Anyone making liquid air or nitrogen at home? Topic is solved

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Downunder35m
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Anyone making liquid air or nitrogen at home?

Post by Downunder35m »

I was looking into the options of making my own liquid nitrogen or at least air.
And it seems to be a basic matter of using some refrigeration compressor and a heat exchange together with a good pressure control of the system.
Basic however does not look that basic anymore once you go into the requirements for the heat exchanger(s) alone.

We only need "moderate" pressures to liquify air, our real enemy is heat.
If you have some something simple in use I would love to hear about it!
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Orngrimm
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Re: Anyone making liquid air or nitrogen at home?

Post by Orngrimm »

Didnt Veritassium make something like that once?

Edit: Found it:


Alter, he also makes liquid oxygen, but thats MUCH harder.

Also, the late King of Random once showcased his setup:
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Re: Anyone making liquid air or nitrogen at home?

Post by Downunder35m »

Bit expensive just for a bit of home use....

But nice to see the King once more :)
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Re: Anyone making liquid air or nitrogen at home?

Post by Jack A Lopez »

I was thinking the historical method for this, was invented by someone named Linde,

https://en.wikipedia.org/wiki/Hampson%E ... inde_cycle

and involved a very long coiled tube, with another tube inside of it... essentially a tube-in-tube heat exchanger, and a large amount of compressed air driven through this. Although I am not sure how much compressed air is needed, in terms of volume and pressure, and mechanical work required. Like, how big of an air compressor would I have to buy, and what lengths of copper tubing?

Also I have noticed there are Youtube DIYers making liquid air, but I am not sure they are using the same method.

For example, that guy at the "Applied Science" channel was also making liquid air, but I think he was using some kind of cryogenic cooler, that he bought used, or surplus or something, to do it.

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Re: Anyone making liquid air or nitrogen at home?

Post by Downunder35m »

I checked some of the variations of the Linde cycle.
There are people abusing heat pipes for this.
Appearently there is a big, industrial type that has channel inside to fit a heat pipe from a CPU cooler or similar in it.
No clue where the guys got it, only that even 30cm lengths with an O.D. of 20mm are already very expensive.
In some old book, yes the paper stuff, I found a crude drawing of a heat exchanger similar to what our condeser type cloth dryers use.
Lots of thin copper plates with spacers and all 4 sides with connecting channels.
Resulting in a cross cooler.
In this contraption the cold or already liquid gas will cirluated between one set of disks while the hot inlet gas goes through the other sets.
However this cooler is only meant for the final cooling stage.
The first stage is more like a twisted airconditioner.
The gas is compressed and discharged through a metering device, resulting in very cold gas.
The evaporator for this state uses the energa to pre-cool the gas for the second stage.
The heat from the cormpression part is cooled by water.
A total lack of moving parts and the "simple" design makes it attractive.
The downside is that the process takes a very long time.
Original was able to produce about 50ml or liquid air per day....

After now seeing even more videos that I thought required to get an idea I think I might have at least dreamt up an alternative approach.
We know it takes only moderate pressure to liquify air, nothing a simple fridge compressor coudn't handle.
We also know that heat is our biggest enemy as air needs insane amount of pressure to liquify at room temperature.
But a shotgun condenser is not only easy to make and insulate but also able to handle quite some pressure.
I should be possible to just line several of them up with expansion tubes and metering devices bewteen them.
Every single stage can get down to -30°C with ease and some good insulation.
That measn our, dry, air supply to be liquified could start at -30° and also be cooled back to this temp after the compression.
The required "cold finger" to condense the gas could designed by a reflector for the compressed gas coming out - it will turn the surface very cold indeed.
The big question remaining is how to properly circulate this really cold gas back and to remove the added heat from the compression - the good old circle of impossible things.
A continued cycle with a third compressor to keep the pressure even means that at some stage we need a heat exchanger that uses the liquid air or nitrogen produced.
So why not turn the complicated stuff around and make so dirt simple that it can't possibly work ?
We need to cycle the gas to be liquified anyway unless you want go for the openy cycle approach.
Then why not use it for the cooling throughout the system?
It would still have to be a two stage system with a propane cooling or so in the first stage.
But, and I know it is a big but due to a lack of experience with it, what if we use a "split metering device" at the start of the second stage?
Like a ventrui system...
Only the coldest fraction of the gas goes into the second stage for cooling.
The rest circulates back the cooling tubes of the first stage.
I got the basic idea from these vortex cooling tubes - one end give hot air, the other very cold air.
No moving parts, just a matter of pressure and flow ratios.
We would end with a super cool stream used to cool whatever gas circulates.
And we use the gas stream to get it even cooler while the hot parts is forcefully cooled back to -30°.
But can we design a vortex cooling tube that would operate under these conditions?
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Re: Anyone making liquid air or nitrogen at home?

Post by Orngrimm »

The Hampson–Linde cycle needs high pressures in the beginning (Wiki notes 200 bar / 2900 PSI). Doable, but a challenge for sure ;)

Edit: Yeah... Ben Krasnow (Applied science) has a good source for all sort of cool and weird stuff. if HE gets something surplus, remember, that it may not be available surplus for you as well ;)
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Downunder35m
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Re: Anyone making liquid air or nitrogen at home?

Post by Downunder35m »

I looked around for a while and although it is possible to spot some cryo components every now and then it is too costly on a hobby level.
Heat is our biggest problem, pressure is relative and depends on production method.
In theory you could just dry up air and let it condense on something cold enough.
But this won't work without keeping things cool and/or pressurised.
What still strikes me is the temp difference of up 80° for these vortex cooling tubes.
No one yet claims to actually fully understand why it works, we are currently as far as suspecting quantum level effects....
Only fact is that they don't really care about the inlet temps.
If you feed 10° air you get -40 or so on the cold end, feed with -20 and you get -60....
And funny enough those things only really work if both pressure and flow rate are optimised.
Same as any HVAC or refrigeration system.

Yesterday I go bored, really bored....
So I hooked up the best working vortex tube I have to my shop compressor and fed the cold air outlet into a big can and from there to a small fridge compressor.
Actually a tiny propane compressor from a water cooler....
I only had about 5 minutes until the flow rate of my shop compressor craps out.
So I set the fridge compressor to 12 PSI on the crappy China regulator and let it warm up for about half an hour.
Housing temp leveled out at 62°C.
Ambient air temp was 22°C.
The air temp in the catch can for the vortex tube was about -8 degrees, it frosted up on the outside.
The warmed up fridge compressor had an outlet air temp of 35°C - measured in another, open catch can to avoid false readings.
Means the incoming air warmed up by about 13° with these regulator settings.
Feeding the cold air instead of amient air resulted in an outlet temp of roughly 5°C going slowly down.
Could not see how far it might go down as my shop compressor tank no longer had enough pressure.
But it seems to confirm that the heat added by the fridge compressor is a more or less linear thing based on the flow rate and work the compressor has to do.
Might sound stupid, what stops us from drilling some holes and adding an external oil cooler to keep the motor and actual compressor cooler?
Can't be too hard to find a suitable oil for low temperatures.
A refrigerant can only be compressed until it is a liquid, which is a factor of temperature and pressure.
Please correct me if I am wrong:
If we have a multi stage sytem where by means of pressure differential and seperation similar to a vortex tube only the cold streams travel furthe while the hot streams fully circulate back while being cooled by the colder streams.....
Would that mean that we only have to remove the added heat heat from the compressore plus just a little bit extra along the way to keep the inlet fow of the hot return gasses below of what we started with?
Means since in the really cold regions our cheap options are limited we would need an additional cooling system to get whatever comes out of our compressor into a good negative range the rest should should literally slowly get to proper operating temps by itself !?
I know I made Newton very agnry somewhere but where?
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Re: Anyone making liquid air or nitrogen at home?

Post by Jack A Lopez »

At Downunder35m, regarding this question of

"...what stops us from drilling some holes and adding an external oil cooler to keep the motor and actual compressor cooler?"

I think the secrets to making a vapor-compression refrigerator work, or making an existing one work better, are to be found in the T-s diagram for the refrigerant swirling around inside it.

https://www.qrg.northwestern.edu/thermo ... efrig.html

If cooling the compressor and condenser, causes pressure to drop inside the evaporator (and I am guessing it might) then the temperature at the evaporator is going to drop too. But I cannot guess by how much.


At everyone reading this, I found a link to a free book (free from copyright since it was written over 100 years ago) titled, "Industrial Gases"

https://archive.org/details/industrialgases00greerich

It was cited in a blurb in a Wikipedia article, specifically "Liquefaction of gases," section titled, "Claude Process"

https://en.wikipedia.org/wiki/Liquefact ... 7s_process

You can read the blurb yourself, but what it suggests to me, is there are a variety of methods (at least two) for removing heat from air. One is the mysterious Joule-Thomson effect, that occurs in free expansion. Another is expansion by doing work, e.g. by pushing against a so-called "expansion turbine."

The trouble with the Joule-Thomson effect, is it really does work best with a high, around 200 atm, starting pressure, when starting with room temperature (around 300 K) air. For "proof" this is true, look at the section of the Wiki article for "Joule-Thomson effect" in the section labeled, "Throttling in the T-s diagram" Following one of those little iso-enthalpy contours, from one iso-pressure contour to another (e.g. the paths a-to-b, or c-to-d, labeled in the diagram) is a kind of voodoo for predicting what the actual temperature change will be.

https://en.wikipedia.org/wiki/Joule%E2% ... -s_diagram

The trouble with an expansion turbine is, I think, mainly that it is necessary to build one, and that it must lack friction, otherwise the friction just turns the work instantly back into heat.
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Re: Anyone making liquid air or nitrogen at home?

Post by Downunder35m »

I hate it how Newton always makes things complicated LOL
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Re: Anyone making liquid air or nitrogen at home?

Post by Orngrimm »

Q is answered as it seems... Ill mark it as answered per viewtopic.php?f=3&t=42
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