Liquid Nitrogen Generator

[laywill]

Ok, I'm looking for a spot of : I want a source of liquid nitrogen - mostly for overclocking - but cannot afford to pay the huge costs involved with registerigng with a gas company, having a risk / Helath & Safety inspection done, deposits on a dewar, cost onf the LN2 itself etc. I therefore think it may be a possibility to build myself a liqid nitrogen generator.

The basic principle of liquid nitrogen generation (or liquification of any gases for that matter) is as follows:

• Filter out dust
• compress air
• water cool in a heat exchanger to ~ambient temperature
• (possibly vent/tap out liquid/frozen water and CO2
• Cool in a heat exchanger cooled by non-condensed by sub-zero gasses produced in later stage
• Feed pressurised gas (@ ambient temperature) into an evacuation chamber, where it decompresses to atmospheric pressure and cools to very cold temperatures
• Some gasses will liquify and sink to the bottom of the container, others will remain as gasses (albeit at sub 0oC temperatures and can be tapped off the top of the chamber)
• These gasses are used to cool the pressuriesed air in the second heat exchanger, then are fed back into the input of the compressor

This system creates a positive feedback loop, where the longer the system is running, the colder the output can reach becasue the input gasses are colder. This is the recognised method of reaching sub-ambient cooling for liquification of gasses. The method can be streamlined by refridgerating the water in the heat exchanger and the input air, bu these are minor details.

Now then; I have done a few calculations and reckon that to realistically be in with a chance of liqifying oxygen (yes the stuff that will corrode anything nearly instantly) I would need to have the output liquids at <-190oC or 90K. Similarly, the output temperature for Liquid nitrogen (the nice, colder, more inert stuff) would need to be <-210oC or 77K (figures from wikipedia).

This would require a pressure of roughly 20 atmospheres (2030000 Pascals - 294 PSI - 20.7 kg/cm2). This is not unreasonably high because gas mains run at a similar pressure. I therefoore reckon that I can use gas main pipe as a container for the air to be compressed into, and the evacuation chamber need only be any old tank or cylinder because it operates at room temperaure. The problem I have is finding somewhere that supplies a compressor capable of supplying air at 300PSI and heat exchangers which can take that pressure too.

A diesel engine has a compression ratio (normally) of about 1:25 (meaning peak pressure of about 25atm if we assume input to reach atmospheric pressure); and a 4 stroke petrol engine has a compression ration of about 1:20 (meaing peak pressure of apx 20atm if we assume input reaches atmospheric pressure).

Using all of the above information, I have formulated a very basic and initial plan:
- take 1 petrol 4 stroke or deisel engine
- modify so it acts as a compressor
- connect prop shaft to a largish electric motor to power it (16 Amp 230V supply available at home)
- Take output of "compressor" through a run of gas main pipe immersed in refridgerated water to try and cool it off a bit.
- Pass gas main run then through a larger gas main pipe containing the sub-ambient gasses from the evacuation chamber
- Finally, take gas main run into a chamber, releasing the gas @ 300PSI through either a pressure pop-valve or a jet nozzle to increase release pressure.
- Tap off liquid gasses from bottom, and flush non-liquified gasses back through heat exchanger system and finally back into input of compressor

Now, I say all this as if I know what I am doing and have it al figured out. BUT, I am talking about huge figures here: 300PSI air being released, pipes and air at <-210oC or 77K and all this for as little money as possible I have no experience of working with any of this sort of stuff before and want to try and build it in my field.

If anybody has any tips pointers or experience of anything like this then please, please drop a note here. I am somewhat doubtful of this project's ability to even get off paper, let alon to prove itself working! However, if it does work, I should have liquid ntirogen on tap! Advice appreciated.

[PeterStoba]

I'll be honest, that's a massive project to undertake and will cost alot. If you did manage to build this, surely you'd need to be checked anyway, as you're working with dangerous substances.

If you can't afford the dewars and cost of Ln2 from BOC then i'd say using Ln2 isn't for you.

If you do want more advice, join the BenchTec UK forums here - http://www.benchtec.co.uk/forums/

They will be able to advise you better and your hwbot points can go to good use too

[Fraz]

Well, good luck to you. It's certainly ambitious...

All I can say is: beware of confined spaces. So for instance if you do ever successfully make some LN2 it's not a good idea to transport it around in a car...

Oh, and LOX is really not a nice substance at all. Although it's a really awesome shade of blue.

[laywill]

Thanks for the comments.

When I say that LN2 is too expensive for me, I was thinking more in the long run, and particularly as a student, I don't really want tying onto long term exenses. I can probably pinch most of the stuff from friends and family, so its more about checking the basic principals are sound really.

And who, may I ask are BOC?

[Fraz]

And who, may I ask are BOC?

http://tinyurl.com/y8r58zt

If you are a uni student, just make friends with some physicists/chemists. I used to be able to lay my hands on more LN2 than I could shake a very cold stick at when I was a student.

[TAKTAK]

And who, may I ask are BOC?

British Oxygen Company



Lots and lots of explosives to crash into where you drive around their sites

[PeterStoba]

Thanks for the comments.

When I say that LN2 is too expensive for me, I was thinking more in the long run, and particularly as a student, I don't really want tying onto long term exenses. I can probably pinch most of the stuff from friends and family, so its more about checking the basic principals are sound really.

And who, may I ask are BOC?

The fact you're planning to create an Ln2 generator and don't know who BOC are worries me

http://en.wikipedia.org/wiki/The_BOC_Group

One of the largest suppliers of gases

[laywill]

As I say, I have very little experience in this field. The actual engineering to produce this is reasonably simple, its just the details as to fitting s and pressures which requires minute attention on the milling and fitting.

If you are a uni student

Erm... actually im only just about to turn 17

[Flibb]

This thread might interest you as it concerns obtaining LN2 for hobyist use
http://www.britishblades.com/forums/...light=nitrogen

[laywill]

Thanks for the link Flibb.
I'll have a natter to the farmer, but I can't thik why he would have any. Nor for that matter can I think how I would get hold of any from anybody else I know... my thoughts revert back to a generator...

[Flibb]

have you got any links to people using converted engines to compress gas down this far? I work with cryogenics and all our kit has to be stainless or PTFE to avoid seizing up, or falling apart, also when using high pressures lubrication becomes a big problem.

Have you looked in to alternative methods of getting resonably cold? Ice and salt can be used, another option is solid carbon dioxide pellets mixed with acetone or alcohol, look for poor mans liquid nitrogen.

[laywill]

Ok, thanks. I'll do some more googling. When you say that lubrication becomes a problem, is that purely because the oil is forced to the bottom of the cylinder by the pressure, resulting in no lubrication for the piston?

Also, why the need for the special melatals for the compressor?

[badass]

£10 says you never complete this project

[Lucio]

Ok, thanks. I'll do some more googling. When you say that lubrication becomes a problem, is that purely because the oil is forced to the bottom of the cylinder by the pressure, resulting in no lubrication for the piston?

Also, why the need for the special melatals for the compressor?

I'll be honest, this thread scares me!!

Seriously, you're talking about mucking around with life threatening chemicals and procedures just to try and save a few quid?? If you're smart enough to understand that principles involved with generating these chemicals, you should be smart enough to realise that it's dangerous and really shouldn't be attempted without proper equipment.

[laywill]

I'll be honest, this thread scares me!!

Seriously, you're talking about mucking around with life threatening chemicals and procedures just to try and save a few quid?? If you're smart enough to understand that principles involved with generating these chemicals, you should be smart enough to realise that it's dangerous and really shouldn't be attempted without proper equipment.

I fully understand your concern. If I was truly seriouis with myself, I could probably find the cash to stump up for liquid nitrogen to be delivered in a dewar from a reputable source. However, I would qute like to have a go at this, even if it was just to see if it can be done.

There are many forums on the net, in which somebody discusses the possibility of this, and I would like to see if that possibility can be realised. If it can't, then oh well. If it can't and it goes bang, that will make a good video for youtube. If it works, I'll be amazed.

[Pob255]

If it goes bang you'll probably not have any form of video to show, there's some very high pressures involved.

I'd look at the alcohol&dry ice poor mans liquid nightrogen, it should work well and getting hold of dry ice should be simplier.