"Bradlee, Michael" wrote:
> Dear AMMRL's:
>
> For anyone who has had experience in designing/installing a ventilation system around an existing instrument:
>
> We recently installed a 300 MHz R2D2 in a room specifically dedicated to this one instrument (area~252 sq. ft). Two Questions:
>
> 1.) For the ventilation system/ air exchanger: Is stainless steel OK or is aluminum the better choice?
>
> 2.) The engineers want to design the exhaust vent to be positioned directly above the NMR. Like literally inches from the N2/He Vent Tubes. I think this may be over-engineering. A system that is flush with the ceiling seems sufficient as long as it can exchange enough air/ cycle.
>
> Any help/ advice is greatly appreciated.
>
I don't claim to be an absolute authority on this, but I do sometimes feel that we are being pushed into doing inappropriate things in the name of safety and it worries me. So here's my opinion (for what it's worth) on your situation.
It sounds to me like the engineers don't understand the problem. In the event of a spontaneous quench with the magnet full, the most you can boil off from that magnet is 30 litres of liquid He. This converts to about 22 cubic metres of gas at STP, which I calculate to be less than half the
volume of your room (I make it about 50 cubic metres assuming a fairly stingy 2 metres ceiling height). The point is that the room fills with helium from the ceiling down as anyone who has ever witnessed a quench can testify, so you actually want the extraction at ceiling height. If you put
the exhaust vent lower, and for some reason not all the helium gas goes into the vent it will float up to the ceiling anyway.
In a quench you generally don't boil off much nitrogen (at least not very quickly like the helium). But if you are worried about nitrogen (in the event of a dewar failure, say) then you have the opposite problem because cold nitrogen gas sinks and the oxygen is displaced starting from the floor
and working up. However, this tends to be much less of a problem unless you are lying on the floor.
As long as you have a reasonable level of through ventillation in your lab there should be no hazard from normal boil-off and no major hazzard even in the event of a quench providing that you exercise common sense. If you see a magnet quenching evacuate the room immediately and if the room is
filling with fog from the ceiling down keep your head well out of the fog (bend down if necessary). Do not re-enter the room until you are sure that there is adequate ventillation (oxygen) - a portable oxygen monitor can be useful, but if you haven't got one go for an extra cup of coffee
(you're not going to do much NMR for the rest of the day anyway!).
Of course, I would be very interested to hear the opinions of other members of the NMR community on what I think is becoming more of a problem for all of us.
Best regards,
--
Alan.
Alan M Kenwright Phone +44-191-334-2095
Senior Research Officer (NMR)
Department of Chemistry
University of Durham
Durham DH1 3LE UK
Received on Fri Jun 27 2003 - 15:09:36 MST