refrigerants for cascade system?

[rebmevon]

Hi, just new in RE forum... need some inputs & ideas regarding the recommended refrigerants in terms of performance for the cascaded system for extreme cooling application.
For Low Stage system, R-23 or R-508b?
For High Stage system, R-507 or R-404a?
Thanks.:)

[keisersosei]

What do you call extreme and for what purposes?

There are lots of options to consider what about R744???

cheers

[rebmevon]

for 10-hp cascade system of temperature cycle chambers with operating temperature range of +180ºC to -75ºC (don't know if it can be considered as extreme).
i'm not familiar with R744 system.