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Keeping Your Cool Takes Planning
Alan Earls, Contributing Editor -- Design News, September 17, 2008
Heat is the enemy of electronics. While design engineers probably thought they were going to be able to say goodbye to heat problems when they traded vacuum tubes for transistors half a century ago, in fact, heat problems are here to stay. For one thing, while solid state devices might not emit much heat in relative terms, when they are combined in the tremendous densities found on chips - and in the packed circuit boards and enclosures of today, they produce a lot of thermal energy. What's more, they are often very sensitive to temperature excesses so they need protection. In fact, in years past, some mainframe computer makers even resorted to running chilled water through their enclosures.
Today, though, the big challenge is to simply develop a better understanding of heat challenges and then apply the right improvements to air flow to regain control over the "heat monster."
For instance, while wide ribbon connectors, which used to be one of the impediments to clean air flow, are largely a thing of the past in most equipment, new challenges have emerged. For instance, the popularity of PCI Mezzanine cards (PMCs) is causing problems because they protrude into space that could formerly be counted on for maintaining air flow. Heat sinks have also gotten larger and more numerous.
But, in fact, any look at a typical enclosure will often reveal that there are in fact innumerable other impediments to smooth and even air flow. Some of this, of course, can be addressed in the design phase, by making air flow a consideration from the start. Fan placement and power are also important. Most cooling systems, for instance, try to take advantage of the elementary principle that warm air rises - so ambient (and presumably cooler) air is taken in at a low point in the enclosure and warmer air is exhausted near the top.
Sizing fans appropriately is also important. Fans that have low capacity may not be able to maintain necessary airflow when confronted with the many obstacles presented within the enclosure. In fact, larger scale air handling equipment designers have rules of thumb regarding the number of turns and the total distance air must travel that help them appropriately size fans and ducts to room and building requirements. To date, no one has come up with similar rules of thumb for enclosures so common sense and experimentation may be the best guides.
But don't assume that just because a fan has a higher rating (more cubic feet per minute) that it will actually deliver better cooling. That's because typical fans don't provide much pressure. So, when they are, in effect, funneling air into an area that is blocked by components and characterized by twists and turns, there's no guarantee air flow will really improve.
In fact, in enclosure applications a fan with twice the air flow might only deliver a 50 percent increase in airflow.
So, in short, it's cool to keep your enclosure cool. However, achieving that state takes some real engineering effort. Reconsider component placements, look for opportunities to open up air flow to and from the outside, consider ways to avoid dust contamination (a challenge in actual operation, over time), and, of course, look for appropriate fan characteristics. Recent advances in fan designs have helped to overcome some of the challenges described here. Check with your fan vendor's technical support department for advice on handling thermal challenges as well as suggestions on optimized fan/enclosure solutions.
