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A case can be confining. It can keep just about everything from escaping, including the heat that electronic circuits produce as a by-product of performing their normal functions. Some of the electricity in any circuit (except one made from superconductors) is turned into heat by the unavoidable electrical resistance of the circuit. Heat is also generated whenever an element of a computer circuit changes state. In fact, nearly all the electricity consumed by a computer eventually turns into heat.

Inside the protective (and confining) case of the computer, that heat builds up, thus driving up the temperature. Heat is the worst enemy of semiconductor circuits; it can shorten their lives considerably or even cause their catastrophic failure. Some means of escape must be provided for the excess heat. In truth, the heat build-up in most computers may not be immediately fatal to semiconductor circuits. For example, most microprocessors shut down (or simply generate errors that shut down your computer) before any permanent damage occurs to them or the rest of the components inside your computer. However, heat can cause circuits to age prematurely and can trim the lives of circuit components.

The design of the case of a computer affects how well the machine deals with its heat build-up. A case that's effective in keeping its internal electronics cool can prolong the life of the system.

Passive Convection

The obvious way to make a computer run cooler is to punch holes in its case to let the heat out—but to keep the holes small enough so that other things such as mice and milkshakes can't get in. In due time, passive convection—less dense hot air rising with denser cool air flowing in to take its place—lets the excess thermal energy drift out of the case.

Any impediment to the free flow of air slows the passive cooling effect. In general, the more holes in the case, the merrier the computer will be. Remove the lid, and the heat can waft away along with temperature worries.

Unfortunately, your computer's case should be closed. Keeping a lid on it does more than just restrict cooling—it is also the only effective way to deal with interference. It also keeps your computer quieter, prevents foreign objects and liquids from plummeting in, and gives your monitor a lift.

Moreover, passive cooling is often not enough. Only low-power designs (such as notebook and Green computers) generate little enough heat that convection can be entirely successful. Other systems generate more heat than naturally goes away on its own.

Active Cooling

The alternative to passive cooling is, hardly unexpectedly, active cooling, which uses a force of some kind to move the heat away from the circuits. The force of choice in most computers is a fan.

Usually tucked inside the power supply, the computer's fan forces air to circulate both inside the power supply and the computer. It sucks cool air in to circulate and blows the heated air out.

The cooling systems of early computers, however, were particularly ill-conceived for active cooling. The fans were designed mostly to cool off the heat-generating circuitry inside the power supply itself and only incidentally cooled the inside of the computer. Moreover, the chance design of the system resulted in most of the cool air getting sucked in through the floppy disk drive slots. Along with the air came all the dust and grime floating around in the environment, polluting whatever media you had sitting in the drive. At least enough air coursed through the machine to cool off the small amount of circuitry that the meager power supply of the computer could provide.

Modern computers do much better. Many have carefully designed air channels to route cooling air over the places most apt to get work (such as memory chips). Some manufacturers opt to put one or more additional fans (besides the one in the power supply) into their systems to keep the overall chassis cool.

Modern microprocessors generate so much waste heat that a single system fan can't keep them cool. They require an additional means to keep air circulating. Most microprocessors in desktop computers now use fans integrated into their heatsinks to keep the chips cool. Each heatsink design requires a fan custom tailored to it.

Large cooling fans make noise. Some computer manufacturers try to minimize this noise by making their fans thermostatically controlled. They switch on only when your computer needs cooling. Sometimes this feature is controlled through the BIOS; other times it is an invariant feature. If you suspect fan failure (discussed later in this chapter in more detail), be sure your system does not have a thermostatically controlled fan that simply has not switched on.

Many people believe you cannot overdo cooling—more fans is always better. In fact, the cooler you keep your chips, the longer they will last. Heat is their worst enemy. The only downside to blowing hurricanes through your computer is that air is inevitably fouled with dust and lint. A substantial amount of foreign material may accumulate inside your computer, and it may alter or stop airflow in some places in the system, causing them to overheat. To prevent such problems, fans may be equipped with filters. In any case, whenever a computer is operated in an environment likely to cause dust and lint contamination (say, for instance, you have a cat), you should vacuum the inside of your computer periodically to remove any accumulation.

Fan Failure

The fan inside a computer power supply is a necessity, not a luxury. If it fails to operate, your computer won't falter—at least not at first. But temperatures build up inside. The machine—the power supply in particular—may even fail catastrophically from overheating.

The symptoms of fan failure are subtle but hard to miss. You hear the difference in the noise your system makes. You may even be able to smell components warming past their safe operating temperature.

Should you detect either symptom, hold you hand near where the air usually emerges from your computer. (On most computers, that's near the big round opening that the fan peers through.) If you feel no breeze, you can be certain your fan is no longer doing its job.

A fan failure constitutes an emergency. If it happens to your system, immediately save your work and shut the machine off. Although you can safely use it for short periods, the better strategy is to replace the fan or power supply as soon as you possibly can.

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