STC-1000 controller + craigslist freezer for Ultra Low Freezer temps? -30F to -40F?

[square_root_pharms]

Has anyone wired in a Temp controller to bypass a factory t-stat to get their cheap deep freezers colder? What is the chilling “limits” of standard R134a refrigerant in a chest freezer? ive read online on different websites and PC benchmark / overclocking crowds that getting below -20F could potentially create a vacuum on the low pressure side of the compressor? i have also read that people have gotten R134a window ac’s to cool below -40f but these are often in short bursts bursts during overclocking PC benchmarks. I would be interested to know if its possible to get a chest freezer to cool around -30f to -40f using the stock refrigeration R134a system for a few days at a time simply by bypassing the stock t-stat and wiring in an ebay temp controller (like an stc-1000) to run the compressor longer?

i dont know enough about refrigeration systems to make “safe” decisions but im irish and scottish so i think ill mess around with it a bit anyways. From what ive comprehended from reading online the higher the working pressure the colder it can go (i dont know if this means on the discharge or inlet of the compressor). i wonder if tuning the system (adding a little bit more R134a and bumping the pressure 1psi at a time in the low or high side line) coupled with longer compressor cycle on time would give the desired effect… i guess it couldnt hurt to experiment lol (or could it haha). its simple enough to wire in my own controller and start off with running up the compressor for an extented period of time to see how low it can go in stock trim.

Pressure/temp ratio of 134a
https://www.forane.com/export/share...ls/forane-134a-pressure-temperature-chart.pdf

 

[cegan09]

Throwing an aftermarket controller on will allow you to control the fridge differently than it was designed, but how cold it gets is very dependent on the device and environment. All devices have ranges they were designed for, and limits to what they can pull off. Each is different. At some point you'll hit a limit of just how well the freezer is insulated, and the cold side of the system can only get so cold. As your freezer temp approaches that and the temp difference between the two decreases, it becomes harder and harder to actually go colder. How cold you get get a standard freezer I have no idea. Window AC units have no problem getting to negative temps, but I've never pushed one much below -4F because I had no need. My dorm fridge style fermentation chambers have to run 24/7 to try and maintain temps down in the 20s.

I'm thinking -40F is a long shot, and even if you manage to get there, the life of the refrigeration system will be rather short. Why do you need to go this cold? You can buy freezers that are designed for extreme temps like that, but they aren't cheap.

 

[square_root_pharms]

my current use deep freezer cycles down to about -18f when the compressor runs and heats back up to around -10f when the compressor kicks off. right now at full cold setting with the oem thermostat the compressor runs for about 30 mins then kicks off for 30 mins.

Ive thought about skinning the exterior with 1" or 2" thick foam board for extra insulation, but i wouldnt want to spend the money unless i knew i could push this freezer colder. its taking about 30 minutes to lose 8 degrees so that shows me the freezer could stand for some more insulation. I only need it go operate for a few days at a time every few months.

I am experimenting with pseudo-cryogenic ethanol extraction techniques for different plant oils and terpenes for use in some experimental brews i'm working on. Yes Cryogenic ultra low freezers are very expensive whereas a used craigslist freezer with an stc-1000 and some other odds and ends would be fairly cheap.

 

[TechFanMD]

What kind of freezer is it? Most chest freezers have the coils (to dissipate heat) in the exterior walls - so if you insulate it better you're decreasing the amount of heat that can be removed. I was going to suggest removing the skin and making a better heat sink for those coils, and insulating from the inside.

 

[cegan09]

What kind of freezer is it? Most chest freezers have the coils (to dissipate heat) in the exterior walls - so if you insulate it better you're decreasing the amount of heat that can be removed. I was going to suggest removing the skin and making a better heat sink for those coils, and insulating from the inside.

Beat me to it. It's hard to add insulation without screwing up the heat rejection of most of those chest freezers. You need to make sure the hot side can dissipate heat.

If the one you have is doing -18 without issue, you can probably put an ST-100 on it and get it down into the -20s. There's a setting in the controller for how long it lets the compressor sit off before turning on again. You could shorten it from the 30 min, but don't shorten it too much.

 

[dthompson79]

Doubtful they aren't made to go that low and probably don't have a big enough condenser coil to achieve this without heavily modifying it. Also doubtful with 134a since that is well below its boiling point

(Edit) The second part of the above statement isn't entirely true. Spoke with a friend that does refrigeration and he said the limit of "cheap" refrigeration is ~-20. Any lower than this and you run into a lot of problems. Oil return, liquid slugging of the compressor, etc. These can be overcome but at great manufacturing cost. Hence why lab freezer are so spendy.

 

[cegan09]

Thanks for providing a real world limit. I was under the impression it was a little lower, but I'll believe the figure from someone in the industry.

 

[square_root_pharms]

Doubtful they aren't made to go that low and probably don't have a big enough condenser coil to achieve this without heavily modifying it. Also doubtful with 134a since that is well below its boiling point

(Edit) The second part of the above statement isn't entirely true. Spoke with a friend that does refrigeration and he said the limit of "cheap" refrigeration is ~-20. Any lower than this and you run into a lot of problems. Oil return, liquid slugging of the compressor, etc. These can be overcome but at great manufacturing cost. Hence why lab freezer are so spendy.

understandable, which is why the expensive ultra low freezers are 2 stage refrigeration loops i guess.

 

[square_root_pharms]

pulled the tstat thermocouple from the sheath so it would read warmer than it was. overnight the freezer was able to get down to -25F this was as cold as i could get it.