How cold realistically can a domestic freezer go?

Pommie said:

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.

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I think I will give this a go, the units are pretty cheap now from China. -100C is kind of a holy grail for some things. Thanks for the explanation I couldnt visualize it as i have mostly used them for power generation. In that arrangement ice formation is a problem, however for this it looks ideal. I wasnt aware you get them to go anywhere near that kind of temperature!! When I look at the lengths some have gone too just to get a acetone bath down to -60C............ I am a bit mystified why this isnt more common in a armature chemist environment.

Looking at ebay second hand small cryo freezers, the price is shocking, 1ltr solvent baths down to -100C are around £2000 second hand. the methods mentioned here are extremely reasonable cost wise.

Little Ghostman said:

I am a bit mystified why this isnt more common in a armature chemist environment.

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Most people don't have a highly diversified technical background so they tend to not think that far outside their known areas of expertise.

I could write a book or several on all my life experiences I have had from dealing with people who waste a fortune on overpriced items and services simply because they refuse to think about and accept that there is likely a different way to accomplish some task they need done.

Little Ghostman said:

I think I will give this a go,

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Could you keep us updated with the design and your progress? Keen to see how it evolves & what results you get.

flatfootskier said:

Could you keep us updated with the design and your progress? Keen to see how it evolves & what results you get.

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Same here.

Mike.

bumped into this. Might be of use
https://www.banggood.com/12V-6A-DIY...r-Cooling-Equipment-p-1074404.html?utmid=1881

High and thanks guys.

Been busy with other things, but have done a few things!! Got the inside of the freezer sorted (or so I thought), rule number 1) NEVER seal things up until your finished! Got the insulation sorted, got things in place and sealed up, only then to discover the peltier plates hadnt been fitted! (trust me i been busy and i was tired lol).

I started the freezer up and it didnt sound right from the off, bearings are long gone on the hot side fan. Doubt the freezer is going to hurt without it, it is sitting in a building outside and average temp of building is 6C. No thermostat and the freezer drops to -31C. Now this is a 1997 freezer, and all the digging I have done says there should be NO ammonia as the coolant in it, it also says there should be a plate that tells me what the refrigerant actually is, i cant find that info anywhere. Not sure it actually matters, but my understanding was it would freeze the lines up under -29C, unless it was ammonia. I cant see this being ammonia as by 1997 i am pretty sure they had all changed.

Anyway dosnt matter. I think i have the cells wired up wrong, the first 4 seem ok, then i get a ice layer and thats it! Nothing happens past that point and I am stuck at -31. Its got to be stupid simple mistake somewhere, but at the moment i cant face taking the insulation out and getting to everything!! No idea why i sealed it up TBH.

Its on my to finish list, but i have something pretty urgent going on first. I tried Chlorine under reduced pressure and got a fog but no liquid, few more degrees and I am there. I wont be running under reduced pressure though, its too hairy if it goes wrong.
So why the big deal with liquid chlorine? I need iodine monochloride, by far the easiest way is liquid chlorine, i also make chlorinated solvents as getting them in the UK is really not easy. And they cost a fortune.

I could use TCCA but it dosnt work well with wijis solution. Slight mishap with the chlorine generator, the valve stuck open and HCL poured into the flask. I stood smugly in my respirator by the fume hood and watched it clear. Not that i am particularly happy i vented chlorine into the environment through a mistake. I will take some pics once I get the time!
I didnt go back to school after the summer break, instead I am now working for myself. You know the thing, long lunches, no one telling you what to do, work when you feel like it...........................Yeah i got all that wrong as well , having fun but never worked so hard!! nearly a month gone and still not a millionaire! That needs a tweak for sure .

flatfootskier said:

bumped into this. Might be of use
https://www.banggood.com/12V-6A-DIY...r-Cooling-Equipment-p-1074404.html?utmid=1881

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Funny you found that, i saw them, and having a shed load old pc parts i have built something similar. The evacuated copper tubes make alot of difference, but getting several cells to work together seems a problem. Mike says 50W per cell, so thats around 6 cells all told i need, stacking them hot-cold leads to plate 4 freezing up solid. But these were cheapo cells, could be one the cells has had it.

From your description I'm not sure how you have them stacked. A single peltier (12705) at DT of 30°C will move about 30W and consume 4A at 10V (40W). Therefore the hot side will have to cool 70W. To do this you will need at least two peltiers cooling it. These in turn will need to cool around 140W and so on. I've partly constructed a little test rig with just two stages (3 peltiers) but haven't managed to do any testing yet.

I'm going to PWM them at 10Hz so 1mS is 1% and during the off time I should be able to measure the voltage produced and work out the Delta T.

For your info, datasheet attached - take a good look at the performance graphs.

Mike.

Little Ghostman said:

Now this is a 1997 freezer, and all the digging I have done says there should be NO ammonia as the coolant in it, it also says there should be a plate that tells me what the refrigerant actually is, i cant find that info anywhere. Not sure it actually matters, but my understanding was it would freeze the lines up under -29C, unless it was ammonia. I cant see this being ammonia as by 1997 i am pretty sure they had all changed.

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I see a LOT of old fridges and freezers (because we deliver new ones), I've NEVER seen any mention of Ammonia, the most common label on the back (not a plate, just printed large on the back) is Pentane. There can't be many of the old CFC ones left out there, I've not seen one for a good few years now.

Pommie said:

From your description I'm not sure how you have them stacked. A single peltier (12705) at DT of 30°C will move about 30W and consume 4A at 10V (40W). Therefore the hot side will have to cool 70W. To do this you will need at least two peltiers cooling it. These in turn will need to cool around 140W and so on. I've partly constructed a little test rig with just two stages (3 peltiers) but haven't managed to do any testing yet.

I'm going to PWM them at 10Hz so 1mS is 1% and during the off time I should be able to measure the voltage produced and work out the Delta T.

For your info, datasheet attached - take a good look at the performance graphs.

Mike.

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Thanks Mike, i will get some pictures. I have to take it apart anyway, I am sure I have messed up the stack somehow. Thanks for the datasheet, no idea where I got my 50W from!! I am pretty sure i need more cells, one of them is very suspect, it gets hot both sides.

Nigel Goodwin said:

I see a LOT of old fridges and freezers (because we deliver new ones), I've NEVER seen any mention of Ammonia, the most common label on the back (not a plate, just printed large on the back) is Pentane. There can't be many of the old CFC ones left out there, I've not seen one for a good few years now.

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There isnt any info on this one, i doubt its ammonia or pentane. the few i seen lately all had a metal plate with the refrigerant number on them, mine is a domestic one, but most the ones i get hold of are the commercial type with all glass fronts. they come from shops etc and used for keeping drinks cold, they make wicked incubators etc, when stripped out.

In the US pretty much every freezer of that age would have still been running R-22. As for your condenser fan, even at lower ambient temps they still make a difference in cold side performance so there's a fair chance that a working fan will gain you another few degrees on the cold side.

Doing a strip down and rebuild this weekend, something definitely very wrong somewhere. Going by Mikes figures I should be hitting temperature no problem, the other issue is the compressor is very unhappy after 40mins running, it gets very hot and the noise dosnt sound good.

I spoke with a fridge guy, he thinks it might need re-gassing, me thinks he just wants some money. I am going to put a huge twin fan from a scania lorry radiator, on the hot side to take some heat out on the return to the compressor. Seeing as this is alot more work than i intended, i am now aiming for -100c. If i got to put this much effort in, then i am going to make it worth it.

I will make sure i grab pictures this time, i am rubbish at taking pictures, i get so engrossed in what i am doing, i forget to take pics!! I have ordered some more cells, they seem pretty cheap at the moment. Also i have scrounged some the copper evacuated tubes from old laptops, those things move some heat in a hurry.

One small mod is to alter the position of the solvent bath, i want access to it from outside, this wont make sense until i do pictures. the other thing I also have to do now, is alter the inside so I can 'pre chill' glassware. If I start getting past -35C i am going to start coming up against thermal shock problems. I use decent glass but have had issues with this now and then. Mainly when boiling Sulphuric acid, I can do 3-4 runs before I start to get star cracks.

Apparently a common problem. Devil and deep blue sea though, very hot acid or not so hot acid but high Vacuum! my preference when i get one, is to go with the heat and not use the vacuum, having had a 2ltr flask implode (was harmless contents), i dont want to experience what would still be hot acid under vacuum going bang!

Little Ghostman said:

the other issue is the compressor is very unhappy after 40mins running, it gets very hot and the noise dosnt sound good.

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Possibly low on refrigerant but if it had a fan on the condenser system and you don't have it working that would be my first guess.

Hot loud inefficient compressors tend to be a standard sign of excess high side pressure which comes from lack of cooling on the condenser.

Well MKII ready Sunday. Has a dual fan set up from a BMW car, no idea age of car etc, got it at a garage free. Its a pretty big unit so should knock some heat out the cooling fins. Tempted to have it temperature controlled, but at the moment time is really limited, so it will be decided by how long i can stand the fans on for!! If its a limited time then I will stick thermostat control on them.

Might be MK!! or MK IV. I have lost count already

I am waiting for a couple more cells from China to arrive. It seemed fairly easy to start with, getting past -40C seems to be the sticking point. I think its likely the ice between the cell plates or something equally silly. Not sure how far down it will go when done, but I did notice getting down to -25C is easy enough. After -30C the drop really slows pace, I am wondering if its a kind of sticking point and once past it.........

But might be alot like cars, upto 200mph is easy (ish) but takes alot of power for every 1mph past that. No idea but I guess we will find out soon enough