X-Message-Number: 1994
Date: Fri, 19 Mar 93 13:12:12 CST
From: Brian Wowk <>
Subject: CRYONICS A nifty idea

        One of the reasons for going from dewars to foam-insulated rooms 
for patient storage is fault tolerance: immunity from catastrophic fail 
modes.  The design proposed by Mike Darwin would keep the room cold by 
venting cold nitrogen gas from a small LN2 dewar within the room.  
Unfortunately this still leaves one possible catastrophic fail mode.  
What if the small dewar vacuum fails?  LN2 would boiloff too fast, 
cooling the room below the desired -130'C temperature.  Since we are 
going to be pushing the edge of the envelope (storing very close to the 
temperature at which fracturing occurs) this is a very serious concern.
 
        I was thinking we should foam insulate the dewar (yes, my 
obsession with foam again) so that even if vacuum failed the boiloff 
rate would remain within acceptable bounds.  Then it hit me: If foam 
could insulate a vacuum-failed dewar enough for it to function properly 
in that room, then why use a vacuum dewar at all?
 
        I ran the numbers and concluded that 1000 liters (a cubic meter) 
of LN2 would boiloff at the correct rate if we surround it with only 5 
centimeters of foam!  This assumes our benchmark 200 square foot room 
insulated with 2 meters of foam, which will require 70 liters/day LN2 
boiloff to maintain -130'C.  A thousand liters of LN2 would last two 
weeks in this design.  This is similar to the maximum time Alcor's 
present patient storage system can function unattended.
 
        So, all we need to do is get a 200 gallon steel drum (or perhaps 
a cluster of four 50 gallon drums), insulate it with 5 cm of foam, and 
we're in business.  If foam is at risk of cracking because of the high 
temp gradient, then 10 cm of fiber glass insulation will do just as 
well.  Normally the room temperature will be maintained by an active 
control system that will increase LN2 boiloff beyond the intrinsic rate 
that is determined by the insulation alone.  However, we can empirically 
adjust the container insulation so that the intrinsic boiloff rate would 
maintain, say, -110'C even if the thermostatic system failed completely.
 
        The final engineering step will be to design the thermostatic 
control system so that all possible fail modes result in too little LN2 
boiloff, not too much.  We will then have a completely fail-safe -130'C 
patient storage system.
 
                                                --- Brian Wowk         

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