I have read up on the distillation process and can’t see why a low rate machine should be huge. You need to get down to about -200C and then you end up with a vertical stack which you gently heat at the bottom and you extract the various gases at different heights. If you are making only nitrogen and don’t care too much about purity, it can probably be simplified. What is the nitrogen supposed to be used for?
Freezing then smashing things. Is there any other use?
In aviation, tyres perhaps?
We have come to the conclusion that the best way (for us, in a big aircraft with plenty of spare weight carrying capability) is to use a large welding oxygen cylinder. This is much cheaper than an aviation cylinder, has a much larger capacity and is much cheaper to refill. We are using it with three O2D2s.
The cylinder fits very neatly behind the spar, exactly filling the width between the side walls (a remarkable, almost interference, fit).
Apart from weight, can anyone see any problem with this approach?
Apart from the huge weight (I have one at home and no way can I lift even one end), no problem 
But maybe yours is the shorter version?
One little challenge is fitting the pressure gauge within sight of the pilot seat. This is normally done (in installed systems) with a pipe that has a tiny bore – something like 0.5mm ID and perhaps 3mm OD.
Our cylinder weighs 30Kg, so it can be manhandled with some effort, and has a built in gauge that we can see with some effort from the pilot’s seat, but it is so big that, short of a massive leak, it is not going to need monitoring for a long time. It is so cheap that I imagine we will exchange it as soon as the gauge shows 50%.
It will need to be preflight checked each time, because if e.g. you leave one of the regulators switched on, and there is a leak in it (normally, with power off, the O2D2 valve shuts, more or less) then the cylinder could leak out over a few days.
Also you will need to have a valve on the cylinder itself, to open pre-flight and shut post-flight. The valves on the BOC cylinders are not particularly nice things to operate
It’s quite funny that a TB21 pilot we know, Paul (no longer flying AFAIK), spent some 4 or 5 figures certifying a composite cylinder for his TB21, to replace the heavy steel one. He was G-reg and it was a massive job, involving the destructive testing of a second cylinder.
This cylinder has a reasonable plastic knurled cut off valve.
Paul (no longer flying AFAIK), spent some 4 or 5 figures certifying a composite cylinder for his TB21, to replace the heavy steel one.
That just shows the stupidity of certification process. Cylinder is a cylinder and it’s always stressed in the same way regardless the application (flying, diving or firefighting).
Emir wrote:
it’s always stressed in the same way regardless the application (flying, diving or firefighting).
Um. Really though? I mean you’ve just given three very different applications – Low ambient pressure, High ambient pressure and high temperature.
