How cold will it get?

[mad fridgie]

Sublimation can already start with -1 degree C of evaporator coil but for good transfer you need very low residue of air in coil area and heat inlet to item under freezer-drying or item cool down to same temperature as evaporator coil and moisture transport stops.

at 30 pa, sublimation point (condense) is below -30C, so it will not form on the evaporator at -1C, if heat is added to the product, then the product will sublime (boil) "freeze drying" is not about the process, but keeping the product frozen during the process (cell wall structure in place)
The purpose of the evap in a freeze drier is to convert a high volume into a very small volume. The vacuum pump is to remove non condensables from the chamber, not the water vapour!

[Thisisausername]

at 30 pa, sublimation point (condense) is below -30C, so it will not form on the evaporator at -1C, if heat is added to the product, then the product will sublime (boil) "freeze drying" is not about the process, but keeping the product frozen during the process (cell wall structure in place)
The purpose of the evap in a freeze drier is to convert a high volume into a very small volume. The vacuum pump is to remove non condensables from the chamber, not the water vapour!

You speak the truth, sir. I should mention though, that for the exact reasons you mentioned (given that at 30 pascals, the ice interface temperature is something like -60) The pump will not be running continuously, but will bring the chamber down to something like 200 pascals, where the interface temp is around 0F, and then the isolation valve will close and the pump will shut off. According to the literature, you want a coil temp 20 degrees below the interface temperature to achieve deposition, and have it freeze on the coils. I want it colder than this, however, because the chamber itself needs to be very cold indeed to start with, so I'm shooting for -40 chamber temp. I actually have looked at having a cascade refrigeration unit, and just buying one used, but it was far too expensive even for a scrappy one.