How do CPU coolers work?

10th Oct 2010 | 09:00

How do CPU coolers work?

Keeping processors cool is a major part of PC performance

An electrical current produces heat. It's rather annoying really, but for this little bit of physics you could run your PC processor at breakneck speeds.

Heat is kinetic energy, which cannot be created or destroyed, all you can do is move it about or transform it. The art of processing cooling is one of absorbing energy and moving it far enough away that it won't do any damage.

Much of the technology in a PC is almost incredibly complicated, and yet on top of it all sits a simple heatsink and fan, which dictates how fast it'll run.

Spreaders and fans

The most basic form of cooling is the heat spreader, which uses passive cooling since there is no power consumption involved. This is simply a material with a high thermal conductivity (copper usually) in contact with your processor which conducts the heat away from it and into a much larger mass with a greater surface area, where ultimately it is transferred into the air; relying on convection to keep air moving.

The next step is to add a fan to your spreader to get the airflow going, since air is such a poor conductor of heat. Get more air circulating over a bigger area and you increase the cooling, although there are practical limits inside a PC. This works well enough for the crowd, but it's not enough for high-performance rigs.

The next step for spreaders is to use better materials. The idea of using carbon has been around for a while, either as carbon fibre nano tubes, which have excellent heat conductivity along their length, graphite or foamed carbon. These materials can easily reach a thermal conductivity of up to one and a half times that of copper or more. Plus they can be made to have huge surface areas relative to the size.

However, despite the promises we've yet to see carbon nano tubes in heatsinks, copper is pretty good and an awful lot cheaper.

Heat pipes

A simple but effective way to transfer large amounts of heat is to use a heat pipe. When a material changes state from gas to liquid, liquid to solid or such like, large amounts of heat are either absorbed or emitted.

For example, when water boils and changes from water to a vapour, it requires 2,260 joules of energy per gram, known as the specific latent heat of vaporisation. This is against just 4.18 joules per gram to heat water by 1°C. The water doesn't get any hotter during the phase change, it just absorbs lots of energy.

Changing from a gas back to a liquid emits the same amount of energy, do this elsewhere in your system and you've an effective heat pump, this is the same basic principle used in your fridge.

CPU cooler diagram

The heat pipes in PC coolers are sealed pipes with a small amount of liquid, usually distilled water, this is under low pressure to bring down the boiling point (which is why you can't make a decent cup of tea on Everest). The inside of the pipe is lined with a layer that acts as a wick, a woven wire mesh or sintered copper (copper foam is promised soon).

The water is boiled at the source of the heat, the gas diffuses to the cooler ends of the pipes where it condenses into the wick and capillary action takes it back to the site of the heat again. Only a tiny amount of liquid is moving, but the relatively large amount of energy required for the change of state means its shifting lots of joules. All with no maintenance or power required.

The vapour chamber is a variation on the heat pipe, and has been around a while but not been taken up much. Here instead of a set of pipes passing though the heat spreader, the whole heat pipe is flattened out and shaped to fit over the processor, with the rest of the heatsink built on top.

Liquid cooling

Liquid cooling systems for PCs work almost the same way as car radiators do. Water is circulated through a block that sits on top of the processor and pumped to a radiator where the heat is conducted into the fins and then into the air.

Early attempts were distinctly hit and miss home made efforts, now you can buy off the shelf coolers which are no more hassle than regular heatsinks to fit and run. Water cooling is very effective, water has a reasonable thermal conductivity, compared to air anyway, which means heat is sucked away from the processor quickly, plus water's massive specific heat capacity means it makes an excellent heat sink in itself.

The final part of the heat exchange, the radiator, is still quite inefficient, but at least you can fit a big radiator with multiple fans if needed, plus it can be mounted well away from your hotspot. Mercury would work even better, given its higher thermal conductivity, but the namby-pamby do-gooders at the heath and safety department have some issues. Apparently it's poisonous.

Thermo-electric coolers

Now we are starting to get exotic, Thermo-electric coolers use the Peltier effect to transfer heat from one side of a material to the other. It consists of a sandwich made from two dissimilar semiconductors across which a current is applied. This causes heat to be transferred from one side to the other. Change the polarity of the current and it reverses the flow.

How it works is similar to the way heat pipes work, only using electrons. It's not terribly efficient (50 per cent or so) but it's solid state and maintenance-free. A TEC cooler can also cool below the ambient temperature, which you can't do with heat sinks, but this has the disadvantage of condensation, not something you want in your PC.

TEC coolers haven't caught on hugely yet. The fact that Coolermaster's V10 can chew through 70W at full power is one obvious reason. The other problem is that is doesn't take the heat very far, just across the thickness of the layer of semiconductors, which means you've still got to dissipate it from there using a traditional design.

TEC coolers remain a specialist and expensive option. Using a combination of TEC cooler and liquid cooling is proper geek and can produce impressive results, using the TEC to cool either the chip or the radiator (which is safer).

What else?

Well, there is always brute force – stick what is effectively the working bits from an air-conditioning unit onto your PC – a vapour-compression unit. It's a little excessive for home use plus it can lead to problems with condensation.

Other over the top solutions include immersing the whole computer in a non-conductive liquid, but it's hardly practical.

Keeping a processor cool sounds like a simple job and you would have thought that we would have some whizzo high-tech solutions waiting in the wings by now. Unfortunately we don't.

Shifting heat in such a confined space still isn't easy – it has to run from a PC power supply, not produce strong magnetic fields or radio frequencies and not interfere with the PC's operation in any way. Tough.


First published in PC Format Issue 244

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