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 Post subject: Does it make sense to have two fans in series?
PostPosted: Wed Nov 22, 2006 1:27 am 
This might be a dumb question, but anyhow, having two identical fans instead of one should result in doubled airflow. What if they're placed serially?

I thought I could use two slow fans for my Scythe Ninja to cut down the noise. One would blow normally towards the ducted heatsink and the other would blow away from it as a case fan. I'm just not sure if the other fan will make any extra airflow as, thanks to the first fan, the air is already flowing and the other fan can't accelerate it as much as the first one.

So, does it make any sense doing this?


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PostPosted: Wed Nov 22, 2006 2:54 am 
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Quote:
This might be a dumb question, but anyhow, having two identical fans instead of one should result in doubled airflow. What if they're placed serially?


There are no dumb questions—only the questions we did not ask. :wink: In free air, fans in parallel doubles airflow, fans in series doubles static pressure.

http://www.nmbtech.com/engineering_101.html

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Two fans in parallel double the airflow only in the hypothetical free air situation. If the enclosure has a high static pressure, this arrangement provides very little increase in flow. Two fans in series double the static pressure at shut-off (when no air flows), but do not increase the airflow in the free air situation. An additional fan in parallel to the first increases airflow in a low static pressure situation, whereas an additional fan in series increases the airflow in a higher static-pressure enclosure.


The Ninja was designed to offer low resistance to flow, but experiment, the only way to know for sure is by trying it! :wink:


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PostPosted: Wed Nov 22, 2006 3:12 am 
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As jaganath stated, you'll have to try out your idea to see the results. There are no hard rules here, because of the many factors. Blowing two fans at each other is usually a bad idea, from a noise standpoint. But blowing both in the same direction can be successful, particularly at low fan speeds.

I have tried a number of setups like this, with success. Try running fan#2 with a thermal controller. When you turn on fan#1 in the airflow path, fan #2 can actually slow down. This is caused by the positive pressure on fan#2, which allows it to blow enough cfm to keep the sensor/CPU at the same temp at a lower rpm. Hard to explain, I know...... :lol:

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PostPosted: Wed Nov 22, 2006 7:18 am 
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Hello & welcome to SPCR!

Some folks have reported (small?) improvements by using the frame of a second fan to form a short tube. This apparently helps the air flow. But the only fans that do this make a lot more noise, with the only improvement being higher pressure, as others have already noted.

Cutting out the stamped metal grills probably helps more, both for lower noise and better air flow.

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PostPosted: Wed Nov 22, 2006 7:26 am 
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You could even try counter-rotating fans if you want really high pressure (and noise!) :lol:


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 Post subject:
PostPosted: Thu Nov 23, 2006 7:08 am 
NeilBlanchard wrote:
Hello & welcome to SPCR!

Thanks you, though I've already been lurking for quite a while.

Quote:
I have tried a number of setups like this, with success. Try running fan#2 with a thermal controller. When you turn on fan#1 in the airflow path, fan #2 can actually slow down. This is caused by the positive pressure on fan#2, which allows it to blow enough cfm to keep the sensor/CPU at the same temp at a lower rpm. Hard to explain, I know

That's pretty much what I was thinking. The 2nd fan would kind of help exhaust the slowed down air coming from the heatsink and make it easier for the 1st one to blow, or something like that. I was just hoping that someone would know for sure if it helps at all, but looks like I need to experiment a bit or just go for one fan.

Quote:
Cutting out the stamped metal grills probably helps more, both for lower noise and better air flow.

I know, but I'm still a bit afraid of cutting my newish p150. I currently have the Antec stock fan at the slowest position with Ninja, and Ninja's fan, slowed down to maybe ~6-8 volts with resistors, cooling my hard disk. I think I'm going to replace those with Nexus fans first and see how they do.


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 Post subject:
PostPosted: Thu Nov 23, 2006 9:32 am 
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I have just had an interesting thought for anyone who has a really hot cpu (Prescot), has anyone tried mounting 2 fans on a ninja on 2 sides that meet at a corner.???

Just an interesting idea, my thoughts were along the lines of watercooling, turbulance has a huge POSITIVE effect on watercooling, one fan from the bottom, and one from the side would create turbulance which means that all of the surface area of the fins would recieve airflow, and as 2 or the 4 sides are essentially blocked all of the airflow must leave the other 2 sides (and the hole in the middle). Of course this will increase noise, and is not going to be easy to mount, and would be pointless for anything less than a Prescot.


Andy

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