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A small correction regarding the laws of thermodynamics and convection. - Page 3

post #21 of 108
The only accurate way to tell what airflow pattern, fan locations and fan direction works best in YOUR PC case with YOUR PC hardware, is to accurately test. There are so many variables that it's impossible to know the actual cooling performance without proper testing. In most cases the difference between optimal and poor is a few degrees C, not 10-15C unless you've created a perpetual heat cycle.

FYI- Hot air rises no matter how you try to push it down with fans. It mixes with the surrounding air but the hot air always rises, regardless. The laws of physics have not changed... wink.gif

The bottom line is test if you want to know what really works best for YOUR application.
post #22 of 108
Quote:
Originally Posted by AMD4ME View Post

The only accurate way to tell what airflow pattern, fan locations and fan direction works best in YOUR PC case with YOUR PC hardware, is to accurately test.
The bottom line is test if you want to know what really works best for YOUR application.

This, and if something is massively overheating... Thats the only way... Other than that i wouldnt worry if my temps are under control on the most demanding applications that are run in a daily basics in that particular pc...

I wouldnt worry too much on fan placement or adding fans for a couple of c...
post #23 of 108
Quote:
Originally Posted by AMD4ME View Post

FYI- Hot air rises no matter how you try to push it down with fans. It mixes with the surrounding air but the hot air always rises, regardless. The laws of physics have not changed... wink.gif.
Low density air rises over high density air... and warm air is less dense then cold air. Adding fans into the equation changes air flow and all bets are off. HVAC systems are example of moving and circulating air in and out of buildings/rooms with total disregard of "hot air rises".

Although my philosophy is different from zGunBLADEz's his does work.
Edited by doyll - 6/11/12 at 3:12am
post #24 of 108
hot air NATURALLY rises, but if you add fan that are strong enough then it can alter the NATURAL direction of air, i agree with that, although the fans can move air better if it's going with the natural flow than going the other way around.
post #25 of 108
Quote:
Originally Posted by Snowmen View Post


1) If you have your case in an open space, the amount of air is so large that the probability that exactly the same air is pulled back into the case is minimal at best. Anyway, because of the laws of thermodynamics, the heat would have a tendency to dissipate itself into the rest of the air to maintain a certain balance.
2) If your have your case in a small space like the little compartments that they put in many desks, air will be recycled anyway and you should just get it out of there if you want better temps.

well... you are kinda taking the 2 extremes approach here.

1. if the case is in open air. then absolutely there is little to no mixing of warm/cold air since the momentum of warm air as it is pushed by the fan would be far enough away that it would not likely mix with the more immediate source of cool intake air. however, i'm willing to bet that most of the computers that are in homes have the back of the case (psu exhaust, gpu exhaust, cpu exhaust) around 6 to 8 inches away from a wall of some sort (behind the desk or actual wall of the room) that limits the travel and dissipation the warm air can go and would eventually build up enough warm air to mix with the cool intake air (maybe after 1hr of gaming?)

2. again another extreme that unfortunately I know actually happens. they are the machines i usually have to replace in a client's house due to HDD or PSU failure most of the time biggrin.gif they put the machine inside a completely enclosed portion of the desk and expect it to survive the heat biggrin.gif that's not the kind of situation i'm talking about either. i'm talking about putting a computer under the desk with the back side of the computer trapped by either the back of the desk or the wall, and one side of the computer pushed up against the side of the desk. so essentially the warm exhaust air is trapped under the desktop with front intake open and one side of the computer open to the user's legs. it wont recirculate the warm air immediately. but the ambient temp under the desk builds up over time.

as i have mentioned. cooling is a function of how loud the fan becomes. it's not that it's impossible to cool things down, but how loud it needs to be to accomplish the same goal.
post #26 of 108
Quote:
Originally Posted by psyclum View Post

the importance and relevance of convection cooling is often OUTSIDE the box.... to discount convection entirely is uneducated at best... yes inside a box, any kind of fan would overpower the action of convection, however, this does not negate for physics behind heat rising to the top. with a top side intake, you are often recirculating hot exhaust from PSU/GPU/CPU right back into the case and causing further inefficiency in your overall cooling solution. this problem is especially acute if the computer is in a semi enclosed area such as under a desk where hot exhaust is trapped by the desktop and then sucked back into the case from the top side intake.
recirculation of hot exhaust (thermal short circuit) creates inefficiency and as a result higher decibel levels then necessary to achieve the same goal. overall cooling performance may not be very noticeable however the real discussion in PC cooling really lies in long term decibel level. anyone can cool anything if they used enough 5000rpm delta fans, however, to keep it under acceptable decibel levels, one must search for efficiency and this is where consideration in convection comes into play
a bottom intake and top exhaust ensures maximum separation between cool intake air and hot exhaust air because OUTSIDE the box, hot air WILL rise and convection DOES work...
Quote:
Originally Posted by Snowmen View Post

I know the thread is very old but I just wanted to answer your comment. The air outside the case doesn't matter either for these reasons:
1) If you have your case in an open space, the amount of air is so large that the probability that exactly the same air is pulled back into the case is minimal at best. Anyway, because of the laws of thermodynamics, the heat would have a tendency to dissipate itself into the rest of the air to maintain a certain balance.
2) If your have your case in a small space like the little compartments that they put in many desks, air will be recycled anyway and you should just get it out of there if you want better temps.

I would tend to agree with Psyclum, and the problem is much worse with top-exhausting cases, e.g., http://www.hardocp.com/article/2010/11/09/nvidia_geforce_gtx_580_video_card_review/8

Different circumstances will result in different risks, but many of us have tower cases underneath desks/tables that aren't very well ventilated, so hot air will tend to collect underneath the desk/table and have a chance to go back inside if there is a top intake fan.

I'm currently re-designing my airflow and plan to yank out my top 200mm exhaust fan and seal the entire top in order to complete my top "wind tunnel." I'll have an intake 120mm fan in my top three 5.25" bays throwing air at my Hyper 212 Evo with push-pull fans directing air out the back (no exhaust fan, just a gaping hole courtesy of some nibbling). Down below there will be a 200mm intake fan, a 140mm bottom intake feeding a 7970 with dual blower fans, and all the rest of my PCI covers taken off to encourage a second "wind tunnel."

There will be no need for top exhaust OR intake, and sealing them will ensure no dust falls through the top when the computer is off.

Wild card: my 200mm side intake fan. I don't think it's as problematic as a top fan intake when it comes to external air temperatures, so I intend to continue using it as an intake fan, but it merits experimentation with my infrared thermometer. smile.gif

Edit: Btw, to tie this back to OP, the OP is correct. At short distances fans will overpower gravity and convection. That doesn't mean the Stack Effect doesn't exist with computer cases, but it does mean that if you were to build a case that takes advantage of hot air rising, it would have only a very modest advantage over traditional front-to-back designs, assuming all cases were using fans (which overpower convection). If fans are banned, a stack effect cooler would have a greater advantage: http://www.youtube.com/watch?v=MXKe7bdBiGc But stack effect cases have other problems, such as a greater risk of higher perceived noise thanks to the exhaust being placed closer to the user (as opposed to facing the wall away from the user).
Edited by unifiedshader - 6/11/12 at 4:44am
post #27 of 108
Hot air rise....

Take of top and put a reducer to 8" diameter chimney about 4' tall... taller is better... to draw heat out of case. smile.gif
post #28 of 108
Why is thermodynamics a whole law while gravity is still only a theory?

But I see the point here. Fans are more powerful than convection, that is why we use em. So flow is important not what direction it is in. I like down front to top back in cases that have normal power placement onatoppa. Once you put the power on the bottom it may be worth looking at other directions.
 
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post #29 of 108
Quote:
Originally Posted by Kip69 View Post

Once you put the power on the bottom it may be worth looking at other directions.
PSU on bottom back is air in bottom out back. So no problem with air in front/bottom and out top/back smile.gif
post #30 of 108
having a rad as intake in the top of your case also highly depends on the physical location of your case, for example, if you have it near the floor of your room, it's going to be cooler than say if you have it on a dresser that's closer to the top of your room.

I can feel a physical temperature difference between the top and bottom of my room, so I can't put my rad as intake because it will draw hot air rather than cool air if i had it closer to the floor
   
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