X-Message-Number: 2083
Date: 09 Apr 93 07:09:49 EDT
From: "Steven B. Harris" <>
Subject: CRYONICS Fancy Thermal Ballasts
Dear Cryonet:
We'd need a conductive plate 1 meter thick, eh, Brian? Okay,
I give up. The cooling fluid is going to need distribution; I
believe you.
I'm probably biased in my assessment of ethyl chloride from
the fact that this liquid is used in medicine clinically as a
local skin "chill" anaesthetic, and so I've gotten to work with
it some. It has to be pressurized at room temp to stay liquid,
but not by much (vapor pressure is 2 atm at 33 C, so says my
Merck index). Commercially it's supplied in pretty ordinary
glass bottles, which are (obviously) safe enough to ship (no, it
doesn't come packed in ice). To use it you just lift a spring-
loaded plug which seals a little siphon metal-spigot-tube that
goes through the cap, and when you do this the liquid comes
shooting out under mild pressure. Vaporization is very smooth--
no boiling or bumping in the bottle. It has a mild ether-like
odor, and when you get it on your skin it evaporates quickly,
leaving the skin very cold, but not frozen. All in all it seems
rather benign stuff, at least if packaged in small containers. I
wouldn't want to ride herd on a 5000 liter reservoir of it,
certainly, but 100 10-liter sealed plastic containers distributed
through a large room might not be too scary a proposition.
I agree with Brian that it's too difficult to operate rooms
above the ballast temperature, and so we'll have to use something
other than ethyl chloride if -136 proves to be too cold. Ethyl
bromide, maybe, as suggested. The heats of fusion are dis-
appointing, aren't they? I don't have the numbers, but I can
predict that heat of fusion for ethyl bromide will be similar to
ethyl chloride per MOLE and per VOLUME, but (mostly because
bromine is heavier) only about 60% as much per weight. Darn if I
know how much the stuff costs commercially.
Alcohols don't crystalize as well as halogenated hydrocarbons,
I've found, and don't seem to have as well defined melting
points, but they generally have higher heats of fusion. I'm
thinking that if we can tolerate 5 or 10 degrees of warmup from
the glass transition temp in an emergency (why not?) it may even
be worth it to tolerate some fuzzy melting behavior to get a lot
of ballast mass reduction and expense reduction. Remains to be
seen; this is one experiment that just has to be done, and can't
be predicted. If we can store as high as -117, or store at lower
temps and dare to use a ballast at as high as -112 to -117, we
can use plain old ethyl alcohol (shades of Ben Franklin and his
preservative cask....), a compound which is dirt cheap, non-
toxic, and theoretically nearly 60% better by weight (50% better
by volume) than ethyl chloride as a thermal ballast. The other
main candidate is n-propanol, which melts at -126 C and has 25%
better performance then ethyl chloride on heat of fusion (I have
to say, however, that this compound had a really lousy looking
phase transition in my experiments).
As has been mentioned already on the forum, there are
additional simple (alkane or alkene) hydrocarbons which melt in
the range we seek (like n-pentane), and most of these compounds
have even better performance than alcohols on heat of fusion
(this is a little weird, but I suppose that no additional
hydrogen bonds to speak of are formed when alcohols freeze).
However, all these exquisitely flammable hydrocarbons are too
dangerously volatile for what we have in mind (you do break
hydrogen bonds when alcohols evaporate, and that gives alcohols
lower and safer vapor pressures by comparison with gasoline or
pentane).
Even with alcohols, however, and *especially* with ethyl
chloride, our facility should have a Halon gas (CF3Br & CF2ClBr =
Freons 1301 & 1211) fire extinguisher system on hand. Halons,
like gasoline antiknock additives, damp ignition by releasing
heavy halogen radicals which interfere with propagation of free
radical chain reactions in flames (such compounds destroy the
ozone layer in somewhat the same way), and because of this
mechanism they work at very low concentrations (too low to be
toxic-- many of us remember the Dupont commercial of years back
where the man in a room with a little Halon in the atmosphere
can't successfully strike a match). In fact, it occurs to me
that the temperatures we propose to use in our system permit us
to have the world's simplest and most foolproof Halon system
ever: just tap off some Halon from commercial extinquishers,
solidify it, and leave chunks lying around in strategic places
(like near the walls) inside our cold room, along with the
ballast and elsewhere. In the event of any accidental warmup and
increase in flammable ballast vapor pressure toward flashpoint
temperatures and concentrations, we'll then get automatic fire-
/ignition damping as the Halon melts and vaporizes too. Fool-
proof!
Interestingly, one of the least volatile Halons, CHFClBr
(Freon 2111), has a very low melting point of -115 combined with
a relatively high boiling point of + 36 C, making it the best
candidate of all the Freons to use for *ballast.* I suspect
it's too expensive, however. Too bad, because a large amount of
vented ballast Halon from a cold room under thermal stress might
even put out a structure fire above it. The stuff is really
potent.
Steve
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