Keras
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Making a dry-ice cold bath without dry ice :p
The usual way of cooling, say, acetone, to -76 °C circa is to use dry ice freshly belched out by a fire extinguisher.
I was wondering if that type of lighter spray can couldn't be used instead. Has anyone ever tried one of those? It can't go all the way to -76 °C, but is much less bulky.
EDIT: It would have to be bubbled through the solution, which can make it quite inconvenient, on second thought.
[Edited on 6-6-2019 by Keras]
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Sulaiman
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I've used freeze-spray many times to fault find electronics,
but I guess it would be expensive and suitable only for small sample volumes,
in which case a modified refrigerator and/or peltier cooling may be feasible ?
CAUTION : Hobby Chemist, not Professional or even Amateur
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Keras
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I never think about Peltier elements to cool things. Can they really work that low?
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Sulaiman
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How low can you go with peltiers = how much spare cash do you have 
https://www.google.com/search?q=cryogenic+peltier&oq=cry...
P.S. Probably cheaper to recycle/liquefy air, propane, refrigerant gasses .. - or freeze spray gas !
[Edited on 6-6-2019 by Sulaiman]
CAUTION : Hobby Chemist, not Professional or even Amateur
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Ubya
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Quote: Originally posted by Keras  | | The usual way of cooling, say, acetone, to -76 °C circa is to use dry ice freshly belched out by a fire extinguisher.] |
Really? I thought it was easy to just buy dry ice (not where I live though).
If you need to cool small masses you could use cascading peltier, but you would need a big power supply for that. If you can get your hands on a
freezer, a water chiller, or anything really that uses a vapor compression cycle, you could tweak it to go to a quite low temperature
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Keras
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| Quote: Originally posted by Ubya | | Quote: Originally posted by Keras | | The usual way of cooling, say, acetone, to -76 °C circa is to use dry ice freshly belched out by a fire extinguisher.] |
Really? I thought it was easy to just buy dry ice (not where I live though). |
Even if I could buy dry ice, where and how could I store it?
| Quote: Originally posted by Ubya |
If you need to cool small masses you could use cascading Peltier, but you would need a big power supply for that. If you can get your hands on a
freezer, a water chiller, or anything really that uses a vapor compression cycle, you could tweak it to go to a quite low temperature
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Tweaking a fridge would mean either increase the duty cycle of the compressor, but I'm not sure it would work for long, because of quicker wear-out.
Or increase the compression ratio, but is that feasible? – and even then, I wonder if it wouldn't end up cranking up the pressure at the cold side
too. Dialling too low could also result in coolant gas condensation.
And that's not even taking thermal leaks (freezers are not that well isolated) into account.
BTW, why did the old fridges use CFC and not, say, nitrogen or argon, as coolant?
[Edited on 6-6-2019 by Keras]
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SWIM
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There are big advantages to a coolant that is condensed to a liquid.
The latent heat of evaporation and all that guff.
Before CFCs they used stuff like ammonia, SO2, etc for similar reasons.
Those were some pretty mean items to deal with if your condenser coils got damaged and things started leaking.
How big are the items, or samples, or reaction mixtures you want to cool off?
Could be some answers are more suitable for certain scales than others, so clarifying that might help the discussion.
My impression is those coolant sprays are good for small items that only need cooling for a little while, but not so cost efficient for bigger things,
or longer times.
[Edited on 6-6-2019 by SWIM]
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Ubya
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| Quote: Originally posted by Keras |
Even if I could buy dry ice, where and how could I store it?
Tweaking a fridge would mean either increase the duty cycle of the compressor, but I'm not sure it would work for long, because of quicker wear-out.
Or increase the compression ratio, but is that feasible? – and even then, I wonder if it wouldn't end up cranking up the pressure at the cold side
too. Dialling too low could also result in coolant gas condensation.
And that's not even taking thermal leaks (freezers are not that well isolated) into account.
BTW, why did the old fridges use CFC and not, say, nitrogen or argon, as coolant?
[Edited on 6-6-2019 by Keras] |
You can't store dry ice for long periods, a big chunk my last a few days in a well insulated box, but usually you buy it when you need it.
As for the vapor compression cycle circuit you can tweak many things, first of all a fridge is not ideal as the condenser coil is passively cooled,
the load must be small or the duty cycle must be short. The best would be a system with a big actively cooled condenser, like an ice machine or an air
conditioner. As for the tweakings, an air conditioner, a dehumidifier or an ice maker machine already work with a 100% duty cycle, you could lower the
temperature of the low side by increasing the pressure difference (longer capillary tubing), cooling more the condenser, changing the refrigerant for
one with a lower boiling point, or even putting two systems in series (evaporator of one cycle cooling the condenser of the other cycle, obviously
with a different refrigerant).
In a normal system refrigerant condenses in the condenser, and evaporates in the evaporator (removing heat), if liquid goes back at the compressor it
could ruin it (liquids are not compressible), but if a tiny bit of liquid reaches the compressor is ok(subcooled system) as it evaporates and cools
the compressor making possible to run it continuously without overheating
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Keras
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Oh, I just wanted to make NaNH2 out of liquid ammonia and sodium, and maybe try a Birch reduction. So microscale experiment.
| Quote: Originally posted by Ubya |
You can't store dry ice for long periods, a big chunk my last a few days in a well insulated box, but usually you buy it when you need it.
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Agreed.
| Quote: Originally posted by Ubya |
As for the tweakings, an air conditioner, a dehumidifier or an ice maker machine already work with a 100% duty cycle, you could lower the temperature
of the low side by increasing the pressure difference (longer capillary tubing), cooling more the condenser, changing the refrigerant for one with a
lower boiling point, or even putting two systems in series (evaporator of one cycle cooling the condenser of the other cycle, obviously with a
different refrigerant). |
I mean, yeah, but that's not really tweaking. What you do here is scavenging a system to buy something similar but with different parts. What I
thought you meant is that you could get away with it simply by, say, cranking a trimpot up or changing a resistor. Not that sort of overhaul.
[Edited on 7-6-2019 by Keras]
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