THE FAN ZONE
Fans are simple enough, right? Not quite. There are a few things you need to learn before you can pick with confidence. By Chris Lloyd
Fans are simple, right? Not quite. There are a few things you need to learn before you can pick with confidence.
PASS ELECTRICITY through a conductor, and it gets hot. All the little electrons bump into things and transfer energy. This bit of basic physics is useful for an electric fire, but a real annoyance for your silicon, which needs to be kept cool. Back in the day, the only fan you might have had on a computer was in the power supply, blowing hot air out. The Intel 486 was the first processor to need a fan of its own, on the faster versions anyway. The original Pentium could just about manage on passive cooling in its slower incarnations, but was likely to pop under load if you weren’t careful (no universal fail-safe thermal cut-offs back then—if you overheated it, you may well have killed it). Since then, every chip has needed dedicated cooling. Graphics cards soon started to get too hot to run unaided, too. The fans got bigger, and spread. Now, some motherboard chipsets, drives, and even memory modules sport fans. And case fans have proliferated into banks of spinning LED lights all over the case.
If you run an unmodified rig, the fans it came with will work well enough, albeit too noisily for comfort. If nothing else, a good fan is quieter. If you start upgrading and overclocking, adequate cooling is imperative—time to add new fans. At least they’re simple. Actually, like every aspect of PCs, fans have their own specifications and variations in design and function, so there are a few things it would be useful to know first. Although getting the “right” number and position is still something of a dark art.
THE SPECIFICATIONS
The primary stats to look for are how much air the fan can shift, and how much noise it makes. Airflow is measured in CFM—cubic feet per minute. The fan’s “push” or static pressure is usually quoted using the delightfully archaic measure of millimeters of water. Higher is better. Noise level is measured in dB, a similarly obscure value, which takes careful reading (see box below). Among the specs you’ll also find its maximum rotational speed, bearing type, control method, MTBF, blade design, and anything else the marketing men think you might be interested in, or impressed by, including fancy rubber mounts, color schemes, and LED lights.
The most basic stat is size. Case and processor fans have been getting bigger—80 and 92mm soon grew to 120 and 140mm, then 200 and 230mm. A bigger fan means lower speed for the same airflow, so if you have room, go big. Unless you fancy cutting holes, your case defines how many and what size you can fit. A decent case offers a good choice of number and position.
Next is the most basic function of any fan: moving air. CFM is an intuitive measurement. A 120mm fan can shift 40–100 CFM, depending on its blade design and rotational speed. This is a wide range, which isn’t terribly helpful information on its own. The big 200 and 230mm case fans can move more air—100–150 CFM—and, more importantly, do it at lower speeds.
How much air do you need to shift? Opinion is divided; there’s a huge number of variables at play here, but, very roughly, around 50 CFM or so should keep everything frosty enough on a fairly standard rig. If you’ve been tinkering and upgrading, you might like to bump that up a bit. You won’t need hundreds, though.
The internal layout has a huge impact on airflow: The tighter and more restrictive the interior space is, the harder the fans have to work, and the more likely it is that you’ll get hotspots. The CFM figure is measured on a completely unobstructed fan, too—as soon as you add some back pressure, the performance drops off.
It’s also worth remembering to match your input and output fans. Tweaking these values can give you a negative or positive pressure inside your case. The jury is out on which is best, or whether it makes much difference at all. The idea is that if you push more air into your PC, it’s pressurized a little, which stops air finding its way through cracks and gaps, and bringing in dust. In theory, it keeps the inside nice and clean, assuming you’ve got effective air filters over the input fans. Detractors point out that it can lead to a build up of heat, and doesn’t really work well anyway.
A fan’s static pressure is how much force it can apply to the airflow. You don’t need to know how or why this is measured in millimeters of water, just know that more is better. A typical 120mm case fan runs to 1.0–2.0mm. A high static pressure fan is good for forcing air into all the nooks and crannies of your machine, or through a radiator attached to a water cooler, say.
Airflow, air pressure, and rotational speed are all related, and there’s a trade-off involved. Fans are often designated as “static pressure” or “airflow,” designed to maximize
static pressure or CFM respectively. This boils down to the design of the rotor blades. Static pressure fans have thick blades with narrow gaps, leaving the air little place to go but through the fan, but less air is shifted with each rotation. On the exhaust side, the airflow tends to spread out into a cone. These are usually recommended for front case input fans, or if you have to mount the fan in a confined space.
Blades designed for maximum airflow are much thinner—you can get your fingers between them on a 120mm. The exhaust flow is tighter and more directional—handy if you have a specific hotspot to cool. They are usually recommended as rear-mounted exhaust fans, good at shifting large volumes quickly against minimal back pressure. Hybrid designs mix blade shapes as a compromise between the two approaches, and often recommended as CPU fans.
The maximum quoted rpm is not quite as important as it sounds. Depending on the blade design, it defines the maximum airflow and pressure values. To keep things quiet and unstressed, you don’t really want to run at full tilt anyway. The noise it makes at typical operating speeds is important, though; a noisy fan is always an annoying fan. Better to have a big one or multiples working lazily, than one blowing its heart out to reach the cooling you need. On variable speed fans, you have a maximum and minimum dB level. The lower figure is as useful to know as the maximum—it gives you a good idea of how much quieter it is running below its maximum.
BEARING DOWN
Now for bearings. Old-school sleeve bearings have been around in one