Whether you're using a desktop or laptop computer, there's a good chance that if you stop what you're doing and listen carefully, you'll hear the whirring of a small fan. If your computer has a high-end video card and lots of processing power, you might even hear more than one.

In most computers, fans do a pretty good job of keeping electronic components cool. But for people who want to use high-end hardware or coax their PCs into running faster, a fan might not have enough power for the job. If a computer generates too much heat, liquid cooling, also known as water cooling, can be a better solution. It might seem a little counterintuitive to put liquids near delicate electronic equipment, but cooling with water is far more efficient than cooling with air.

Image courtesy Darrin Gatewood A liquid-cooled PC in a clear case. See more liquid-cooled PC pictures.

A liquid-cooling system for a PC works a lot like the cooling system of a car. Both take advantage of a basic principle of thermodynamics - that heat moves from warmer objects to cooler objects. As the cooler object gets warmer, the warmer object gets cooler. You can experience this principle firsthand by putting your hand flat on a cool spot on your desk for several seconds. When you lift your hand, your palm will be a little cooler, and the spot where your hand was will be a little warmer.

Liquid cooling is a very common process. A car's cooling system circulates water, usually mixed with antifreeze, through the engine. Hot surfaces in the engine warm the water, cooling themselves off in the process.

Click on "Start" to see the fluid flow through the engine as the engine warms up.

The water circulates from the engine to the radiator, a system of fins and tubes with a lot of exterior surface area. Heat moves from the hot water to the radiator, causing the water to cool off. The cool water then heads back to the engine. At the same time, a fan moves air over the outside of the radiator. The radiator warms the air, cooling itself off at the same time. In this way, the engine's heat moves out of the cooling system and into the surrounding air. Without the radiator's surfaces making contact with the air and dispelling the heat, the system would just move the heat around instead of getting rid of it.

A car engine generates heat as a byproduct of burning fuel. Computer components, on the other hand, generate heat as a byproduct of moving electrons around. A computer's microchips are full of electrical transistors, which are basically electrical switches that are either on or off. As transistors change their states between on and off, electricity moves around in the microchip. The more transistors a chip contains and the faster they change states, the hotter the chip gets. Like a car engine, if the chip gets too hot, it will fail.

Most computers dispel this heat with heat sinks and fans. Heat sinks are basically pieces of metal that provide lots of surface area for the air to touch. The chip warms the heat sink, the heat sink warms the air, and the fan moves the warm air out of the PC case.

Image courtesy HowStuffWorks Shopper A heat sink uses lots of surface area to transfer heat from electronic components to the air.

This system works most of the time, but sometimes, electronic components produce more heat than simple air circulation can dispel. High-end chips with lots of transistors can overwhelm an air-cooling system. So can chips that have been overclocked, or manually set to work at faster than their default speed.

Cooling with a Terminator T-1000 In 2005, computing magazines reported that a liquid-metal cooled graphics card called the Radeon Blizzard X850XTPE would soon hit the market. Preliminary reports suggested that cooling with a liquid metal alloy was even more efficient than cooling with water. However, the card never made it to market, most likely because the cost of materials and production outweighed any improvement in cooling performance.

That's where water cooling comes in. Water has a higher thermal conductivity than air - it can move heat faster than air can. Water also has a higher specific heat capacity. It can absorb more heat before it starts to feel hot.

There are two reasons why a computer might need the increased thermal conductivity and heat capacity of water:

• Its electronic components produce more heat than the air around them can absorb
• The fans required to move enough air to cool all the components make too much noise or use too much electricity

In other words, there are two reasons why you might need to cool a computer with a liquid instead of air:

• The components inside your computer need more cooling than air alone can provide
• You want your system to be quieter

Next, we'll look at the components of a liquid-cooled system and how they work together.

All-in-one Units and Kits If you like the idea of liquid cooling but don't want to research individual components, you can buy a ready-to-use unit or kit. Self-contained units can plug directly into a computer's expansion slots or power supply and provide liquid cooling to one specific chip. Kits include all the parts you need and instructions for assembling them - just make sure the parts included are compatible with your computer's hardware. Some companies also sell high-end PCs with liquid cooling factory-installed.