X-Message-Number: 1796
Date: 22 Feb 93 03:51:08 EST
From: Mike Darwin <>
Subject: CRYONICS Use of Liquid Nitrogen

From: Mike Darwin
To: Michael Riskin, All
Re: Use of liquid nitrogen
Date: 21 February, 1993

     Michael Riskin asks if it wouldn't make more sense to determine the 
temperature you want and then find the right coolant.  The answer is yes. 
The longer answer is that this has already been done.  Not that many 
things which are safe to use or are economical to use boil at the right 
temperatures.  Noble gases such as argon, xenon, etc., are not cheap.  
Other compounds with suitable boiling points are toxic and/or corrosive.  
Nitrogen is cheap, readily available, nontoxic and inert.  The fact that 
it is colder than we'd like poses some engineering difficulties, but not 
in my opinion insurmountable ones.

     Why continue to use liquid nitrogen?  Because it is so safe in so 
many ways.  The alternative is a mechanical system employing a recycled 
refrigerant.  These systems are very difficult to operate: they are 
costly, they break down all the time, they require a duplicate unit to sit 
idle as a back-up...and they are unsafe because of the problems associated 
with them, including the problem of power failure.  Liquid nitrogen 
doesn't break, doesn't suffer from power failures, etc.  In principle it 
should be possible to build a room-sized system with the patients racked 
floor to ceiling in pods with access aisles that staff could walk around 
in (in well insulated and electrically heated clothing + an air supply).  
The only electricity that would be required would be to operate fans, 
(plus the thermostat and solenoids) to keep the gas stirred and back-up 
fans using compressed air motors would be present which could operate from  
LN2 in the event of a power failure.  The solenoids and thermostat could 
operate from battery power for a long while.  

     Basically what I'm describing here is a large insulated room with a 
medium pressure cryogenic dewar in it.  The dewar would be inside the room 
in order to increase the effeciency of the system by minimizing heat leak 
into the LN2 reservoir (in effect using the vapor temperature room as a 
heat shield).  The dewar would be a medium pressure unit (50 to 100 psi?) 
so that in the event of a power failure a valve requiring power to keep it 
closed, would open and dump liquid to an external heat exchanger to 
generate compressed nitrogen to run the fans.  The nice thing about this 
design is that it is all off the shelf: dewars, heat exchangers and so on 
are all standard equipment used by hospitals to drive their medical 
oxygen system by liquifying and "compressing" LOX.  What's more, this 
stuff turns up used or surplused for almost nothing out here.  In fact, at 
the moment there is an incredible glut of such hardware as the aerospace 
industry is melted down.

     For sometime I have wanted to test the workability of working in a
-135*C environment.  I have repeatedly asked Alcor management for 
permission to do this, but they are too afraid of liability and have 
refused to allow me to carry out the experiment.  Soon, however I should 
be able to try it.  We will soon have a large insulated cargo container 
dropped off here (4 inches of urethane foam on all sides) and it is 
already plumbed for liquid nitrogen refrigeration.  I plan to go into this 
unit after cooling it to -135*C wearing  heavily insulated garb over a 
heated flight suit.  Of course I will also bring my own air supply!  I 
want to see how long I can work and stay in such an environment before 
becoming uncomfortable.  

     I have been very frustrated by the irrational fears I've heard 
expressed about conducting this experiment because I have already had my 
hands (with and without gloves) and indeed my whole upper body in -140*C 
environments (and at Alcor I might add!) many times: this was achieved by 
leaning into partially filled whole body dewars during patient transfers 
practically up to the waist.  Yes, it gets very cold very fast, but it 
isn't instant death as long as you don't breathe. (In fact, I have had my 
arms without gloves or any protection up to the elbow in liquid nitrogen 
and am none the worse for wear.)  The key here is solid respiratory 
support.  Good gloves greatly extend the working time.  Heated gloves 
might prolong it more or less indefinitely.

     If it can be demonstrated that short periods of useful work can be 
done in this environment by persons who are properly clothed and 
outfitted, then a major barrier to the use of room-sized enclosures will 
have been overcome.

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