[SOLVED] calculating for slight positive pressure

[kluchgts]

I'm getting the arctic liquid freezer ii 360. The p12 fans that come with them are 56.3 cfm but they have to go through the radiator. When choosing case fans how much lower in cfm should I be looking for if I want slight positive pressure?

 

[geofelt]

What is your case, and where will you mount the radiator?

 

[JohnBonhamsGhost]

that depends on a lot. the CFM rating isn't the only factor that comes into play here.

even if you have different airflow and\or air pressure with your intake or exhaust fans,
using more of them or having some of them running at higher or lower speeds would negate this stat.

if you are using the cooler in exhaust mode with proper ventilation placed in the top of your case,
having an extra rear exhaust plus 3 or 4 intakes with the same or higher stats than the P12 should give you a positive case pressure.

 

[Paperdoc]

VERY rough guess to start, about 10 to 20% less for the max air flow. And of course that's TOTAL capacity for all of your exhaust fans, versus the combined capacity (somewhat reduced) for THREE of those rad fans. But use that only as a starting point - the calcs are VERY rough. Sometimes you cannot find exactly the right fans, so some that give too much air flow for exhaust are better, because you can always slow them down.

Then what? Once you have it all running with OS and applications installed so you can experiment with workloads, try this IF you want to check and maybe tweak it. First, I will assume that your mobo, like many today, has a feature on each SYS_FAN header usually called "Manual" or "Custom" fan curves in the header Profile choices. This is the place where you can set your own graph of what speed the fan should run for several measured temperatures. That is how you can "tweak" the fan air flows to what you think best for various workloads. Make sure that the SYS_FAN header in use for the exhaust fans is set to use the motherboard temperature sensor, and not the one inside the CPU chip. To start his, set the header to use the Standard (pre-set) fan curve.

How to judge the "right" speeds? A smoke tracer test. You need a small source of smoke, like a smouldering cigarette or an incense stick. With the system running some workload, use the system monitoring tool it comes with to show you on-screen the temperature of the motherboard sensor that guides the SYS_FAN headers, and the fan speed. Now go around the case to wherever there is a small hole or gap that air can flow through - near a front optical drive or cover plate, or case side plate edges, etc. Put the smoke source near that and observe which way the smoke flows, and roughly how fast. Ideally you want that smoke to blow AWAY from the case slowly. Record smoke flow, mobo temp, and fan speed. Change the workload and repeat. Try to cover three or more workloads. With that info you can decide whether the Standard fan curve in use is right or needs to be altered in the range of temperatures you have observed. Assuming you have not covered ALL of that temp range, make your own extrapolations. If you decide to change, go into BIOS Setup, change to using a custom fan curve, and repeat the process to decide if your new settings are better. Repeat if necessary.

 

[kluchgts]

What is your case, and where will you mount the radiator?

it's the corsair air 540 and I'm putting it on the front as intake. Shopping for my exhaust fans

edit: I tried putting it on top as exhaust with a 280 and it was blocking basically everything up top so I can't do that

 

[USAFRet]

it's the corsair air 540 and I'm putting it on the front as intake. Shopping for my exhaust fans

edit: I tried putting it on top as exhaust with a 280 and it was blocking basically everything up top so I can't do that

I have an Air 540, with a Cryorig A80 (280mm) top mounted, and it works just fine.

The 2 included front intake fans, the 1 included rear exhaust, and the Cryorig top mount.

 

[kluchgts]

is that setup for positive pressure? or was it something you just put together

 

[USAFRet]

is that setup for positive pressure? or was it something you just put together

Just put together with no calc on pos or neg pressure.

Parts specs below
Sitting here at mostly idle, CPU temp @ 30-33C.
GPU @30C

I don't think I've seen it go over 71 and 73C respectively, even under the most stringent stress test.

 

[Paperdoc]

Thanks for Best Solution.