X-Message-Number: 2146
Date: Fri, 23 Apr 93 12:01:19 CDT
From: Brian Wowk <>
Subject: CRYONICS Frost Freedom
Here are the numbers for the Cold Room frost problem:
Soaking wet (saturated) air at 20'C will have 20 grams of
water per cubic meter. There will be about 100 square meters of
surface area within the Cold Room air circulation system for frost to
condense on. To coat everything with 1 millimeter of frost we will
need 100 kg of water, or 5000 cubic meters of wet room-temperature
air. Even if 10 cubic meters of outside air got sucked into the Cold
Room every day (an extraordinary amount) it would still take *500
days* to accumulate 1 tiny millimeter of frost. With the large air
spaces we will have, you could probably run for 100 years without
trouble.
Notwithstanding, because things have a way of being worse than
we think, I have thought about this problem. I would like to suggest
some interesting changes to the room design.
To reduce frost accumulation, and increase your ability to
remove it, you want all exposed surfaces (except the heat exchangers)
insulated as much as possible. To this end, I propose that the entire
internal structure of the room be built from WOOD. No I'm not
kidding. This would include the floor and false floor, which can be
made from 2" x 6" planks with lots of bracing under the false floor.
Dry softwood, such as white pine, is an excellent insulator (1/4 as
good as foam) and is super strong for its weight. I also vaguely
recall that it has a small coefficient of thermal expansion (and
contraction) which is important given that the room will be cooled
150'K after it is built.
You want your ballast and patients insulated from the room air
flow so that you can warm the air during defrost operations. I
therefore propose that the bottom of the room (top of the false floor)
be covered with 3" Styrofoam panels (a common construction material).
Also the top of the ballast and patients should be covered with 6"
fiberglass batting to insulate them from the overhead air flow.
Fiberglass will be better than foam at the top because it can be made
to conform to nooks and crannies to cover things well. You just peel
it back when you want to work with ballast or patients underneath.
You can also throw foam panels on top of it if you want to stand on
ballast while working (just better made sure you're standing on
ballast, not patients).
I know you're all wondering right now whether all this wood,
foam, and fiberglass is going to impair heat flow from the patients to
the room air, and give rise to temperature non-uniformities (defeating
the purpose of air circulation). The answer is no. The only heat
source in an operating Cold Room is the outside world, which sends in
heat through the ceiling and floor. The over and under airflow
intercepts this heat, insuring -130'C is always maintained over and
under the insulated patients and ballast. The only exception to this
is patients undergoing cooldown from -80'C to -130'C. These patients
(heavily insulated to slow heat transfer) will be layed down in the
air flow, on top of the ballast, in a part of the room set aside for
this purpose.
Then there are the sides of the room. Heat flow coming in
through the sides must be intercepted as well. Doing this with air
flow would be complicated and space-inefficient. The best way to do
it is with 3mm sheet aluminum along the outer walls (the only non-
wooden part of the room). The aluminum makes thermal contact with the
-130'C air flow at the top and bottom air circulation spaces. This
ensures that even farthest from the air circulation, the sides will
not get warmer than -129'C.
During defrost operations, the aluminum outer walls will be
warmed by the hot circulating air. You therefore have to insulate
your patients and ballast from the outer walls just as you do against
the over/under air flow. Above the false floor, the aluminum walls
will be covered with 3" Stryrofoam up to 18" short of the ceiling (to
allow contact with the overhead air flow).
To defrost, you blast 0'C air through the air ciculation
space. The air is heated by 15kW electric heating elements in the
heat exchange cells. 0'C (or a bit below) insures you will sublime,
not melt the ice. In about one minute the alumimum outer walls will
reach 0'C, and you are defrosting.
Water vapor that sublimes off the ice in the room is captured
(condensed) onto special LN2-filled cold traps in the heat exchange
cells. The air leaves the heating elements at 0'C, traverses the
room, and hits the next heat exchanger at -10'C (cooled by 3kW losses
into the ballast and patients). It is then cooled to -50'C by the LN2
condensers (designed to cool that much) and suddenly warmed to 0'C
again at the heating elements.
How long could this be sustained? With the ballast and
patients insulated by 3" foam or equivalent on all sides, you will
heat them at a rate of about 3kW during defrosting. This will raise
their temperature by only 0.5'C per hour. So you can defrost for a
long time. At least an hour or more. According to some rough
calculations of mine, blowing dry 0'C air over ice will sublime off
about 1mm of solid ice per hour. This is pretty good considering that
we don't expect to accumulate that much frost in less than a year.
The frost problem is thus definitively solved, and we've made
some good design improvements along the way. Extra insulation around
the ballast and patients will make the ballast work better, and
temperature stability will be better if the room is open for long
periods. Wooden supports for the foam ceiling blocks will
dramatically reduce radiative losses through the open spaces between
blocks. Also, having lots of wood and other insulation in the room
will make it much safer to work over, and even in.
An interesting aside to the frost buildup question is the
question of how you freeze your ballast. Someone suggested awhile
back pre-freezing ballast before placement in the room. This
absolutely, positively must not be done. Pre-frozen ballast will
acquire condensation that will freeze all the containers together when
the room is finally cooled. You would never be able to neatly move
containers afterward.
By the way, while everybody seems to carry ethanol and
2-propanol (isopropanol), nobody has been able to sell me (or even
find me a supplier) for 1-propanol in industrial quantities. I was
ultimately refered to a petroleum refiner (which I did not follow up
on). It appears that the bottom layer of ballast is going to have to
be ethanol/water. Maybe the impoverished FSU can give us a deal on
Vodka. :)
--- Brian Wowk
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