Thermal chimney to replace case fans - calculations here

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coteesh
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Thermal chimney to replace case fans - calculations here

Post by coteesh » Mon Aug 22, 2005 9:18 pm

There's a lack of thermal chimney data on this site from what I can tell, it seems to be mostly experimental, and some bad experiences seem to have made them unpopular to some extent. I've never made one, however I have done engineering projects at work where I am required to calculate the air flow rate inside of incinerators (like a huge chimney with huge amount of heat injected from waste fuel). I believe the thermal chimney is feasible as a silent source of air flow, as my calculations suggest:

http://www.naiad.ca/chimney/

Previous calculations I've seen on this site only use the internal case temperature as a given parameter... in my calculations, I use the estimated heat dissipation inside the case - this is a much better starting point to work from. I've included some graphs with a 60" high chimney at various heats and diameters, results are CFM and temperatures of chimney/case air. I've included the Excel calculation sheet as well in case anyone wants to play.

The key is to have a sealed and insulated case and chimney so that there is just one intake and one exhaust. I also think a fanless PSU should be placed inside the sealed case and not outside of it, to help boost the chimney effect (preferably at the top of the case near the chimney opening so that the heat from the PSU mostly goes up the chimney and doesn't get spread around inside the case). A personal goal of mine is to come up with a fanless modern system that runs at high clock rates, where the hard drives are the only moving parts, and the system remains very stable, using off the shelf heatsinks. http://forums.silentpcreview.com/viewto ... 811#205811

Anyhow, the reason I'm posting is because I made these calculations and figured somebody that visits this site might be able to make use of them, thus increasing the chances of somebody making a completely fanless computer with minimal moving parts at a relatively low cost.... and then hopefully they will share their success story with us ;)

I wonder if anyone else has experimented with large thermal chimneys? In a text file located at the first link above I discuss "air changes per minute" and how that is important... Apple has done some very careful engineering work with the Cube and this is a great example to use. My thoughts are to put a bigger chimney on a relatively large case and minimize internal case volume (i.e. stuff the case with foam) to pump up the air-changes-per-minute, and we may be able to get a high performance modern PC with almost no moving parts... and a big ugly pipe in the living room.

cotdt
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Post by cotdt » Mon Aug 22, 2005 10:05 pm

i couldn't find any info on this topic. what is it exactly?

coteesh
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Location: Calgary, Alberta, Canada

Post by coteesh » Tue Aug 23, 2005 6:55 am

It’s easiest to explain using an analogy. Things that are less dense than water float, for example, certain oils float on top of water. The same is true of any fluid – some gasses are less dense (or lighter than) air, and they rise up in it.

Air becomes less dense when it is heated, and rises, much like how a hot air balloon operates. This is called the buoyant force, and it can drive an updraft flow from a hot source. (this is how a chimney in a fireplace works).

The bigger the chimney, the stronger the updraft… and the calculations I’ve included here are simply measuring how big a chimney I need to get the CFM flow rates I want. So far I think I may be able to get the same flow rate of about 2 case fans using a 12” chimney that’s 5 ft high.

EndoSteel
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Post by EndoSteel » Fri Aug 26, 2005 10:50 am

I guess, placing a heater in the upper section of a chimney would improve it's performance greatly. However, in this case you'll have to direct the exaust somewhere outside your room unless you don't want to make it a sauna :).

cotdt
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Post by cotdt » Fri Aug 26, 2005 11:12 am

ahhh... good idea. you can even make it 50 ft high out of thick cardboard.

Pjotor
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Post by Pjotor » Sun Aug 28, 2005 9:54 am

cotdt wrote:ahhh... good idea. you can even make it 50 ft high out of thick cardboard.
I'd just love to see someone lug a 50' chimney to a lan party.

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