A word of caution if you’re planning to try this cryocooler method for making liquid nitrogen: not only does it involve toxic and flammable gasses and pressures high enough to turn the works into a bomb, but you’re likely to deplete your rent account with money you’ll shell out for all the copper tubing and fittings. You’ve been warned.
In theory, making liquid nitrogen should be as easy as getting something cold enough that nitrogen in the air condenses. The “cold enough” part is the trick, and it’s where [Hyperspace Pirate]’s cryocooler expertise comes into play. His setup uses recycled compressors from cast-off air conditioners and relies on a mixed-gas Joule-Thomson cycle. He plays with several mixtures of propane, ethylene, methane, argon, and nitrogen, with the best results coming from argon and propane in a 70:30 percent ratio. A regenerative counterflow heat exchanger, where the cooled expanding gas flows over the incoming compressed gas to cool it, does most of the heavy lifting here, and is bolstered by a separate compressor that pre-cools the gas mixture to about -30°C before it enters the regenerative system.
There’s also a third compressor system that pre-cools the nitrogen process gas, which is currently supplied by a tank but will eventually be pulled right from thin air by a pressure swing adsorption system — basically an oxygen concentrator where you keep the nitrogen instead of the oxygen. There are a ton of complications in the finished system, including doodads like oil separators and needle valves to control the flow of liquid nitrogen, plus an Arduino to monitor and control the cycle. It works well enough to produce fun amounts of LN2 on the cheap — about a quarter of the cost of commercially made stuff — with the promise of efficiency gains to come.
It does need to be said that there’s ample room for peril here, especially containing high pressures within copper plumbing. Confidence in one’s brazing skills is a must here, as is proper hydro testing of components. That said, [Hyperspace Pirate] has done some interesting work here, not least of which is keeping expenses for the cryocooler to a minimum.
Best intro paragraph on HaD ever. Definitely gonna read the rest!!!
“Say no more, I’m in”
DIY cooling is always of interest, but isn’t this quite a complicated way of liquefying air? IIUC industrial air liquefaction uses Joule-Thomson cooling, as do those cooling spray cans, so if that’s the solution at both ends of the cheapness spectrum, how come it doesn’t make sense here?
Second paragraph. Third sentence.
I shoulda said, “uses Joule-Thomson cooling of the nitrogen /directly/“; i.e. what is the advantage of cooling an argon-propane mixture first
Many gases cool down much more than nitrogen across an equal pressure drop, even though they condense at high enough temperatures that they can’t take you all the way to liquid nitrogen temperatures, so they need to be mixed with other gases that don’t condense at such high temperatures, which can finish the job. If you went with pure nitrogen all the way without precooling, you’d need way more power, way higher pressures, etc. Precooling with other refrigerants is way more efficient, and makes the remaining job easier. As does doing the nitrogen condensation under pressure, so that it doesn’t need quite the same temperature from your refrigeration equipment, and it can just boil to reach the normal temperature instead, as a final step.
You clearly didn’t read/understand the comment.
Is the article correct when it states the precooling is done BY the refrigerant, TO the refrigerant?
If yes.
What is the benefit? I could see it being an efficiency gain. The pre-expansion gas gets colder. The post-expansion gets warmer. That warmer gas getting compressed results in a higher temperature, which is easier to cool with ambient air because it’s all about temperature differential.
If NO.
Is the precooling being done BY the refrigerant (post expansion)TO the nitrogen input. And what benefit does that impart?
You have a different username than who I was replying to, so I have reason to doubt that you have a better idea what I did and did not understand about what someone else said than I do, but even if you did you could be more polite about it.
Regardless, the purpose of the counterflow heat exchanger with the expansion valve is because it is a good way to reach the necessary insanely cold peak temperature. If you expand gas that was at ambient temperature, then you only get to a certain temperature, and can only cool other things down to that new temperature. The leftover cold gas doesn’t get to do much, because like you kind of touched on, the gas coming out of the compressor can be fairly easily cooled back down to ambient even if it starts out higher, so cooling the incoming gas isn’t so important as long as the compressor is happy.
Precooling using a separate refrigeration cycle is a straightforward thing to do; it changes the fixed temperature you’re starting from to a lower value that’s closer to the goal. You could, if you wanted, have a bajillion separate refrigeration cycles, each with different sizes but probably not a lot of different gases, where each one precooled the gas that was about to be expanded in the next one. It would suck to do very many stages though.
This way is to use the counterflow heat exchanger so that the temperature of the incoming gas at the expander is reduced, as you know. But the temperature of the outgoing gas from the expander is also reduced, not increased. The temperature doesn’t increase again until it’s returning through the heat exchanger, where at least when it increases it’s because it swapped temperatures with the gas that was going the other direction, just like what some other gas did for it a few moments before. This does, at the big picture level, increase the efficiency of ln2 production, but that phrasing kind of hides how the efficiency started at zero, and this made it possible to do.
Also all this cooling was done in a closed loop with sealed gases, whereas the nitrogen has to be an open loop if you’re going to remove it once it’s a liquid. The nitrogen does get pressurized in this design so that it doesn’t need to get as cold to liquify, which means it does act as a sort of extra stage when you remove it to atmosphere, but it may or may not count as one depending on semantics.
I’m the closing paragraph, “Confidence in one’s brazing skills is a must here” might better read “A high level of demonstrated expertise in…”. Lots of people are confident in their skills right up to the moment something goes terribly, terribly wrong.
Absolutely! And he we all know Dunning – Kruger. Confidence is ~ inversely proportional to skill. On top of that, those who are more confident are less likely to do proper testing, get someone else to review their work, put safety measures in place, etc.
I was the new guy on a professional fireworks shoot. The lead pyrotechnician paid attention when I pointed out a safety concern with his process – he wasn’t overconfident in his skills and knowledge, but ready to listen to concerns. That attitude is why he’s had many safe and successful shows.
“A high level of demonstrated expertise in…”
Absolutely.
Assuming the requisite level of robust copper tubing / piping [“…It does need to be said that there’s ample room for peril here, especially containing high pressures within copper plumbing…”], then—
a) If one’s first inclination is to not use a 15% silver-solder alloy (Sil-Fos®15, e.g.), one does not have the required level of expertise.
b) Alternatively–if one’s first inclination is to use some form of lead (-based) solder for one’s joining operations, one most definitely does not have the required level of expertise.
I am very happy to see this featured, but please excuse me as I think one or the other of us has made some significant misinterpretations as to what’s going on in that video. If I’m right, it explains why he’s doing things the way he is.
My summary understanding is that the process that he’s currently using is as follows.
The first refrigeration cycle is where the mix of 5 gases (*which he says is more efficient than the argon/propane mix) is compressed, put thru a radiator, then precooled to -30C by a second refrigeration cycle. This precooled mix is put thru a regenerative heat exchanger with joule thomson expansion, with the cold end inside a sealed insulated tank, and reaches good cryogenic temperatures with decent cooling power as a result.
Nitrogen gas, (**which he tried to precool but currently isn’t precooling because of equipment failure), is fed into the cold tank at 28-30 bar and perhaps ambient temperature. It condenses in the tank and more is repeatedly added until the tank is full of liquid. (***This condensation requires only around -150C due to the elevated pressure, not the -196C it takes to do it with atmospheric air.) The tank is then discharged gently into a dewar where boiling proceeds as the pressure has been reduced to atmospheric, and his yield is the fraction of the nitrogen that remains liquid in the dewar having reached the normal -196C.
As for peril, there’s a lot of moving parts, cold stuff, various handmade pressure vessels, etc. Definitely agree that it’s something to make sure you do right, and with a plan for what happens if you made a mistake. But at the same time, very rewarding, allowing you to do things that can’t be done any other way.
The “first cycle/stage” refers to the outer most stage. This is the one that needs to happen first. It’s the first one you turn on, even though it does nothing to the target.
On a 3 stage cooler example…
Stage 1 hot side is cooled by ambient air.
Stage 1 cold cools stage 2 hot.
Stage 2 cold cools stage 3 hot.
Stage 3 cold cools the target.
What’s your point? Call the cycles/stages Moe, Larry, and Curly for all I care. I’m more concerned with being correct about what actually happens in each one than numbering them in any particular order.
“will deplete your rent money”
haha
Why is it always the renters?
Actually the best mix I tried was 25/15/20/20/20 Propane / Ethylene / Methane / Argon / Nitrogen.
The 30/70 Propane/Argon mix is simpler to set up for people who don’t want to go tapping their own natural gas lines or making ethylene, but not quite as efficient.
And the copper didn’t cost that much tbh
I bought a 2 foot long, 1.5 inch diameter copper pipe for a domestic water heater a few months ago, $40!
Fortunately, it wasn’t needed, and the box store refunded the purchase.
Homemade LOX made with a tank (or two) of Liquid nitrogen.
Attach Metal cone/tube to unregulated nitrogen tank.
Open valve.
Liquid air will for form on outside of tube from cold gas blowing out of it.
Put thermos under tube to collect drops of liquid air.
When thermos is full, put open thermos into _deep_ freezer.
Wait about 1 week.
Old liquid air is almost pure LOX.
LOX is a controlled substance in the USA.
Too much fun potential, like machine guns and lawn darts.
Best dispose of it ‘properly’…Don’t get caught.