The idea seems simple - if more air is desired to move through a unit, stack another fan on top of the first one to increase airflow - but is this how it really works? This article will answer a common question we receive: does stacking fans double the CFM (cubic feet/minute)?
In short, stacking 2 (or more) fans will not double the airflow. Here are a few principles that explain why:
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The same amount of air is going from the first fan to all subsequent fans, and the air is traveling off the first fan’s blades at approximately a 45-degree angle, which increases turbulence on the intake side of any extra fans.
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Stacking fans increases the static pressure, which may cause difficulties in more closed systems like a computer case. Creating high pressure will make it more difficult for fans to fight this pressure to get air across the fins - this can be minimized by removing panels/increasing openings in the case.
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To counter some turbulence of stacking fans, you would have to pair a clockwise rotating fan with a counterclockwise rotating fan (such as the counterclockwise-rotating EBM Papst 700F series) - note the performance yields from this are varied and typically aren’t any more than about 20-30%.
Stacking fans directly on top of each other generally increases turbulence, static pressure, and cost without having a definite positive impact on CFM and cooling.
Ideas to increase CFM would include:
- Purchase a fan that has more CFM
- Put two fans side by side, they should move roughly twice the amount of air that a single fan is rated at.
Here’s a video that shows some of the principles discussed above:
If you have any comments or questions about this article, feel free to post your responses below.
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As a general rule, two fans in series (one behind the other) will nearly double static pressure capability but will not significantly increase air flow in a low impedance airflow system. Two fans in parallel (positioned side by side) will approach double the air flow capability but will not increase it very much in high impedance airflow systems.
However, in an enclosure system with high system airflow impedance (minimal venting, multiple obstacles, baffles, etc.) the higher static pressure provided by two fans in series can increase air flow relative to a single fan.
Here is a good source from NMB for further reading on the topic.
So what do you think is going on here then? This configuration of stacked 40x40x20 fans lasted at least two generations of Dell PowerEdge servers. Granted, they were EXTREMELY loud, but in a NOC low decibels aren’t always the highest priority. I believe they are 10,000rpm fans.
There must have been some legitimacy to the stacking of the fans in the array for the engineers to have kept this configuration for so long. Thoughts?
That looks like a very high airflow impedance environment (high static pressure due to multiple obstacles in the airflow path). As stated above, two fans stacked (especially when they are specifically designed to be stacked together) will nearly double the static pressure, and this will help push the air through the high airflow impedance system. So it makes sense to do this in such an environment.
In my experience, if there is a lot of back-pressure due to obstacles, a single fan might spin much slower than it otherwise would because of the load. When you stack two fans in its place, it is indeed correct that you only increase the static pressure, but there is a side-effect that has not been recognized or observed here. By increasing the static pressure, the pair of fans is under less load, and the pair will then spin a bit quicker than either one alone would when under load. So, increasing the static pressure results in less resistance (lower load) and that results in an increase in speed, and that, results in more airflow. Certainly not double, and maybe no increase at all if the fans spin at the same speed when stacked. Most fans are designed (and/or controlled) to spin at a particular RPM, but despite that, a load can still slow them down a bit. The increase in speed won’t be substantial, but any increase does result in increased airflow. I think David is correct, for the most part, but there is a subtle possible side effect that might give you a slight boost in airflow. However, if the fan is not under a load due to obstacles or back pressure, then stacking them won’t make any difference to the airflow at all since the fan is already spinning freely at full speed.
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