X-Message-Number: 2251
Date: Sat, 15 May 93 10:37:48 CDT
From: Brian Wowk <>
Subject: CRYONICS Dewar Insulation!

        In my last message I wrote that a vacuum-failed Bigfoot dewar 
in a -130'C coldroom would draw heat at a rate of about 10kW.  While 
we're on the subject, let's discuss the problem of vacuum failure 
*outside* the coldroom in a typical room-temperature cryonics 
facility.
 
        Because of the greater temperature gradient, we can expect a 
failed Bigfoot dewar in Alcor's patient care bay to draw heat at
30 kilowatts instead of 10.  This is very, very serious.  The outside 
of the dewar would quickly cool to something near -190'C, creating an 
extreme burn hazard and "cold zone" near the dewar because of thermal 
radiation disequilibrium.  LN2 in the dewar would boil at about 10 
liters *per minute*, generating 10,000 gaseous liters of nitrogen per 
minute-- enough to pose an asphyxiation risk to personnel.  In fact 
the entire patient care bay (if sealed) would fill with nitrogen in 
under an hour.  During this time the dewar LN2 level would drop by one 
meter, and begin to expose patients.  One hour is thus the maximum 
time available to transfer four Bigfoot patients to other dewars 
following a vacuum failure.
 
        Consider that the most likely time for a dewar failure is 
during an earthquake.  Can you imagine a worse time to be faced with 
an emergency patient transfer than after your facility (and possibly 
staff) have been trashed by an earthquake?
 
        What can be done about this?  Well, all the above heat flow 
calculations assume free convection around the outside steel wall of 
the dewar.  It turns out that adding as little as 1 inch of foam 
insulation around the dewar will reduce heat flow during a vacuum 
failure by a factor of *TEN*.  This means one tenth the hazard, and 
ten hours to transfer patients instead of one hour.
 
***>>   I urgently recommend that any cryonics organizations storing 
patients in vacuum dewars cover the sides and undersides of dewars 
with at least one inch of foam insulation (or several inches of 
fiberglass).  The shiny steel dewars will no longer look as pretty, 
but they will be much safer in an emergency.  The insulation will also 
provide modest mechanical protection against shocks and projectiles, 
thereby reducing the risk of a vacuum failure in the first place. <<**
 
        Getting back to the coldroom, it is now apparent that putting 
at least neuro dewars in the coldroom may be feasible after all.  
Since even modest amounts of foam insulation reduce vacuum-failure 
heat flows dramatically, these dewars need no longer pose a risk to 
coldroom patients.  An XLC-1520 neuro dewar with one inch foam 
insulation around it would draw less than 100 watts from a coldroom if 
its vacuum failed.  If the dewar was situated so that the room 
airstream circulated around it, a vacuum failure would simply put the 
room into de-facto LN2 backup mode.  The room thermostat would 
automatically reduce the mechanical refrigerators from pumping 500 
watts down to 400 watts to compensate.  With LN2 boiloff going at 
about 50 liters per day (ten times normal room temp boiloff), staff 
would have up to several days to remove the dewar and transfer 
patients.
 
        The LN2 savings of coldroom-stored dewars are potentially 
dramatic.  Conductive losses would be reduced by 75% due to the 
smaller temperature difference and reduced mobility of residual gas 
molecules in the dewar vacuum.  Radiative losses (proportional to the 
4th power of ambient temperature) would be reduced by 95%.  Depending 
on the relative contribution of radiative losses, we can expect at 
least a 75% LN2 saving by storing dewars in the coldroom.  For two 
neuro dewars, this adds up to savings of $1000 per year, which would 
pay 20% of the coldroom electric bill.  There would also be a floor 
space saving because the bulky concrete neuro vaults would no longer 
be needed (the coldroom itself is a vault).
 
        My only remaining concern is the added design complications, 
which may negate the $1000 annual savings.  We shall see.
 
                                                --- Brian Wowk

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