X-Message-Number: 2124
Date: Sun, 18 Apr 93 16:26:07 CDT
From: Brian Wowk <>
Subject: CRYONICS Flexible and Efficient
This posting is intended to address the concerns recently
expressed by Tim Freeman about how to get 30,000 liters of water into
a Cold Room without a lot of mess or ice damage. My thinking on this
matter has changed considerably since my last posting. The design I
now envision is extremely safe, simple, and inexpensive.
To begin with, we get rid of of the square meter cell concept.
The room will still be accessed by lifting cubic meter foam blocks
from above, but the interior will only have a few long walls as shown
in this downward-looking view.
|-------|---------------------------------------|
| P-550 | Heat | | |
| cooler| Xchg 1| | |
|-------|-------| | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
| | | | |-------|-------|
| | | Heat | P-550 |
| | | Xchg 2| cooler|
|---------------------------------------|-------|
The walls provide structural support for the ceiling, and force air
flow originating at Heat Exchanger #1 to follow a zig-zag path through
the room until Heat Exchanger #2 in the far corner is reached. Air
passes down through Heat Exchanger #2 into an air circulation space
beneath the room (about 2 feet high). There the air retraces a zig-
zag path back to the bottom of Heat Exchanger #1, and the cycle
repeats. The air flow is driven by fans near the top of each heat
exchange cell. These fans (one for each cell) are driven by vertical
shafts that pass up through the foam blocks over each cell so that
ordinary room-temperature electric motors can drive them. I've
abandoned the idea of cryogenic motors since I really don't know
anything about them, and because there is a certain sense of security
in seeing a motor working right in front of your eyes.
In this design, air flows both over and under patients
ensuring a uniform temperature distribution. I've done the forced
convection heat flow calculations (the subject of a future posting)
and have concluded that even a very modest air flow (less than 1 meter
per second) will keep temperature differences in the room less than
1'C. The only points in the room that can get a little warm are the
midpoints of the outside walls. Using 3mm thick aluminum will keep
these differences less than 1'C.
Where does the ballast go? Everywhere. We fill about 1000
ten gallon cans 85% full with water. (I think ten gallons is the
maximum size that could be easily lowered in and out of the room) We
cover them with lids to prevent water from spilling during handling.
A small hole in the lid will allow the 15% air content inside to
escape as the water freezes and expands. (Speaking as a Canadian who
has seen water freeze in barrels many times, I can testify that
lateral expansion will not occur provided that vertical expansion
space exists above the water.) We then fill the room with these water
cans to a height about 2 feet short of the ceiling (to allow air
circulation). This will be about 40,000 liters of water. With both
cryocoolers running, and lots of LN2 boost in the heat exchanger
tanks, we should be able freeze it all down to -130'C in about one
month.
Where do the patients go? We create patient storage slots by
selectively lifting out ballast cans. This is why we don't really
need prefabricated storage cells. We can make storage cells of any
size anywhere we want by removing ballast. This is a tremendously
flexible and efficient system. No longer bound by fixed cell sizes,
we can pack patients right next to each other with no wasted space in
between. Space savings that occur for smaller patients will accrue
throughout the room rather than end in a single cell. Using this
strategy, I can now state uneqivocally that this room will hold 100
whole-body patients, and more likely 150 (1500 neuropatients).
When patients are inserted, the bottom-most layer of ballast
cans will remain beneath them. These cans (and only these cans) are
half filled with submerged ethanol cannisters. Once the room is
completely filled with patients, only this bottom layer of cans will
remain. The final ballast load will consist of 5000 liters of water
(in the patients), 5000 liters water (in the cans), and 5000 liters
ethanol (in the cans). This will hold the room below -110'C for two
weeks after a cooling failure.
This system is also adaptable for use with the more expensive
(and superior) propanol ballast. The expense can be overcome by only
adding propanol as patients are added to the room. You start with the
room full of only water ballast cans. When a slot is being created
for a new patient, all the ballast cans in the slot, including the
bottom layer, are lifted out. The bottom layer is then replaced with
pre-cooled cans of submerged propanol in water ice. Once the room is
completely full of patients, you will have enough propanol under them
to hold the room at -125'C for 10 days after a cooling failure.
The water-submerged propanol cans must be precooled to -130'C
because they would heat their vicinity enormously if you dropped them
in at room temperature without insulation. The best way to precool
them would be to lower them into the room with enough insulation that
they would take one month to freeze. Since newly-suspended patients
cannot be kept waiting that long, we would have to maintain a small
inventory of pre-cooled propanol ballast in the room.
I really like the idea of propanol ballast because it holds
things well below Tg for a long time. If 5000 liters isn't too
expensive, we might consider installing a complete water/propanol
ballast layer at the very beginning (as in the ethanol system). Of
course, before we buy any propanol we better make darn sure these
cryocoolers can hold the room below -127'C.
--- Brian Wowk
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