Cascade Refrigeration System

[Jingle]

Hi everybody,
Eventhough I recently joined, I have been visiting this site for quite some time. I am in the process of designing a cascade refrigetation system for -80 deg. C chamber. I think R-23 in the low stage and R-404a in the high stage is a good solution. In this regard I need somebody to through some light on the following.
1) What exactly is gliding problem associated with R-404a?
2)Why some manufacturer use few circuits in the aircooled condenser of R-404a (whose disch temperature is about +70 deg C) for low stage (R-23) discharge whose discharge temperature is +20 deg C( I hope this is correct)
3) Is copper coil with aluminium fins evaporator OK for -80 deg.C
4) What fans to be used for -80 deg.C.
5)Is there any special precaution to be taken for oil return for R-23 and R-404a.
6) I remeber somebody suggesting capillary tubes instead of Thermostatic expansion valve. How to size capillary for these refrigerants.
7) How to calculate the refrigerant charge needed. I think this is needed to design expansion tank.
8) The refrigeration load is around 3 Kw. Can somebody suggest the size of compressors and other equipments needed?

(The ambient temperature in our part of the country is +40 deg.C)
Regds,
Jingle