How is positive/negative pressure calculated?

[Grishnaackh]

Hi!

I hope this post is under the right sub forum. I'm wondering how negative and positive pressure is calculated inside the case. Is it the total amount of static pressure that is relevant, the total airflow or is it an equation consisting of both these factors? There is no real reason why I'm aksing, besides curiosity. But to make a practical example; Let's say you have two 140mm AF-fans as intake and a radiator with two 140mm SP-fans as exhaust. Would the case have a negative or positive pressure,  given that both type of fans are top of the line?

 

I know it's a strange question, and it might even be hard to say without having any fan specs. But I'm wondering what the result would be in general.

[Billy_Mays]

Having one exhaust and two intake is positive, have two intakes and exhaust that's neutral, and if there more on intake it's positive and more on exhuast it's negative 

[manikyath]

calculated is probably the wrong word, the pressure is more so a result of your work than it is an outcome of a formula.

 

even just minor dust buildup can change the outcome.

[Hawoezeka]

sp fans move less air so positive

 

as long as the intakes are not restricted by anything like fan filters

Edited August 5, 2017 by Hawoezeka

[Grishnaackh]

16 minutes ago, Hawoezeka said:

sp fans move less air so positive

 

as long as the intakes are not restricted by anything like fan filters

Thanks! That was the answer I was looking for.

[Jonathan Lemmens]

18 minutes ago, Grishnaackh said:

Hi!

I hope this post is under the right sub forum. I'm wondering how negative and positive pressure is calculated inside the case. Is it the total amount of static pressure that is relevant, the total airflow or is it an equation consisting of both these factors? There is no real reason why I'm aksing, besides curiosity. But to make a practical example; Let's say you have two 140mm AF-fans as intake and a radiator with two 140mm SP-fans as exhaust. Would the case have a negative or positive pressure,  given that both type of fans are top of the line?

 

I know it's a strange question, and it might even be hard to say without having any fan specs. But I'm wondering what the result would be in general.

In the scenario you posted I guess it'll be close to neutral, maybe leaning towards positive. The problem is that the numbers on the box don't mean very much, because in the factory static pressure is measured at 0 airflow, and max airflow is measured at 0 resistance. Neither is a real world scenario.

 

In general a static pressure rated fan will perform decent to very well as an airflow fan, simply because they need to be able to move lots of air to even build up the pressure they are rated for. Typically an SP-fan has fewer larger blades, slightly stronger/bigger motor and draws more power at similar RPM than an AF-fan.

 

On the other hand, an airflow rated fan will not do very well in front of a radiator or a thick fan filter. The blades are designed such that they have the least air resistance in order to move more of it in a non restricted environment. AF-fan typically have more, skinnier blades and larger spaces between the blades, and are often quieter than their SP counterparts at similar speeds.

 

I hope this satisfied your curiosity  

A little more info on how corsair tests their fans:

 

[Grishnaackh]

1 hour ago, Jonathan Lemmens said:

In the scenario you posted I guess it'll be close to neutral, maybe leaning towards positive. The problem is that the numbers on the box don't mean very much, because in the factory static pressure is measured at 0 airflow, and max airflow is measured at 0 resistance. Neither is a real world scenario.

 

In general a static pressure rated fan will perform decent to very well as an airflow fan, simply because they need to be able to move lots of air to even build up the pressure they are rated for. Typically an SP-fan has fewer larger blades, slightly stronger/bigger motor and draws more power at similar RPM than an AF-fan.

 

On the other hand, an airflow rated fan will not do very well in front of a radiator or a thick fan filter. The blades are designed such that they have the least air resistance in order to move more of it in a non restricted environment. AF-fan typically have more, skinnier blades and larger spaces between the blades, and are often quieter than their SP counterparts at similar speeds.

 

I hope this satisfied your curiosity  

A little more info on how corsair tests their fans:

 

Thank you for the more elaborate answer!

[LogicalDrm]

In simple way: Air going in - air going out. If there is more air going in than coming out, you have positive pressure. If there is more air coming out than going in, you have negative pressure. If these are about same, you have neutral pressure.

 

In practice. If you have box with only fan mounts and nothing inside and no other holes, more fans or fans moving more air works just like that. But everything obstructing airflow at any point will have effect.

 

So continuing with simple things. Having more fans bringing air in than taking it out will be positive. If you have same amount of same size fans, you have negative pressure (because there usually is more obstruction with intakes). If you have same size and same amount of fans, but intake's are spinning faster than exhausts, you have positive.

[ravenshrike]

Buy a set of incense sticks. If the smoke is pulled in from anywhere but your intake fan vents, you have negative pressure. If the smoke is pushed away from anywhere but your exhaust fan vents, you have positive pressure. If the smoke doesn't do either you have close enough to neutral pressure.

[SageOfSpice]

8 hours ago, ravenshrike said:

Buy a set of incense sticks. If the smoke is pulled in from anywhere but your intake fan vents, you have negative pressure. If the smoke is pushed away from anywhere but your exhaust fan vents, you have positive pressure. If the smoke doesn't do either you have close enough to neutral pressure.

That seems like a bad plan. You probably don't want a bunch of soot all over your sensitive electronics.

[ravenshrike]

12 minutes ago, SageOfSpice said:

That seems like a bad plan. You probably don't want a bunch of soot all over your sensitive electronics.

The amount of soot that might coat the fans and electronics from 1-3 incense sticks is the definition of negligible.

[SageOfSpice]

2 minutes ago, ravenshrike said:

The amount of soot that might coat the fans and electronics from 1-3 incense sticks is the definition of negligible.

Negligibility is a subjective perspective. You'd be surprised how much smoke those incense put out. Not to mention, incense are commonly made from varying plant matters that when burned, become alkaline in nature. This could lead to corrosion.

 

 

[Jonathan Lemmens]

Just now, SageOfSpice said:

Negligibility is a subjective perspective. You'd be surprised how much smoke those incense put out. Not to mention, incense are commonly made from varying plant matters that when burned, become alkaline in nature. This could lead to corrosion.

If you burn a stick of incense in front of your PC every day, then maybe you'd have a point. 

 

If you do this just once to find the airflow characteristics of your case, there is no danger whatsoever. 

[ravenshrike]

45 minutes ago, Jonathan Lemmens said:

If you burn a stick of incense in front of your PC every day, then maybe you'd have a point. 

 

If you do this just once to find the airflow characteristics of your case, there is no danger whatsoever. 

Especially since you're running the fans which means the soot will have minimal time to settle.