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ATX Chassis Airflow

by Parm Mann on 20 October 2008, 00:00

Quick Link: HEXUS.net/qarrn

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Overview

This HEXUS.help guide explains why good case airflow is essential for a reliable, modern PC system.

The Technology

The modern PC is potentially a mass of heat output and heat production hot spots. With CPUs rated at more than 100W of heat output, single graphics boards carrying similar ratings (and people want to run two!), multiple hard drives the norm, lots of memory and mainboards covered in heatpipes to combat toasty core logic and PWM circuits, a PC appreciably warming up a room when it's working hard is no joke.

All that heat output largely comes from power-hungry devices (that lose power via inefficiency as heat), which means your PSU has to work hard and since that's not perfectly efficient either, well, you get the idea.

So if you run a pair of HDDs, some kind of multi-GPU graphics solution and a fairly high-end CPU in a modern mainboard, your cooling systems have to be up to the task. And the simple truth is that no interior cooling system will work properly without the chassis allowing it.

Poor chassis design will contribute to heat build-up, the chassis unable to deal with the heat dumped into it by the mini ovens churning away inside. However, the door to good chassis design is guarded by the beast of ATX. ATX came at a time (1995!) when the modern PC system was largely inconceivable. A modern GPU makes more heat on its own than most ATX systems did back then. Running CPUs passively wasn't the hilarious novelty it would be today.

So when you look at a modern ATX mainboard these days and see the CPU and memory modules placed almost arbitrarily on the board, core logic ASICs at random and only the expansion slot positions static, you're starting to understand what's up.

Then think about that mainboard in a modern case. Is the chassis a tower or a desktop? How are the hard drives positioned? In-line with the mainboard or opposing it? It's a lottery of component position loosely glued together inside a spec that doesn't really make sense any more.

So to really solve the problem you need to consider component placement and in-line airflow. A chassis needs sensible fan placement that pushes air from front-to-back in large volume, sweeping the heat-producing mainboard area of a case, before ejecting it quickly. Intake and outlet volumes should largely match and the chassis shouldn't present interior obstacles that prevent the air from making an uninterrupted journey.

Logically that means big fans with big blades at front and back to move the volume needed without making the noise unbearable. Drive cages and bays should be placed into the path of intake airflow - given the confines of how ATX lays out a case and if a multi-GPU setup is being used - a chassis must be chosen that takes that configuration into account.

Too many cases think that air is intelligent and will move around inside like JFK's magic bullet, of its own accord, before swiftly departing via some spuriously placed exit fan that's doing no real assisting at all. Airflow must be forced by smart fan placement and open interiors so that blockages are minimised and flow is unrestricted.

And if it sounds like common sense, that's because it is! Common sense is what created BTX, a largely thermally-driven specification created by Intel and its partners, but one which is yet to take off. The thermal modules are largely obnoxious, but the basic tenets of the spec see it place heat-producing parts of a PC system in thermal 'channels', ready for assisted cooling by the chassis.

You can achieve much the same thing with a well designed ATX chassis, too, with the right fan choice factored in. Our recommendations?

Recommended Chassis' for thermal performance

At the very high end you have Cooler Master's Stacker, a huge chassis that takes case cooling and internal capacity seriously. There's so much room inside that effective cooling is almost elementary. Up to 9 120mm fans can be popped inside to get the worst of any heat out effectively.

Gigabyte's 3D Aurora gets similar thumbs up. It tackles thermals using effective 120mm fan placement at front and rear, with a vented side panel for the opportunity of side-vented graphics cooling. Aluminium top to bottom it looks good too. Good stuff from Gigabyte. Silverstone's TJO6S is built on similar foundations, too.

Then you have Akasa's Eclipse-62. Wide enough to get 120mm front and rear with very little to cause problems in between, it's been the choice of the discerning PC enthusiast for some time, despite staid looks that do it no favours in the aesthetic department. A new front section helps that on a little, and adds a door to the drive bays at the top. You lose the useful front I/O slot, though!

What to look out for

For a fairly high-end PC, wide is good. Wide should mean 120mm front and rear (witness the Eclipse-62) and less opportunity for the chassis putting things in the way of the front-to-back airflow. Look for a 120mm exit near the CPU, too, and a separate compartment for PSU can work well, with the right PSU inside.

BTX makes most of the cooling troubles in a modern PC go away, but BTX and faux-BTX chassis are few and far between.

Summary

The best thing is that the really bad cases are quite easy to spot. When choosing a new chassis, keep your wits about you and try and visualise intake-to-exit airflow from the fan mounts available as it passes across your components. That pre-purchase sanity check will let you choose wisely. Put the aesthetic third or fourth on your list of considerations, past thermals and capacity.

Heed the wise words of the HEXUS.help guide and you can't go far wrong.


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