What is Negative Pressure?

[xTempered]

Hey guys I started my build today with the help of my Uncle but after completing the build my uncle said that the inside of the case would experience a thing called negative pressure. He explained it to me a bit but I didnt really understand it.

He then simply said I need another intake fan because I have 3 exhaust and 1 intake. Could someone explain what negative pressure is?

 

[Karsten75]

You suck more air out of the case than you blow into it.

Exhaust cfm (cubic feet/minute) > intake cfm.

 

[ksham]

Negative pressure means that more air is leaving the case than air is entering the case. This means that to maintain equilibrium, air needs to enter your case somehow. That is through any hole that air can get in.

In other words, you are pulling all air that is in the case to out of the case. By doing so, you are indirectly forcing air to enter the case through undesired places and usually not dust-filtered places. That means you can get a lot of dust build-up. It's almost like poking a large hole in a balloon and trying to blow it up. It'll never work because the second you blow into it, it leaves immediately.

To put into perspective, the opposite of negative internal air pressure is positive internal air pressure. This means that more air is entering the case than leaving. So at all times, your case will always be filled with air. This is great to prevent dust build up. Why? Because if your case is filled with so much air, it must leave somehow when more air is pumped into it. So it leaves through any tiny cracks it can find. So dust cannot come in from those tiny cracks. At the same time, if you dust-filter all your intakes, your case internal should be relatively dust-free. It's like poking a small hole in a balloon and blowing it up. Air will be leaving through the small hole and dust cannot get in because air is leaving through that hole.

I prefer positive internal air pressure.

 

[ssddx]

i'll put it very simplistic (perhaps too simplistic... but hey...)

it may be helpful if you thought of it as water instead of air (sometimes we forget that air can have pressure too).

an example would be a swimming pool being filled up with a hose with a hole in the side leaking out water. if the hole is too big and there is more water getting out than in (negative pressure) the swimming pool would empty. if there was more water going in then getting out then it would be negative pressure.

getting back to airflow physics... puff your cheecks up with air then open your mouth a bit. notice how air escapes and blows out? that is because it is under pressure and wants out. likewise if you pucker them in by sucking with your mouth closed and then open your mouth slightly, air rushes in since the positive pressure outside naturally pushes its way into low pressue zones.

negative pressure will pull dust into your case through nooks and crannies which is why positive pressure is better. positive pressure will blow out through these same nooks and crannies so will result in a bit less dust. if you use dust filters on your fans that is.

 

[Karadjgne]

Negative pressure in layman's terms is called a vacuum. So if you run more exhaust than intake, you create a weak vacuum inside the case, will suck in every practical of dust it can find near any crack or unused fan hole. Dust is bad.

2 fans out is ok if fans are the same, if not, then use software or fan controller to turn exhaust lower than intake. 2 fans in and 1 out is better. Dust cannot enter a hole that air is blowing out from.

 

[xTempered]

Negative pressure means that more air is leaving the case than air is entering the case. This means that to maintain equilibrium, air needs to enter your case somehow. That is through any hole that air can get in.

In other words, you are pulling all air that is in the case to out of the case. By doing so, you are indirectly forcing air to enter the case through undesired places and usually not dust-filtered places. That means you can get a lot of dust build-up. It's almost like poking a large hole in a balloon and trying to blow it up. It'll never work because the second you blow into it, it leaves immediately.

To put into perspective, the opposite of negative internal air pressure is positive internal air pressure. This means that more air is entering the case than leaving. So at all times, your case will always be filled with air. This is great to prevent dust build up. Why? Because if your case is filled with so much air, it must leave somehow when more air is pumped into it. So it leaves through any tiny cracks it can find. So dust cannot come in from those tiny cracks. At the same time, if you dust-filter all your intakes, your case internal should be relatively dust-free. It's like poking a small hole in a balloon and blowing it up. Air will be leaving through the small hole and dust cannot get in because air is leaving through that hole.

I prefer positive internal air pressure.

Wow thanks that analogy was really easy to understand. Guess I'll get 2 more fans for intake instead of 1 since it seems like positive pressure is much better

 

[ssddx]

you dont want to get too carried away. you only want "slight" positive pressure.

 

[ksham]

Wow thanks that analogy was really easy to understand. Guess I'll get 2 more fans for intake instead of 1 since it seems like positive pressure is much better

Actually, it's not quite that simple. Positioning of the fans is also important. Here's an example of a bad setup with a lot of intake fans. You would THINK that positive internal air pressure (any kind) is good, but it's not.

Two fans in the front for intake. Two fans on the side for intake. Two fans on the bottom for intake. Two fans at the top for exhaust. And one fan at the rear for exhaust. So that's 6 intake vs 3 exhaust. So very big positive internal air pressure there. But it's not a good setup. Why? The short answer is that there is no good airflow. There's no synergy. The long answer is as follows: the two bottom intake is actually bad. It blocks the air going directly into the back of the case from the front intakes and pushes the air straight up. The side intakes also doesn't help because it just pushes air straight at the GPU but doesn't assist in the bottom or front intakes. And actually, it's worse for the front intakes because it's cut off at two places, each pushing in different direction.

Ideally, you want your air current to flow nicely from the bottom front of the case to the top rear of the case so that it moves the air nicely along. Nothing worse than having a pocket of warm air stuck somewhere because your air current misses it and doesn't move it.

It's like trying putting a bunch of fans in a single room, each blowing in a random direction. That just scrambles the air around aimlessly. You're better off with just one fan.

 

[xTempered]

Wow thanks that analogy was really easy to understand. Guess I'll get 2 more fans for intake instead of 1 since it seems like positive pressure is much better

Actually, it's not quite that simple. Positioning of the fans is also important. Here's an example of a bad setup with a lot of intake fans. You would THINK that positive internal air pressure (any kind) is good, but it's not.

Two fans in the front for intake. Two fans on the side for intake. Two fans on the bottom for intake. Two fans at the top for exhaust. And one fan at the rear for exhaust. So that's 6 intake vs 3 exhaust. So very big positive internal air pressure there. But it's not a good setup. Why? The short answer is that there is no good airflow. There's no synergy. The long answer is as follows: the two bottom intake is actually bad. It blocks the air going directly into the back of the case from the front intakes and pushes the air straight up. The side intakes also doesn't help because it just pushes air straight at the GPU but doesn't assist in the bottom or front intakes. And actually, it's worse for the front intakes because it's cut off at two places, each pushing in different direction.

Ideally, you want your air current to flow nicely from the bottom front of the case to the top rear of the case so that it moves the air nicely along. Nothing worse than having a pocket of warm air stuck somewhere because your air current misses it and doesn't move it.

It's like trying putting a bunch of fans in a single room, each blowing in a random direction. That just scrambles the air around aimlessly. You're better off with just one fan.

Well my case has the normal intake fan that came with it the the front panel with a extra empty space there along w/ space for another 120mm along the bottom of the case about right in the middle so I was planning on putting the fans there.

To me this seems like air would flow fine but im not entirely sure. 6 fans in total 2 front intake pushing air in then a middle buttom intake pushing air up then 2 top mounted exhaust fans w 1 rear exhaust fan

 

[ksham]

The problem with the bottom intake is that it pushes air up so that air will never reach the back of the case or the back of the GPU, which means airflow is not optimal in that area.

 

[xTempered]

The problem with the bottom intake is that it pushes air up so that air will never reach the back of the case or the back of the GPU, which means airflow is not optimal in that area.

Then would it be best just to get the single extra 120mm fan for the front intake making it 2 front panel intake then just have my GPU be the one to direct airflow upward? It has a rear exhuast for itself but it has the twin frozr fans both pushing air up in a sense.

 

[ksham]

The GPU generally doesn't direct airflow upward at all. It intakes from the bottom and spread out to the sides. But two side intakes and one rear is usually good. Hot air naturally rises so leave mother nature to that.