# Can my CPU temperature be less than room temp?

## Main Question or Discussion Point

Is it possible that my cpu temp could be less than room temp with a standard fan and heatsink? I've been hearing that it's completely impossible to go below ambient room temperature because the fan is simply blowing air and not cooling anything.

But what about when you turn the fan on in your room... doesn't it cool you? Is it physically impossible for a fan to cool you below room temperature?

Related Classical Physics News on Phys.org
vanesch
Staff Emeritus
Gold Member
For a normal fan, it is totally impossible to get things below room temperature by just blowing air. However, there are exceptions possible. The simplest one is when the fan makes some liquid evaporate: as such it can be possible to get slightly below room temperature by lowering artificially the partial pressure of the liquid (by removing by convection the air above the liquid), so that evaporation will be favored more than at thermal equilibrium.
A more sophisticated one is when there are high pressure differences because of the "fan" (which became a compressor now), and that one can have a kind of heat pump that way. But a normal fan cannot induce sufficiently high pressure differences to do that.

ok thanks for the response

russ_watters
Mentor
Three points of amplification:
-A fan cools you normally because of convective heat transfer.
-If your room is above body temp, a fan can still cool you because of evaporation of sweat. The surface temperature of your skin will be well below room temperature (at a temp which depends on the humidity).
-Evaporative cooling (ie, a cooling tower) can provide 75% or so wet-bulb suppression. Ie, if the ambient temperature is 75F and the relative humidity is 50%, the wet bulb temperature (the minimum possible temperature due to evaporation) is 55F and a cooling tower can make 75-(75-55)*.75=60F water.

vanesch
Staff Emeritus
Gold Member
Now, to come back to the OP, unless he has a very sophisticated PC with integrated cooling tower and water feed, I think it is safe to say that its CPU can never be cooler than ambient temperature...

Many CPUs on higher-end computers use heat sinks with heat pipes, which are based on evaporative cooling. Having said this, I don't think the CPU surface gets much below 50C.

I have to disagree. I have sitting on my desk a "USB Fridge." It is a simple box with a USB-powered fan which blows on a heat sink. It only takes a few moments and the surface temperature of the flat side (the side that would contact the CPU) gets very cold to the touch.

http://www.thinkgeek.com/computing/accessories/96b3/

sure it is possible!
even without a fan...
it's a metal piece. it's naturally getting cold, even more than the room temperature.
now it's very improbable, cause the cpu is working, and is near to bla bla bla.... but there are definitely materials that goes below rooms temperature by them self.
even after you hit them, they'll come back to a be cool.

all of this bring me to one conclusion. i have to learn thermodynamics.

Staff Emeritus
2019 Award
it's a metal piece. it's naturally getting cold, even more than the room temperature.
i have to learn thermodynamics.
When you learn thermodynamics, you will learn that metal is not "naturally cold".

SpectraCat
I have to disagree. I have sitting on my desk a "USB Fridge." It is a simple box with a USB-powered fan which blows on a heat sink. It only takes a few moments and the surface temperature of the flat side (the side that would contact the CPU) gets very cold to the touch.

http://www.thinkgeek.com/computing/accessories/96b3/
There is more going on here ... there is a thermoelectric Peltier cooler inside that metal block that allows all this to work. You can see how to build one of these for yourself here:

http://www.metacafe.com/watch/944184/diy_5_mini_usb_fridge/

When you learn thermodynamics, you will learn that metal is not "naturally cold".
yes, i know i probably said rubbish regarding technical terms, that why i mentioned i have to learn thermodynamics, but this is an empiric fact :P i have a lot of metal stuff here, and they are all definitely colder than the room temperature..

Borek
Mentor
Your empiric fact is that metal FEELS colder, not that it IS colder.

"metal FEELS colder, not that it IS colder"

Yeah, because my fingers and mouth (when I drink my cool drink) lie to me. :-)

vanesch
Staff Emeritus
Gold Member
I have to disagree. I have sitting on my desk a "USB Fridge." It is a simple box with a USB-powered fan which blows on a heat sink. It only takes a few moments and the surface temperature of the flat side (the side that would contact the CPU) gets very cold to the touch.

http://www.thinkgeek.com/computing/accessories/96b3/
Probably with a Peltier element, no ?

vanesch
Staff Emeritus
Gold Member
"metal FEELS colder, not that it IS colder"

Yeah, because my fingers and mouth (when I drink my cool drink) lie to me. :-)
In a way, yes. Because what happens is that metals are good conductors of heat, and if they are at a temperature below your body temperature, they will evacuate more heat from your fingers than would a bad conductor such as plastic. This results in a local lowering of the temperature of your finger which tells your brain that it is cold. If you do the same with a piece of plastic which is initially at the same temperature, then it evacuates less heat, and the finger will signal that it is less cold.

But if you keep a metal coin a long time in your hand so that it gets at your body temperature, it will not feel "cold".

On the other hand, a piece of metal at higher temperature than your body will feel much "hotter" than a piece of plastic at the same temperature, because now the heat flow is in the other direction.

russ_watters
Mentor
Missed one:
Many CPUs on higher-end computers use heat sinks with heat pipes, which are based on evaporative cooling. Having said this, I don't think the CPU surface gets much below 50C.
Heat pipes will passively transfer energy from the chip to the heat sink, but they operate at constant pressure, so they can't lower the temperature below ambient.