X-Message-Number: 2007
Date: Mon, 22 Mar 1993 12:18:44 MST
From: "Richard Schroeppel" <>
Subject: CRYONICS  Alternative cooling: Solid-liquid phase transition

Current cryonics practice uses the liquid-gas phase transition of LN2
as a thermal buffer to maintain the patient at a constant temperature.
The patient is partly immersed in the LN2.  The temperature of the
system is (roughly) constant as long as any liquid remains to evaporate;
heat that leaks into the dewar goes into the liquid->gas phase change,
and not into warming the patient.

We've been discussing changing the storage temperature to around -130C.
One nuisance is that there are no convenient substances which liquefy
at that temperature.  ("Convenient" abbreviates cost, toxicity,
environmental, & handling considerations.)

I suggest we examine using another phase transition to supply the
thermal buffer:  Solid->liquid.  The patient would still be immersed
in the liquid, but solid lumps of the coolant would be in the bottom
of the dewar.  There would be a smallish air space above the liquid
level.  Incoming heat would go to melting the solid.  To "top up"
the coolant, we could either drain liquid & add solid, or pour LN2
into the dewar to refreeze some liquid.

Advantages:
Another set of cooling substances to choose from.
Less coolant is released to the outside world, so the environmental
constraints can be loosened, and more expensive coolants are feasible.
Liquid-solid freezing points can be varied over a range by mixing
two or three substances.  We can synthesize whatever value we choose.
The venting system on the dewar might be simpler.

Disadvantages:
Solids are harder to handle than fluids: details of the topping-up
process need to be worked out.
The solid-liquid phase transition generally takes fewer calories
per gram (water is 80 to melt, but 500 to boil), so the required
amount of buffer may be larger.
A spill of LN2 is "self-cleaning".  The replacement coolant won't be.
The patient may have to be wrapped more carefully, if the coolant is
a contact toxin.
Measuring the remaining cooling capacity in the system won't be as
easy as looking at the level of LN2 in the dewar.

Rich Schroeppel  

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2007