Can a regular chest freezer be modified for -40C operation?

[noisette123]

Hello All,

I have a question regarding the possible modification of a new chest freezer in order to bring the operating temperature down from -20 to -40C. Is this possible with minimal hardware changes? Here is an answer that i received from Michael St-Pierre from mytekcontrols.com.


Hello Jean-Francois,

If your electronic device that needs to be tested is relatively small, as in low heat producing, it probably would be possible to use a standard refrigerator/freezer with a modified charge. A small dedicated chest freezer would be the best candidate, but it must be fairly new, having POE or PVE type oil in the compressor.

Changing to something like R507 for the refrigerant, and bypassing the original thermostat for lower temperature operation should get you where you want to be. Some other tricks like being sure to wrap or solder bond the cap tube to the returning suction line will insure that the R507 gets fully condensed before being expanded in the freezer's evaporator.

If you start off with the dedicated chest freezer as your basis, the cap tube will likely be close to what is needed. However I can not guarantee that this is true. Many of the guys on the Xtreme Systems forum create systems like this all the time for this temperature range. You might want to join the forum, and then post your proposed project in the phase change cooling section in order to get some good feedback and help with things such as cap tube sizing.

Good luck

Is this project feasible? And if so, what problems should i expect ?

Cheers for your technical insights,
Jean-Francois

[The Viking]

Feasible yes...

Unfortunately only if you got access to A) An experienced R&D (refrigeration) engineer. B) Time. C) A bucket full of money.


But what are you going to test, how many items and for how long?
If it is a small run your easiest option would be to create a chamber based on dry ice or liquid nitrogen.



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[noisette123]

I invent regularly new small pcbs (4 x 4 inches) that generate less than 5 watts of heat for clients that need to make sure the boards can work in cold weather (-40C) . The time i need to cold soak them before power up varies from projects to project (anywhere from 1 hour to more than 48hours)

That is why a dedicated small chest freezer that can reach -40C would be a great tool and simplify obtaining dry ice or liquid nitrogen each time i need to test.

What would be your way of handling this? purchasing an old medical ultra-low temp freezer and modifying it to warmer temperatures? I'd like to keep the costs as low as possible. I can invest a lot of time, and seek refrigeration engineering help (here if possible if someone can offer their design services)

Any ideas ?

Have a great day,
Jean-Francois

[The Viking]

Personally, me with 25+ years experience in the trade a fair few of which was spent inventing specialist solutions?

Unless I could find a ULT chest freezer really cheap I would get a suitable thermostat, a solenoid, some pipe, a bottle of liquid nitrogen and a cool box (like the ones you take for a picnic) large enough to divide in to 2 sections.
Then I would arrange it so that the thermostat probe and the PCBs were in one section of the cool box and the liquid nitrogen being injected in to the other. There will need to be some holes in the dividing wall and some sort of pressure relief "valve" on the chamber with the PCB. (For the PRV think 1/2" hole with a small flap on top).

For similar systems in full sized chest freezers a bottle of liquid nitrogen will keep it at -90ºC for about 4 days, your small scale version at -40ºC... most likely several weeks (and you can shut it off so you don't have to use the weeks in one go)



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[monkey spanners]

Would a stack peltier coolers get to -40C at only 5watts load? Would be easy to make an insulated box with polystyrene on expanable faom to mount it all in.

[install monkey]

former scientific,sanyo and others do small -40 ultra low temp freezers- with data loggers to prove youve tested the pcbs to -40