X-Message-Number: 2255
Date: Sun, 16 May 93 01:29:10 CDT
From: Brian Wowk <>
Subject: CRYONICS Reply to Mark Voelker
Mark Voelker:
> *"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?
You bet. Any engineering text on heat transfer will tell you
that the film coefficient (convective heat transfer rate) for vigorous
free convection is about 10 watts per square meter per degree C. The
outside area of a Bigfoot dewar is about 10 square meters, and the
temperature difference between -196'C and the coldroom is about 60'C.
This works out to 6kW (which became rounded to 10kW in my original
mental arithmetic).
Of course all this is academic since (as I admitted in my
recent message) free convection is not a good model for heat transfer
in a coldroom because even a modest amount of insulation around the
dewar makes a big difference. However free convection *is* the
correct model for bare dewars in a patient care bay, which is why
Alcor should wrap some polyurethane foam around its whole body dewars
right away. Bare steel dewars are a disaster waiting to happen.
> Firstly, removal of the dewar is not necessary to halt the sudden
> cooling process--only removal of the rest of the LN2. This can be
> done by sucking the LN2 out of the dewar with a pipe dipping into
> the dewar;...
Three thousand liters of LN2 would have to be removed in a
timely fashion. Does Alcor own equipment able to pump several liters
of LN2 per second?
> 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.
This repair work *would* be done in a crisis setting. I am
now convinced that warming LN2 patients to -135'C would be a disaster.
Major trauma could result from LN2 trapped in intra-cranial spaces
such as the sinuses. For this reason we want to ensure LN2 patients
do not rise above -186'C, let alone -135'C.
The best way to buy time to cope with a vacuum failure (in or
out of a coldroom) is to surround the dewar with insulation. Simply
adding one inch of foam to a Bigfoot dewar would, for example, extend
your time-to-respond to ten hours from one hour.
Another way to extend your time-to-respond is (paradoxically)
to keep your dewar out of the coldroom. This is because coldroom
patients, not dewar patients, are the limiting factor for response
time when dewars fail in a coldroom. This is true for any amount of
dewar insulation, and any response time. To see why, let us define
loss of 1000 liters of LN2 (about 1/3 the Bigfoot capacity) as the
critical response point for dewar patients. What would happen to
coldroom patients during loss of 1000 liters of LN2? Well, the heat
to vaporize those 1000 liters (175000 kW) would come almost entirely
from the coldroom. Since the heat capacity of water ice at -130'C is
1 kW/liter/degC, the 30,000 liter volume of patients and ballast would
cool 6'C to provide this heat. But wait! It is the patients and
ballast immediately adjacent to the dewar that would cool the most,
and they would cool far more than just 5'C during loss of 1000 liters
from the dewar. Thus, even though the dewar patient response time
increases by a factor of three when you move to the coldroom, the
coldroom patient response time is so much shorter that you are better
off keeping the dewar out of the coldroom to start with.
I trust I have now beaten the Bigfoot question to death.
Under no circumstances should a Bigfoot dewar be put in a 500 square
foot coldroom. The heat capacity of this room is just too small to
handle a vacuum failure in a dewar that big. We'll put the Bigfoots
in next time around, when we build our 5000 square foot room.
> "Putting fragile dewars in the coldroom, with the associated
> requisite of constant human monitoring, is not in keeping with the
> design 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:...
> 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.
By "failsafe" I mean that the coldroom has no catastrophic
fail modes. (I consider "catastrophic fail mode" to be a failure
which requires response within 24 hours.) The coldroom would warm at
0.5'C per day during a refrigeration failure (and even less during the
ballast phase transition). This means that the coldroom could be left
unattended as long as two weeks after a cooling failure before
patients approached Tg, and danger of devitrification. Terminology
aside, putting Bigfoots in the coldroom would shorten this "risk-free"
time by one or even two orders of magnitude.
> *"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.
It may indeed be feasible to put foam insulated neuro dewars
into the air circulation stream of the current coldroom design. It
does, however, introduce certain design complications and space
inefficiencies that concern me. Nevertheless, it may be worthwhile.
In any case, your dogged determination has convinced me that all Alcor
LN2 patients and their dewars should eventually be put in -130'C
storage. If not in this coldroom, then in the next bigger one.
--- Brian Wowk
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