Does heat always rise?

[Kryten]

We all know under ideal conditions that heat rises (if you don't go to the bathroom and turn on the hot tap and you will have a fogged up mirror) BUT inside your computer case is not an ideal situation you have air flow and in some cases quite a bit of it, if you were to have a large fan at the top blowing in and a large fan at the bottom blowing out...would hot air rise?

 

[Door Knob]

Often people's reasoning for blowholes is that heat rises. My feeling is that with descent well placed case fans the heat really never gets a chance to rise as it is quickly expelled through the back of the case. What the blowhole ends up doing is disrupting airflow, creating circulation, and actually hampering cooling. However in a real small case this is sometimes the only option available and will work well for those particular instances.

 

[Shadow ÒÓ]

why fight a natural force? The top is where the PSU and CD Rom's are.......and they can stand more heat than the lower half which has the CPU and HD's. Yea it naturally rises, so why not give it a boost instead of fight it and force it down? (Shadow logic........don't pay no attention)

 

[vandersl]

Heat doesn't rise - hot air rises, since it is less dense than cold air. The process is fairly slow, hopefully much slower than airflow induced by case fans.

 

[Metaxas]

I have one of those Lian-Li PC60 cases. And I am just going to use a 131CFM 120mm fan that I intalled into the plexi on the side, and set all the other fans for an exhhaust. and I should have a positive pressure...

 

[Door Knob]

vandersl (May 07, 2001 07:47 a.m.):
Heat doesn't rise - hot air rises, since it is less dense than cold air. The process is fairly slow, hopefully much slower than airflow induced by case fans.

My thoughts exactly.

 

[Kryten]

Door Knob (May 07, 2001 08:59 a.m.):

vandersl (May 07, 2001 07:47 a.m.):
Heat doesn't rise - hot air rises, since it is less dense than cold air. The process is fairly slow, hopefully much slower than airflow induced by case fans.

My thoughts exactly.

This is what I'm thinking if the case is semi sealed and air flowing through it what little HOT AIR there is, should bend to the will of the fans, I am no rocket scientist but I figure what little heat there would be rising (figuring that it will radiate in all directions off the CPU, I feel that the fans would take care of multiple times, please correct me if you figure I'm wrong.
Yes shadow why fight nature but in this case I feel it's a weak fight it's putting up.

 

[William]

vandersl (May 07, 2001 07:47 a.m.):
Heat doesn't rise - hot air rises, since it is less dense than cold air. The process is fairly slow, hopefully much slower than airflow induced by case fans.

ok, well if we are going to get technical here . Here is the basic thing you remember to get good case cooling. Air coming in = Air coming out. Example, you will have awful cooling if you have five fans blowing in, but only one blowing out unless of course, the cfms work out. Use that equation, and set up some nice airflow, and go from there. Blowholes may not be a huge deal, but they don't hurt(well, maybe some fingers, but that is different.

 

[Shadow ÒÓ]

aaight....lets get real technical.

you actually want your out cfm to be slightly less than your in. If you had a say 60 cfm fan blowing in......wouldn't you have a big restriction (due to the internals of the system) and actually want slightly less (say 55 cfm) going out? I see it as possibly having a negative air pressure at the out side.....and a positive pressure on the in side.

We hashed this once but I think a slight positive pressure should be maintained inside the case. negative pressure would cause dust to be sucked in every nook and cranny. This is of course assuming you are using filters on all of your "in" fans.

 

[Door Knob]

I have little circular vents along the side of my case. When I have more airflow in it leaves dust dots on the two bay holders. When I have more air comming in I don't have that problem. Point - More air in gets rid of a good amount of dust. Only slightly though, say 5-10cfm difference is good.

 

[compaq_ML530]

I definately like positive pressure...it's cool when you insert a floppy and can feel a breeze coming out of the drive. I think I need to add another exhaust fan in my rig.

 

[Kryten]

I have a slightly positive air flow, but my aim of the post was.. to get your thoughts on.. "is hot air rising and needed to be vented from the top maybe a little overkill when the fan will blow what little there is out the fan..."

 

[William]

Shadow ÒÓ (May 07, 2001 06:23 p.m.):
aaight....lets get real technical.

you actually want your out cfm to be slightly less than your in. If you had a say 60 cfm fan blowing in......wouldn't you have a big restriction (due to the internals of the system) and actually want slightly less (say 55 cfm) going out? I see it as possibly having a negative air pressure at the out side.....and a positive pressure on the in side.

We hashed this once but I think a slight positive pressure should be maintained inside the case. negative pressure would cause dust to be sucked in every nook and cranny. This is of course assuming you are using filters on all of your "in" fans.

yes, i remember that topic well. Seemed to be evenly split to three groups, positive pressure, negative pressure, and it doesn't matter. I side with the positive group, but that is me.

 

[Hoot]

Kryten (May 07, 2001 09:17 p.m.):
I have a slightly positive air flow, but my aim of the post was.. to get your thoughts on.. "is hot air rising and needed to be vented from the top maybe a little overkill when the fan will blow what little there is out the fan..."

The bottom line. Yes, hot air rises, but it is a very subtle current, especially at the temps encountered in the case. If you have anything remotely resembling fan induced airflow through your case, the impact of rising hot air will be negated. Watch the smoke rising from a cigarette (right or left handed ;D) and in the presence of even a slight lateral breeze, it goes with the breeze. In a strong wind, like the flow I maintain in my mid-tower (55cfm), this phonemenon becomes a moot point.

Hoot

 

[Kryten]

Thanks for the input guys you basically confirmed my suspicions....