DIY LN2 generator

[Fry-kun]

Ambitious, isn't it?

I was initially inspired by a blog post here tinyurl.com/yg3xhsn

For now I'm just wondering how/if it's possible.
Looking around I don't see any reason why it shouldn't be doable. Which is what makes me a naive newbie, of course

Thing is, the basic schematics look very simple, e.g. Carl von Linde apparatus here tinyurl.com/yjnz25k
Also, there are plenty of inventions that have been made since 100 years ago to increase efficiency and decrease the cost, you'd think an individual would be able to build a machine for fairly cheap, even if it's 100 times less efficient than industrial counterparts. For instance, this book tinyurl.com/yfu57q4 from 12 years ago claims Kleemenko cascade cryocooler can be implemented with ~$300 worth of parts.

P.S. sorry for tinyurl, apparently the forums here don't allow me to post urls for being a new user

[mad fridgie]

In the past a had a side line of cryogenic tempering, originally I used LN, but I was being charged a fortune.
So I made my own system (did not really need the liquid just the refrigeration effect, how ever my kids loved freezing thing in the liquid, so I used to liquify air fairly regular.
I went back in time, and used joule thompson effect.
I got a dive compressor (220bara), a small refrigeration unit, additional drier, pressure relief valve, a thermos flask and heat exchanger (hand made). So simply build the pressure, dry, refrigerate, dry, heat exchange, expand into the thermos flask, return the cold via to heat exchanger inlet of the compressor (not completley sealed here) walla "liquid air"
Only problems lie with how clean you can keep the air (water and oil).
To get around this, I would have a small bleed of high pressure nitrogen feedin the suction, and 2 relief valves

[Fry-kun]

220 bar sounds like a heavy duty compressor, though. I just looked it up on Froogle, it's >$3k new, and one for $650 on ebay
The challenge is to go below $100 for the project, although I'd be happy if I get below $300 claimed in the book I linked in the original post.

Did you make your own JT valve? Or just a small hole/bunch of small holes?
How did you make the heat exchanger?
Do you have any design/project notes, photos? Any tips greatly appreciated

I've heard of someone making a cheapo oil filter by shoving some copper wool in a container to act as a filter. Would that be enough?

[mad fridgie]

Hi, yes mine was a heavy duty compressor, nett cooling 250W efficiency a massive 5%. (ran 48hrs non stop per product batch)
I just bought a pressure relief valve (2) I found at these pressures, the hole had to be to small.
If you are looking at lower pressures than pherhaps you can get away with a bigger hole (thickness of a hair)
The heat exchanger simply I used 3/16th copper tube, made a big slinky (many small coils like a spring) I then inserted this into some plastic pipe (I also had some plastic pipe up the middle of the slinky) This heat exchanger was put into a box then foamed. (the box was designed firstly for resistance to the pipe exploding)
If you are going to dry-ice instead of a refrigeration unit, you maybe able to get away with 20Bar.
In this case I would make the dryice heat exchanger of a bigger pipe size to allow for fouling (freezing water and oil)
Sorry no photos, great machine, but not a great business (market to small in NZ)

[Fry-kun]

That's weird, I thought for some reason that with higher pressure the expansion hole could be bigger o_O

In tinyurl.com/yfu57q4 they claim to be able to use a refrigerator compressor; I'm guessing that would be in the 20bar range.
Their design is almost exactly the same (positive feedback loop), except with addition of liquid/vapor separators.

It should work without dry ice too, right? I'm not going for an efficiency record here

[mad fridgie]

You can use an enthaply chart, see how it calculates out, at the lower pressure, getting it going could be your problem.