Radical Case Fans Idea

StopSign

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Dec 15, 2006
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http://img338.imageshack.us/img338/3986/untitled1gs3.jpg

The basic idea is to use side and top fans only. The front fan is only for hard drives if your case mounts them horizontally at the front (like my case).

Have two low/med speed side fans (120 mm) blowing directly onto the motherboard and hitting pretty much everything on the motherboard. I think this is a lot more effective, in theory, than front intake or a small side intake because moving air is always cooler and the faster the air is moving upon impact, the better it should cool the components. With front intake the air isn't really felt by the time it reaches the back of the case; it just kinda draws in cool air which slowly mixes with hot air inside the case.

All exhaust will be at the top. With a med/high speed fan (120 mm). Of course, it will have to be covered with something to prevent stuff from falling in. A lot of modders do this already because hot air rises and the most logical to place exhaust fans is at the top of the case, not at the back. One exhaust fan should be enough because grooves or holes will be cut out from the top to let hot air flow out naturally, in addition to the exhaust fan's draw.

You may say the air flow is not balanced here because there's clearly more intake than exhaust, but I don't think that would be a problem because the air inside the case is always warmer than outside and with enough openings (the fan grill at the back of the base is an opening) the air inside will be forced out by the large amount of intake. The power supply can also act as an exhaust if it's one of those dual-fan ones.

Anyone have input on this idea?
 

RallyMaster

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Dec 28, 2004
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Not sure how well it will work compared to a 250mm side fan. Where would you put the PSU BTW?
 

StopSign

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Dec 15, 2006
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A 250mm fan would cost much more. I don't know the performance difference either but two (or three, if you want) 120mm fans would be fairly cheap.

The PSU would go at the top in the normal spot. I have a rather large case so there's some room above, but if you don't have room up there you can drill some holes and mount the fan outside the case (on top). It will be covered anyway so whether the fan is inside or outside shouldn't matter. By cover I'm referring to a "spoiler-like" thing as found on the Antel P180.

And I'm not saying you shouldn't put fans at the back. You can put one if you like because it shouldn't have any negative effects aside from maybe additional noise.
 

Fraggable

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Jul 20, 2005
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The idea that fast moving air is cooler is wrong, that's a sensation we feel through our nerves that doesn't really apply to metals. The idea that higher airflow through a heatsink (CFM) will result in better cooling is correct.

You can stick a giant fan in a hot room and blow all day, but unless there's cooler air entering the room, the temperature of the air is not going to change just because it's moving faster ;)
 

StopSign

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Dec 15, 2006
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Yeah I know. However, what you're thinking of is having fans inside the case blowing case temperature air onto components. Of course this wouldn't lower the case temperature since you don't have any intake/outtake.

Fast moving air is a lower pressure stream than standing air, and if the temperature of that moving air is the same or lower than the temperature of the standing air inside the case, it will draw the warmer air away from the components. It's Bernoulli's Principle. Also, it should help that you are blowing a high volume of room temperature air into the case. If you keep blowing cool air into the case, you are lowering the temperature.

My only concern is having enough exhaust without using too many fans. You're already looking at 5 fans if you put a rear one in.
 

StopSign

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Dec 15, 2006
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Ok here's a few questions: How important is a balanced intake/outtake? I've seen cases that have a bunch of front 120mm intakes but not nearly as much exhaust. By theory, shouldn't it be fine as long as there is some way for the warm air to escape the case and the method doesn't necessarily have to be via a fan?
 

herbiehancock

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May 11, 2006
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I'm sorry, but I've got to ask a couple of questions about assumptions you made in your OP.

front intake the air isn't really felt by the time it reaches the back of the case; it just kinda draws in cool air which slowly mixes with hot air inside the case.

because hot air rises


I just have an observation really about those two statements....the cool air mixing slowly in the case with the warm air and the notion that warm air rises in a case....with fans going.


First, take a mid-tower case 18" x 18" x 8"....pretty standard case size for mid-tower. That gives you 1.5 cu. ft. of case space inside.

Now, I have two intake fans: one 120mm front, one 80mm side, and two exhaust fans: one 120mm rear and one 120mm in the power supply.

Let's also give these fans a CFM of air they can move.....the average I've seen on low speed 120mm fans is around 35CFM....so let's give the front fan a 35CFM. Give the side fan 22 CFM and for the exhaust, bump it up a bit.....we want to exhaust the hot air.....so 55 CFM. The power supply probably isn't going to exhaust huge amounts.....temp controlled and all....let's forget it for a moment.

So, we have 55 CFM of air moving through the case from front and side to the rear. That means we exchange the air in the case approx. 36 times a minute. But let's downgrade that due to inefficiency of the fans and also due to the fans having restricted access to air....the CFM ratings are typically done in free air, so we cannot expect them to produce their full rated air movement.

So, let's decrease the volume of air moved by 1/3.....a reasonable figure.

Now we're exchanging the case's air 23 times a minute. That's still quite a volume of air moving through the case. So where's the slow mixture of hot and cold air. Seems to me that there is a definite movement of air through the case.....strong enough to quickly mix hot and cold air, not slowly. If it were slow mixture as you allude to, air cooled cpus would not be quite as cooled to temps. as low as they are.

The air temp surrounding the cpu and its cooler would gradually increase leading to climbing cpu temps that would not decrease until you shut the system down.....idle or load.....it wouldn't matter....the cpu and other associated electronics in the case, i.e. video card, memory, capacitors on the motherboard, etc., are all generating heat well in excess of ambient temps........but we do see that temps do in fact stabilize due to ..... good movement of air through the case.

Second.....I know hot air rises...in a static environment. But when you are exchanging the air in an enclosed box 23 times a minute, I'd think there's little time for hot air to rise and settle at the top. In fact, if you search out some case reviews on Anandtech and look at the internal case temps taken in enclosed cases (done to demonstrate how well or poorly their ventilation straight from the manufacturer has been done), I think you'll see that the concentration of hot air is centered around the heat producing devices and out the rear exhaust fan......not concentrating at the top of the case.

But, if we followed the "hot air rises" mantra, we'd see the concentration of air at the top of the case being the hottest....because hot air rises, so as the heat is generated, it rises and collects, creating a hotter and hotter area, eventually maybe coming close to matching the cpu.

In fact, follow the link below......there are thermographs of working cases representing interior case temps....done during an el cheapo case roundup.

http://www.anandtech.com/casecooling/showdoc.aspx?i=2336&p=18

Notice anything strange? No big buildup of heat along the top......the top of almost every case is just barely 3C over the ambient cool air entering the front of the case. The heat is concentrated around the gpu, cpu, and towards the rear cooling fan.

We have so much air flow through the case there is no time for air to rise to the top and collect....instead it's being churned inside the case and, if properly designed and with enough intake and exhaust, expelled before it can collect at the top. Then, also remember, in conventional cases, there is the power supply providing some exhaust there, too.

Now, one may bring up the special case of the inverted case.....and while it's true that the heat producing items are much closer to the case's top, typically in those the exhaust fan is located much closer to the case's top...negating any hot air collection again.

So, I do dispute the "hot air rises so we must have a top exhaust fan" notion. I hope I presented my theories well enough.

Peace....out.

Alan
 

StopSign

Senior member
Dec 15, 2006
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Thanks a lot for this info. It makes me wonder if a single 120mm side intake would be enough.

My idea was based on my case which is quite a bit bigger than the standard mid-tower but only houses 80mm fans.