So what if hot air rises? Convection and PC cases

pcfoo_convection_600pOur physics teacher in high school taught us that hot air is rising, and there is convection in the atmosphere, much like there is in a pot of boiling water that forces hotter molecules to rise, while the colder ones sink till those get warmer again, in an ever-lasting circle. Few people understand the scale of these movements, leading to misunderstandings and self-assumptions about proper PC case designs. Does it really matter in a properly ventilated case?You weren’t lied. Natural convection is true. Yes, hot air rises and hot air balloons do fly as those are lighter than the ambient air around them. But how relative is it to PC cases, electronic device enclosures or even cars – machines that usually deploy high heat intensities, in small enclosures? To understand it a tad better, lets go to the other extreme: huge enclosures that by comparison house relatively low heat intensities: buildings.

Natural ventilation in buildings

Engineers and Architects can take advantage of natural phenomena around convection in building design, but the scale is way different: buildings are huge, have thousands or square feet of facades that get pushed or pulled away by thousands of lbs/Nm of wind forces on each direction, creating significant pressure differences that create drafts through pairs of openings facing different pressure conditions.

Air volumes are also huge, especially in multi-story common spaces like courtyards and atriums, so you can realistically talk about stack ventilation or chimney effects as you have much larger volumes for air to stratify, and an envelope (enclosing structure) big enough for pressure differences between the different sides of a building to assist – with proper design – the passive ventilation of such spaces. The airflow in such scenarios is pretty low, but the effect of even the slightest breeze around our skin, in conjunction with the large air volumes that get displaced – even at slow speeds – is significant…

All that to achieve what?

A few air-changes per hour (i.e. replacing 100% of the interior air with fresh air).

The Building Codes specify the requirements for a building to be considered properly ventilated: 30 air changes per hour for busy places like bars, to 40-60 air Changes/hr for commercial Kitchens. Normal housing / offices etc are usually in the middle single digits, 4~6. The latter is what the law recognizes attainable with natural ventilation (i.e. combination effect of pressure differences and natural convection). For spaces and occupancies that require more air-changes per hour

 How big are our PC cases?

Good question that few people think in terms of volume: A case is what? 5 cubic feet of space average? Nah…much less!…

A massive full tower PC case, like the Corsair Obsidian 900D is a tad less than 4 cu ft. (113lt). A midi cube like the Corsair Carbide AIR 540 is 2.2 cu. ft.(62 lt), most midi towers like the ever popular Cooler Master 690 II Adv float around the 2 cu.ft (60 lt) while a m-ITX case like the Cooler Master Elite 130 is a small as 0.65 (18.5 lt)  cu ft.

Of course these are the overall volume figures, the drive bays, the fans, the drives and all the rest computer components inside take up a big portion of that space – surely a smaller % of this volume is the actual air that is left to be pushed around in a crammed m-ITX vs a roomy full tower, but let’s not get into that detail.

The game changer, is that in PC cases, we have 120 and 140mm fans as standard equipment. Modern fans, displace (nominal) 20-30cu ft./minute at really low rpm, and even at really low noise levels.

The AF140s like those in the 540 AIR claim 68 cfm…that means that in an ideal condition, and with a heavy, 50% restriction due to mesh/filters/internals we will still have some 30 cu. ft moved around for each “paired” intake / exhaust fan set per minute. In theory just a single exhaust fan can achieve more than that being unrestricted (i.e. filters)

30 cu ft / minute, translates to roughly 800 air changes per hour, using just 2x 140mm fans…

Even an effective 20 cfm real airflow through the case, would still result to more than 540 air changes per hour, which is orders of magnitude bigger than the single digit air-changes that natural convection can achieve.

I hope you realize that the natural buoyancy of hot air, convection and gravity forces are simply not even in the ballpark of “resisting” forced ventilation. The effect of those in the bigger picture, falls within the “measurement error/tolerances”. So, the next time someone sings praises on how well their PC, Mac Pro or whichever system with active fans and a clear airflow path takes advantage of the hot air “rising”, challenge them to place their machine upside down for a short period, and see how much worse it will be (if any).

One thought on “So what if hot air rises? Convection and PC cases

  1. Thank you for this great article. You provided the numbers behind why “hot air rises” is irrelevant to PCs with case fans that force air much faster than convection can even touch.

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