X-Message-Number: 1799
Date: 23 Feb 93 05:45:44 EST
From: "Steven B. Harris" <>
Subject: CRYONICS "High" Temp Storage


Dear Cryonet:

   Brian Wowk presents some calculations on the heat transfer
cost of having a layer of nitrogen gas insulating liquid nitrogen
from a patient at 138 K, and he's right so far as he goes *if
convection is negligible.*  In the real world, however, I would
expect that with this kind of gradient there should be at least
some breakup of such a big laminar thermal gradient into convec-
tive cells of the sort that you see in hot tea with milk, or over
a valley on a cloudy day (all right, you see them in your mind's
eye-- every one of those big cumulus thunderheads is sitting on
top of a cell).  With such cells come attendant heat pumping, and
inefficiency.  The bottom line is that I just do not trust this
setup!   The disruption of gradients you get whenever you fill
the tank is emblematic of the general problem you get in having
to rely on the delicate thermal stability of gas layers.  

    If we're to rely totally on passively generated thermal
gradients, actually, I'd much rather see them present with a
vengeance and no convection, i.e., a patient cassette embedded in
a foam block, with LN2 on one side and room air on the other. 
This design would bring a lot of attendant heat loss, of course,
but it's actually not a whole lot worse than if you had to foam
insulate an entire room kept at 138 K, which is the next step
(Mike Darwin's proposal).   I've been thinking along those lines
for some time, and I will admit that I once did a few ideal
calculations on the thermal efficiency of a room-sized 138 K
freezer (see _Cryonics_, June, 1986).  Unfortunately, I've since
been disabused of the idea that thick blocks of foam under
gigantic temperature gradients act as they should in ideal
calculations.  In the real world it seems they crack, and this
way, I'm afraid, lies financial disaster.

   So what's the ideal setup for cooling patients to 138 K using
LN2?  Well, obviously the ideal setup has the patient at 138 K
insulated as well as possible from the LN2 at 77 K, so as to
minimize heat transfer and LN2 boil-off rate.  So what we really
want is vacuum between LN2 and patient (as in Mike's design), and
also vacuum (not foam) between patient and room air.   One
solution (which reminds me of Baked Alaska, my previous term for
Alcor patients after the Big California Earthquake) would have
the patient in the center of a double jacket Dewar (four walls)
at 138 K, with LN2 circulating between walls 2 and 3 at 77 K, and
two vacuum spaces, one insulating LN2 from the outside, and the
other insulating the patient from the LN2.  Radiative heat
transfer in worst case here is (Stephan-Boltzmann relation) about
18.5 watts/square meter (about 40 watts/whole body patient), and
in practice this would be cut considerably with silvering.  This
heat would be supplied by a (12 volt-camper type) electric
blanket wrapping the patient (damned ironic for cryonicists to
wind up in electric blankets, eh?), which would in turn be under
thermostatic control.   Of course, we'd want to include some
sealed ethyl chloride packets with each patient, for thermal
ballast (Gee, I hope my name's included on that patent applica-
tion, Mike...).  With loses kept to a few 10's of watts or less
per patient, I see no reason why the whole thing couldn't be
backed up (in case of power failure) with a rack of car batteries
which would hold out as long as the LN2.  

   Oddly enough, Professor Ettinger, with his custom fabricated
soft vacuum Dewar technology, is probably the logical person to
build and test a man-sized double Dewar system of this sort. 
Maybe we can convince him to run one along side a pontoon system,
for comparison.

                                  Steve Harris

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