Negative Pressure

[Bona Fide]

Well this isn't orthodox "Highly Technical" material, but it isn't primarily about computers.

Let's say you have a case and the cooling system is as follows:

- 1x exhaust fan in the back
- 1x exhaust fan on top
- bottom half of front is slitted with with air holes

Now, when the system is on, both exhaust fans will be taking hot air out of the chassis. This should create negative pressure inside the case, forcing colder air through the slits in front. How viable is this? My main question is whether or not the front intakes will supply cold air as fast as the exhaust fans are ejecting hot air. Will the air system always tend towards stable pressure? Or will there always be slight negative pressure inside the chassis while the system is running?

 

[onix]

It is only through a pressure difference that you will have fluid/air transport.

 

[BigPoppa]

Negative pressure?

 

[TuxDave]

Originally posted by: BigPoppa
Negative pressure?

Yeah.. you know.. it's like when you fill a volume with negative air

 

[CycloWizard]

Originally posted by: Bona Fide
Well this isn't orthodox "Highly Technical" material, but it isn't primarily about computers.

Let's say you have a case and the cooling system is as follows:

- 1x exhaust fan in the back
- 1x exhaust fan on top
- bottom half of front is slitted with with air holes

Now, when the system is on, both exhaust fans will be taking hot air out of the chassis. This should create negative pressure inside the case, forcing colder air through the slits in front. How viable is this? My main question is whether or not the front intakes will supply cold air as fast as the exhaust fans are ejecting hot air. Will the air system always tend towards stable pressure? Or will there always be slight negative pressure inside the chassis while the system is running?

This is just conjecture on my part (I turn off that part of my brain for the weekend), but I would think you'd run the case at near-ambient pressure, maybe slightly below. I think it would depend on the fans you were using, what speed they're running at, and how big the slits in the front of the case were. That said, the viscosity of air is so low that it readily moves to offset pressure gradients, so you'd really have to do something special to create a significantly negative pressure in the case.

Originally posted by: BigPoppa
Negative pressure?

He's talking about a negative gauge pressure - where the pressure within the case is below that of the surroundings. This is different than absolute pressure, which is what you're probably used to dealing with, which can never be negative.

 

[TraumaRN]

Well I know that in the hospital we use negative pressure isolation for TB and other highly infectious diseased patients, and yes negative pressure is when the air pressure is different from the surrounding air, for example the way the system works in the hospital is there are 2 doors you have to go through to enter the patients room, open one door enter small antechamber, close door behind you, open other door, however if you open BOTH doors at once you'll feel air whoosh in as it equalizes the pressure in the room...

But getting to your question, you'd have to have an EXTREMELY airtight case and probably a complicated set of cooling fans to actually achieve a negative pressure etc etc,

It could work in theory, however it would definitely be complicated, and like all air cooling the inside of the case wouldnt be any 'colder' than the ambient air temperature unless you hooked it onto an air conditioner or something of the sort.

 

[Bona Fide]

Originally posted by: DeathBUA
Well I know that in the hospital we use negative pressure isolation for TB and other highly infectious diseased patients, and yes negative pressure is when the air pressure is different from the surrounding air, for example the way the system works in the hospital is there are 2 doors you have to go through to enter the patients room, open one door enter small antechamber, close door behind you, open other door, however if you open BOTH doors at once you'll feel air whoosh in as it equalizes the pressure in the room...

But getting to your question, you'd have to have an EXTREMELY airtight case and probably a complicated set of cooling fans to actually achieve a negative pressure etc etc,

It could work in theory, however it would definitely be complicated, and like all air cooling the inside of the case wouldnt be any 'colder' than the ambient air temperature unless you hooked it onto an air conditioner or something of the sort.

Yeah, well I don't expect it to be any cooler than ambient air, but I'm hoping that it will create a relatively weak vacuum effect so that there won't be any need for front intake fans.

Btw...I found a good reference for what I meant. The Antec P180 with the front door opened fully.

 

[damonpip]

Well, it certainly would work. However, you usually want positive pressure in a computer case so that you do not have dust building up everywhere.

 

[Mday]

air out > air in = negative pressure...

some cases are designed for that. IMHO, after seeing some, they are a hot bed for dust. Air will be sucked in through all the cracks and crevices. That 1 mm line outside your optical drive = intake = air filter. An assload of dust will collect there. If you have any holes, period, from buttons, drives, front panels, etc. they will become the intake. you will have a lot of dust collect there. Since they are not really filters, the dust will also accumulate inside your case, providing a nice layer of insulation increasing the temperature of components (not actual case) by quite a few degrees.

 

[Bona Fide]

Originally posted by: Mday
air out > air in = negative pressure...

some cases are designed for that. IMHO, after seeing some, they are a hot bed for dust. Air will be sucked in through all the cracks and crevices. That 1 mm line outside your optical drive = intake = air filter. An assload of dust will collect there. If you have any holes, period, from buttons, drives, front panels, etc. they will become the intake. you will have a lot of dust collect there. Since they are not really filters, the dust will also accumulate inside your case, providing a nice layer of insulation increasing the temperature of components (not actual case) by quite a few degrees.

Ouch...so should the case be airtight, save for the intake/exhaust fans? Or should I just try to keep the pressure equalized?

 

[TraumaRN]

Originally posted by: Bona Fide

Originally posted by: Mday
air out > air in = negative pressure...

some cases are designed for that. IMHO, after seeing some, they are a hot bed for dust. Air will be sucked in through all the cracks and crevices. That 1 mm line outside your optical drive = intake = air filter. An assload of dust will collect there. If you have any holes, period, from buttons, drives, front panels, etc. they will become the intake. you will have a lot of dust collect there. Since they are not really filters, the dust will also accumulate inside your case, providing a nice layer of insulation increasing the temperature of components (not actual case) by quite a few degrees.

Ouch...so should the case be airtight, save for the intake/exhaust fans? Or should I just try to keep the pressure equalized?

Bona Fide, like I was explaining about a person in isolation in a hospital, that room is AIRTIGHT, it has it's own separate heating and cooling and exhaust unit all separate from the main hospital ductwork and filters, it's even equipped with a system that can detect if air is leaking from somewhere other than the what air the system is pushing into the room. Your case would have to be airtight for this negative pressure deal to work I'm thinking, but as other stated it's going to draw lots of dust

 

[CycloWizard]

Originally posted by: Bona Fide
Yeah, well I don't expect it to be any cooler than ambient air, but I'm hoping that it will create a relatively weak vacuum effect so that there won't be any need for front intake fans.

Btw...I found a good reference for what I meant. The Antec P180 with the front door opened fully.

Ah, if you're just doing it to get rid of the intake fan, then you're asking the wrong question. The intake fan is just to make sure air is circulated throughout the case to prevent stagnation and the related formation of hot spots. Using your idea, there will not be much circulation through much of the case unless the exhaust fans are positioned just right.

 

[dkozloski]

Do a search on "vortex tube".

 

[Mday]

no such thing as an airtight computer case. anything you do to make it airtight will pretty much be counter productive.

best to have in = out, erring on the side of more in than out. it's not a matter of how much air you are moving, it's where you are moving the air from, and to, etc.

 

[0roo0roo]

buy plexi glass. tape to side of open case. buy fog/smoke machine. watch the flow.... more restrictive the openings the less airflow. its not rocket science.

 

[iwantanewcomputer]

that's not negative pressure, just a lower pressure than outside. it would work better to have roughly equal in/out take to move more air because in your case the fans would ave to work harder to push air from low to high pressure.

2 out 1 in is good

 

[Powermoloch]

Originally posted by: Bona Fide
Well this isn't orthodox "Highly Technical" material, but it isn't primarily about computers.

Let's say you have a case and the cooling system is as follows:

- 1x exhaust fan in the back
- 1x exhaust fan on top
- bottom half of front is slitted with with air holes

Now, when the system is on, both exhaust fans will be taking hot air out of the chassis. This should create negative pressure inside the case, forcing colder air through the slits in front. How viable is this? My main question is whether or not the front intakes will supply cold air as fast as the exhaust fans are ejecting hot air. Will the air system always tend towards stable pressure? Or will there always be slight negative pressure inside the chassis while the system is running?

you just answered your question slight negative pressure inside the chassis while the system is running