X-Message-Number: 2057
Date: Sat, 3 Apr 93 18:50:21 CST
From: Brian Wowk <>
Subject: CRYONICS Mechanical Freezers

Perry Metzger:
 
> 1. a simple-minded question. I was under the impression that you could
> buy commercial freezer units that operated at the temperatures in
> question. Wouldn't it be simpler and cheaper either to buy such units,
> or, if they are too small, to commission a manufacturer of such units
> to build extra-large units?
 
        There are -130'C body-size freezers ("Queue" freezers) 
commercially available.  I don't know the exact cost, but as I recall 
the people who looked into them a few years ago concluded the per-person 
costs would be very high.  
 
        To maximize economy you should store all your patients in a 
single foam-insulated room, not a bunch of separate freezers.  The next 
question is then cooling.  Thermoelectric cooling (no moving parts) is 
still my first choice.  However if further investigation reveals it to 
be uneconomical, then multi-stage mechanical freezer(s) operating 
between the room and outside may be indicated.  (Of course internal 
thermal ballast and external backup such as LN2 would be essential.)  
This system should NOT be custom built.  It should be made from readily-
available off-the-shelf cryogenic freezers with tons of spare parts 
available.    
 
        Bear in mind that every mechanical part added to this room 
decreases robustness and increases maintenance requirements, quickly 
eating up savings resulting from using electricity instead of LN2.  It 
also increases dependence on the outside world for spare parts and 
expertise.  The economics of mechanical refrigeration will have to be 
looked at very carefully if this route is chosen.
 
> 2. Another issue I haven't seen addressed too much is this: precision
> of temperature regulation. The systems that have all been mentioned
> thus far sound like they would expose patients to relatively large
> temperature swings compared to the "stay at the boiling point of LN2
> forever plus or minus a few tenths" system that we have now.
 
        A system based on melting reserviors of ethyl chloride (as Steve 
Harris suggests) would have essentially zero temperature fluctations.  
It would have even less temperature variation than LN2 submersion 
because freezing points vary less with pressure than boiling points.
 
        If a more active thermoelectric temperature control system is 
used, I wouldn't expect temperatures to vary more than ~0.1'C (and very 
slowly).  This is similar to what patients in LN2 are currently exposed 
to because of atmospheric pressure variations. 
 
> Also has anyone actually done experiments on mammalian brain
> tissues to determine the extent of cracking damage at LN2
> temperatures?
 
        Alcor froze some rabbits with 3M glycerol a few years ago, and 
their brains were badly cracked as I recall.
 
> 3. Lastly, it seems that people have dismissed using a working fluid
> other than liquid nitrogen out of hand. It would seem, however, that
> other fluids do exist with an appropriate boiling point and that they
> are not considered primarily because of expense or toxicity. However,
> it is unnecessary to actually expose the patients or employees to the
> working fluid or to consume it -- one could simply use the liquid in a
> closed recirculating system to exploit its boiling as a way of very
> stably maintaining patients at a desired temperature. Patients could
> be surrounded by dry gas or by a liquid with a suitably low freezing
> point. Is this impractical in some way that I haven't thought of?
 
        This is exactly what a mechanical refrigerator does.
 
                                                --- Brian Wowk

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