X-Message-Number: 2253
Date: 15 May 1993 12:33:40 -0700 (MST)
From: 
Subject: CRYONICS dewars in cold room

Dear Brian:  After mulling over your comments in Cryomsg #2244 about the
possible dangers of putting our present dewars in a future cold room, I
have to say that I don't think the problem you mentioned is real. You
constructed a scenario wherein the dewar, inside the room, lost vacuum and
therefore started to boil off its LN2 at a high rate, sucking heat from its
surroundings, and therefore causing a drop in temperature in the rest of the
room. This process, if allowed to proceed unchecked to thermal equilibrium,
would indeed lower the core temperature of the patients below -135C, with a
resulting devitrification of those patients. But both the time scale of these
events and the magnitude of the resulting temperature drop you claim will
occur are not supported by your reasoning or the evidence you present in the
Cryomsg.
*"If a Bigfoot failed inside the room, you [the patient caretaker] would have
all of five minutes to lift a three ton -196C metal object out of the room
before adjacent patients were cooled dangerously."
Firstly, removal of the dewar is not necessary to halt the sudden cooling
process--only removal of the rst of the LN2. This can be done by sucking the
LN2 out of the dewar with a pipe dipping into the dewar; such transfer plumbing
is needed in any case to add LN2 to the dewar and to remove LN2 on a regular
basis to keep the concentration of LOX at acceptably low levels, if that is
deemed to be necessary. Futhermore, the time scale you mention (five minutes)
is unjustifiably short in my opinion. Air is not a good heat transfer medium:
try cooling a bottle of wine by putting it in your home refrigerator's freezer
compartment. (The temperature difference between the room temperature bottle

at 25C and the inside of the freezer at 0C is similar to the difference between
the pateints at -135C and the cold nitrogen vapor at -196C.) The time to cool
the bottle is measured in hours, not minutes, provided you don't bury the
bottle in ice or frost! In addition, our patients are stored inside double
sleeping bags, lowering the termal conductivity between their bodies and the
surrounding air of the cold room. So a mad dash to lift a 6000lb dewar out of
the room in less than five minutes is uncalled for. Appropriate action would
instead be to pump the LN2 out of the failed dewar sometime within an hour of
failure. And if things really got out of hand, a little warm air from outside
the room could be brought in. All of these actions could be taken by a single
competent caretaker. It is true that removing the dewar from the room, and
transfer of the patients within it, would be the work of a several people, but
this repair work would not be done in a crisis setting, since (unlike the
situation with a dewar outside the room) the patients in the dewar would not be
in danger of thawing--they could get no warmer than the -135 temperature of the
cold room.
*"Putting fragile dewars in the coldroom, with the associated requisite of
constant human monitoring, is not in keeping with the dsign philosophy of a
room that would otherwise be failsafe." --- "The question of human supervision
is almost acdemic."  I do not consider the proposed cold room to be "failsafe".
The room must be held at cryogenic temperatures by active means: there is no
way around this requirement, since it is in a nonequilibrium state with the
surrounding environment. The machinery to maintain the cold temperatures
whether onsite in the form of a refrigeration system or offsite in the form of
an LN2 plant, is subject to breakdowns and therefore must be watched over at
all times by a human being. If your design goal is to eliminate the need for
a human caretaker, you will have a tough time convincing me as an Alcor
Director of the feasibility of that goal.
*"Putting dewars......inside the room, while conceptually elegant, is in
practice too complicated and dangerous to consider."   Since we have no
practical experience at putting dewars inside a cold room, the statement that
"in practice [this is] too complicated" is unsupportable. Therefore the only
way to evaluate the proposal is in fact to consider it.
*"A vacuum-failed dewar would suck heat from the room at the ferocious rate of
10kW."  This is a nice round number. Do you have any calculations or data to
back it up?
*"Vaporizing LN2 in pipes under the room would be an explosion hazard." As
pointed out above, the transfer pipes are needed whether the dewar is stored
at 25C or -135C. LN2 is not an explosion hazard unless it is confined to an
enclosed volume while allowed to boil. Pipes are not enclosed volumes: they
have a hole at each end.
Finally, in my opinion, words such as "ferocious" and "failsafe" do not belong
in an engineering vocabulary. They play more to the reader's emotions than to
his or her reason.
Should Alcor decide to keep its pre-coldroom patients at -196C even after
building a coldroom, I think that we should consider putting their dewars into
that room, for reasons of safety to those patients and lowered LN2 costs. No
arguments have been presented yet to convince me otherwise.
Thank you, Brian, for your work so far on this concept.
			Sincerely, Mark Voelker

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