[Splinter]

Okay so I have designed a computer case and was getting some feedback on it over at SPCR. Somone brought up that he thought the fans would be progressivly starved for air the further away from the rear intake hole they were... I wasn't so sure and we went back and forth a bit and I decided to you guys since I KNOW I have seen some smart people in here before...

[Bobboau]

I'm not sure what you mean, but internal fans (like on a CPU or vid card) only work to pull heat off of a small point and push it into the air, the case fans are designed to exchange the air in the case with the room air, so as long as you have flow through the case the internal fans should still work effectively.

but as I said, I'm still not sure what exactly you are asking.

[Splinter]

here look:



the intake fans are all at the bottom of the case in an isolated chamber lined up one after the other and the only entrance for air is at the back of the case. So the question was would the first fan in line suck up all the air and starve the fans further down the line.

[captain-custard]

the first fan would take the majority of air if it was left like this , but if you make the 1st fan less powerfull it would take less air , this is a fluid dynamics question so what is needed is the area of the intake and the tube that follows and the power of each fan to create a mathmatical model, you could try inserting a piece of plastic in the entree angled so that it isolates a part of the entree that isolates the first fan from the others.....

[Herra Tohtori]

I would put filtered intakes to the sides of the bottom chamber. Not because of what you brought up, but because you're going to get a lot of airflow through that hole in the rear, and the smaller the hole, the faster the air will flow through it... and if you don't design the intake well it might start to make turbulence and noise when the pressure differential created by the fans grows big enough (ie. the amount of air they suck in). Depends of the fan capacity as well, of course. I don't think the airflow of the later fans will be meaningfully hampered by the location of the intake hole in this kind of scale, unless you're planning on putting in some jet compressor fans or something. There will be some effect, but how much is a different thing.

Actually what happens is that each fan creates the same pressure differential between the "up" and "down" side of it. If the chamber below is sealed, this will move some air from the lower chamber into upper, and then the fans will work to retain the pressure differential between chambers. If you open the back of the chamber, the pressure differential will start to evan out starting from the rear, and instead of a pressure differential there will be an airflow from the lower chamber to upper chamber. But, because the fans at the back are further from the intake, the air can't get to them as easily as the first ones, so the air pressure in the chamber will reduce the further you get from the intake, and that will slightly reduce the amount of air available for the last fans to transfer, but at the pressure differentials and air transfer volumes that PC fans are capable... I don't think that's the thing you need to concentrate on. Building bigger intakes to the sides is a better solution for other reasons IMO; it would just take care of that potential issue as well.

Making a bigger hole for the air to flow will reduce that (and as a bonus it will pretty much ensure equal airflow for each fan).

Also, think what happens when you place the HD tray above the last fan full of stuff. Same with optical drive location, actually. And all the horizontally placed extension cards like GPU's and sound cards and network interface cards and other stuff. Does the case really need that much fans, and is the vertical airflow really the ideal solution here?

Also... as far as I can see, it would be good to have an equal amount of intake and output fans, at least roughly. If you have more intake fans than outtake fans you'll create an overpressure inside the case and air will start bleeding out from each seam and hole available to it, and you lose some energy to maintaining the overpressure, and you don't get as much control over the airflow... In this case that might not be an issue though, since your case has almost open upside, but in more "sealed" cases it is worth thinking about.


Nota bene: I have no experience building PC cases, but I do have some insight into physics and, by extension, fluid dynamics...

[Splinter]

So if I understand correctly the first fan WILL have easier access to air but nothing significant?

The reason I don't open up the sides is because this case is built around silence as well as cooling. by creating the most indirect path from fans to ears it should be more silent... however. If as you say the demand for air through that hole in the back would be high enough to create a loud noise this defeats the entire purpose and I will have to rethink things. So how can I go about making sure the demand for air through that size hole won't create "turbulence and noise". If they are your run of the mill fans. I have 3 120mm fans lets say each is 50CFM and 1 180mm lets say that's 70CFM... so that 220CFM through the hole that is 2 3/4" x 7" (HxW)... a bit more than 3.5 CFM per second right? now how do I find out if that makes of noise...

None of the extension cards are horizontal... the motherboard is flipped 90 degrees... that's the whole basic premis that started me on this case... I wanted to make use of hot air rising and being able to place all the exhaust fans at the top and all the intake fans at the bottom so they can all be covered by one single dust filter that you slide out the front. So I'm not sure I understand what you meant about "And all the horizontally placed extension cards like GPU's and sound cards and network interface cards and other stuff."

about the ratio between exhaust and intake fans I wanted to go for positive airflow. So Instead of having every extra ventilation hole sucking in dust, air is now being forced out through every possible exit and in conjunction with the fan filter creating a (hopefully) dustless environment.

BTW: the picture I had is obviously without the side panels... and they reach all the way up and all the way down. Dunno if that makes a diffrence.

[Aardwolf]

Just put red paint and racing stripes on it.

[portej05]

If you're going for silence, make sure that you stick a filter on it somewhere - cleaning out the heatsinks on the processor and GPU slowed them down big time (and thus much more quiert) when I was trying to figure out why it was doing 4000rpm on a 30° (C) day

[Scuddie]

Why not just use small multiple side ducts instead of one large rear duct?  If you isolate the fans to their own duct, the air supply will be nearly identical for all fans, and you will be able to control the bottlenecks and airflow that much easier.

[Splinter]

If you're going for silence, make sure that you stick a filter on it somewhere - cleaning out the heatsinks on the processor and GPU slowed them down big time (and thus much more quiert) when I was trying to figure out why it was doing 4000rpm on a 30° (C) day

There is a dust filter that covers the entire area below the intake fans and that you can slide out the front of the case and wash as seen here:

Why not just use small multiple side ducts instead of one large rear duct?  If you isolate the fans to their own duct, the air supply will be nearly identical for all fans, and you will be able to control the bottlenecks and airflow that much easier.

I was trying to avoid, as much as possible, opening direct vents to the fans because this would significantly increase the noise you get from the fans. However I realized the virtue of your and others suggestions and that I had to create more room for the air to enter so I decided that because the feet lift the case up off the floor 1/4" if I ventilated the bottom in addition to having that rear intake this would create enough room for air to flow freely... I hope.