X-Message-Number: 2166
Date: Tue, 27 Apr 93 12:22:43 CDT
From: Brian Wowk <>
Subject: CRYONICS LN2 Backup

Tim Freeman:
 
> The system Wowk is proposing would be able to use delivered LN2 if
> both coolers went down at once or the electricity and the diesel
> generator both failed at once.  The idea is to pour it over the heat
> exchanger coils, if I remember right.
 
        Actually my LN2 backup system is considerably more elegant.  
You have a foam insulated tank in one of the heat exchange cells.  The 
insulation is calculated (and later fine tuned) to conduct 500 watts 
between a -130'C ambient temperature and the -196'C inside.  By moving 
room air past the tank you will then maintain -130'C with perfect 
stability and zero effort.  You just refill the tank once a day.
 
        Here comes the best part: Remember that two diagonally 
opposite heat exchange cells join the over/under air circulation of 
the room.  Fans suck air up one cell and blow it down the other.  Now
imagine that the fans fail while in LN2 backup mode.  This is a 
potentially dangerous situation because parts of the room (near the 
LN2) might get too cold.  Well, this doesn't happen with over/under 
air circulation.  The cold LN2 reservoir in only one heat exchange 
cell (compared to the warm opposing cell) will form a thermo-siphon 
that will mildly drive the over/under air circulation even if the fans
fail.
 
        This phenomenon is too beautiful to be an accident.  I take it 
as a sign from God that this is the way He intended a Cold Room to be 
built.  In fact, this LN2 "backup" mode is so simple and failsafe it 
is how I would now build a pure LN2-cooled room if I was told to do 
so.  Forget all the vertical pipes, plumbing and insulation of 
previous schemes.  Just build a wooden room with lots of thermal 
ballast, over/under air circulation, and a small daily-fill LN2 tank 
in one corner.
 
        It should be reassuring to know that even in its *backup* 
mode the Cold Room will be operating as the best possible LN2-cooled
room you could devise.
 
                                                --- Brian Wowk   

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