Cooling technologies explained

Cooling systems in PCs rely on two mechanisms – conduction and convection. Efficient conduction requires materials that have low resistance to the transfer of energy from one atom to the next, or low thermal resistance (which can also be referred to as high thermal conductivity).

Gold (Au), silver (Ag), copper (Cu) and aluminium (Al) all have high thermal conductivity (Au = 3.18, Ag = 4.29, Cu = 4.01 and Al = 2.37 W/cm-ºC). Unfortunately, air has a very poor thermal conductivity (air = 0.000262), so a good CPU cooler design needs a heatsink with a big surface area to allow heat transfer from the metal to the air.

It will also need good airflow to ensure that the heated air is moved quickly away from the heatsink surface. Heatsinks are one instance where a turbulent air flow turns out to be a good thing and actually improves the heat transfer.

Water conducts heat more efficiently than air and several CPU coolers are available that use a closed loop of water, with a convection cooling tower mounted outside the PC. Although these systems are silent and do a good job of cooling, they are also mechanically complex, relatively difficult to install and cumbersome.

Most cooling systems are limited by the ambient temperature of the surrounding air. There have been attempts to market cooling systems using similar principles to the domestic refrigerator, but these have turned out to be too expensive and complex.

There are also companies making systems using Peltier elements to cool the air intake. In these systems the case air inlet is divided into two by one or more Peltier elements. These are heat pumps that absorb heat energy on one side and emit it on the other, under the influence of an electric current.

The hot side of the element is isolated in its own fan-cooled air path outside the PC case and the cooling side is used to cool the air passing into the case. Syrtin Corp makes the Nextherm ICS 8200 case with a built-in Peltier cooler, temperature control and status panel.

Heat pipes
The simplest form of heat pipe consists of a vertical sealed metal tube, with the bottom of the tube in good thermal contact with the heat source and the top end of the tube thermally connected to cooling fins.

A small amount of liquid (often water or alcohol) in the tube is heated by the heat source and evaporates, cooling the hot end of the pipe. The heated vapour rises up the tube to the top, where it is air-cooled by the fins, re-condenses and drips down to the bottom of the tube to start the cycle over again.

The problem with this simple design is that it relies on gravity-driven convection to work and must be vertically oriented. Commercial heat pipes use the capillary action of a metal mesh or sintered metal powder ‘wick’, to return the condensed fluid. Although these capillary designs are less dependent on gravity for their operation, it nonetheless has a significant influence on their efficiency.

Heat pipes are usually made from copper, but are much more efficient than solid copper of the same cross-sectional area. The best of the current CPU coolers use two or three heat pipes rooted in a copper block, which is clamped to the CPU. The pipes are bent into a circle and thermally connected to a very large number of thin copper plates, or fins, which are force convection-cooled by a fan.

Heat pipes operate efficiently only within certain temperature bands, depending on their design, and they require a sufficient temperature differential between the ‘hot’ and ‘cool’ ends (see attached pdf).

See also Creating a computer cooling system and Testing your cooling system