[SOLVED] How does passive cooling work?

[rok_hribar]

Hello,

I am interested in how does passive cooling work.
Here are just some questions i have:

Why is some sort of metal used? Would not that make it hotter?
What is the difference between using cooper and aluminium? Does one conduct more heat?
Does the heatsink need to be high or is it okay if it is just low and long?

Thank you for your reply,
Rok.

 

[SkyNetRising]

Passive cooling works the same as active. One uses fans, other - doesn't.

Metal is used to conduct and dissipate heat.
Copper has better heat conductivity.
Surface area of heatsink determines amount of heat, it is able to dissipate.

 

[little_me]

Metals are usually used because they conduct heat better. So yes, they heat up easier, this is a desired effect since heat then transfers from item being cooled (processor) to item doing the cooling.

Copper has better heat conductivity than aluminium, at same time it is also more expensive, which is why most coolers have copper "pipes" which are then attached to aluminium fins Another matter is weight, if all coolers would be pure copper, they'd be a LOT more heavy and would pretty much require separate support along with motherboard mounting.
Heatsink as such relies on surface area, bigger area, more heat dissipates, better it is. which is why they are pretty much always full of fins next to each other. the shape itself has no real meaning unless you consider outside factors like airflow, in which case having air flow between said fins cools them down better.
said airflow caused/increased by additional fan is pretty much only difference between passive and active cooling.
Passive coolers tend to be bigger than active if they are rated for same cooling capacity (often measured by watts) since airflow helps a lot in cooling.

Additionally orientation of cooling fins is not as important in active since the fan will push the air through anyway, while in passive, heated air tends to rise up.
so for passive coolers, having fins in vertical orientation is better since it doesn't hamper the warmed up air from rising upwards where it naturally wants to go.

 

[rok_hribar]

Thank you for your reply.

Would you say that the more surface area of a heatsink is necessarily propotional to the cooling capacity (in watts)?
If for instance the passive cooler would only be about 3 cm thin but it would have a surface of 200 cm2.

What about the distance from one fin to another? Is it better that they are closer or further apart. Or does it vary, and you just need to experiment with it to find out which is better.

Is airflow a neccesity while having a passive cooler?

Rok

 

[little_me]

more surface area is needed to better dissipate more heat, yes.

The fin spacing is a matter of too close together, they get clogged up easier by dust and too much apart, the distance from point of origin is too far and fin doesn't heat up as much anymore and thus also cools the point of origin less.

Experiment with a cup of something hot and metal spoon, you want to attach the fins as close to surface of the cup as possible since it's most hot there, thus the fins heat up most and also dissipate heat the most.

Airflow is usually necessary since in tiny enclosed box, even tiny amounts of power can heat it up considerably. The positive side is that the heated air rises up and if there are small ventilation holes on top and few at bottom for replacement air, it creates it's own flow.

I would say something like shoebox size can't take much over 20W without ventilation holes. If the box is metal, that also acts as a weak cooler (well, all materials do but metals yet again transfer heat better than say.. cardboard or plastic)

passive cooler transfers heat from A to B (B being air inside the case) and case transfers heat from B to C (C being air outside case)

The bigger the difference between the temperatures is, better it cools. So naturally you would want the B to stay cooler by adding those ventilation holes to lower it's temperature.

 

[rok_hribar]

I want my pc (i have a small pc) to lie vertically such as nucs. Are nucs raised from the bottom because there is air intake?

So my pc would need ventilation holes there. But how does the air come through the nuc since there is a motherboard in a way?

Moreover is it better then that the chassis is metal rather than plastic.

Sorry, i know this is kinda incoherent and messy, because even i dont know atm what i want.

 

[tennis2]

It would be helpful to know your system specs as a start. CPU, mobo, storage, case, psu etc etc.

Assuming you're looking into passive cooling for absolute silence?

 

[little_me]

Yes, Intel's (and others) NUC computers have those small rubber pads on bottom for two reasons, to prevent sliding AND to raise it off desk surface. (in 90% of cases there are ventilation holes at top and bottom and they also usually have a fan, thus not all of them are truly quiet/passive cooled)
Also since motherboard is not fixed directly without gap to bottom, air can get in, get around the motherboard from sides (and slightly cool that too) and then go from there where it will.
Metal chassis is better for cooling since it heats up easier and also dissipates heat better. As Tennis2 said though, majority of fully passive cooling solutions need to take into account both total power of things inside the chassis (the load) and space inside the chassis.