What is Phase cooling?
CyrixInstead
Stoke-on-Trent, England Icrontian
I know this is probably a stupid question, but what is phase cooling?
Just saw it on the main short-media page. 'Cooling - Air, Water or Phase', and was curious.
Just saw it on the main short-media page. 'Cooling - Air, Water or Phase', and was curious.
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Phase-change cooling is the most powerful long-term solution for cooling available. We're talking temperatures in the negatives.
Widely used? No, not really.
Water cooling isn't exactly phase change cooling.
Starting from the most simple...heatsink and fan. A processor generates a tremendous amount of heat in a ver small area. The heatsink provides a medium for that heat to be transferred to...so now where does it go.
Into the air.
A fan provides airflow past the fins of the heatsink which, because of the surface area of the many fins, air molecules have the ability to carry away some of the heat.
It's like having 1000 people build a brick wall instead of one.
Water cooling is more efficient. Heat transfers to water much better than metal to air. So the water cooling heatsink is only providing a larger area to "wick" away the heat from the processor core. The larger the area...the more water can come in contact with the warm heatsink plate.
Since heat is a more efficient transfer medium than air...the water is better at "sucking away" the heat from the heatsink plate...there is a constant exchange of water therefore the warmed water is circulated out to be replaced by cooler water from the reservoir.
Note: on the way back to the reservoir the water will have to pass through a radiator, which like your car, has a fan blade...the same principle as a fan and heatsink applies. The water is given a tremendous area in which to "lose" it's kenetic or heat energy by way of passing through the many loops of the radiator which cooler air is passing through.
In both cases room temperature is the threshold. You can't get near room temperature with air but you can get closer to it with water. But since the water has no active cooling...such as phase change (refridgeration) then obviously it can't go below room temperature. BUT water is much more efficient at cooling the processor than air.
Phase change cooling ads the element of refridgeration to the liquid that is passing through the waterblock that sits on the core. Now you have cold liquid not only wicking away the heat but actively COOLING the processor core.
Like an ice cube in a drink.
Peltier devices can be introduced because when current is applied to the Peltier "slab"...one side goes cold...and the other side becomes hot. So the cold side sits on the processor core cooling it...but then you have to have some device like a water block to wick away the heat from the hot side. Otherwise the Peltier device just burns up.
There have been many home made contraption devices discussed to cheaply cool a reservoir. One such discussion involved putting the reservoir in one of those 12 volt mini coolers or inside a mini-bar fridge.
This is, as you may have guessed, introducing phase change cooling.
Hope this helps.
The reason that you do it (phase change) is that a liquid requires much more heat to boil it than it does to just heat it up. heating a pound of water from room temp to boiling takes 140 BTUs. To boil that same pound (at 212F, just change from liquid to gas) takes 970 BTU.
You use the boiling phase change to absorb the energy and then you recompress and condense the fluid in an external system and send it back again.
A heat pipe is a passive phase change cooling system.
You need low boiling fluids to build systems to cool at low temps. There is some temperature control in the pressure that you use. Take R12 freon for example. At 10 psi it boils at -37F , at 100psi it boils at 81F and at 400 psi it boils at 193F.
Systems like Vapochill and Promedia use mechanical compressors like any freezer or AC unit. There are also some chillers that use acustic compressors. Think powerful speakers. These have no moving parts.
Compressors are more commonly used in computer phase-change applications. These work by taking a refrigerant that would normally exist as a gas at room temperature and compressing it into liquid form. The liquid refrigerant then passes through an expansion valve or passes by a capillary tube where it is allowed to return to its gaseous state. In doing so, the refrigerant draws in heat energy from the surrounding materials. It all operates on the principal that gas holds far more heat energy than a liquid. After the gas absorbs heat energy it eventually makes its way back to the compressor, where it is liquidified and releases most of the heat energy that it was holding. The lower the boiling point of a refrigerant, the more heat energy it will absorb when it is vaporized. These are the same concepts used in home refrigerators and the like.
There are essentially two forms of computer refrigeration: On die and resevoir refrigeration. On die refrigeration means that the vaporization takes place in what would be the 'water block' of a watercooling solution. In other words, it takes place right at the site of the heat source (CPU). This is generally more effective and more expensive than other forms of refrigeration, and it is employed in the commerical systems like Prometia and VapoChill. Resevoir type systems use insulated watercooling setups that run a mixture of coolant and water. The refrigeration takes place in the resevoir. While on die systems are more efficient, resevoir systems can be made more powerful because the size constraints of the system are minimized. As long as you have the space for a giant, dual-phase (Two compressors, two evaporators, etc) refrigeration setup, then it's possible. The problem then becomes finding a solution that remains a liquid at temperatures approaching -60C.
Lastly, and in my mind, most importantly, phase change doesn't have to be expensive. Anybody can build an in-resevoir phase change cooler for almost nothing. Find an old air conditioner and steal its compressor. Compressors from air conditioners usually work a lot better than those from refrigerators or freezers because they're more powerful and are designed to work with refrigerants that have lower boiling points. Then you just need to learn as much as you can on the subject and you should be able to build your own system relatively cheaply. If that seems like too much, then you can BUY an air conditioner for a couple of hundred bucks and easily adapt it to cool a resevoir by essentially sticking the evaporator in a bucket. From there on out, it's a matter of insulation, insulation, insulation and setting things up like a normal watercooler (Sans radiator).
Anyway, I love phase change discussions, so I just thought I'd add in my own crash course and build on MM's and ed's contributions.
The issue is which prefabbed refrigeration unit is the best one for the job. Air conditioning units are, IMHO, best because they generally use R22 as a refrigerant and have more powerful compressors than refrigerators or freezers, and dehumidifiers at that. You see, fridges, non-commerical freezers and dehumidifiers aren't really made to cool something quickly. They are intended to gradually bring something to a low temperature or condense moisture from the air. Thus, they are relatively low powered compared to a commerical deep freezer or an air conditioner.
You've also got to look at the form that they come in. For our uses, air conditioners and dehumidifiers are way more attractive than refrigerators and freezers because they come in a small package. Fridges often have the condensor waaaay spread out throughout the insulation material, and they are very bulky. If the evaporator won't fit in a reasonably sized resevoir, then it won't do you much good. Likewise, if the condensor (Like the opposite of the evaporator) is the size of a fridge and totally spread out, then it's not going to do you any good without modification - And if you're skilled enough to modify the condensor, why'd you even bother ripping the system out of the fridge in the first place? Just snag the compressor and go from there...
The other nice thing about air conditioners is that they are plainly rated in terms of BTU's, so it's easy to judge how well they will cool - Not necessarily in terms of actual temperature, but relatively speaking compared to other air conditioners - It's easy to pick one out that will cool better than another.
Directly cooling a cpu with phase change is just like a small version of your a/c with the evaporator (the part that gets really cold) sitting right on the socket. I used phase change indirectly by using a dehumidifier as a chiller in my water-cooling loop, pictured below
Damn, that price is crazy for the new model. With pricing like that maybe Kryotech will consider making some barebones kits in the future.
for $1000 it'd better get under -50C
Phase change cooling involves using the phase change of a working fluid to remove the heat from the source. External compressor cooled devices and 'bongs' may involve phase changes, but in those cases it isn't the primary cooling. It is secondary.