What is phase change....

[Perl]

What is phase change cooling? someone please give me a link to somewhere that i could i read about it

[Mr Evil]

A 'phase' is a state of matter such as solid, liquid or gas.

To change phase, a material must absorb or release some heat. This heat is called 'latent heat'.

For instance, for liquid water at 100oC to change to steam at 100oC it must absorb some energy, even though it doesn't change temperature. It must then release that heat to change back to liquid.

This is the priniple on which phase change cooling is based: It can (theoretically) move heat at a temperature difference of zero.

in other words, a phase change cooler is the same thing you find in a air conditioner or refrigerator, uses a compressor and a special type of coolant. (as I understand it) you have the compressor pump coolant into a radiator or some other kind of hot side. The quick increase in pressure causes it to condense, which releases alot of heat. Then it goes through a very narrow tube (creating a pressure differential) into another radiator or 'cold side'. The sudden drop in pressure causes it to turn to vapor, and going from liquid to vapor is endothermic (absorbs/sucks up heat). So that side gets very cold. Then it goes back to the compressor and around again.

[redleader]

The difference in energy to heat water from 0C to 1C compared to 2C to 3C is huge. When ice melts, or liquid evaporates, or any phase change occurs, intermolecular forces are disturbed. Doing this requires a TON of energy. Furthermore this energy does not get stored as heat.

I forget the exact number, but melting ice requires more then enough heat to make the same amount of iron glow hot (I think it was iron, don't quote me on that). The idea is to use some sort of liquid that will phase change right arounf CPU temp and then again around air temp. THat way it will suck heat really fast of the die, evaporate (and take the heat it absorbed with it without actually being warmed itself) and then can condense again by dropping heat to the air in the room once its well away from the CPU.

It similar to a bong cooler, but the same coolant it used over and over and the unit would probably be selfcontained without a pump.

Sorry if that was long and confusing.

[newbie]

redleader is almost right. The energy transition takes place entirely at 0 degrees, not 0 to 1 degree C.

It takes 80 calories/gram to change ice into water. It takes 600 calories to make a gram of liquid water vaporize. It takes 1 calorie to raise 1 gram of water 1 degree C. The phase change coolers I've seen go for the 600 calorie transition by boiling water. Water coolers move 600 times as much liquid water as vapor change coolers do to achieve the same cooling effect.

Since water boils at 100 degrees centigrade at sea level pressure, you would think the cpu would melt. The trick to a phase change cooler is they lower the pressure inside the cooler to about 1/4 atmosphere so that the water boils around 25 degrees C. It still takes 600 calories to change the water from a liquid to vapor but it happens at a lower temperature due to the lower pressure.

Besides being able to absorb lots of heat quickly, water-based phase change coolers have the advantage of being able to move heat away from the source. When water vaporizes, the molecules take the heat of vaporization along with them. Since they're vapor instead liquid, they can move much faster and hence, they can move heat faster and further. If you look at a thermal picture of a phase change cooler vs. a block of copper, you'll see an even distribution of heat in the former and a hot spot where the cpu is in the later.

There's a lot more info at [url =http://www.thermacore.com ] Thermacore.[/url] If you're not scared of math, try this paper on cooling lasers with heat pipes Skip the section on lasers and focus on the heat pipe discussion. Good stuff.