So I have been working on a cascade TEC cooling unit that can get the cold side to well bellow -100c at idle and when a heat load is applied it sits at around -70 at 230 watt load. My plan is to use an ac to cool a water loop as the first loop that will cool a cascade system that I build years ago that can achieve a delta of ~30c from the hot side. My ac unit brings the water down to about -20c which means that if it works as planned the first stage will get down to -50c or so. I plan to use 4 of them to cool pure ethanol to cool my final stage which should get down bellow -100c. My problem right now is any tubing I have tried starts to break at anything bellow -40c. Any suggestions on what I can use would be very helpful.

30 minutes ago, Mythical tech said:

TEC cooling  230 watt load.

That means ~700W of heat for second stage (-45c) and ~2000W of heat for first (-20c) stage. Your AC unit is too weak for this I think. Classic gas cascade unit will be cheaper and easier to build.

As for cost I already have the TECs except for a few 400 watt ones and I am using dual 40k btu ac units that I got for free. They get 15 gallons down to - 20 in about half an hour. I am going to using it to bin CPU's for super pi 1 million so it will only have load for about 30 seconds at a time. I am not good enough at welding to safely build a r1150 system that won't kill me. 

It will be interesting project.

You need to make a quiet huge 2kW heat exchanger anyway. And another one for the second stage large enough for 4-5 250W or what you've got TEC modules. As for tubes - copper ones usually are used.

34 minutes ago, Mythical tech said:

build a r1150 system that won't kill me. 

You can use R13/R23/R502 or R508b. Although they still can kill you, if the pressure rises high enough

So after doing some math -70 idle seems more doable. The system will still draw over 3000 watts but I don't pay for electricity where I live so that won't be a problem.