Can I increase cooling efficiency with evaporative cooling?

Physics Asked by tyler mackenzie on December 11, 2020

I had an idea recently. But I don’t have the physics knowledge to know if it makes sense or not. I’ve become a bit of a PC enthusiast lately and one thing that is big in the PC world lately is water cooling.

Here is my theory. If I take a regular PC water cooling loop, and lower the air pressure (custom loops typically have a bit of air at the top of the reservoir). I could lower the boiling point of the water to just above room temp (lets say about 30°C). Then when the system is running, the liquid water will hit the CPU block and vaporize, since CPU’s can easily be 75°C. The water vapor will then travel to the radiator and re-condense to liquid. And continue back to the reservoir, etc.

Lets just assume the PC cooling parts can handle the lower pressure for now. Also I assume the system pressure would increase once some water is turned to vapor, but I should be able to continue lowering the pressure while the system is running until it reaches a steady state, right? Since heat pipes evidently don’t have this problem.

Would this system remove heat from the CPU any faster then a regular water loop? From what I understand, the phase change will remove heat much faster then normal. It’s the basic principal that heat pipes use, which are already widely used in PC’s. But heat pipes don’t have a pump to force the circulation of a large amount of water.

Are there factors I’m not aware of. Will the pressure in the system just skyrocket once water begins to vaporize? Would it require some kind of active pressure regulator mechanism? I’m guessing not since this isn’t a problem for heat pipes.

I was thinking of testing the idea out in the next fews months, though maybe using a small heater element as a stand-in for a CPU, just in case things go sideways.

Also, if you are thinking that all this might be needlessly complicated for no real gain. Well, that’s the PC enthusiast world right now! regular PC water cooling already isn’t necessary right now. many air coolers are cheaper, quieter, more reliable and keep things cooler as it is. it’s just fun to play with.

basically, what you are describing here is a closed-circuit phase-change refrigeration system that runs below atmospheric pressure and uses water as the working fluid. this was common 100 years ago before the invention of working fluids (freons of various molecular weights) more suited to temperature ranges that are typical for systems intended to freeze foods; it is called the steam refrigeration cycle and is extensively documented on the web and elsewhere.

assembling from scratch a working miniature steam cycle refrigerator and installing its cold side inside a computer case will be a difficult engineering task. You would be better off, in my opinion, considering a peltier-effect thermojunction cooler or a once-through water system that runs on the cold water tap and exhausts the warm water through a drain hose into a flower bed in your back yard.

Answered by niels nielsen on December 11, 2020

You are right in thinking that the pressure would shoot back up when the water vaporises, as the amount of gas in the tube is related to the pressure (in a rough approximation by the ideal gas law). If you kept pumping it to maintain the pressure, you’ll be pulling the water vapour out of your system, and it wouldn’t go back to your reservoir.

Moreover, while you might be able to remove more heat per unit mass of water used, the rate of heat removal shouldn’t be visibly affected, as it depends more on the contact surface and thermal conductivity of your exchanger.

Answered by LvdT on December 11, 2020

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