[Beowulf] HVAC and room cooling...

Jim Lux james.p.lux at jpl.nasa.gov
Sat Jan 31 18:26:55 EST 2004

----- Original Message -----
From: "Daniel Pfenniger" <daniel.pfenniger at obs.unige.ch>
To: "John Hearns" <john.hearns at clustervision.com>
Cc: <rossini at u.washington.edu>; <beowulf at beowulf.org>
Sent: Saturday, January 31, 2004 9:40 AM
Subject: Re: [Beowulf] HVAC and room cooling...

> John Hearns wrote:
> > On Fri, 30 Jan 2004, A.J. Rossini wrote:
> >
> > Daft idea.
> > What happens when the liquid nitrogen vapourises? It turns to nitrogen
> > gas. And if you are unlucky anyone in the room chokes...
> > I was reminded of this in the lift of a chemistry department where
> > we have an opteron cluster - a large sign reminding that LNO2 and
> > dangerous chemicals are not allowed in passenger lifts.
> When liquid N2 vaporizes in a closed room the natural fraction of N2 in
> air, already 78% by volume, just increases.  N2 is thus not toxic, the
> only risk by increasing N2 volume is to reach a too low O2 concentration
> for mammals beings breathing in the room.

Which is precisely the problem. You can fill a room with nitrogen and
there's no obvious sign of it. Someone opens the door, walks in, and is
unconscious in a remarkably short time.  The problem is that you've also
displaced the CO2, so there's no "breathe reflex", and you just breath
faster trying to get the PO2 in your blood up, which aggravates the problem,
then you pass out. All that oxygen poor air is now flowing out through the
door so eventually you'll get oxygen into the room, as well, but you're
really relying on diffusion (there's not a huge density difference).  This
is one of the classic "silent but deadly" hazards.

This is what "confined space" training is all about.  People have climbed
down into a tank or crawlspace and not climbed back out, for instance.  Here
at JPL, even in ordinary labs, if there's nitrogen plumbed in (we use it to
purge "dry boxes" or temperature test chambers, e.g. to prevent frost from
forming when it's cooled), we have oxygen monitors and big placards on the
door.  The risk is that the hose comes off, and that even a fairly low flow
(say, 1-2 CFM, typically) can flush all the oxygen out overnight, for
instance. Say the lab is 20x20x8 feet.. that's 3200 cubic feet, less a few
hundred for the stuff that's in the lab (shelves, file cabinets, equipment
racks, benches, etc).  The hose pops off unnoticed and 12 hours later at a
couple CGM, and the lab's full of nitrogen when you arrive the next morning.

I can't recall the exact number for where you go unconscious, but certainly,
somewhere around 10% O2 would probably be sufficient.  You'd only have to
displace half the air to get there.

With LN2 there's also the problem that the gas is cold, so it pools more
readily. LN2 and elevators is a bad combination if you get a catastrophic
leak, because the elevator shaft fills.

People do, in fact, die from this, every year.  (though, thankfully, not at
JPL).. There's cases of stagehands quietly suffocating below stage when
they're using dryice foggers (CO2 is very much more dense and pools quite
nicely).  In stage shows where they use large quantities of LN2 for making
fog (we're talking hundreds of gallons here!), they generally use a mix of
LN2 and LO2 for just this reason.

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