Friday, December 30, 2011

CryoBUG breaks the -150°C barrier

Yes you heard that right, CryoBUG not only made it to my target temperature (-150°C), but it actually surpassed it by a small margin.

-151°C Evap-In, -148°C Evap-Out (not shown)

22 psig Suction, 235 psig Discharge

Evaporator Inlet (final cap tube feed)

Now to be honest, my temperatures and pressures were not completely steady in all cases (although some temperatures such as the evaporator outlet were rock solid at -148°C).

Here's the specs:
Compressor Suction Pressure: 17-23 psig
Compressor Discharge Pressure: 235-240 psig
Compressor Current Draw: 2.65-2.80 amps
Evap-In Temperature: -149 to -154°C (average -151.5°C)
Evap-Out Temperature: -148°C

The compressor discharge temperature was pretty steady at +57°C, and the Liquid Line equaled +20°C with an ambient room temperature of 19.5°C.

Now how did I do this?

Well I made a few hardware modifications to the HX Stack. First I chopped off about half of the Auxiliary Condenser's length...

Then I re-established the liquid and suction connections that connect to the condensing unit...

And then I took the coils I chopped off, trimmed off a small amount, and added this to the end of the Cascade Condenser's SLHX section, thereby creating an even longer final sub cooling stage..

Extending the Cascade Condenser's SLHX

And finally I brazed everything up, and got it ready to go back into it's orange Home Depot bucket.

Modifications Completed

Boy this thing is getting pretty ugly with all the cutting, hacking, and re-brazing going on. But it is a bread-board prototype, and that just happens to be it's fate in life.

Now back to the test results...

Well I started off with basically the same charge as I used before this particular set of modifications. But after seeing it take longer to pull-down, and also seeing some jerky jumps in temperature at the Phase Separator, I assumed that it was the result of an insufficient supply of refrigerant condensate. So based on this conclusion, I decided to up the R-134A by another 30 Grams. This really made the difference, and the temps quickly dropped, bringing the evaporator down into the -143 to -144°C range. So seeing things starting to level off, I gave it another 20 Grams of R-134A. This helped a tad bit more, but then I started seeing a bit more fluctuation in the pressures and at some temperature points including the evaporator inlet. I then went on to add a tiny bit more R-23 without any noticeable benefits.

My final addition was to add about 5 psi more Argon, and although this did jack up the compressor discharge pressure, it really delivered and pushed me past -150°C.

Aside from the continued fluctuations, I think I'm getting pretty close to a final design. And most likely with the extra mass of a machined copper evaporator fed by the final cap tube, the temperatures will tend to even out. I also think that I will add back to the Auxiliary Condenser half of what I previously removed.


Anonymous said...

Congrat Mike. Now you have got to get this on a video and onto you tube! Even a timelaps of it from off to on and full pull down! How much adding/modding then do you think to get this to hold 300W?

Michael St. Pierre said...

Yep I'll have to get on that video, but I think it'll have to wait until I finish a few more tests.

As to how to get 300 watts out of this design, and still obtain -150 C, I think it would take scaling everything up by a factor of 10 or more. So in other words, at least a 50-60cc/rev displacement rotary (or equivalent reciprocating compressor). Basically something that would handle about 50,000+ Btu/Hr. and of course upsizing the heat exchangers proportionally. It would be quite a beast.

Anonymous said...

Shame nothing could be done with a 28,000BTU unit! A vid is needed still asap

Sergio Menchaca said...

hi mike

i have a system in cascade I only get -114c how will can down more the temperature ?

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