Case Airflow: Positive or Negative Pressure?

[randomizer]

Its not really fence riding, but I see your point about the poll. I would go for negative if I need to, but I would rather balance it out. Not completely, but at least in the CFM pulled by intake and exhaust fans, so that

CFM intake = CFM exhaust

As for gaps etc in the case, oh well. I'm not silicon sealing my dvd-rom drive. :lol:

 

[TMSter]

What if i like a little of both? No other option.lol

 

[Findail]

CFM intake = CFM exhaust would be great, but I'm having trouble figuring out CFM exhaust due to the PSU and GPU. The 8800GTS (EVGA) has what I'm guessing is about a 60mm fan on it. I have no idea how much it pulls, or if it is variable. The PSU is even more of a pain. It's a Corsair CMPSU-620HX with a 120mm fan that varies speed based on PSU temp and that isn't controllable by software AFAIK. It's because of those variables that I started this thread in the first place. I need to figure out if I want to overshoot on exhaust or intake.

What if i like a little of both?

Then you'd be a normal guy. Who doesn't like a little of both suck and blow on occassion? But that's not a topic for these forums... :twisted:

 

[eric54]

I don't mean to steal attention from the OP, but I think my questions can help us both out since we are under similar situations.


So, i'd like to add a question into the mix. In a antec 900 case with all the fans replaced by noctua nf-s12's and the 200mm top fan on low. Would the two front intake, and one back exhaust, and the top 200mm fan create a negative, mostly balanced, or positive airflow situation?

Given the answer to my first question, what direction should the optional side fan blow?

Thirdly, the noctua's can be slowed down to 800rpm with the ULNA, is there a method you'd recommend i stack this to even things out, such as intake@800 and exhaust@1200 or vice versa. What negative airflow situation would most closely cover the exhaust of a galaxy psu?

From the information i've gathered from this forum is that overall negative is better than positive, negative allows for maximum cooling when variables such as psu fans spin up and slow down, neutral is ideal but hard to attain due to extraneous uncontrolled fan spinning. Positive seems to be best for older, cooler running equipment where you dont want dust in the case and the temperature isnt a high concern.


Edit: LOL AT FINDAIL
changed/added questions

 

[I800C0LLECT]

It seems Mpilch nailed it in his earlier posts.

There are pros and cons to negative or positive, but with an open case it would seem best to neutralize the pressure in order to maximize flow.

I have only ever seen a positive pressure case work efficiently if it was sealed. You can imagine that if you'd seal it properly then it's quite the engineering effort. Anyways, I apologize for not giving links to examples but I'm sure those interested are more than willing to google.

Pros: Noiseless system while creating vacuum with cool air.

Negative air pressure would seem to be the norm for people who care nothing for system noise or lots of swirling fans with extra wires.

Either way, I'm quite lazy myself and shoot for the balanced approach.

The best example I ever witnessed of a positive air flow case was something of a speaker box type enclosure added underneath a sealed case utilizing a 120mm fan for intake.

 

[leo2kp]

"Negative air pressure would seem to be the norm for people who care nothing for system noise or lots of swirling fans with extra wires."

Certainly you aren't suggesting that negative pressure requires more fans, are you? Negative pressure requires only more exhaust than intake, in which case you can have negative pressure, drawing in cold air, with a single exhaust fan as long as you have no intake fans. Including the PSU and a single exhaust fan, you can have negative pressure with one intake fan blowing over the HDD as long as it has equal or less CFM as the exhaust fan (PSU would be also an exhaust, adding more exhaust than intake in this case).

In fact, negative pressure requires absolutely zero additional fans than you will have by default since the PSU acts as an exhaust, although not recommended obviously, while positive pressure REQUIRES at least one intake fan to overpower the CFM of the PSU and possibly the video card (if you have that type of setup), which means you're looking for a louder, more powerful fan for the most minimal requirements in a positive-pressure system.

And you'll have air pockets

 

[I800C0LLECT]

I apologize...


I didn't mean to mislead anybody. I was only referring to many of the poorly configured "gaming" rigs I've run into.

I appreciate the clarification on my behalf. Sorry :/

 

[T8RR8R]

Overall pressure should be balanced though as long as fans are in the right positions. For example it doesn't make sense to exhaust at the bottom of the case and intake at the top no matter what pressure (=,-,+) you have, but cool air at the bottom and hot air out at the top makes the most sense. So to me balanced pressure with correct positioning should work the best.

 

[Findail]

I still find dust buildup on the rear vents from air being sucked in from those vents.

Interesting. Since I'm unable to determine the CFM on two of my exhaust fans, I'm guessing this may be the best way to tell if the case is running positive or negative. I don't have anything in the way of plain vents (I think), so I guess I can check around seams and drive bays to see if dust builds up. If it does, it's still running negative. If it doesn't, it's either positive or I do a good job of cleaning the house.

 

[leo2kp]

I have negative pressure and I find dust on all my fans, even exhaust. Dust is going to get in no matter what, and it's going to stick to all your fan blades weather it's positive or negative pressure. Dust should not be a deciding factor for how you set up your airflow since you WILL get dust with any design except for 100% passive cooling, or if you use great filters and seal every hole.

 

[eric54]

Firstly, thank you very much for your informative answers. I did a little research this morning and found out the cfm of the 200mm fan. Here's a quantitative analysis of the airflow in the 900, however the principle can be applied to any case.

High 800 RPM: 134 CFM @ ~30dbA
Medium 600 RPM: 108 CFM @ ~27 dbA
Low 400 RPM: 83 CFM @ ~24 dbA

The Noctua:
600 RPM(ULNA): 24.1CFM @ <6dbA
1200 RPM: 48 CFM @ ~17dbA

Intake
2x120mm+120mm side fan@1200RPM= (48x3) = 144 CFM

Exhaust
1x200mm+120mm @ 1200RPM = (83)+(48)=131 CFM

*PSU: average 120mmfan pushes from about 40 CFM to 80 CFM.

With the PSU Variable:

Intake = 144 CFM

Exhaust = 131+~40 = 171CFM @ low RPM/low draw
~56 = 187CFM @ Medium RPM/medium draw
~80 = 211CFM @ High/Heavy draw

Overall, with PSU, net negative CFM airflow of:

+27 CFM exhaust on Low
+43 CFM exhaust on Medium
+67 CFM exhaust on High

The PSU makes a big difference in the pressure since before it the case was slightly positive and after installation it turned the case to negative. The PSU in regards to airflow should be counted as an exhaust moving anywhere from 30 CFM to 80 CFM. So if you set up your case to have a slight tolerance towards positive before the PSU, you will net a smaller CFM difference from neutral to negative.

As for my case, here is the setup I will be applying.

Intake: 2x120mm + side 120mm @ 48 CFM/1200RPM = 144 CFM
*all fans at stock 1200RPM

Exhaust: 1x120mm + Top 200mm + PSU fan low = 147 CFM
*120mm exhaust @ 600RPM or 24.1 CFM

Negative Pressure Delta With PSU:

3 to 43 CFM

Conclusion:

Even with equal cfm pushed by fans the PSU and gfx cards are going to create negative airflow. Therefore I would recommend intake fans move more CFM than exhaust to get as close to balanced as possible. Rule of thumb, treat the PSU as an exhaust case fan.



*The figures from the 120mm fans assumes the PSU uses a 120mm fan, and the CFM on that fan is roughly equal to that of Tri-Cool fans.

 

[Problem]

Yes dust ends up on all the fans. If you have strong negative presure in the case then you will find dust around most every crevise in the case. Anywhere that air can get sucked in will have dust. If your neutral or on the positive side then you won't have this result.

My point exactly. When building this new system I chose a case based on its capacity to hold the optical and HDD's I planned to fill it with. It had to have the right form factor and look, and it also had to fit into the budget. I knew when I bought it that I would need/want to modify the case for improved airflow and cooling. I removed the air deflectors from the front of the case and added some black nylon window screen to improve the look of the front of the case.

For airflow and cooling I have the air flowing where it needs to go, and slightly more capacity on intake than exhaust. The front fan 120 blows across the HDD and towards the CPU and chip set. The 80 side fan blows in directly down on top of the CPU and Video card. There is a 80 fan on exhaust on the back panel of the chassis and the PSU fan is also blowing out. The only hot spot in the case is on top between the DVD drive and the PSU. That will be addressed with a 5-1/4" drive bay cooler for the purpose of stimulating airflow to eliminate hot spots.

Over all I think I have a well balanced system with good cooling capacity. I just try to put more air in than fans to suck it out and allow the vent holes exhaust the rest of it. Airflow is primarily front to back with the side fan blowing directly on the hottest components.

Oh yeah I also clean my cases at least twice a year.

Problem

 

[leo2kp]

Positive and negative pressure allow dust to come in either way you look at it. Positive pressure will push it away from the cracks, but you're getting dust from the intake fan unless it's filtered but you still get air pockets. Neutral will also give you dust from intakes unless filtered as well.

 

[HotFoot]

I would say the best situation is where you exhaust the hot air as directly as possible. Good examples of this are video cards that exhuast to the outside as well as PSUs. The problem is that CPUs tend not to provide this option unless you do some custom duct work.

Barring that, I would say that it's a very good idea to have good exhaust removing heat from the top-rear section of the case. The PSU tends to be in this area, so it nicely fits this bill. Cases that put the PSU on the bottom tend to have exhaust fans at the top and/or rear.

This heat extraction requires a thermal convection to bring the hot air up and away from all passively-cooled components. This is because suction is not a very directional process. When you breathe in the air comes from all directions around your mouth/nose. Blowing, however, is very directional. This is why you can blow a candle out from far away but you can't suck hard enough to blow a candle out. For this reason, if you have known hotspots that don't have active cooling of their own, it's a good idea to have case fans blowing cool air over these. This works for HDD, RAM, and NB/SB/VRM components on the motherboard.

I don't understand the proposition that negative-pressure setups would avoid dead air pockets while positive-pressure setups would allow these. So long as your case had good open vents/screens at the top/rear section, plenty of intake air would flush the heat out of the case as well as an equivalent system of exhaust fans at the top/rear with passive vents/screens at the bottom/front. Inside the case, the only difference would be that the intake-fan system will cause higher local velocities at the intakes.

I prefer exhaust fans - negative pressure - mostly because the hottest components of the computer have been designed to be towards the rear/top of the case. This design makes sense because, for example, HDDs don't have to be cooled by air that's already warmed by the CPU. This is why the intakes aren't placed at the rear/bottom with exhausts at the top/front. I preffer exhaust fans because they are closer to the hotspots. An intake fan that is far away from the electronic hotspots would have to overcome all the leakage through the case to guarantee good flow out the vents in the hotter sections of the case.

Unfortunately, this means that you will be having dust come in through all the cracks in your case, but c'est la vie.

Also, in a negative-pressure arrangement, you can provide extra cooling to an area such as the rear of the CPU socket area on the motherboard by making a vent-hole in this location. Cool air will be drawn in through this hole and cool the motherboard. I don't know how effective that could be, but it's an idea.

Side note: you will never have, in a steady-state situation (so, other than the first fraction of a second of operation), more mass flow in that mass flow out. At the speeds we're talking about, air is also functionally incompressible, however the heat will cause it to expand somewhat. You will have a slightly higher volume flow rate out than in.

Also relevant is that fan's rated CFM doesn't mean that that fan will produce that much flow when installed. Restrictions and adverse pressure gradients reduce the total volume flow rate. Consider a simple situation: a tube with one high-CFM-rated fan at the exhaust with a low-CFM fan at the intake. The total mass rate through each fan is going to be the same (and in this case, also the volume flow rate since heating is negligible). The same happens in your computer case, though you will likely have many more fans involved as well as passive vents and leaks.

 

[olgraybeard]

My thoughts on this. The real issue is cooling. For cooling, you need to remove the heat. If the air does not move across the item that requires cooling, than you fail to meet your needs. You want to control where the air flows.

Some facts:
If the fan does not have sufficient air to move, it will move less air.

The higher the net positive suction head (pressure) on a pump (fan), the more flow it will achieve.

If a pump (fan) does not have enough air to move (negative pressure), it will seek to get it from where ever it can, any crack, opening or whatever. This may mean, some of the air flow is coming from a area that results in no cooling, (like say a pci slot cover on the back plane of the case and flows along the case to the exhaust not resulting in any cooling)

So, My take on all of this:
I want to control the flow path of my air flow and have all of the air I exhaust (using my exhaust fans as my design optimal flow for achieving required cooling) to aid in ensuring my air flows effectively over all of the components I want cooled.

The purpose of the location of my fans is to create a flow path to the powered exhaust points that effectively cools all required components requiring cooling. Any extra positive pressure will result in air loss out of those paths that though no effectively cooling because I am still exhausting my desired level of air movement, will not effect the level of cooling I want.

I would seek an overall positive pressure based on my on purpose of achieving the level of cooling by setting exhaust flow as my bases and ensuring nothing impacts that cooling.

 

[jitpublisher]

I think there is no way you can have "positive" or "negative" airflow pressure unless you hook up a GMC 6-71 blower to your case. Simply put, those pitiful little case fans are not going to positive or negatively flow anything (with any degree that actually makes any difference at all) The rate of airflow thru your case will be exactly equal to whatever the slowest fan(s) are able to move- whether they be on the intake or exhaust side of things.

 

[geofelt]

Well, you certainly opened up a hot topic. Here are some of my thoughts:
1) The PSU is designed to take care of itself. It's fans will not spin until it gers hot enough, either from generating power, or from recieving hot internal air. If you have a cool case, the PSU cooling will not be contributing much.
2) I personally like positive flow because you can reduce dust with filters on the inputs.
3) The vga cards have replaced the cpu as the biggest heat generators. The dual slot direct exhaust cooling of the 8800gts/gtx is a good idea. There are, however four slots near the exhaust that send a lot of hot air back into the case to be recirculated. I have reduced my case, cpu and vga temperatures measureably by installing a slot cooler under the vga card. http://www.newegg.com/Product/Product.aspx?Item=N82E16811999704
4) I like a quiet case, and my fans are on low. It seems that the cooling vs noise equation is very much dependent on how fast the fans are running. Look at this comparison of the antec 900, solo, and lian-li: http://www.bjorn3d.com/read_pf.php?cID=996 What is interesting is that at equal noise levels, all three cases cooled equally well. At highest noise levels, the 900 was best at cooling, and at lowest noise levels, the solo was quietest. Pick your poison.
5) A good strong airflow might bypass some areas that need cooling. Has anybody installed an internal fan just to mix up any internal hot spots? Did it help?

 

[T8RR8R]

Positive pressure seems to be the way to go. Here is what I did. I took the side panel off my Armor case and place a window fan where the panel would go. Then I turn the fan on so it was sucking out all the heat from the case causing negative pressure. I then turned the fan around and created positive pressure. Between the two the positive pressure seemed to cool things better by a 1-4 degrees celius than negative pressure. Not that a huge fan like this is what we would use everyday but it creates huge CFM that no matter what's in the case or where the air is coming from it creates more air than anything else possible. We could go on about air flow but that's not what this topic is about. Since heat rises it only makes sense to put the exhaust fans near the top and intake fans near the bottom.

For you people that want near equal pressure. I did a little test for you too. My living room has 2 windows of equal size on opposite sides of the room. So I put a fan in one window blowing in and one in the other window blowing out. Then I turned one around so both were blowing in and then I turned them so they were both blowing out. I know that the outside temperature didn't change all that much...maybe 1-2F. I tried each way for 15 minutes each and the one with the equal air flow dropped the room temp the fastest. Neither the positive or negative pressure setups cooled the room any better, but also neither of them cooled the room as fast as the equal pressure setup. Uncontrollable outside temps probably played enough of a factor to mess this test up so I can't really say you'll get the same results.

So in my not so scientific and best I could do with what I have methods, I seemed to almost prove that positive pressure in a case has a slight edge on temps over negative pressure. Also near equal pressure setups would be best because they can cools the case and exhaust hot air at the same time with equal effort.

 

[olgraybeard]

I like your test. And applaud your efforts.

However, the temp change may relate to the side fan blowing air directly on the major components when trying to make the case positive.

Case positive or negative is not going to really a major concern but rather air flow and where it goes. And you can more effectively control the flow direction if an over abundance of air is available and the fans work more efficiently. That is why I side on the positive side.

JITPublisher says a case can not be positive or negative. Yes, a case can be positive as long as sufficient flow in requires flow to find paths to exit. However slight the positive pressure is, it will be there.

If T8RR8R had gone to his door to the outside and opened it with both fans blowing in, he would have felt an ever so slight pressure drop in his ears and would all of a sudden have air flowing out his door. If both of the fans where blowing out, he would experience air rushing into the door. This can only occur if a pressure differential is present. Science says so.

 

[darksidedragon]

I personally tend to having more intake CFM than exhaust, simply because you get less dust build up. My next case will actually have 1 12cm intake, 1 12cm exhaust, and of course, the PSU. Although this goes against what I've just said, it's an uber quiet case, and has a dust filter, so happy overall. I think ideal would be to have more intake than exhaust so that you don't get dust build up, but have an exhaust on the roof (to assist natural convection).

 

[senor_bob]

A lot depends on how the case was designed to operate - the TT Armor is designed for neutral to positive pressure with the side fan and 2 front intakes, and you get the best results that way. Also note that the side fan is blowing cool air right on the components that need cooling.

On the other hand look at a case with a mesh side like a CM Stacker (without side fans) or a Gigabyte 3D Aurora. On these cases positive pressure won't do much because a lot of the air you blow in from the front will just exhaust through the mesh side before even getting to the CPU and graphics card(s). These cases work better with negative pressure as the exhaust fans pull cool air in through the mesh, and they are in fact overloaded with exhaust fans by default to provide negative pressure.

 

[olgraybeard]

Geofelt brought up a good point about putting in a fan to mix up flow.

Actually, in heat transfer and fluid flow theory, laminar flow is less effective at removing heat than turbulent flow. A steady flow through an area without something that causes mixing (an example, some water heaters have what they call turbulators or some such other vague term, these are just something in the water stream that cause the flow pattern of the convection currents to mix).

Also, with laminar flow, the flow closest to the surface is slower than the flow away from the surface (caused by surface friction). Some Heat sinks have bent edges ( I think their term is wings) and the purpose is to break up the laminar flow.

This is why the side fan blowing into the air flow path can be so effective because it causes turbulence in the air flowing front to rear.

 

[olgraybeard]

Senior_Bob says you can not get positive pressure nor an air flow front to rear in the CM830. I for one would disagree. Also, the Stacker only comes with 2 fans, a front and a rear. You could put a total of 9 fans on this monster, just do not know why you would (have to try that and see if my case can fly, hehe).

And as for flow, front to rear, yep, it does that. If you have ever been in the ocean at the beach, you can have a flow path at the surface as the wave comes in as well as a flow going out below the surface at the same time. Now, if you think about that, I think you will realize that a flow can be established between any high pressure point going to a low pressure point (listen to your weather person). And as for cases, the sky is the limit for air flow from one point to another outside with no boundaries. Imagin that will ya :lol:

 

[darksidedragon]

lol, this topic reminds me too much of the Heat Transfer Theory and Design exam I just took today. Scary stuff, although most of what I've done is 2D, and has simplifying assumptions (stuff like material properties being uniform).

 

[olgraybeard]

Than this should really scare you, I tech this stuff, hahahahaha

And dont forget to include the transitional interface when laying out your heat transfer profile for each material and its respective heat transfer coefficient, :lol: :lol: :lol: :lol: :lol: :lol: :lol: