Calculating Air Pressure

[Bobe]

I'm still researching my new build and I'm now trying to simulate the airflow in the case to ensure I get positive pressure. The case is a 750D and currently I'm planning to use this configuration:

 

2x SP140 front intake

2x AF120 bottom intake

1x AF140 rear exhaust

2x SP120 top rad exhaust (H100i)

1x AF120 top exhaust

 

What is the correct way of estimating the total pressure inside the system? What I did was add up the CFM and mmH2O (separately) of the intake and exhaust (based on the tech specs on the Corsair website), subtracted the latter from the former and came up with the following:

 

-29.71 CFM

-0.63 mmH2O

 

Is this valid or am I just making up nonsensical arithmetic?

 

If I am doing it right and the result shows a negative pressure system, should I just remove the top AF120 fan (above the 5 1/4" bays)?

[blackadder]

I'm still researching my new build and I'm now trying to simulate the airflow in the case to ensure I get positive pressure. The case is a 750D and currently I'm planning to use this configuration:

 

2x SP140 front intake

2x AF120 bottom intake

1x AF140 rear exhaust

2x SP120 top rad exhaust (H100i)

1x AF120 top exhaust

 

What is the correct way of estimating the total pressure inside the system? What I did was add up the CFM and mmH2O (separately) of the intake and exhaust (based on the tech specs on the Corsair website), subtracted the latter from the former and came up with the following:

 

-29.71 CFM

-0.63 mmH2O

 

Is this valid or am I just making up nonsensical arithmetic?

 

If I am doing it right and the result shows a negative pressure system, should I just remove the top AF120 fan (above the 5 1/4" bays)?

the correct way is using software and MATH! or having pressure sensors for a cheap way. adding them isn't the worst way to think about it. you will get no where close to what the manufacture says on the box, those are under the best  testing conditions. for a rough estimate you could use your volume of your case then the flow rate of fans. ft^3/CFM would give you the amount of air is being displaced per minute in your case, you are looking at a ratio so something like 1/1 is great and you may not notice any improvement over this. also time is a big factor, we are rating air flow in feet per min and we would also do the same for heat and you can loose this much heat, normally measured in watts. all this is really a distribution equations and take a beastly computer to do accurately.

 

tl:dr if you have high temps after you build, check and make sure nothing is wrong, then think about getting more fans for better air or less noise. don't waste money. with all that being said i bough more fans than i needed. temps won't decrease by much unless you are overheating to begin with.    dust will be worse with more fans even with positive pressure. 

[Elapo]

I usually just look at CFM.

 

If more goes in than comes out you have positive pressure. 

[Bobe]

dust will be worse with more fans even with positive pressure. 

 

That's interesting, I don't ever see that being brought up in these discussions. Would you say then that the two 120 bottom intakes are a bad idea? I figured with the 750D's restricted front panel that the extra airflow would be a big help. I would have the case on a clean desk away from any potentially dusty areas.

 

As a side question, I'm still debating between the H100i with two SP120 exhaust and an extra AF120 (as described above) or to use a H110i which would mean just two SP140s at the top. The latter is $40 more (factoring in the fan changes)

[blackadder]

That's interesting, I don't ever see that being brought up in these discussions. Would you say then that the two 120 bottom intakes are a bad idea? I figured with the 750D's restricted front panel that the extra airflow would be a big help. I would have the case on a clean desk away from any potentially dusty areas.

As a side question, I'm still debating between the H100i with two SP120 exhaust and an extra AF120 (as described above) or to use a H110i which would mean just two SP140s at the top. The latter is $40 more (factoring in the fan changes)

for the price you can fine better fans, i just reccomended a few manufacturers in a topic below. The bottom fans i have never used, i feel like they may mess up the flow from the front fans and, normally that aren't blowing at anything important. Dust is of little concern if you clean one a month, air can 5 mins or so, no matter how many fans. I currently have 7 running, smoke in my room occasionally, and dust hasn't been a big problem as long as intakes are filtered. Now the fans placements you said in op are stellar besides the bottom (not needed) you could run fans at a pretty low rpm and still get the needed airflow,unless your components are room heaters you should be fine with stock or slighly more fans 3 seems to be the sweetspot when air cooling, pump noise might be one of the louder things at idle.

Rant about corsair fans. For just a little more you can get much better fans that will last longer. I feel the price doesn't justify their performance. They make good looking fans and great cases.

[Bobe]

For just a little more you can get much better fans that will last longer. I feel the price doesn't justify their performance. They make good looking fans and great cases.

 

I hear that a lot, but my local retailer doesn't have a huge range. I don't have a great deal of experience with fans, but I feel as though one of the only brands that would perform significantly better are Noctua, but I'm willing to sacrifice the performance and longevity to avoid their horrible aesthetic.

[ThomasD]

Math is a good guestimate at best because everything changes from rated specs once they begin interacting with each other and the exact specifics of your case.  Also, being a rigid container there can be localized areas of negative and positive pressure (e.g. an "exhaust" fan will often pull air in through a nearby unrestricted vent.)

 

The best indicator of total flow is system temperature - if you are getting good temps you are probably getting good flow.

 

A good way to test for positive/negative pressure is some dry ice in a mug of water to generate water vapor.  You hold that near vents/gaps and see which way the vapor moves.  Here in the States dry ice can often be found in grocery stores.