Thanks again to all respondents - your help is always greatly appreciated.
I received requests to summarize, so I have done so below.
bob
Does anyone know the volume of LHe and LN2 that are held in a standard
400 MHz magnet (narrow bore, with active shielding)? Our safety group
will not sign off for use until adequate ventilation is provided in the
event of a quench. As a result, I am trying to determine the volume of
gas generated by complete boil off of both LHe and LN2 from my magnet.
bob
----------------------
Surely you have a 'magnet book' particular to your magnet and the
information is in there...Bruker certainly provides this also in their
installation manual!
------------
Well, if PV = nRT
And there are 36.75 moles/L of LHe
And 28.8 moles/L of LN2
And the temp is approximately 290K
Then each L of LHe would be 0.082*36.75*290 or 873.9L of gas
And each L ol LN2 would be 0.082*28.8*290 or 684.9 L of gas
If the safety folks need cubic feet, the value of the gas constant is
0.7302 atm-cuft/K/mol
-------------------------------------
According to the Varian site planning manual Pub No. 01-999038-00 a
400/54 AS holds a maximum volume of 123L of liquid He. "The expansion
ratio of liquid at room temperature is about 740:1, which means one
litre of liquid will expand to 740 L of gas."
"For fans rated in LPM (litres per minute), multiply the LHe maximum
volume by 740 to get an idea of helium gas the fan should be able to
displace."
"For fans rated in CFM (cubic feet per minute), multiply the LHe maximum
volume by 26.13 (includes expansion ration) to get an idea of the total
amount of helium that the fan should be capable of displacing (e.g. A
magnet that holds 30 litres of LHe will require a fan that can displace
about 784 ft3 of He gas)."
Helium alone is considered when discussing the issue of quenches and
emergency ventilation. Helium would be the first thing to vent during a
quench, and depending on the scenario and severity of the event it may
be the only cryogen to vent. During the quench of our 800MHz system the
nitrogen bath remained intact for 4 days until the vacuum container was
compromised, then it simply boiled away at a more rigorous pace (it did
not vaporize quickly like the He). The LN2 boil off did not produce
dangerous N2 gas levels within the room, I don't believe.
---------------------
53 cubic meters for a bruker 400 54 mm US standard hold. The N2 loss in
a quench is probably minimal. But even in a worse case, i.e., the dewer
explodes, the N2 volume will be similar. The new bruker install manuals
spell this all out.
-----------------------------
These numbers are from the Varian installation manual, so they apply to
an Oxford 400/54
AS. The liquid helium number is for ventilation calculations, so should
be the total
liquid helium volume. The liquid nitrogen is from the refill table and
is usually 95% of
the total liquid nitrogen volume.
Liquid helium 123 liters
Liquid nitrogen 67 liters
The manual gives the liquid helium to room temperature gas expansion
ratio as 740:1. They
don't give a number for liquid nitrogen as it usually doesn't boil off
in a quench.
--------------------------
For our Bruker shielded 400 long hold magnet, installed in 1998, the
magnet book lists:
Liquid Helium = 80 liters {hold time 220 days}
Liquid N2 = 84.5 liters
-----------------------------
The volumes of both the Helium and Nitrogen chambers, along with the nominal
refill volumes, should be in your magnet manual. This is liable to be at least
slightly different for different vintage and vendor models. If you don't have/
can't find the manual, the vendor's magnet group should be able to find that
info for your system.
--------------------------
According to the Varian Installation Planning manual, for an actively
shielded narrow bore Oxford 400 the maximum amount of liquid helium that
can be displaced in a quench is 123 litres (1 litre liquid = 740
litres gas at STP). As far as I understand it, liquid nitrogen is not
usually displaced rapidly in a quench (boil off does go up, but not to
the point of going whoosh, it's the helium you need to worry about).
Note that the helium which is boiled off goes to the ceiling and oxygen
is diplaced from the ceiling downwards, so you want to think about the
volume of space in your lab above head height, and any additional
ventillation should go in at ceiling level. In the much less likely
event of major nitrogen displacement, the (cold) nitrogen gas sinks to
the floor and oxygen is displace from the floor up. In either case, the
best procedure is to walk out of the lab (keeping your head out of any
visible fog cloud), increase ventillation in the area and wait for the
oxygen level to re-establish (time for a cup of coffee).
--------------------------
In a quench situation, only the He will boil off quickly.
-------------------------------
We had a similar problem with the safety people here when we
remodeled a room for a new Varian 600. I told them that the normal
boiloff is so miniscule that there is no need for ventilation (we
actually pipe the boiloff N2 and He out of the room) and that there
is no ventilation system which would quickly remove all the He gas from
a quench. They still insisted on putting the room under negative pressure,
even though they accepted that it would not help in a quench. Now we
find that the there are big vibration peaks in our lineshape spectrum
at 36 and 48 Hz which go away when the ventilation roof fan is shut off.
We can't just turn it off because it also vents the welding and sandblasting
areas in the machine shop.
I would find out the He gas volume for a total boiloff and thencalculate
how far you have to go down from the ceiling to equal that volume in the
room. Since the He gas will go up and collect against the ceiling, the
important thing is how much air will be left down below. Another thing
that might placate them would be to install an oxygen sensor and alarm.
Also, I don't think there is any N2 boiloff in a quench, only the He
can is affected.
-------------------
Use the ideal gas law. Get the volume of liquid from the
magnet manual, get the mass of liquid from the standard density
(CRC handbook, e.g.) of lHE, lN2. Then you can get number of moles
n from the molar mass, and V=nRT/P gives you the volume, with P being
atmospheric pressure, T being room temperature and R being the gas
constant.
--------------------------
For a Bruker 400NB Ultrashield:
Per our spec sheet the magnet holds 80 liters liquid helium and 84
liters liquid nitrogen.
Received on Thu Nov 06 2003 - 16:11:42 MST