Clemmaster, When I made the statement:
Actually I think I can condense it at reasonable pressures with only R-600 (43 suction/240 discharge)
I really meant to say "I know" instead of "I think". The reason I say this is, that I know beyond a shadow of a doubt, that the only way I could have obtained -100C or colder with a charge based on R-600, R-170, R-14 -and- with a 240 psig condensing pressure -while- evaporating into a 43 psig suction, would be by condensing at least the R-14, which would have minimally required condensing and evaporating the R-170.

My questions to you are: Have you studied, or have been working with autocascades? Do you understand that an autocascade is not the same as a standard multi-compressor cascade, and that in an autocascade a mixture of refrigerants are fractionated out and used to cool successive stages? If not, then this may be the source of your dilemma about why I was able to condense R-170 (and R-14 which has a much lower boiling point), while using only one other refrigerant (R-600).

One last question for you: Have you read through this entire thread? If not, then I would highly recommend that you do so, since much has been covered on the topic of autocascades and how we think they work.

I'll be the first one to admit that there has been much theorizing on the subject, and I'm sure you'll find that some of the later posts will be in conflict with what had been thought to be fact in earlier ones. But overall I believe it has been a very productive effort, and hopefully is allowing others to have the knowledge to build their own autocascades, and have their own successes at condensing R-170, and/or R-1150, and/or R-14.