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Case Fans: Should You Worry About Positive Or Negative Pressure?

How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 2
By

Much Ado About Dust

If your case is more or less airtight (fan openings excluded), the ratio between intake and exhausted air determines whether positive or negative pressure develops inside. Let’s take a look at schematic drawings showing the use of fans with different air flow capabilities, which also applies to groups of fans with different cumulative totals:

Negative Pressure

Benefits

• Good cooling performance for performance-oriented situations
• Amplification of natural convection
• Linear and direct air flow
• Compatible with all direct heat exhaust graphics cards (the ones that blow dissipated heat out of a vent built into the shroud)
• Augments the cooling performance of downward-facing CPU heat sink fans

Drawbacks

• Without a dust filter, dust gathers inside the case
• Graphics cards without the ability to exhaust heat don’t benefit much

Positive Pressure

Benefits

• Only average cooling performance in enthusiast-oriented configurations
• Better support for graphics cards without the ability to exhaust dissipated heat
• All case openings contribute to getting heat out
• Less dust in the case

Drawbacks

• Counters convection
• Air flow is determined by size/position of case openings
• Counteracts the cooling performance of downward-facing processor coolers
• Graphics cards with DHE (direct heat exhaust) can partly counter the overpressure

Analysis

If you want maximum PC performance, which means turning massive amounts of electricity into heat, you should carefully consider the cooling strategy you want to adopt and the best-suited case for it.

Keep in mind that you cannot achieve positive pressure in a case with meshed surfaces or lots of openings. If you want to go that route, you should also be sure that the heat sink on your processor isn't being artificially handicapped.

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