Phase 1

Ok, this is Phase 1, so don't expect much. :-) I have been doing a lot of research trying to come up with a good design. I looked at some people that were using phase change (liquid to a gas) systems to cool a liquid and then circulate that liquid around the CPU. My goal is to cool the CPU down to -120° C and I could not find a pump that would pump a liquid that cold. So, what I am going to try to do is have the phase change occur right on the chip.

I now needed to pick a refrigerant, I looked at R-134a, R-22, R-502, and R-404a. I ended up going with R-404a this was chosen for several reasons, it is easily obtainable, has a low boiling point at 0 PSIg (-52° F), and is environmentally safe. Don't get me wrong on the last one, it is not that I am a hug a tree guy or that I believe CFCs are a big part of the "greenhouse effect", but why contribute to it. Anyway, enough with the commercial, lets get back to the project

Perhaps the most important part of my project is the evaporator. Most evaporators are fairly large, I wanted the phase change to occur on the CPU so my evaporator had to be small and yet still be able to provide proper heat absorption, I came up with the following design.

Above you can see the design for the evaporator. I was tried to turn my drill press into a milling machine, but that did not work so well. I ended up building the evaporator using 1/2 soft copper, 1/4 soft copper, few end caps, and 1/16 plate copper. I soldered it all together, I am sure it is not efficient as my design, but it would do for testing. The main 1/2 flare connection connects to a Constant Pressure Expansion (CPE) Valve. The two outputs connects back to the suction side of the compressor. Two hoses were needed because there is move volume that needs to be removed after the liquid flashes to a gas in the evaporator.

To save time I used a off the shelf condenser unit instead of building one, I selected a Copeland condenser unit. The unite comes with a compressor, condenser coil, liquid receiver, and is charged with R-404a.

The system is very straightforward, the compressor compresses the gas into a liquid. The liquid is then cooled off in the condenser and sent to a liquid receiver. The liquid then enters the CPE valve that is set to maintain a evaporator pressure of around 10 Hg. As the liquid flashes to a gas it absorbs heat from the evaporator, it is then discharged to the compressor to start the process all over again. To lower the CPU temp farther I decided to use Thermo Electric Coolers (TECs) to drop the temp further. The evaporator is designed to fit on top of two Intel Celeron CPUs.

As you can see I was able to cool a sensor down to -73° F with a suction pressure of about 5 PSIg, not bad for a first crack at it, but that was without a load. I ran into a big problem when I tried to put it on a real CPU. The problem is that the 500 MHz CPUs that I wanted to use have the clocks locked, so the only way to up the CPU speed is to up the buss speed. I wanted to reach 1GHz, to do that on a 100 MHz system buss I would need to overclock the buss to 200 MHz. Even if I could cool the CPU, I would not be cooling the memory, video card, etc.

So now we move to phase 2.