physics/fan question?

[Sushiman]

Ok guys, i've bene working on modding my case to be ultra cool (thats being temp cool and looking cool). Here is my question. I have installed one fan (120mm) on the case side panel over the cpu (flow in 38cfm). Now i also installed one on top of the case (again 120mm flow out 38cfm). Now the temp has dropped by about 10 degrees celcius, which i was extremely suprised/pleased with.

*interupt note... im writing this letter, and my power unit appears to be crapping out, but i guess that's what happens when you run a computer for 3 years straight without really turning it off for more than a day per year. end interupt note*

Now i noticed another drop in temp when i added a 3rd fan. but i have not carved out the case yet, and the airflow was somewhat restricted. So assuming a completely sealed case (which it is)

1)if i have one fan blowing in 38cfm, and just an open hole somewhere, does that mean its coming out of the hole at 38cfm

2)in reality i have one blowing in at 38cfm and one blowing outat 38cfm, does that actually bring the cfm up higher since the outflow is "pulling" air out.

3)would it be useful to have 2 inflow and 1 outflow? or will i stress the outflow fan, and/or cause a buildup of pressurized air inside the case.

4)vice versa, would it be useful to have 1 inflow and 2 outflow, so would this pull the "hot" air out quicker.

I would test the last two... but i cant reverse flow without pulling it all apart and reconnecting everything, which takes approx 30 mins to do... then testing temps in between would take at least 12 hours to allow for proper cooling testing of everything. Thanks for answers to any and/or all of the questions. I know you can help

[gaccm]

http://www.amdmb.com/article-display.php?ArticleID=132


it holds all the answers

[Sushiman]

Ok, well that article answered questions 1 and 2 quite well. But i'm still a little confused on 3 and 4. Having only 1 year of college physics under my belt and no engineering.. i am a little confused at the article.

It said that having higher case pressure is good, how do i do this? I have a sealed environment at the moment, every hole and crack is sealed with tape or some compound. The only openings are for the power supply fan, and the two fans which at the moment do not have filters (which i guess i should add so i dont have dust coming in like crazy... and does that mean i should only have it on the intake fan right?)

[Sonny]

Originally posted by Sushiman
1)if i have one fan blowing in 38cfm, and just an open hole somewhere, does that mean its coming out of the hole at 38cfm

2)in reality i have one blowing in at 38cfm and one blowing outat 38cfm, does that actually bring the cfm up higher since the outflow is "pulling" air out.

3)would it be useful to have 2 inflow and 1 outflow? or will i stress the outflow fan, and/or cause a buildup of pressurized air inside the case.

4)vice versa, would it be useful to have 1 inflow and 2 outflow, so would this pull the "hot" air out quicker.1. Not really unless you have perfect airflow & that is impossible.

2. No it doesnt. What it gives you is an opportunity to exhaust the hot air more efficiently.

3. Always strive for equal CFM on your intake & exhaust. There will be a really long list of people that would say otherwise.

a. Some say more intake drops their temps. I would think the reason for that would be because the exhaust flow is more efficient so an increase in intake CFM would be called for.

b. Some say more exhaust drop their temps. Again this is probably that the intake is more efficient so an increase in exhaust will be needed.

If you got a PSU that has exhaust fans then that too have to be factored in your computations.

4. Positive air pressure can help with dust but that is another can of worms that I don't want to get into right now but if you want to then just ask.

Try to draw your case flow on paint then save it a .jpg & post it here. We can discuss this further if your wish.

[nihili]

Originally posted by sonny
2. No it doesnt. What it gives you is an opportunity to exhaust the hot air more efficiently.

I would just add that in practice having an exhaust fan will increase the air flow through your case somewhat. It won't take it over 38cfm in this case. But as Sonny pointed out, you only get 38cfm if you have perfect airflow. The exhaust fan serves two purposes. First, if the exhaust is near the cpu, it help to make sure that the hottest air is exhausted, thus leaving the average air temp inside the case cooler. Second, the exhaust fan helps to deacrease the static pressure inherent in the case. Thus it helps you get closer to the perfect airflow of 38cfm than you would without an exhaust.

At least that's my understanding of it.

nihili