As a Practicing Engineer for almost 40 years and owner of an engineering consulting firm, I can tell you that we don't design buildings with significant heat sources "for slightly positive pressure to ensure circulation". We design buildings to get the heat out as quickly as possible and that is not by blowing it around the interior.
Most simple building ventilation (talking simple air exchange here not AC or heating) is exhaust, tho supply fans are sometimes provided to augment this. Things get a bit more complicated in muti-story buildings focusing on human habitation rather than heat mitigation, but a PC case is more akin to free standing restaurant, manufacturing facility, power plant or emergency generator room but even the house you live in operates on negative pressure.
- The most common example is that in every house with an attic fan, ventilation operates under negative pressure. Hot Air is sucked outta the house, pushed into the attic and out ..... fresh air comes in thru windows because of negative pressure and, if used, intake vents.
- Staying in the house for a moment, how does hot air get out of the kitchen ? Is there an intake fan blowing air in ? Now that would just blow all the hot air around the kitchen. Instead there is an exhaust fan which creates negative air pressure to quickly remove heat from the room.
Having positive pressure does help circulate air, which in turn helps remove heat from the hot components
- No it doesn't. Your building analogy falls apart here because where heat is being produced it's just the opposite. Commercial Kitchens are always designed with negative pressure because the important design criteria is to get heat (and fumes) out in order to keep the important components (the cooks) at a reasonable working temperature..... just like a PC case. Machinery rooms, power plants, engine rooms all have high side wall or roof exhaust ventilation. Blowing air in pushes the heat around the room and is counter productive. As the heat is exhausted fresh, cool air enters the space and cools the components .... as the hot air is vacated, cool air moves in to take its place.
Look at the product lines here:
http://www.greenheck.com/
Why are product lines are predominately exhaust if the primary means of building ventilation is positive pressure ?
- Instances of positive pressure design are confined spaces. I teach a training course on the subject and students oft suggest that "isn't it more important to get the hazardous gases out rather than get fresh air in ?. While getting the hazardous gases out is important, the primary concern that we have when entering a confined space is that there is oxygen available for the men entering the space. Exhausting gases out in no way insures that oxygen is getting into the work area.... blowing air in does. Another issue with regard to positive ventilation in homes is that warmer air contains more moisture and this is normally augmented by humidifiers especially in cold climates with long winters and long heating systems. With positive pressure ventilation that warm moist air escapes thru outlets cracks and seams until it reached the outside wall and condenses leading to mold and other problems.
This also has relevance again to your house where positive ventilation may be desired in a basement in areas where radon intrusion thru the foundation is an issue. There are other benefits to positive ventilation but as in the example above, the purpose here is to avoid radon exposure, nor increase circulation or exhaust heat.
I didn't say anything about positive or negative pressure causing implosions or explosions. I said " If air in didn't equal air out the case would implode / explode". Exhausting air from a storm drainage chamber with rotting leaves which create hazardous gases in now way insures the presence of oxygen as the exhausted gases could easily be replaced by more heavier than air gases from the connecting pipelines.
Say you have a 2 Cubic foot case ..... Ya have 2 fans pushing air in at the rate of 50 cfm. Ya have one fan pulling air out at 50 cfm. So someone measures and comes up with the following answers, pick which is feasible. (Assume 0 friction loss)
a) There is 100 cfm going out and 50 cfm going in
b) There is 100 cfm going in and 50 cfm going out
c) There is 100 cfm going out and 100 cfm going in
The answer is C.
-Under a), each minute there would be 50 more cfm going our than going in and the case would implode.
-Under b), each minute there would be 50 more cfm going in than going out and the case would explode
-Under c) .... there is 100 cfm going in thru the 2 fans, 50 cfm going out thru the 1 fan and 50 cfm going out the rear grille ... aka equilibrium.... for every unit of air that enters a case, a unit must leave the case. Think of a pressure tank with pressure relive valve set at 50 psi.... Pump air in ....48, 49 50 ..... you can keep putting more air in but the relief valve should open ... for every cfm in, 1 cfm goes out the valve .... equilibrium. Disable the valve and have what happens ? Air in must = Air out or Big Badaboom !
Another reason this method is beneficial in building design is that as workers and customers are in and out all day long and negative pressure inside the building helps keep doors closed tight. I am reminded of this each day as I'd enter the corner delicatessen for my morning coffee and you have to give the door a good tug to get the door to start opening and then once you create an opening the slight pressure differences immediately equalizes and the door swings freely.
However a case has much more air turnover than a typical building. Where a simple building with a significant heat source might have 6 to 30 air changes an hour. A PC with just 3 fans might have 30+ air changes
a minute. Take a very simple Best Buy Type PC with no grilles or other openings except for one 120mm exhaust fan mount at rear top and and one intake fan at the bottom front.
Now take this fog machine
And test the air flow with the fan in front as intake and then the fan in back as exhaust ..... the air flow pattern is identical. With one place to go in and one to go out "circulation" does not change one way or another. Been there, done that using that exact same fog machine.
You can create instances where either negative or positive will presents issues. But the positioning of the intakes and exhaust has far more influence on circulation patters than positive or negative pressure.
Take this room plan (looking Down)
AOOOOOOB
AOOOOOOB
OAAOOBBO
OOAA BBOO
The exterior wall is A-B
In scenario 1, we mount a 150 cfm fan at point A on the outside of the room blowing in..... the air velocity would move in a U pattern shown by the AB lettering, around the room and out the vent B. Now mount the same fan at point B as an exhaust fan and the same 150 cfm would enter thru A at the same velocity and continue thru the exact same pattern. A person inside that room would in no way be able to tell or measure a difference in circulation of air patterns.
Similarly, the air pattern within case is determined solely by the quantity of air entering / exiting each intake / exhaust port. Take my case..... I can mount 4 x 120mm fans on top and 4 x 120mm fans on bottom... using no filters since we have a whole house filter it's not needed and all other openings are sealed.
4 x 120mm on bottom each pushing 65 cfm or air in from the bottom..... since we must maintain equilibrium, that will push 4 x 65 cfm out the top
let's switch....
4 x 120mm on top each pulling 65 cfm or air out the top..... since we must maintain equilibrium, they must pull 4 x 65 cfm in at the bottom
No difference whatsoever.... the intake air is coming in at the same rate and at the same velocity whether it's pushed in or pulled in and with therefore have identical circulation patterns. Introduce from and reap vent openings and things get much more complicated but again, it's the positioning of the ports that matters.
However, having 1 fan on the middle of the top blowing out, replacing the middle of my front fans blowing in, and replacing the back fan blowing out with the new fans actually helped the temps a bit better, but it looks like I'll be running 60hz outside of gaming for now.
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