Ours is exhausted out the top of a one story building.
Even if you are worried about random pedestrians, it would not seem
necessary to exhaust any higher than head height, since any exhausted helium
is going to rise rapidly ...
Based on my experience in the past year and a half with a system like
this, I would not do the automatic emergency exhaust at all unless it is
absolutely required. The sensors have a relatively short lifetime, they are
expensive to replace, and as they age, they read lower and lower oxygen
content even though the air is fine in the room. Eventually they drop
below the trigger level, the fan starts, and because it is the sensor's
problem, unlimited amounts of fresh air don't help--i.e. the fan will run
forever or until someone sees it and gets the facilities people to
disable it. This is a problem for us in the winter when the outside
temperatures are low. Magnets use the same o-ring material as the space
shuttle, so you don't want the room temperature to drop below 39 degrees,
ever.
If I were doing it over, I would have manual fan controls both inside the
room and on the outside, and skip the sensors entirely. If somebody sees
a quench, they can turn on the fan with the nearest switch. If there is
nobody there to see it, helium diffuses readily through concrete, and the
problem will probably take care of itself quickly. This is the direction
we are going in all our labs with the exception of our 800. I have seen a
600 quench and I do not think it is violent enough that you really have to
worry about someone being incapacitated during the quench and unable to
exit safely.
Dave Vander Velde, University of Kansas
Received on Wed Dec 07 2005 - 11:21:45 MST