X-Message-Number: 2086
Date: 09 Apr 93 08:57:27 EDT
From: "Steven B. Harris" <>
Subject: CRYONICS CF4 at -129 C.

Brian:  >>  My design description was insufficiently detailed. 
The 1 inch pipes pass >6 inches under the floor of the room in
troughs  cut in the Trymer foam.  Remaining space in the troughs
is then filled  with perlite.  In other words the pipe run under
the room is heavily  insulated, with little heat transfer
occurring down through the floor.   In fact most of the floor
heat transfer is *upward* from the ground  below.  The insulation
around each vertical pipe run is adjusted to  sink about 10 watts
over its 3 meter rise.  The 20 vertical pipes around the outer
room walls thus sink a total of about 200 watts,  which is the
approximate amount expected to flow through the room  sides. 
Finally, at the top of their vertical run, the pipes bend to  
carry -196'C vapor through horizontal pipes running along the top 
edges of cell walls.  Vapor from the LN2 reservoirs is similarly 
routed.  The insulation around these pipes and the reservoir
tanks within the room will be designed to sink the expected heat
flow coming in through the ceiling.  If anything, an excessively
*warm floor* may occur in this design. <<

   Comment:  Okay, but with the availability of a good -135 C
mechanical refrigeration unit (which you just discovered), what
you want to do is adapt this same design, minus insulation
between pipes and pods, to a working fluid that boils at higher
temperatures.  To wit: CF4 (carbon tetrafluoride, Freon-14),
which boils at -129 C.  With small pipe diameters, our system
volume might be low enough that we could afford the capital cost
of the CF4, especially since the system will run "semiclosed"
(subject to multiple failsafe pressure relief valves, of course). 
CF4 also has the charm of being one of the few environmentally
safe fluorocarbon gases, so the authorities will never bug us
about it.  We can buy and valve off as much as we can afford.

    I propose dual "small" CF4 reservoirs (or two independent
sets of 2 interlinked ones, to make four total, if you must),
each of the 2 reservoir systems with its own -135 refrigerator. 
The key is that we have two complete and independent sets of
cooling pipes running side by side everywhere, so that if one
springs a leak we have the other (this is exactly the time-tested
principle of the dual hydraulic brake system in your car, and
it's a good one).  By "small" I mean that we make the CF4
reservoirs as large as we can afford to.  Each refrigeration unit
will be large enough to do the job of cooling the whole room on
its own, and we'll run them alternately or at half power, or
whatever, so that we always have one for backup in any failure.  

   To start, you just turn on both refrigerators with cooling
elements in the reservoirs, wait till you're down to -135 in the
reservoirs, and valve in CF4 gas, letting it liquify on the
refrigerator elements and boil again as it encounters the lower
passive pipe system and spreads out to cool the room.  Keep it up
until the level of liquid comes up all the way in your reservoirs
and the room is at -125 or whatever, and there you are.  Cold
vapor cooling and boiling liquid to equalize temperatures works
as with the LN2.   At these temps we'll have to use ethyl bromide
or ethanol ballast.  The CF4 will act as something of a ballast,
but I don't think we can afford enough to be cost effective at
this-- cheaper stuff needs to be added inside.  We really need
only the minimum volume of CF4 it takes to fill the conductor
pipes and not give us gas bubble trapping.  We'll be boiling only
about 1.4 grams per second of CF4 at 200 watts, for about 0.2
liters/sec total gas flow (rough est), which is 100 cc/sec to
each reservoir, 5 cc/sec though each pipe.  Your pipes only have
to be large enough to handle that comfortably.

   Backup is diesel generators, as before.  Another possibility
(and we can have both) is a simple metal coil in one of the
reservoirs through which you can valve LN2 from a LS-160 to
liquify CF4, if need be.  This can be done by hand.  If all
fails, the CF4 boils off until the pressure relief valves pop
off, and the system runs dry.  Then you have to recharge it again
with Freon, just like your auto air conditioner <g>.

                                          Steve

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