enough to start with.

  • Drives. 3 to 5 percent of throughput. An 11 kW drive at full load is around 400 watts.
  • Switched mode power supplies. Roughly 10 percent of rated output.
  • Transformers. 5 to 10 percent depending on size and loading.
  • PLC and I/O. Tens of watts, usually not significant unless the rack is large.
  • Contactors and relays. Coil power, small individually and worth counting if there are many.
Use the actual duty rather than the nameplate. Three drives that never run together are not three drives' worth of heat, and sizing for a case that cannot occur is how panels end up with cooling that short cycles.

Step two: what the enclosure gives you free

A steel enclosure dissipates roughly 5.5 watts per square metre of free surface area, per degree Celsius of difference between inside and outside. Stainless is poorer, around 3.7; plastic is poorer still.

Free surface means surfaces exposed to air. A panel against a wall loses that face, and a bank of panels side by side loses both sides.

So: a 2 m by 0.8 m by 0.4 m enclosure, freestanding, has about 5.5 square metres of free surface. At 5.5 W per square metre per degree, that is about 30 watts per degree.

Step three: the sum

The temperature rise is heat load divided by the dissipation coefficient.

With 900 watts of load and 30 watts per degree, the rise is 30 degrees. At a site maximum ambient of 35, the internal temperature reaches 65.

Compare that with the lowest maximum ambient of anything inside. A PLC rated to 60 is now out of specification, and the drives are derating.

That is the whole calculation, and it is where the decision is made.

Choosing the cooling

Natural convection with vents. Cheapest, and it drops the IP rating to whatever the vents allow. Fine indoors and clean.

Filter fan and exit filter. Brings outside air in. Effective, cheap, and it cannot cool below ambient. It also brings the outside in: dust, moisture, oil mist. Filters need changing, and a blocked filter turns a cooled panel into an oven with no warning.

Air to air heat exchanger. Two separate circuits, internal air and external air, exchanging heat through a matrix. Sealed, so the IP rating is kept, and it cannot cool below ambient either.

Air conditioner. Refrigeration. Cools below ambient, sealed, expensive, and it produces condensate that has to drain somewhere sensible.

Air to water heat exchanger. Where chilled water exists on site. Compact and very effective.

The heater

The other direction, and it is forgotten more often than cooling.

In an unheated building or outdoors, a panel cools overnight and warm moist air condenses inside it in the morning. Water on terminals is worse than cold.

A small anti-condensation heater with a hygrostat, or a thermostat set a few degrees above the minimum, costs very little and prevents a failure mode that is difficult to diagnose because the evidence dries.

The mistakes worth avoiding

  • Sizing for the average rather than the worst case. The worst case is a hot afternoon with everything running and a filter that is due for changing.
  • Ignoring solar gain on an outdoor panel. Direct sun on a dark enclosure adds a lot, and a sunshade is cheaper than the cooling it saves.
  • Mounting the fan so it fights convection. Air in low, out high, always.
  • Forgetting the filter change interval exists, and that nobody will do it unless it is on a schedule.

Common questions

How much heat does a VFD produce?
Roughly 3 to 5 percent of its throughput power as heat, so an 11 kW drive at full load dissipates around 400 watts into the panel. Manufacturers publish the figure and it is worth using rather than the rule of thumb, particularly for several drives, where the total is often the dominant load.
When is a filter fan not enough?
When the outside air is dirtier, wetter or hotter than the inside needs to be. A filter fan brings outside air in, so it cannot cool below ambient and it brings whatever is in the air with it. In a dusty or washdown environment, or where ambient approaches the internal limit, you need a closed loop heat exchanger or a cooler.
Does an enclosure need a heater?
In an unheated or outdoor location, usually yes, sized for condensation rather than for cold. Equipment tolerates low temperature better than it tolerates the water that forms on it when warm moist air meets cold metal. A small anti-condensation heater with a hygrostat is cheap insurance.

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