Understanding Case Fans

Considerations that Help You Improve Cooling Performance

by Fantronic.com


Fan Dimensions

Case Fans are available in a wide variety of sizes. There are small 30mm fans and 40mm fans such as you’d find in a laptop or car audio amplifier, and there are larger 80mm fans and 40mm fans which are commonly used for cooling computer cases. All case fans have one thing in common – their outer dimensions are square, so if a fan is referred to as a 120mm fan, the 120mm is the measure of the outer length of the fan’s square case. Case fans vary in depth, which is why you commonly see notations such as “120x25mm Fans” or “40x10mm fans”. The number following the “x” is referring to the fan’s depth. As a general rule, selecting the largest fan that fits your application, both in terms of length and depth, will result in optimal cooling with reduced noise.

Fan Blade Design

There are a lot of misconceptions out there about how the number of fan blades and their shape affects a case fan’s cooling abilities. Some case fans have as many as 10 blades, while others have only 3 blades. As more blades are added to a case fan of a given size, the width of the blades decreases. The narrower blades can operate at higher speeds without making as much noise, but contrary to common belief – more fan blades reduce the fan’s overall efficiency. This is one reason a silent fan is so quiet, even if it is operating at the same RPM as a louder fan. On the other hand, a case fan with only 3 blades will sound loud and choppy as the huge blades slice through the air, but the larger blades move more air per revolution and have a higher overall efficiency. Fan blades are rarely a consideration for overclockers, but in an application where flow efficiency outweighs noise levels, a fan with fewer blades will get the job done better.

Airflow and Noise Levels

Fan noise increases proportionally with air flow, and the noise level to air flow ratio is something that must be considered when the application will be used in close proximity to yourself or other people. 80mm fans used to be the most popular size, but in the past few years, people have made more use of 120mm fans due to their ability to move more air at drastically lower noise levels. There are some instances where bigger is not better, such as with CPU cooling. The reason is that the majority of the airflow generated by a fan comes from its outer edges, and the center part has a much lower flow rate. The area around the fan’s motor hub has zero airflow. Whenever directional cooling is required, it is typically better to use a smaller 80mm fan or even a 92mm fan, even if the noise levels are greater.

Static Air Pressure

The relationship between a PC fan and its static air pressure rating is a murky topic for many. To simplify and make it easier to understand, a fan that has a higher air pressure rating can maintain its rated air flow at higher resistances. A resistance for a fan would be something like a radiator or having to pull or push air through a fan filter (static pressure is not an issue with fan grills, which offer no appreciable flow resistance). Water cooling is a popular application where you need to consider a case fan’s static air pressure rating. If the pressure rating of the fan is too low, the fan will not be able to maintain an adequate air flow through the resistance imposed by the radiator. Yate Loon fans are a great example of case fans that have high static air pressure, yet are affordably priced. If you plan to use a foam fan filter, you will probably want a fan with a higher static pressure rating. Mesh Fan Filters offer much better airflow but do not filter as well.

Intake Fan or Exhaust Fan

A lot of PC enthusiasts opt to use both intake and exhaust fans to cool their case, and they refer to that configuration as a push-pull setup. When it comes to cooling a PC case effectively and without excessive noise, using one or more high mounted exhaust fans tends to work the best. The hot air from the components in the case will rise, and having the fans mounted in the upper-rear of the case will yield the most efficient flow of air. Most modern PC cases have front intake fans as well – the front fans are not so much for lowering the case’s internal temperature, rather they are directed to cool hard drives. Intake fans can help improve overall case cooling, however it is possible to achieve fully optimal cooling with properly placed exhaust fans. A single 120mm fan works great, or on smaller cases you can use two low noise 80mm fans such as the Panaflo fan or Enermax Enlobal fan. If you are building a rackmount server or need powerful cooling in a tight space, blower fans may be the answer.