Deep freeze,Help please!

[mouse]

Hi,
I have one problem and i need Your help,Please!
I want to cool one small volume 5x5 inch only at
-70C,-90C will be great.For this deep freze i need two stage refrigeration units.R404a cant do this job?Second stage refrigeration units cool the liquid for the first stage.First stage compressor work on suction line in low pressure(vaccum ?),second stage compressor work in suction line in high pressure.
Becose its talking about 5x5 inch volume only, hermetic compressor R404a use in domestic refrigeration cant do this job?
I add to first stage compressor on high pressure line one oil separator becose of low presure in suction line make the oil from the compressor to be easy lost in the high pressure line and prevent the compressor failure.
Sorry for my very bad English,please.
Correct me Please if im wrong.
Thank You for Your time,
Respectfully,
mouse.

[Peter_1]

You will run very far outside allowable specs of compressor.
You can use Peltier elements in 1st stage and R404a like a NZ member did it and explained it here very recently.

[norseman]

Dear Mouse.

The easy way would may be to buy a ready cold trap.I have seen a lot of them over the Years. 5 litres steelbottle with a temperature to -104c.
If you want to make a low temperature system like that, you need to get a heat exchanger or make one as a tube in tube type which is cheap and simple.
I would also used a coalecent oilseparator to get a good oil return. The separator is also good to get some space/volume for the static charge on the low stage. I have seen such units been running for 15 Years in an all time run.(No thermostate involved)
The compressors as simple as Danfoss SC types and similar. R404A in the first/high stage and if it is possible a hydrocarbone type in the second/low stage due to the good oil return those create.
R170 or R1150 is commonly used. Otherwise R23 or 508A/508B could be a choice. I prefer the 508 serie due to safer/colder discharge. The task would be to adjust the capillary tubes to match the both stages load. I have seen the use of an over and under cooling thermostate for the HX, that would hold the HX inside a good working temperature. I think you need some time and effort to make it run. I belive most standard compressors would take the duty, but it is important to cool them properly with a fan or two who really boost a lot of air over them.

Good luck from Norseman

[mouse]

Thank You Mr. norseman
Yours help its more usefull for me. I work on transport refrigeration but i dont have experience in two stage refrigeration units.With Yours help now i understand what its talking about! Thanks!
I make the heat exchanger from a old TK or a CT receiver tank and inside i add 5 meters of 12 mm copper pipe,double pack inside.Must be enough to a proper heat exchanger.Yours ideea to make a pipe in pipe its great and cheep too.About oil sepparator its not a problem for me i have enough to modified to desired capacity.Becose the compressor must run nonstop withaut a termostat i choose to cool down the compressors with water to keep the oil temperature to a safe point,a guarantee for a long life.
About the refrigerant R404a,its enough,problem i have to found R508b the best choice for me .I must found this refrigerant somewhere in my country,i hope.
About the capillary tube i have enough patiente to make the correct adjustment.I love refrigeration and its like a hobby for me,and to achive -104C its great.
Thank You Mr.norseman for Yours usefull help,
Respectfully
mouse.