Alternate dual exhaust case fan idea for better silencing

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Alternate dual exhaust case fan idea for better silencing

Post by cmanley » Tue May 12, 2009 10:56 am

Some modern silent cases have 2 12cm exhaust fans (one at the top rear and one at the top) and a separate chamber for the PSU at the bottom. The Antec P182 is such a case.

When I think about a few factors in a silent case, this doesn't seem like an optimal combination to me. I think a better set up would be to close the top exhaust fan hole and place 2 fans in series at the top rear hole separated by at least 1cm space and ducted together (but staying clear of the CPU cooler).

These are the reasons I believe that this will make a more efficient (and therefore silent) set up:

1. Most cases resist intake airflow because of filters and small intake holes. The static pressure rating of fans becomes an increasingly important factor when air has to flow under resistance. This is an often underestimated and overseen fact.
2. The standard dual exhaust fan set up means that these fans are competing with each other. The more the intake airflow resistance the more the competition. This means a waste of rpm's and noise, and a greater chance of airflow stalling around the blades.
3. Fans in parallel can only create a maximum static pressure difference equivalent to that of a single fan.
4. The maximum pressure difference between inside and outside of the case for fans ducted in series is that of the sum of the static pressure pressure rating of each individual fan.

I'ld like to know if anybody has tried ducting fans and what the results were.

It is important to not be misled into using certain silent fans that have a relatively good air flow rating but due to their small blade surface area a very low static pressure rating. The effective airflow will be poor with such fans mounted in the case. I believe that fans that have a large surface area (most do) and a small angle of attack are the best choice in most situations because that will prevent the relative airflow from stalling around the blades. This happens when the blades angle of incidence relative to the airflow exceeds about 15 degrees. When airflow stalls, lift drops (=efficiency) and turbulence (=woosh noise) increases suddenly.

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Post by blackworx » Tue May 12, 2009 12:18 pm

You're right about the top/rear "competition" thing, but why replace that with 2 rear fans in series when 1 will do? I don't think static pressure brings about a need for 2 fans in series. Besides, by placing them in series you'll get all sorts of audible intermodulation effects because the 2 fans will never run at exactly the same speed.

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Post by Olle P » Wed May 13, 2009 12:32 am

I agree with Blackworkx. If airflow restriction is a problem it's better to have the second fan directly at the intake instead.
Another good option for more airflow through restrictions is to use a more powerful fan.

With two fans working in parallel it's also better to have them either side by side or more separated to avoid the competing effect.
In my case I have three 12cm fans side by side sucking air in through the bottom, and two fans side by side evacuating it at the upper rear.


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Post by Hypernova » Wed May 13, 2009 2:22 am

If you want effective filtered intake you need to concentrate on intake fans. More exhaust fan will hurt the filter's effectiveness.

See my build for an example that focus on filtered intakes.

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Post by cmanley » Wed May 13, 2009 4:02 am

You're all right about it being best to have a fan at the intake and a fan at the exhaust. That was my initial configuration but I forgot to mention that in my P182 case, I have 2 IcyBox fanless trays at the top that rely on airflow through the small gaps when hard disks are inserted. Using the optional fan at the intake area of the case (below those trays), actually decreases airflow around them.

In any case (excuse the pun), I think it's best to close the top fan hole. Of course 1 fan at the rear exhaust can move enough air on it's own. It's just that 2 in series can move just as much air with probably half the rpm's. However as blackworx says, there may be some new audible side affects.

The top exhaust hole is the only weak part of the P182's case in my opinion for the reason I mentioned and also because noise has a more direct path the user's ear from that hole than from the rear hole.

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Post by pcy » Wed May 13, 2009 5:02 am


I can't comment on other cases, but in the PaQ case the air filter has no measustrable effect on airflow.

The PaQ air filter is placed immediately in front of the main muffler, just inside teh front door, so you can easily access it to clean it. It is 168mm x 168mm (233mm in the new bigger case) and 5mm thick, 30ppi.

In general, I can find no evidence that the backpressure from the airflow through a computer case is ever significant (except in a laptop). In consequence there is no point at all in mounting fans in series (except accross the CPU cooler of course). The arguements about Inlet and Outlet fans, or about +ve vs -ve pressure, are all in fact irrelevant. The deviation from room pressure is far too small.

By the same token, two exhaust fans in close proximity are not competing with eachother - that would only be true of the level of airflow was large enough to mean there was significant reduction in air pressure aruond the fans.

What matters is that all the case ventialtion fans should operate in parallel, they can be all input, or all output, or all in the middle., and it makes very little difference so long as they are all in parallel, not in series.

Stictly speaking all in teh middle is best, but only because that means any noise they do make is muffled by acioutic material inside the case - nothing to do with airflow.


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good points...

Post by Rebellious » Thu May 21, 2009 8:01 am

I tried double rear exhaust fans in my build here, but couldn't measure any advantage -- viewtopic.php?t=51531

However, competition between exhaust fans happens more often than people realize. If the rear exhaust fan runs fast enough it can create sufficient negative pressure so that the slow-spinning PSU fan is defeated and heat from the PSU starts backing INTO the case.

Because of that I prefer positive case pressure, and so I'm also undecided about the top 140mm blowhole fan in my Lian-Li A17 (similar to Antec P182). Reversing the top fan (into an intake) makes a huge change in the A17.

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