X-Message-Number: 2067
Date: Tue, 6 Apr 93 14:07:49 CDT
From: Brian Wowk <>
Subject: CRYONICS New LN2 System
While I recover from my thermoelectric embarrassment let's get
on with the business of designing an LN2 Cold Room.
One of the problems of Cold Room design is achieving a decent
temperature distribution for the case of air circulation failure. My
old design with a central LN2 reservoir connected to adjacent metal
cell wells would create a terrible temperature gradient without air
circulation. I now have a new design that definitively solves this
problem.
Returning to our prototypical 5m x 5m room with square meter
cells 3m high, we distribute four insulated LN2 reservoirs in four
cells as shown below. This leaves 21 cells for patients, giving a
21 x 6 =126 patient capacity. Allowing space for insulation, I
estimate we can squeeze 1000 liters in each reservoir for 4000 liters
total, giving 20 days worth of cooling when full.
O-------O-------O-------O-------O------O
| | | | | |
| | | | | |
O--------------------------------------O
| | | | | |
| | LN2 | | LN2 | |
O--------------------------------------O
| | | | | |
| | | | | |
O--------------------------------------O
| | | | | |
| | LN2 | | LN2 | |
O--------------------------------------O
| | | | | |
| | | | | |
O-------O-------O-------O-------O------O
Now here comes the clever part. The "O"s along the outer wall are
lightly insulated vertical pipes holding LN2. The pipes bend and pass
underneath the floor of the room connecting to the bottom of the
nearest LN2 reservoir. No pumps are required required to keep LN2 in
the pipes; hydrostatic pressure automatically keeps the LN2 level in
the pipes the same as in the connecting reservoir.
There are many beautiful aspects to this design. Because the
pipes and reservior walls are metal (with a high thermal conductivity)
the temperature underneath the insulation remains at -196'C along the
entire vertical length of each. (Rising -196'C vapor from boiling LN2
in the pipes also helps this happen.) This means that the temperature
distribution in the room remains very stable independent of the LN2
level. Also, the temperature of the room can be "fine tuned" very
easily. By designing insulation around the reservoirs and pipes in
the form of removable rods you can remove and replace rods (like in a
nuclear reactor :) ) to achieve whatever temperature or temperature
distribution you want. This would be done during the first few weeks
of room operation (with fans turned off) as part of the commissioning
process. As a final touch you might want to add electronically
controlled heating elements in the reservoirs for continuous super-
fine temperature control.
Since the reservoirs are permanent, purging accumulated LOX
(if this is a real concern) is somewhat problematic in this design.
The only way to do this would be to peroidically (every year?) allow a
reservoir to boil dry, relying on the remaining three reservoirs and
fan-driven air circulation to keep that quadrant cold for the few
hours that the LN2 is very low.
I think this design is definitely on the right track. I await
your feedback.
--- Brian Wowk
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