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Fan Speeds and Case Air Pressure


RetroGamesIn4k
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Hi there! Probably has been asked somewhere at some point, but I have a question. So I’m making my build with the 570x mirror black. I have the H150i Pro in the front as intake push/pull, I have the Corsair hydro series gfx gtx 1080ti with its aio cooler as exhaust in the rear fan slot and I have 2 exhaust fans on top. All fans are ML 120s and 120 Pros.

 

I was wondering what type of pressure is better, Positive or Negative? Does it make a real difference? What is the static pressure that is marketed on the ML 120s? If all my fans and rads are linked through Corsair Link with the Commander Pro is there presets that take care of this for me?

 

Also if I switched my rear exhaust fan that is my radiator for my gpu to push/pull exhaust, will that affect my air pressure as well?

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There are two different types of "pressure" you are asking about.

 

"Static Pressure" as it is labeled on many fan specifications, is the measure of how much resistance it takes to slow a fan speed to zero RPM. That means the value is extremely RPM dependent. The greatest fan in the world will have a piddly static pressure value at 150 rpm and the worst radiator fan of all time will have a flattering value at 3000 rpm. The example I used to offer is this fan that has a static pressure value of around 2.00mm H20. That rates better than many 140mm fan. However, this one is 40x40x20mm and runs at 4000 rpm. You don't want it on your radiator. So why do people reference this at all? What you really want on your radiator (or other high resistance pathway) is a fan with a fewer number of wide, flat blades. These push air in a more direct path and are less affected by heavier resistance like radiators. The opposite is something like a AF140, with 11-15 thin, steeply rakes blades. These fans are more efficient at moving air and will do it with less noise than the flat blade fan. However, their performance is substantially affected by resistance. The static pressure value is a consequence of that blade design.

 

How does static pressure fit into this? It doesn't, other than serving a rough measure of how much a fan is affected by resistance. So, the conundrum is is something like this: Which moves more air through a radiator? A high static pressure fan that can move less air, but is more efficient at pushing it through a radiator? Or a high airflow fan than is less efficient at moving air through the radiator? So pretend a AF140 can move 60 cfm through free air, but only 30 cfm on a radiator (at max speed). SP140 can move 45 cfm at max speed in free air, but 30 cfm through the radiator. OK, so the same? More or less, when at max speed. Most fans will overcome the resistance at higher speeds, but it is the lower end of the spectrum where the static pressure fan will be better. Since most of us don't like high fan speed on the radiator from a noise perspective, the static pressure models are a better choice so we can run them slower and still get good cooling. So how much is a significant difference in static pressure? I would say about 0.30mm of H20 at the same RPM. It is the last bit that causes issues. You will have to get out the calculator and do a little math to adjust static pressure values to a like RPM across fans. Technically, P-Q curves for fans are not strictly linear and this is an approximation, however that is all we can do and it works well enough. Be careful, 120mm fans in particular tend to have a little lift at the end of the P-Q curve. This means a fan with a 1800 rpm max may appear to have a much better SP number than a 1600 rpm fan just below it. However, that advantage is likely insignificant when you adjust both fans to the 1200 rpm you want to run. Just to make this messy, a lot of current fans do not fit clearly into an "airflow/AF" or "static pressure/SP" category. Many of them are hybrids in between the two archtypes. This is a good thing and makes them useful in all locations and possibly the most efficient at medium speeds where most of us want to run. The ML, HD, and LL series are all of that "hybrid" type. You can use these on anything.

 

The "air pressure" inside your case is another matter altogether, but an easier one to understand. There is a ton of internet babble of wanting a "high pressure" case and laughably some take this to the extreme with all intake fans. Funny enough, that is what I usually suggest to people who are bent on creating a high pressure case -- turn all your fans inward. That is usually when a moment of common sense takes over. The general idea has merit, but gets stretched out too far. If you have more intake air flow than exhaust, pressure will build in the case. In a heavily vented case, this will push air out of those vents. This is useful for weird shaped cases or in higher heat cases to force localized GPU waste heat out the back (where you find most venting). However, that also means the opposite is true. If you have more exhaust flow compared to intake, air will be pulled into the case through the venting. Not overly disadvantageous, but it might bring dust(!) into your case. Oddly, a lot of enthusiasts have a real thing against dusting or cleaning their case, so this gets a lot of attention. In a tight, sealed case, this may have significantly less value and the only way out is through the exhaust fans. Where this becomes problematic is when people start doing crazy things like using 5 intake fans and 1 exhaust and thinking the pressure will take care of it. It does not. Your goal with case fan management is still to remove warm air from the case and bring cooler air in to replace it. As such, if you take 60 cfm out, you want to bring 60 cfm in to replace it. When you get extremely unbalanced, you run into issues. Even if you set up 5 intake fans at 60 cfm each for 300 cfm in, the one exhaust fan is still 60 cfm out. You cannot force 240 cubic feet of air out the venting. You will hear your door hinges squeak before that happens. You are looking for balance, and if that balance is slightly tilted toward intake, then fine. But you still want to set the case up in balance, 3 in/4out or the reverse. Where you tip the balance in your favor is with FAN SPEED. This is the easiest way to control the flow rates, not by setting up a 5 in/1 out configuration. Keep in mind a fan on radiator likely can only move half of its rated free air volume of air when it faces resistance like that. Also, this is a clearly an estimation game unless you have a lot scientific tools to do your measuring. You don't need them. Estimation works fine and fan speeds tend to be variable. No reason to go crazy trying to calculate dynamic air flow loads.

 

Most people who set-up their 570x case with the radiator on the front in and the top and rear as exhaust. Since you can't bring a ton of air in through the radiator, you don't need a lot of exhaust speed either and those fans can run slow. Some people might experiment with running the top fans as intake and forcing it all out the back. That's one you would have to try and see, but I would favor the first. Your situation is a little different since the GPU in on a AIO cooler and that also means you don't have a lot of heat to get rid of inside the case (i.e, you don't need a lot of fan speed). What might be an interesting thing to try is setting up the H150i Pro on the front rail (only place it can go), but turn the fans to exhaust. GPU radiator on the back, also set to exhaust. This means the top fans become intake. This dumps all of your heat out of the case and would maintain a balance of intake/exhaust. You can always tip the pressure in your favor with a little more top fan speed. The more traditional method would be H150i front intake, top fans exhaust to get the CPU waste heat out, then the GPU radiator as exhaust at the rear. You keep moderate fan speed on the front at all times to keep the air exchange rate up and don't go too wild on top exhaust fan speed. I can't tell you which one will be more advantageous. Most of the time, you need to set it up and see for yourself. Either way, you are not going to see critical temperature differences. You can let visual appeal be part of the decision making.

Edited by c-attack
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  • 5 weeks later...
This case has alot of air flow, so creating as much as a positive i think is the best bet. I put every fan i had positive. Having the filters on the front and top keeps dust out. I did put one fan on the rear as exhaust although I don't think it does much. I also mounted that one on the outside of the case so as not to cover the cool rgb. This is a great case, but if you had a negative pressure you get a lot of dust in the case.
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I have the 570x and have tried all the methods you can for cooling and airflow in it ;)

 

You certainly need positive pressure in the 570x or it will fill it's self up with dust and fluff,,

 

Both Front and Top are filtered,, use both these as intake and the rear as exhaust.. excess air escapes from all the gaps around the glass!.. technically it's an open case ;)..

 

I also run push/pull in the front

 

 

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Note...

 

the fan on PSU shroud is now on the back of my new H150i :)

Edited by Zotty
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