X-Message-Number: 2113
Date: Thu, 15 Apr 93 17:56:45 CDT
From: Brian Wowk <>
Subject: CRYONICS 30 Tons of Ballast
Here is a summary of the thermal ballast materials that have
been suggested for use in a Cold Room:
melting point boiling point heat of fusion
-------------------------------------------------------------
Ethyl chloride -136'C 12'C 62 kJ per liter
Ethyl bromide -119'C 38'C 79 kJ per liter
Ethanol -115'C 78'C 86 kJ per liter
I think ethyl chloride can be ruled out because it is too volatile and
the melting point is too cold. The melting point of ethanol is only
slightly higher than ethyl bromide. Ethanol is also less volatile,
less flammable, less toxic, and less expensive than ethyl bromide. It
appears to be the best of the three.
Another possibility is 1-propanol with a melting point of
-127'C and boiling point of 97'C. It is less volatile (and presumably
less flammable) than ethanol, and the temperature is ideal. It is
probably more expensive, though. I don't know the heat of fusion.
Let's calculate the performance of a Cold Room with 5000
liters of ethanol ballast. What happens if we turn the refrigerator
off? The heat capacity of ethanol liquid is 1.5 kJ/liter/degC at
-100'C. If we assume that like water the heat capacity gets cut in
half in half when frozen, then the heat capacity of ethanol ice will
be 0.75 kJ/liter/degC. Is this the only heat capacity to consider?
If the room is full of patients, we can expect over 5000 liters of
water ice (in the patients) to contribute additional thermal ballast.
If the room isn't full, we can simulate patients by keeping frozen
barrels of water in unused cells to give a total of, say, 10000 liters
of water ice. The heat capacity of water ice at -130'C is 1.3
kJ/liter/degC. The total heat capacity of the room (ethanol and water
ice) will be 16750 kJ/degC. If we turn the refrigerator off at
-130'C, the room will take 5 days to warm to -115'C. The ethanol then
starts to melt, holding the room at -115'C for 10 days. This gives a
total of 15 days before vitrified patients are in real trouble.
In the example above, water makes a major contribution to
thermal ballast by slowing the rate of temperature rise to -115'C.
This suggests an interesting possibility: What if ONLY WATER is used
as thermal ballast? Water is cheap, safe, and the local fire
marshalls won't mind a bit. We could put 10000 liters (in sealed
containers) in the four centrally-located cells as shown below.
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
X----------------------------------------X X = ice around
X| | | | | |X the outside
X| | | | | |X walls
X----------------------------------------X
X| | Water | | Water | |X
X| | Ice | | ice | |X
X----------------------------------------X
X| | | | | |X
X| | | | | |X
X----------------------------------------X
X| | Water | | Water | |X
X| | Ice | | Ice | |X
X----------------------------------------X
X| | | | | |X
X| | | | | |X
X----------------------------------------X
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
We could put another 5000 liters around the outside walls, and stuff
the patient cells with at least 15000 liters. This would be 30000
liters with a thermal capacity of 40000 kJ/degC. If you turn the
refrigerator off now, your room will warm up at 1'C PER DAY. It would
take two weeks to break -115'C (same as the ethanol case), and the
rise would be very gradual instead of a sudden jump to a plateau.
I can hear critics saying that I should really compare 30000
liters of water to 30000 liters of ethanol or propanol. This is
academic since we couldn't *afford* 30000 liters of an organic
solvent, and regulators would never let us store it in an occuppied
building no matter how much halon we put on top of it.
There is another advantage to lots of water (as opposed to
eutectic) thermal ballast. Large quantities of water ice will
strongly resist temperature swings in either direction. This will
make the temperature in the room very, very stable, even if
refrigerator power is cycled or if a cell cap is open for long
peroids.
A mechanical system (with simple LN2 backup capability) using
30000 liters (30 tons) of water ice for ballast is the best design I
can think of right now. Comments?
--- Brian Wowk
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