In the past few years, water cooling is being realized for cooling computer components, especially the CPU. Water cooling usually consists of a CPU water block, a water pump, and a heat exchanger (usually a radiator with a fan attached). Water cooling not only allows for quieter operation and improved overclocking, but with improved heat handling capabilities hotter processors can be supported. Less commonly, GPUs, Northbridges, hard drives, memory, VRM, and even power supplies are also water cooled.
Water coolers for computers (other than mainframes) were, up until the end of the '90s, homemade. They were put together using car radiators (or more commonly, a car's heater core), aquarium pumps and home made water blocks. In conjunction with these automotive items users would pair laboratory-grade PVC and Silicone tubing and various reservoirs (home made using plastic bottles, or constructed using cylindrical acrylic or sheets of acrylic, usually clear) and or a T-Line. More recently a growing number of companies are manufacturing pre-made, specialised components, allowing water cooling to be compact enough to fit inside a computer case. This, coupled with the growing amount of heat coming from the CPU has greatly increased the popularity of water cooling. However it is still a very niche market.
Dedicated overclockers will occasionally use vapor-compression refrigeration or thermoelectric coolers in place of more common standard heat exchangers. Water cooling systems in which water is cooled directly by the evaporator coil of a phase change system are able to chill the circulating coolant below the ambient air temperature (an impossible feat using a standard heat exchanger) and, as a result, generally provide superior cooling of the computer's heat-generating components. The downside of phase-change or thermoelectric cooling is that it uses much more electricity and antifreeze must be added due to the low temperature. Additionally, insulation, usually in the form of lagging around water pipes and neoprene pads around the components to be cooled, must be used in order to prevent damage caused by condensation of water vapour from the air on the surfaces at below ambient temperature. Common places from which to borrow the required phase change systems are a household dehumidifier or air conditioner.
An alternative cooling system, which enables components to be cooled below the ambient temperature, but which obviates the requirement for antifreeze and lagged pipes, is to place a thermoelectric device (commonly referred to as a 'Peltier junction' or 'pelt' after Jean Peltier, who documented the effect) between the heat-generating component and the water block. Because the only sub-ambient temperature zone now is at the interface with the heat-generating component itself, insulation is required only in that localized area. The disadvantage to such a system is that pelts typically draw a large amount of power, and the water cooling system is required to remove this power, in addition to the heat generated by the component. Another possible danger is condensation, resulting from the ambient air right around the pelt being cold. This condensation could cause a short-circuit, shutting the computer down or possibly permanent damage. A proper installation requires that the Peltier be "potted" with silicone epoxy. The epoxy is applied around the edges of the device, preventing air from entering or leaving the interior.
Apple's Power Mac G5 was the first mainstream desktop computer to have water cooling as standard, and Dell later followed suit by shipping their XPS computers with liquid cooling, using thermoelectric cooling to help cool the liquid.