The following will describe what measures to take and when to take them as far as computer thermoregulation goes. You will learn everything there is to know about computer fans, how and where to mount them.
The width and height of computer fans are measured in millimeters; common sizes for case fans include 60 mm, 80 mm, 92 mm and 120 mm. The amount of airflow which fans generate is typically measured in cubic feet per minute (CFM), and the speed of rotation is measured in revolutions per minute (RPM). Fan speeds may be controlled manually (a simple potentiometer control, for example), thermally, or by the computer hardware or by software. It is also possible to run many 12V fans from the 5 V supply, at the expense of airflow, but with reduced noise levels. Fan size and RPM also affect noise output, measured in decibels (db). As a bench a small hydro-bearing 25.5db rated hard-disk fan that spins at 3500rpm will only emit a barely noticeable hum if your case is open and your ear is next to it.
Bearings
The type of bearing used in a fan can affect its performance and noise output. Most computer fans use one of the following bearing types:
- Sleeve bearings use two surfaces lubricated with oil or grease as a friction contact. Sleeve bearings are less durable as the contact surfaces can become rough and/or the lubricant dry up, eventually leading to failure. Sleeve bearings may be more likely to fail at higher temperatures, and may perform poorly when mounted in any orientation other than vertical. The lifespan of a sleeve bearing fan may be around 40,000 hours at 50 °C. Fans that use sleeve bearings are generally cheaper than fans that use ball bearings, and are quieter at lower speeds early in their life, but can grow noisier as they age.
- Rifle bearings are similar to sleeve bearings, but are quieter and have almost as much lifespan as ball bearings. The bearing has a spiral groove in it that pumps fluid from a reservoir. This allows them to be safely mounted horizontally (unlike sleeve bearings), since the fluid being pumped lubricates the top of the shaft. The pumping also ensures sufficient lubricant on the shaft, reducing noise, and increasing lifespan.
- Ball bearings: Though generally more expensive, ball bearing fans do not suffer the same orientation limitations as sleeve bearing fans, are more durable especially at higher temperatures, and are quieter than sleeve bearing fans at higher rotation speeds. The lifespan of a ball bearing fan may be around 63,000 hours at 50 °C.
- Fluid bearings have the advantages of near-silent operation and high life expectancy (comparable to ball bearings). However, fans using fluid bearings tend to be the most expensive.
- Magnetic bearings or maglev bearings, in which the fan is repelled from the bearing by magnetism.
The standard connectors for computer fans are:
- 3-pin Molex KK connector: This connector is used when connecting a fan to the motherboard or other circuit board. It is a small thick rectangular in-line female connector with two tabs on the outer-most edge of one long side. The three pins are used for ground, +12 V power, and a tachometer signal. You may have a couple of these male connectors on your motherboard. If it’s important for you to monitor fan RPM then use fans with this connector.
- 4-pin Molex KK connector: This is a special variant of the Molex KK connector with four pins but with the locking/polarization features of a 3-pin connector. The additional pin is used to provide variable speed control. These can be plugged into 3-pin headers, but will lose their fan speed control.
- 4-pin Molex connector: This connector is used when connecting the fan directly to the power supply. It consists of two wires (red/12V and black/ground) leading to and splicing into a large in-line 4-pin male-to-female Molex connector. Every power supply comes with sufficient standard 4-pin Molex connectors so if you don’t want to figure out where your motherboard supplies the 3-pin KK family then you’ll stick with these.
The general rule of thumb for proper airflow is as follows: Front and side fans blow air inside the case (intake) while back side fans suck air out of the case (exhaust). It is very important to properly take airflow under consideration since a couple of properly placed case fans might do the job of 4 or more that circulate air in opposite directions.
Another aspect that you should take into consideration with case fans is noise. Because they rotate at above 1500rpm the metal you screw them to will tend to vibrate. While you won’t be able to visually notice this high frequency low amplitude vibration you will most definitely hear it. The first step in avoiding such unpleasantness is to use silicon or rubber screws instead of regular ones. You can find these in the same shop that you bought the fans from. Having a sturdy computer case will also work in your favor (yes, a higher price for that tower is justified by something else other than the exterior aspect of it).
Standard 80mm case fans start at less than 2€ and the cheapest hard-disk fans start at just under 4€. Depending on your case design (see picture to the left) you may have the option of mounting case fans in front of your hard disk bay. This trick might save you some money, especially if you have multiple hard drives that require cooling. The alternative would be to purchase a dedicated cooler for every HDD. Hard-disk fans have a casing the size of the drive you want cooled and attaching them is done with screws provided in the package. It’s a very easy procedure and normally you should not require additional shock absorption.
How cool is cool ?
Because just about every other computer component is made of silicone the cooler it is the better. Hard disk drives, however, store data on disk platters made of aluminum, glass and ceramic which require a rather narrow thermal operating range. A common misconception is that a colder hard drive will last longer than a hotter hard drive. A Google study seems to imply the reverse--"lower temperatures are associated with higher failure rates". Hard drives with S.M.A.R.T. reported average temperatures below 27 °C (80.6 °F) had higher failure rates than hard drives with the highest reported average temperature of 50 °C (122 °F), failure rates at least twice as high as the optimum S.M.A.R.T. reported temperature range of 36 °C (96.8 °F) to 47 °C (116.6 °F). Considering Google holds ⅓ of the entire Interweb at any given moment in time, on their own servers, I’m putting my money where Google does.
In conclusion, if at the end of the day your hard drives are colder than ideal maybe you should go back to the drawing board or throw a HDD cozy on them or snuggle them together for warmth, like I did.