* One reason desktop computers are inexpensive to operate is that, like the old Volkswagen bug, they're air-cooled. They don't need air-conditioned rooms, as mainframes did. Just add an internal fan and adequate ventilation and that desktop marvel should whir away with no problem.

Well, almost no problem.

The denser microprocessors are, the hotter they get. Ever since Intel introduced its 486-class chip, heat has become an important concern. With the introduction of the next-generation Pentium chip earlier this year, the topic is, well, heating up.

Several computer magazines in recent editions have mentioned the tremendous heat generated by the Pentium chip. Norman Bailey, president of a chip-cooling company called PCubid Computer Technology, likens the Pentium to a soldering iron.

"Today's chips generate about 10 times as much heat as does a cooking surface of comparable size," writes International Business Machines researcher Robert W. Keyes in the June issue of Scientific American.

If computers are ventilated correctly, the excess heat doesn't pose a problem. Mr. Keyes says the heat from chips could rise 10-fold without exhausting known cooling technologies.

Intel, which makes the Pentium microprocessor, publishes strict guidelines on how to cool it. Most computer manufacturers appear to be following those guidelines.

"I can't say we're actually seeing any huge problem," says Lew Paceley, marketing manager for Intel's future chips.

The potential problem, Mr. Bailey says, is in the 486-class machines that can be upgraded to use the Pentium chip.

"The Pentium is so hot that you really need to do a thermal systems design," he says. Bailey sells his add-on cooling device - one of many on the market - for $39.95.

The Pentium runs hotter because it packs 3.1 million transistors into a package not much larger than its 486-class predecessor, which had only 1.2 million transistors. The Pentium runs at 14 watts; Intel's fastest 486-class chip - the DX2-66 - runs at only 6 watts.

Will Intel's next-generation chip have to rely on something other than air cooling? Probably not, Mr. Paceley says. "We're not at the limits of esoteric technology yet."

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