Any news, mytek? Every morning I am looking forward to see progress on your new AC...
PhilippF,
I just got done fabricating all 4 heat exchangers last weekend, and I am now assembling them into a complete stack. Probably wont get a chance to do the brazing until my Christmas vacation. So far it's looking good, although I did have a fight with the 1/2" bender on Cascade #1 (and I think the bender won ). Nothing that will affect operation, but there are some deep grooves on some of the bends.

I'll post pictures when the stack has been brazed

quintus,
-80C can be obtained with only 2 refrigerants R600 (n-Butane) and R170 (Ethane), which are hydrocarbons, so might be easier to come by (and cheaper). Will probably work with only 2 heat exchangers (auxiliary condenser and cascade condenser), and a single phase separator. Just be sure to make the phase separator fairly large, and stuff the upper end with either steel or copper wool. Having fewer stages does require that you do a better job of phase separation, otherwise too much of the high boiling component gets through to contaminate the final stage. !!!Important Note!!! When fabricating your phase separator, be sure it is constructed to handle the working pressure on the discharge side of your system (350+ psi). Also implement a high side pressure switch to limit over pressure. Either 1 5/8" or 2 1/8" OD ACR hard drawn copper tubing with copper end caps works well.