X-Message-Number: 2089
Date: Sat, 10 Apr 93 03:49:37 CDT
From: Brian Wowk <>
Subject: CRYONICS Cold Progress

Steve Harris:
 
> If we can store as high as -117, or store at lower
> temps and dare to use a ballast at as high as -112 to -117, we
> can use plain old ethyl alcohol (shades of Ben Franklin and his
> preservative cask....), a compound which is dirt cheap, non-
> toxic, and theoretically nearly 60% better by weight (50% better
> by volume) than ethyl chloride as a thermal ballast.
 
        I vote for ethanol.  Since the ballast is for emergencies only 
(staff hauled off to jail, refrigerator unplugged?) it's better to 
have three weeks of stability at -112'C than one week at -119'C.  The 
price difference would probably do that for us.
 
        By the way, do these organics expand when they freeze, or 
contract?  The distinction is important.  If they contract, we can 
package them in anything and not worry about it. 
 
> In fact, it occurs to me
> that the temperatures we propose to use in our system permit us
> to have the world's simplest and most foolproof Halon system
> ever: just tap off some Halon from commercial extinquishers,
> solidify it, and leave chunks lying around in strategic places
> (like near the walls) inside our cold room, along with the
> ballast and elsewhere.  In the event of any accidental warmup and
> increase in flammable ballast vapor pressure toward flashpoint
> temperatures and concentrations, we'll then get automatic fire-
> /ignition damping as the Halon melts and vaporizes too.  Fool-
> proof!
 
        I love it!
 
> Perhaps best then to keep the poor sods who are in LN2 there
> until resurrection day, unless we are absolutely forced to move
> them (to -130'C) by financial constraints.  
 
        You're probably right.  (Similar concerns were expressed at 
the recent Chicago meeting, by the way.)  Too bad.  Our 100+ patient 
Cold Room will not break-even with our current system until we have 20 
whole-body patients in it.  Our present LN2 patient load would have 
put us halfway there.  We are also still stuck with the problem of 
vault protection for our LN2 whole-body patients.  
 
 
Perry Metzger:
 
> Hmmm. What figures are you using for the thermal losses in the room?
> Presumably, the better our insulation, the lower our annual cost but
> the higher our capital costs. I realize that you have already
> described this a couple of times before, but it would be good to to
> bring it up again.
 
        A couple of times?  The discussion of room cooling and 
insulation cost optimization (including my computer program and 
spherical room approximation) runs over 50K!  Seriously, though, I 
have since developed a very simple, common-sense, conservative 
approach to room heat flow calculations.  Simply use the Newton 
formula for heat flow though a infinite planar slab of insulation, to 
wit:
 
        Heat Flow = K * (Area) * (Temp Diff) / (Insulation Thickness)
 
where K is the thermal conductivity of the insulation.  Use the K 
value at room temperature (its most pessimistic value), and instead of 
using the inside surface area of the room (with fancy "spherical room" 
correction factors) just use the *outside* surface area of the 
insulation.  This will give a worst-case upper bound on the heat flow 
regardless of fancy gradient effects beneath the surface.
 
        Our prototypical 5m x 5m x 3m interior Cold Room with 1 meter 
foam insulation on all sides will have an outside surface area of 189 
square meters.  The room temp K value of Trymer foam is 0.021 
watts/m/degC (calculated from the spec sheets kindly faxed to me by 
Hugh Hixon).  And the temperature difference between -130'C and 20'C 
is 150'K.
 
        0.021 * 189 * 150 / 1  =  595 watts
 
Because of all the pessimistic assumptions being made, I think 500 
watts is a round, reasonable, and still-conservative figure, and this 
is what I have been using in my calculations.
 
        As you may recall, I made very detailed calculations balancing 
off capital amortization and on-going operating costs to arrive at a 
cost-minimizing insulation thickness.  For an LN2 cooled room of the 
size we are considering, this thickness was 1.4 meters.  I cut this 
down to 1 meter because it didn't make that much difference.
 
        With cooling costs even lower now with the refrigeration 
system we are considering, a further reduction in insulation thickness 
might be indicated.  I'm not going to do it for the following reasons: 
1) Reducing insulation thickness increases thermal ballast 
requirements.  2) Reducing insulation thickness increases emergency 
backup LN2 requirements.  3) I'm tired of doing complex optimization 
calculations that don't make much difference.  Let's just use one 
meter.
 
> Oh, and there is one other problem we have to deal with, which is
> sudden warming of the room during patient loading/unloading. These
> things don't chill instantly. The chillers will only remove 500W,
> which means that cooling down patients to "cold room" temperature
> and loading them without disturbing the other patients is still a
> problem.
 
        Don't worry about it.  Patients will be cooled to -130'C in 
our present dewar system before they are put into the room.  If you 
didn't do this, they would warm up the patients beside them (not good) 
as they cooled regardless of the room cooling capacity.  Also, -130'C 
air is twice as dense as room temp air.  It will *want* to stay down 
there when you pull a cell cap off.  Even if you deliberately push a 
little warm nitrogen into the room to maintain positive pressure of 
dry gas as you pull the cap off, it still doesn't matter.  The heat 
capacity of air is so low that you could replace all the -130'C air in 
the room with 20'C air and the rest of the room would warm only 1'C 
before all that air was down to -129'C.  Best to leave things alone 
rather than crudely spray LN2 all over the place.
 
> Given the available budget, we may easily be able to afford to get a
> couple of the units for redundancy and a couple of diesel generators
> and we'd be home free! This sounds great!
 
        I hope the Board is as enthusiastic when we present them with 
this proposal.  Patient Care Fund money (which would have to pay for 
all this) is pretty sacred.
 
>> For example it would take 5000 liters to keep the room cold during one 
>> week of refrigeration failure.  Volume considerations alone suggest we 
>> would have to settle for less than this.
 
> Would we necessarily? I don't know what the density of Ethyl
> Chloride is (my CRC handbook is at home), but 5000 liters of water
> is just five cubic meters, which is not THAT big a space.
 
        I was sleep deprived when I wrote that.  Yes, we would could 
get in 5000 liters of ballast quite easily.
 
> We can also operate exactly AT the freezing point of the balast and
> gain temperature stability from it that way.
 
        No way.  You can never operate exactly at your ballast 
transition temperature.  If your refrigerator one day starts pumping 
450 watts instead of 500 watts, your ballast could completely melt and 
you'd never even know it.
 
> I suspect that keeping the temperature of the
> patients highly stable is a desideratum -- it will minimize thermal
> stresses, for one thing.
 
        We are working on this problem.
 
 
Steve Harris:
 
> 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.
 
        This is a very neat idea, but isn't it needlessly complicated 
compared to an air circulation system (which we will probably need 
anyway)?
 
        I envision a single cell at the room periphery holding a tall 
metal cylinder with lots of vertical heat transfer fins.  This unit is 
the room heat exchanger.  It makes good thermal contact with the 
cooling coils of the refrigerator, and an insulated tank in the middle 
of the cylinder stands ready to hold LN2 for emergeny backup.  The 
insulation is calculated to sink 500 watts between -130'C and LN2 temp 
if the tank is filled with LN2.
 
        Air passes through the heat exchange cell from the bottom up.  
At the top of the cell are three replaceable, redundant fans with 
cryogenic-rated electric motors.  The fans suck air up through the 
cell and drive it into a cell-to-cell maze pattern that ensures air 
flows over the top of every cell in the room.  (Borrowing an idea from 
Steve, I think the tops of cells should have a metal cap with thermal 
ballast on top.  The air circulation maze exists in the space between 
these tops and the foam cap above.)  At the end of the maze, air flows 
into a duct leading downward to ductwork under the metal floor of the 
room.  Thermal ballast is put in this space as well.  This ductwork 
finally connects with the bottom of the heat exchange cell, making a 
complete circuit.
 
        This design would hold 24 x 6 = 144 patients in our 5m x 5m 
room.  Thermal ballast only exists at the cell tops and under the 
floor of the room, leaving maximum space for patients inside.  The 
outer walls of the room are 1 cm aluminum, giving a maximum temp 
difference of 1'C along the outer wall.  We can afford to buy this 
much aluminum because the cell walls within the room can now be 
arbitrarily thin.        
 
    More on this design tomorrow (if it isn't shot down by then :)  ).
 
 
To Richard Schroeppel:
 
        Thanks for the Arizona electricity cost info.  The savings 
between 8.25 cents/KWhr and my assumed 10 cents/KWhr will probably pay 
for building air conditioning (see my "Hidden Cost" message) leaving 
my orginal estimates unchanged.
 
        By the way, it was never my intention to use the cost of 
electricity as a criterion for chosing a facility location.  It's 
probably the last thing we need to worry about.  I just wanted to make 
sure my operating cost estimates were in the ballpark.    
 
> You should be able to look at the age & health of the current Alcor
> membership and form an actuarial estimate of suspension rate; don't 
> build too much overcapacity.
 
        Actuarial analysis of the current membership is useless for 
determining the suspension rate since a large portion of suspensions 
are "last-minute" signups.  A simpler and better method is to just 
assume that the suspension rate will grow in proportion to the 
membership size (which grows at about 30% annually).  I will crunch 
these numbers when I can find time.
 
> (Moving the LN2 dewars into -130C would be *some* saving, until
> your room filled up.
 
        I would never put dewars in the Cold Room.  Besides the 
logistics hassles (bigfoots wouldn't fit, and neuro dewars would use 
space inefficiently) they would endanger the -130'C patients.  A dewar 
vacuum failure would fracture -130'C patients in the vicinity (by 
turning them into -160'C patients).
 
        On the other hand, if we end up building an LN2 Cold Room, we 
could put existing neuropatients in the reservoirs!  Hmmmm.  Maybe I 
should do some design calculations on a refrigerated Cold Room that 
has some LN2 reservoirs in the center for the specific purpose of 
holding Alcor's existing patient load.  The LN2 used to maintain these 
patients would then directly reduce our refrigeration power 
requirements, a bunch of floor space would be freed up, and Alcor's 
whole-body patients would have vault protection!
 
> I think you've settled on EtBr too fast, and should explore
> mixtures.  They'd let you set your melting point at exactly
> -129.5C or whatever.
 
        Being a physicist and not a chemist maybe I'm naive, but I 
thought that mixing two liquids with different freezing points just 
gives an unsharp phase transition.
 
                                                --- Brian Wowk

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2089