The Science of Water Cooling (without too much science)

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How does normal cooling work?

In most PCs, internal components give off heat that is absorbed by ambient air and circulated out of the computer case while cooler air (from outside the case) is circulated back in. When you hear your PC hum, it’s usually (and hopefully!) the sound of several internal fans constantly circulating this air to keep your CPU, video card, motherboard and everything else at functional temperatures.

When is normal cooling not enough?

In most computers, air cooling is sufficient. PC’s that have been overclocked or are packed to the brim with extra components (all drawing power and expelling heat), like redundant hard drives, or SLI/CrossFire multi-video card setups, will simply put out more heat more quickly than the circulated air can absorb.

Thermal Conductivity – Air vs. Water

The ability of a material to conduct heat through (or into) itself is known as thermal conductivity.  Think of holding a hot baking-tray straight from the oven with an over mitt on. As you hold the tray, you can feel the heat from the tray in your hand, but you can comfortably hold it for several seconds without it becoming uncomfortable. Compare this to holding the same tray without an over mitt: the tray would be hot to the touch immediately and your hand would be burned in seconds.

The difference here (among other things) is that the oven mitt is protecting your hand with its relatively-poor thermal conductivity. Trapped loosely within the fabric of this protective mitt are thousands and thousands of tiny pockets of air; this trapped air is actually where most of the protection of the mitt comes from.

For a more direct comparison, consider:

  • Air has a thermal conductivity rating of around .02 at room temperature,
  • Water has a a thermal conductivity rating of around .60 at the same temperature.

What does this mean? It means that water moves heat around 30x faster than air does. This is why we told to run cool water over burns: the cold water whisks away heat much, much faster than blowing on it can.

Specific Heat Capacity, or Holding onto Heat

Water has another benefit over air when it comes to moving heat: an abnormally high capacity for retaining heat without a change in temperature:

  • 4.1813 for watervs. 1.0035 for air.

Water, as you can see from the previous comparison, can hold over 4x more heat than air without a change in temperature. This is why standing in front of an air conditioner running at full-blast for a minute or two might result in some chilly discomfort, while taking a plunge into water at the same temperature for the same amount of time can cause serious hypothermia or even death.

An Oven in a River

Since water not only transfers heat 30x more-efficiently than water AND can hold over 4x as much heat-energy without changing temperature, you can see why people with serious PC cooling-needs choose to go with a water cooling system. If air cooling your PC is like putting an oven in front of an air conditioner, then using a water cooling system is something like dumping the same oven into an icy river.


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