Gargoyle said
Would a bong cooler also be considered phase-change? They're supposed to be effective, both on heat and your wallet. Adds an element of humidity to the house without proper ventilation though.

Yes, a 'bong cooler' is a phase-change cooling system. It differs from the commerical computer cooling productions in that the phase change occurs in the resevoir and it is generally passive, relying on naturally occuring evaporation. This evaporation is accellerated by forcing water through small holes, effectively seperating the water into many tiny droplets which increases convection from air to water and results in some of the water vaporizing.

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.