Mr G H Ireland wrote:
In article , "Daniel A. Mitchell"
wrote:

Your understanding of liquid oxygen containers is essentially correct ..
they are normally 'dewars', or super thermos bottles. They are not
normally pressurized to any great extent ... just enough to force the
liquid out a discharge tube, usually.


I had an interesting experience with a liqud Nitrogen Dewar. We wanted a
couple of litres of liquid N2 in the lab. I went with a colleage to the
spherical N2 bulk tank in the yard, with a 2-litre Dewar. We cracked the
discharge open a tiny way and directed the liquid into our Dewar,
whereupon,it came straight out again. There was no way we could sample that
N2 in the way that we tried. In the end, we went to the research lab, who
had a larger, differently-shaped Dewar kept at atmospheric pressure, from
which we were able to pour our sample. - G.H.Ireland.

In those days, BOC and Air products workers used to discharge full gas cylinders
off the delvery truck by rolling them off, and on to a piece of coconut matting.
We took the cylinders we rquired by polliing them by kicking them along.

This prctice was stopped by Management when the stop valve was knocked off a
cylinder. the rocket effect shot the cylinder out of the works and it is
rumoured that a person was killed.
igor _____________________________________________
Acorn RISC OS4
_____________________________________________




Perhaps larger liquid nitrogen storage dewars are pressurized to some
degree, as you suggest ... I'm not familiar with them.

Smaller lab dewars, up to a couple hundred liters capacity at least, are
NOT usually pressurized to any great extent. Normally they are vented
directly to the atmosphere, and are closed only when a little pressure
build up is desired to force the LN out a transfer tube (usually into a
smaller dewar or cryostat). The transfer tube extends to near the bottom
of the storage dewar, and hence any pressure build up at the top of the
dewar forces the liquid out the tube. When pressurized (usually briefly)
the dewars are protected by safety valves to prevent excessive pressure
build up.

The insulation in the better grades of such dewars is quite remarkable,
as a small dewar of perhaps 10 liters capacity (about the size of a milk
can) will retain a usable quantity of LN (at atmospheric pressure) for a
couple weeks. The sacrificial boil-off of a small part of the LN removes
the heat leaking in, and keeps the remainder cold and in a liquid state.
Typically, you might lose 5%-10% of the stock per day due to boil-off
(depending on the dewar).

COMPRESSED nitrogen cylinders are a whole different 'animal'.

Dan Mitchell
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