How cold realistically can a domestic freezer go?

[large_ghostman]

Ok UK domestic freezer, I dont know much about refrigeration in general. The question is if I disabled the thermostat and replaced with something else, and maybe if I looked at better cooling for the hot side, what is the realistic temp I could get a domestic freezer down to? I am aware life span of compressor etc would be shorter and thats ok.

Type of freezer is a fairly large free standing fridge freezer, freezer section is separate from fridge and fairly large. I can cut bits of it up and that as its in my lab. It has two plates in the freezer section with pipes going through them, these are the plates that get cold. So one idea is to move one nearer the other and heavily insulted the an area, basically aiming to make the space inside to cool alot smaller than now.

My reasoning being, the compressor handles the size of space to cool to -22 at present, perfectly well, so if I decrease the area to cool then the compressor dosnt have to work so hard. With a smaller space to cool I get a tiny amount of leverage on how cool it will go down too with the compressor on for the normal time (thermostat removed obviously).

If I cool the hot side better this should also help should it not??

What else can I do without major **** like re gassing? And while i have no idea what the refrigerant inside is, it must have a limit on what it would cool too. So has anyone played with freezers? And realistically how much past -22 can i get a freezer to go down too? In a ideal world -32 - 38C would make me happy .

I havnt touched the thermostat yet, but moving it as far as it will go gets me down too -24C.

 

[tcmtech]

My mom picked up a old chest freezer years ago at an auction for something like $5. Turned out the thermostat was broke so the compressor never shut off. The thermometer she had in it went to -40F and was buried way bast that to someplace in the likely -50F or better range! It ran like that for a year or so before I put a new thermostat in it.

The Ice cream she had in it was like stone and you could pound a nail with a steak too!

If you could modify a heat pump system from an air conditioner, refrigerator , or freezer to have the evaporator side running at a ~ 12 inch vacuum you can get R-22 to go down to -60F.

Or if you converted it to propane you could conceivably get down to ~-85F at~22 inches of vacuum on the evaporator side which with a oversized compressor on a old freezer that uses a simple capillary tube type refrigeration expansion valve system it would not be that hard.

 

[Nigel Goodwin]

After my daughters graduation the other week she took me to her lab, where she had worked for the last 3+ years, and on the way back (walking round the back of the chemistry building) I noticed a locked chest freezer outside that said (I think?) -37 on a red LED display - it also proclaimed it was a special low temperature freezer. Melissa mentioned it was for storing dry ice in.

 

[large_ghostman]

This is likely to be from 1997..... I happen to know the date because it came from a house clearance, and the house was last done up in 1997.

I might try the easy option first and remove the thermostat, then have it cut out at -36 if it will go that low. I actually need -35 but a bit lower gives me a small margin, it wont be on all the time. I need it for cooling some things during low vac distills. It needs to cool a solvent down low enough to liquefy a gas I want anhydrous.

 

[Pommie]

Ammonia by any chance?

Mike.

 

[cowboybob]

... I might try the easy option first and remove the thermostat, then have it cut out at -36 if it will go that low. I actually need -35 but a bit lower gives me a small margin, it wont be on all the time. ...

Depending on the ΔT needed to condense your gas, not sure the freezer will be able to maintain that 1° buffer.

Although, I suppose that if the freezer contained enough thermal mass(at -36°), it'd probably work fine.

 

[large_ghostman]

Its Anhydrous Chlorine (***** to prepare), and ammonia is the other one now and then. But mostly Chlorine is what I am after.

I will give it shot next week, our freezer at home goes down to -30 on the readout thing, it has electronic setting and readout, but the old one is really old. -30C isnt enough anyway, Anhydrous and -35C works (Just). The school dosnt have a cryo freezer anymore, that was open top alcohol bowl type so wouldnt have worked for me.

 

[large_ghostman]

Depending on the ΔT needed to condense your gas, not sure the freezer will be able to maintain that 1° buffer.

Although, I suppose that if the freezer contained enough thermal mass(at -36°), it'd probably work fine.

The way its condensed uses a alcohol or solvent as the liquid coolant, thermal mass wise 5Ltr of solvent cooled to -38C works for sure and more Chlorine than I am after at a time. If I did it with just an air jacket your probably right, the Delta T would be really scratchy.

Lucky enough I dont have to try and store it long once liquid, so vapor pressure wont be an issue. I have seen it done with Dry Ice but getting that in Scotland where I live............ More chance getting Iodine monochloride in Glacial Acetic Acid

 

[flatfootskier]

Don't forget the minimum temperature is attained as result of not only the temp of the cold side, but also the heat loss (gain) through the walls. Better insulation should give a lower temperature. A few inches of PIR should bring the temp down a bit more. It should also lower the duty cycle £££ significantly

 

[Pommie]

Just a random thought. Could a small peltier cooler placed in the existing freezer give you an even lower temperature?

Mike.

 

[tcmtech]

I might try the easy option first and remove the thermostat, then have it cut out at -36 if it will go that low. I actually need -35 but a bit lower gives me a small margin, it wont be on all the time. I need it for cooling some things during low vac distills. It needs to cool a solvent down low enough to liquefy a gas I want anhydrous.

It will more than likely get there but it might take a while.

Just a random thought. Could a small peltier cooler placed in the existing freezer give you an even lower temperature?

That would probably work pretty well for a smaller container.

The bigger issue would be the freezers actual capability to move heat. most are pretty weak and rely pretty heavy on just being well insulated.

Infact I just picked up a ~ 20 cubic foot freezer this week at an auction. Power input is ~ 100 watts so assuming a large temperature differential of say -30 F inside to +70F outside ambient I would have doubts that it could handle much more than 50 - 100 watts internal additive internal heat load which given Peltier units poor efficiency that would make for a pretty small secondary container to work with.

Upside is it could probably take a one quart metal can and its contents down to -100F or better if given enough time!

 

[large_ghostman]

Just a random thought. Could a small peltier cooler placed in the existing freezer give you an even lower temperature?

Mike.

Interesting idea, I dont know the answer.
I do know when I played with them before I had problems when they froze up. But I was doing energy generation at the time so the situation was different. Something I could try on a small scale. Thanks for the idea.

 

[large_ghostman]

It will more than likely get there but it might take a while.



That would probably work pretty well for a smaller container.

The bigger issue would be the freezers actual capability to move heat. most are pretty weak and rely pretty heavy on just being well insulated.

Infact I just picked up a ~ 20 cubic foot freezer this week at an auction. Power input is ~ 100 watts so assuming a large temperature differential of say -30 F inside to +70F outside ambient I would have doubts that it could handle much more than 50 - 100 watts internal additive internal heat load which given Peltier units poor efficiency that would make for a pretty small secondary container to work with.

Upside is it could probably take a one quart metal can and its contents down to -100F or better if given enough time!

I will start with taking the thermo out for now, like I said the freezer wont be running every day so I am not too bothered about the compressor on it. Ambient temp outside the freezer is around 6C. The freezer is in the store room at the back of my lab, even this time of year when its 27C outside, inside its alot cooler.

I got some kingspan sheets I can put inside the freezer to cust down the area it needs to cool and also help insulate it. I think off the top of my head they are 120mm thick.

 

[flatfootskier]

I got some kingspan sheets I can put inside the freezer to cust down the area it needs to cool and also help insulate it. I think off the top of my head they are 120mm thick.

get yourself a foam gun (and some cleaner at the same time if you want to use it more than once). The control you get vs a simple can is incredible. Allows you to glue together & seal PIR sheets with good results.

 

[tcmtech]

I got some kingspan sheets I can put inside the freezer to cust down the area it needs to cool and also help insulate it. I think off the top of my head they are 120mm thick.

Putting insulation inside the freezer will diminish its cooling ability being the inside walls are where the heat absorbing evaporator tubes/coils are located. Or at least that where they are in US models.

If you ever noticed that when a freezer builds up ice on the inside it typically starts forming in bands that go along the sides. That where the tubes or coils that do the actual cooling run.

 

[flatfootskier]

Putting insulation inside the freezer will diminish its cooling ability being the inside walls are where the heat absorbing evaporator tubes/coils are located. Or at least that where they are in US models.

If you ever noticed that when a freezer builds up ice on the inside it typically starts forming in bands that go along the sides. That where the tubes or coils that do the actual cooling run.

He talks about moving the evaporator coils/plates. Not sure how you'd make this frost free, but i don't that's a concern in this case.

 

[large_ghostman]

Sorry Guys I have had to use my old laptop, i dropped the other and cant get another HDD working in it. Anyway took ages to get logged in again! although I have read the thread not signed in.

TCM

The plates on this type of freezer are horizontal and act like one the shelves inside it. I havnt seen models with plates inside the sides of the freezers, but i expect thats more to do with not seeing many models lol. The plates can be moved if your careful, this is what I intend to do. So adding kingspan insulation hopefully will help. Now I am aware models exist with cooling plates inside the side panels, i will make sure in future I dont picking up any scrap ones with this arrangment.

Frost and ice build up isnt an issue for this set up, the freezing is to liquafy and then bottle or use, the freezer dosnt have to stay on to keep anything liquid. It is also not going to be used for storage. I am trying to find out more about the peltier idea, although i have used them alot I havnt used them like this. Not 100% sure how or where I would locate the the 'hot' side, except maybe outside the cabinet?

Talk of -100C is very very attractive lol. That would open up a new world of possibilities .

 

[tcmtech]

That makes more sense. I have seen the type you're referring to where the evaporator side of of the system is exposed and makes up the shelves in a upright type freezer. The chest type deep freeze ones I have seen almost always had the coils in the inner side walls so that they had maximum internal space.

As for the peltier based super cooler I would build it to be a smaller super freezer (~.5 - 1 cubic foot internal capcity) that sits inside the larger freezer unit so that it's working as a two stage system. I did some reason on it the other week and I think with a modified freezer unit with a smaller peltier based super freezer inside that -100 F is totally within reach. Granted it might take 6 - 12+ hours to get there on a hard run but I don't see any reason it couldn't do it.

 

[Pommie]

The peltier hot side would have to be in the freezer so that the inner "fridge" would be some 30-50C lower than the freezer.

It may be possible to do this with just peltier devices. You'd start with a single one on the innermost box and this hot side would be cooled by two peltier devices, which in turn would be cooled by four more peltier devices. The reason for the multiple devices is that although each device can move 50W it also generates an additional 50W. Therefore to cool the cold side 50W you need to remove 100W from the hot side. The devices could be bonded to lumps of copper/aluminium and only the outer ones would need a fan cooled heatsink. If you can get hold of some 25mm polystyrene sheet (like this) and used 3 layers, you could end up with a 300mm cube with an internal size of 150mm cubed and very cold. Just a thought.

Mike.

 

[flatfootskier]

taking the peltier box inside your freezer idea...

Say it has an area of 1 m2.
-100F = -73C
73-22 = 51 deg C delta between the inner sanctum & the freezer normal.
0.22 W/m2k for 100mm thick PIR
51*0.22 = 11.2 Watt heat loss (gain) through the walls, assuming perfect seal & ignoring a few other factors. Assumes 1m2 area of box - enough space for a decent bottle. Hope that's right, not done it for ages. Hope someone will verify

Seems an easier approach than trying to modify the current freezer
Make sure you use PIR insulation (0.022 W/m.K) not polystyrene. Or you could use aerogel, but you need deep pockets.