X-Message-Number: 1999
Date: Sat, 20 Mar 93 17:45:00 CST
From: Brian Wowk <>
Subject: CRYONICS No moving parts!

        Yesterday I outlined how a -130'C storage room can be made 
failsafe by replacing the LN2 reservoir dewar with a lightly insulated 
steel drum.  Today I will outline how to make the room even more 
failsafe by dispensing with the need for mechanical devices (thermostat 
solenoids, recirculators, etc.) entirely.
 
        One potentially serious problem with vapor cooling systems is 
the problem of temperature non-uniformity, or stratification.  Warm air 
is less dense than cold air.  Therefore the air inside any sealed room 
will tend to stratify, with colder air settling at the bottom of the 
room and warmer air at the top.  Mike Darwin has proposed electric 
stirring fans to prevent this.
 
        There are problems with electric fans.  A 200 square foot 
operating cost-minimized room (one with 2 meters of foam insulation) 
will boil 70 liters of LN2 per day.  This corresponds to a heat flow 
into the room of about 150 watts.  This means that the power dissipated 
by a mere 50 watt fan will increase LN2 consumption by 30% !
 
        As an alternative, consider reducing the two meter insulation 
thickness underneath the room to one meter.  (This will make for easier 
access anyway.)  LN2 consumption will increase by about 10%, and the 
warm floor (compared to the ceiling) will cause convective air 
circulation.  This effect might be enhanced by covering the floor and 
ceiling with metal.  Note that for the price of increased LN2 
consumption, you can make the convection as vigorous as you want by 
continuing to reduce the thickness of the foam under the room.  You can 
even recover some of your LN2 efficiency by moving that foam above the 
room (giving 3 or more meters of foam thickness over the room).  So 
don't use fans!  There is no need for them.
 
        What else can be done to passively combat stratification?  An 
insulated pipe should carry nitrogen vapor from the LN2 resevoir to a 
network of pipes on the ceiling of the room.  The boiling LN2 in the 
200 square foot room (Should I now call this the Standard Room?) will 
produce about 1 liter of gas per second.  This gas should be vented 
uniformly over the whole ceiling through tiny holes in the pipes.  By 
venting cold gas at the ceiling we further enhance convection.
 
        Mike Darwin suggested using the nitrogen gas flow from the 
boiling LN2 to power fans to stir the room air.  There is a physics 
flaw in this idea.  The flowing gas already has its own kinetic energy 
that will stir the room air as it is vented.  By reducing flow energy 
in the vented gas to power fans which in turn increase air flow 
elsewhere you gain nothing.
 
        Having dispensed with the need for fans, what about the 
mechanical complexities of the temperature control system?  I suggest 
the following: Begin with an insulated barrel of LN2 (unpressurized) as 
I have already discussed.  The only way for gas to get out of this 
barrel is through an insulated pipe leading to the ceiling vents 
outlined above.  The insulation around the barrel is adjusted so that 
the instrinsic boiloff rate will keep the room at -110'C (failsafe 
mode).  At the bottom of the barrel, immersed in the LN2, you have a 
electric heating element.  An electronic thermostat (copper-constantin 
thermocouple) controls the current to the heating element.  Applying 
current to the element increases the LN2 boiloff above the intrinsic 
level, causing the room to cool to the desired level of -130'C (or 
whatever).  How does electrically boiling LN2 cause cooling?  Well, the 
heat of vaporization is supplied entirely by the electricity, so you 
get no cooling from that.  However, the extra boiling vents cold -196'C 
gas into the room, and *that* is where you get your cooling.  The 
system may seem inefficient, but it is not.  Remember that you are 
actively controlling only about a few percent extra boiloff beyond the 
intrinsic amount, so the inefficiencies in this few percent do not 
significantly affect overall efficiency of the room.
 
        Voila!  A -130'C storage room with air circulation, active 
temperature control, and not a single moving part!
 
                                                --- Brian Wowk

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