Dear All,
I've summarized the responses we received in regards to designing an appropriate Ventilation System for our NMR facility (I hope I did not lose the essence of the responses in transposing them). Of course depending on the size and number of magnets as well as the size of the facility, specifics may vary, but the insight provided by the responders warrants sharing the info with the whole group (thank you all who responded). By the way, there was also a significant cut in the overall cost of our ventilation system; another excellent reason to consider running this exercise.
Summary of responses:
1.) If there is a 'quench,' most likely only liquid helium will 'boil off.' Helium, being significantly lighter than air will make its way to the ceiling rather quickly. It will displace O2 from the ceiling down. If the exhaust duct is not flush with the ceiling level it may actually be more difficult to remove.
2.) Total volume in NMR if the helium dewar is full (300 Oxford R2D2): 30 liters of liquid = ~22 cubic meters of gas. If our total room volume is 75 - 100 cubic meters, this is approximately 1/3 to 1/4 the total room volume. In the event of a quench, crouching down while quickly exiting the room is the key to making it out alive.
3.) Normal boil-off occurs at a very slow rate and should not present a hazard during routine use with a standard air exchange/ ventilation system. We have been working - sans ventilation/ air exchange system - and monitoring O2 in that room for over a year without any issue. That's over 52 N2 fills (one per week) and about 6 helium fills, with no alarms from the O2 sensors (or unusually high pitch voices).
4.) In the event of a quench or dewar failure the helium or nitrogen will likely exit via the blow-out valves on the lower back of the instrument, not at the boil-off vents at the top of the unit. The cold N2 would drop to the floor and displace O2 from the floor up. Thus a drop down hood directly above the magnet would not necessarily add a significant safety margin for working in the facility and escaping in the event of a release..
5.) Ventilation system in close proximity to the NMR (either flush with the ceiling or a drop-down hood) will create a few hazards of its own.
a.) the vibrations generated may cause an interference with the magnetic field putting a strain on the system.
b.) some Responders have stated that a duct directly above the NMR may actually act as a dust accumulator (especially anything with a magnetic charge) and result in dirtying the sample tube. Generally magnetic dust particle accumulate at the base of the magnet and can be removed with a wad of tape.
c.) in the event of some type of water leakage, the exhaust vent may provide a very convenient viaduct for water to drip right onto and directly into the magnet (water is not a friend of an NMR)
d.) it can also cause eddying air currents that can effect the field around the magnet by effecting the distribution of charged/ magnetic particles, thus affecting the efficiency of the magnet.
e.) the normal maintenance, filling liquid helium, would be obstructed by a vent duct directly over the NMR. A clearance of about 4 feet is needed to insert the stinger transfer line.
6.) Materials of Construction: As long as the stainless steel in non-magnetic (some recommend 304, 316, 321) it's OK. Others say aluminum is what they have.
7.) Normal boil-off is not the problem. A quench is. And in the event of a quench, if it starts while someone is using it, common sense will save the day, (if not base fear) as the Helium vents quickly and loudly producing a smoky white cloud that quickly rises to the ceiling. Leave the room and do not re-enter until air monitoring has been satisfactorily conducted.
8.) Some engineering advice, concerning a continuous extraction system: 'My recommendation (Guillermo Monya, PhD, University of Sciences, Philadelphia) is that if you are going to install an extraction system for safety reasons, have it linked to an O2 sensor that starts it if the O2 level goes below a set value. You can also put an over-ride switch which you can turn it on during LHe fills. I would also have it flushed against the ceiling. In A&M there were several R2D2s in small rooms, and I don't recall any of them having a special extraction system.' Don't over-engineer it, is his (and others) advice.
9.) Exhaust system should be remote from the magnet, for the very reasons presented above. (Dr. Charles Fry, Director MR, U of Wisconsin). p.s. I've never met anyone from Wisconsin that I didn't trust.
I hope I did the responders justice. Those who greater expertise on this matter, I defer to you further comments or elaboration on this summary.
Thanks Again,
Michael J. Bradlee
System Administrator
Rhodes Technologies
-----Original Message-----
> From: Bradlee, Michael
> Sent: Thursday, June 26, 2003 1:50 PM
> To: ammrl_at_chemnmr.colorado.edu
> Subject: Ventilation System
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.
Regards,
Michael J. Bradlee
NMR System Administrator
Rhodes Technologies
Coventry, RI
Received on Mon Jul 07 2003 - 09:36:53 MST