Positive or Negative air pressure?

[Bigmac80]

What's better for cooling positive or negative air pressure? I've read and seen in videos that positive air pressure does make your cpu run cooler and makes your system less dusty. But with Negative air pressure it releases all the hot air in your case faster. But whichone is actually better for overall cooling?

Reason im asking is because i have a Lian-li mid tower case with 1 140mm intake and 1 120mm exhaust fan. I was thinking of either getting the front bezel that comes with a 120mm mounting fan to intake air or a top cover that comes with 2 140mm fans as a exhaust. Also if i did get the top cover with the 2 fans would it be reasonable to put the front 140mm fan a intake and the rear 140mm fan as a exhaust or is that just sucking all the hot air back in from the case? Or should i just get the front bezel and the top cover?

Here's the top cover.

http://www.frozencpu.com/products/10797/cpa-353/Lian_Li_Mid_Tower_Top_Cover_w_Dual_140mm_Fan_Holes_for_PC-60FN_7FN_-_Black_T-LM25B-1.html?tl=g43c241s1113

front bay cover.

http://www.frozencpu.com/products/5433/cpa-155/Lian_Li_Triple_525_Bay_120mm_Fan_Module_-_Black_CCFANMODULE.html?tl=g43c241s612

 

[dirtmountain]

For cooling i'd go with negative air pressure, for cleanliness i'd go with positive air pressure.

 

[Leaps-from-Shadows]

As long as every important area of your system is getting the airflow it needs, the overall pressure doesn't matter.

With the dual-fan top, both should be exhausting. Since heat rises, having a top intake isn't a good idea.

I have equal pressure in my case, with two front and one side for intake and two top and one rear for exhaust.

 

[jaguarskx]

I rely on a simply intake fan (bottom) / exhaust fan (top) case. Additionally, I only buy video cards that exhaust the fan out the rear of the case.

The one exception is my HTPC which has a passively cooled 9600GT for "silent running". I'll probably toss in my HD 5850 just to find out how much louder the HTPC will be because of it.

 

[Ubrales]

I go with positive pressure. All things equal, positive pressure will improve heat transfer and will aid the exhaust fans.

 

[oc_mania]

Negative should be better for cooling because it will get the hot air out of the case, but it will get dust in it.

 

[cjl]

Personally, I'd go for a slight positive pressure. It'll keep the dust down and improve the flow out of the outlet fans.

 

[13thmonkey]

positive, as it will keep it clean and therefore help with keeping the cooling good.

 

[lennyc]

Unless there is a dust filter on the intake with positive pressure, the amount of dust entering the case is proportional to the air flow regardless of positive or negative pressure. ( with negative air pressure there are many air entry points that would need filtering.)

 

[cjl]

The amount entering the case, yes, but with positive pressure, the airflow velocity out will be fairly high, and the inflow velocity will be slow. This will tend to carry the dust all the way through.

 

[lennyc]

Whether you blow in 10cfm or suck out 10cfm the air velocity in a given volume is the same. Look under the cpu fan blowing onto a heat sink (positive pressure) you will find lots of dust there after a period of time.

 

[cjl]

But a "10CFM" fan has a pressure-dependent airflow curve. Usually, the rated flow rate (assuming it's accurate at all) is for a zero pressure differential. If the flow is from a lower pressure region to a higher pressure region, the flow will be below rated, and the velocity will similarly be lower than rated. If the flow is from a higher pressure region to a lower pressure region, then the airflow will be above rated and the velocity will also be higher.

Similarly, if a fan has a pressure rating, that's usually the pressure at zero airflow. The most accurate spec would be a plot of pressure vs airflow. On a positive pressure casing, intake fans will have an adverse pressure gradient across the fan disk, causing lower air velocity and flow, but outlet fans will have a favorable pressure gradient, increasing flow rate and velocity. On a negative pressure case, the intake fans will have a favorable pressure gradient, and the outlet fans will have an adverse gradient.

 

[lennyc]

Or simply stated the more air you blow though the better the cooling ( whether positive or negative pressure). Yes you have to blow more in or suck more out to have a pressure difference. If the inlet and outlet are the same size the same amount of air will flow though each ie. the same air velocity. Hence why either the intake or the outlet fan must work harder to have a pressure difference. It's simpler if you look at a case with only one fan.

 

[HEXiT]

you should always have a very slightly positive pressure in your case or you will suck air in through vents and any crease or fold in your case not to mention your dvd/bdr drives drives. leading to excessive dust and negating any benefit you gain from your filters.
the more air you can get in the case the better as the heat transfers to the air and is then ejected negative pressure as i see it has no place in a pc case.

 

[lennyc]

I will agree only if you have filters installed.

 

[blackhawk1928]

What's better for cooling positive or negative air pressure? I've read and seen in videos that positive air pressure does make your cpu run cooler and makes your system less dusty. But with Negative air pressure it releases all the hot air in your case faster. But whichone is actually better for overall cooling?

Reason im asking is because i have a Lian-li mid tower case with 1 140mm intake and 1 120mm exhaust fan. I was thinking of either getting the front bezel that comes with a 120mm mounting fan to intake air or a top cover that comes with 2 140mm fans as a exhaust. Also if i did get the top cover with the 2 fans would it be reasonable to put the front 140mm fan a intake and the rear 140mm fan as a exhaust or is that just sucking all the hot air back in from the case? Or should i just get the front bezel and the top cover?

Here's the top cover.

http://www.frozencpu.com/products/10797/cpa-353/Lian_Li_Mid_Tower_Top_Cover_w_Dual_140mm_Fan_Holes_for_PC-60FN_7FN_-_Black_T-LM25B-1.html?tl=g43c241s1113

front bay cover.

http://www.frozencpu.com/products/5433/cpa-155/Lian_Li_Triple_525_Bay_120mm_Fan_Module_-_Black_CCFANMODULE.html?tl=g43c241s612

Thats correct, however I recommend positive pressure. If postive pressure cools your components enough, then keep it positive as less dust means it will keep the cooling good for a long time, vs negative pressure will result in quick dust accumulations and unless maintained and cleaned a lot, will take away the benefit of negative pressure.