Changing Fan Speeds

Posted on Feb 5, 2014

The DC Brushless Fan is the best solution for now, but some predictions are that computers will soon be at 70dBA or the same noise level of freeway traffic. In the future we all may be liquid cooling but the heat has to go somewhere. Since the DC Brushless Fan is the mainstay of enclosures now, I researched how it actually operates as most HSF and Case fans are of this type.

Changing Fan Speeds
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There are two types that are used in computers these days. They are the axial fan and the blower fan. The axial fan works best where the static pressure of the enclosure is not too great. The blower works better when the pressures are higher. The tradeoff is that the blower has a higher noise figure for the same air movement. The axial fan can be used in enclosures with higher static pressures if they are used in series, one blowing in and one blowing out. The static pressure of an enclosure is changed by the obstructions in the airflow. From my research it seems to be almost impossible to calculate, and expensive to empirically determine. In other words, experimentation is the key to good results. I will talk about the axial fan as it is adequate for most enclosures, and has lower noise. These days we are varying the speed of our fans to decrease noise while adequately cooling our enclosures. We can achieve the same airflow by using larger fans or more fans at a lower speed, while the noise stays below that of smaller fans at higher speed. Let us start by examining the DC Brushless Fan. Today there are two types of fans generally used. One is a sleeve bearing fan and the other is a ball bearing fan. The ball bearing fan is slightly more expensive than the sleeve bearing fan. The advantages of the ball bearing fan are that it has a longer lifetime when used in an ambient temperature above 20C. It can also be mounted both...

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