Solid Snake wrote:Another thing to note is the current that these fans draw at 12 and 6 volts. My guess is that the fan will draw half the current at 6v (you can easily test this). This amounts to 25% power and although RPM and CFM remain at 50% (I was wrong on the RPM of this fan) something is lost.
When you say "something is lost", I would say "something is saved", basically you save energy because you get roughly the same airflow and consume less electricity.
It might sound too good to be true, but here is my explanation:
the electrical power that a fan consumes is not directly transformed into pressure & airflow. There is a loss.
The output power of a fan is airflow multiplied by pressure (airflow in cubic meters per second, pressure in pascals).
For example, if a fan runs at full speed in a completely open environment, the static pressure it creates is 0, therefore it doesn't give any power at all, but still consumes electricity. In this extreme case, efficienty is 0%
In a computer case, static pressure is relatively small in comparison to what the fan is capable to do, so we are nearly in the same case: fan efficiency is very low.
Let's say that it is 5%, for example( this is arbitrary ). Using 2 fans in parallel at half the voltage might increase efficiency to 10%.
But you can't keep multiplying fans and reducing their voltage like that forever, because at some point the fans will be so weak that the static pressure of the case will be relatively high for them, and then it's a different problem.
Solid Snake wrote:Just as in an electrical system where volts are useless without amps or in a mechanical system where rpm is useless without torque, with fans, CFM is useless without the pressure to back it up. Since we're just blowing air through a case, we don't need extreme pressures, but cases that use ducted CPU exhausts or other restrictive passages need more pressure than others.
I'm sure you know that when you put two fans in parellel their CFM is (roughly) doubled.
When you couple fans, their static pressure is doubled.
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I don't agree. In a computer case, mounting 2 fans in series will not double the static pressure.
In a system with a very high impedance, then yes, it would (roughly).
You can't choose both the pressure and airflow. They depend on each other. For a given pressure, the case will determine what airflow it allows.
For a given airflow, the case will determine what the pressure needs to be.
Solid Snake wrote:Just as you noted, the CFM of a fan goes up somewhat linearly with the voltage. The same is true with static pressure. A fan's air moving ability is somewhat a product of these two numbers.
Have a look at this article:
http://www.pcper.com/article.php?aid=-1 ... pert&pid=8
Solid Snake wrote:Assuming a 6v fan pulls half the current of a 12v fan:
Running the 2 fans at 6v probably pulls half the power of one fan at 12v. Doesn't it seem strange that 2 6v fans each moving 20cfm consume half the wattage of a single 12v fan moving 40cfm? The catch is that you're now running at half the static pressure which will translate to a much lower CFM once the fans are installed than if the 12v fan was installed.
My guess is that if you want the same air moving ability (CFM and pressure) as a 12v Nexus you'll need 4 fans running at 6v in a series/parellel configuration. Two side by side with the other two stacked on top. This will also probably give you the same power consumption as a single 12v Nexus.
You can't control both airflow and pressure indepandently.
Have a look at this page:
www.adda.com.tw/engineering/info5-2-1-s.htm
On the "Fan Curve" graph, the red "System Resistance Curve " shows you that for 1 pressure value, there is only 1 possible airflow value, and vice versa.
Solid Snake wrote:
Check the current draws of the Nexus fan at 12v and 6v. If they're different, that's a red flag that the air moving ability of 2x6v fans will probably not match the performance of a 1x12v fan in anything but a free air environment.
On that same graph there is an efficiency curve. You can see how it varies... it is near 0 in the "free flow" case, at the far right of the graphic. Installing fans in parallel will help get closer to optimum efficiency, so it is not surprising that the power consumption decreases.
Electrical power consumed is not equal to the power transfered to the airflow.
On that same page, there is a graph called "System Flow Rate Adding ", which shows you the benefits of 2 fans in parallel or in series.
On that graph, the system has a high impedance, so the "series" option ( in red ) is preferable to the "parallel" one (in blue) . In PC cases, it's usually the opposite: impedance is relatively low.