X-Message-Number: 2085
Subject: CRYONICS Mechanical Mania
Date: Thu, 08 Apr 1993 20:14:05 -0400
From: "Perry E. Metzger" <>


> From: Brian Wowk <>
> 
>         Yesterday I talked to a company called Polycold in California.  
> (Please, let's not all phone them.)  They sell a 3-stage closed-cycle 
> circulating fluid refrigeration system that will pump 500 watts from 
> -133'C to room temp for an input power of 5kW (10% coefficient of 
> performance).  The capital cost is $16,000.  The system is maintenance 
> free, and has no known mean-time-before-failure.  The sales rep compared 
> the system reliability to that of a "household refrigerator", which is 
> pretty good (MTBF measured in decades).

Hot damn! Or should one say, prehaps, cold damn!

>         So this is beginning to sound interesting.  Cooling our cold 
> room with this unit would consume 44000 kW-hours in one year.  At 10 
> cents per kilowatt hour this represents an annual operating cost of 
> $4400.

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.

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.
Perhaps the LN2 still has a place -- as a way to partially chill down
the room on rare occassions where the air warms up because you have to
open the door for an extended period. One might carefully aerosol LN2
into the room to cool it down if you find yourself having to open the
vault door for a prolonged period.

>         In any case it looks like mechanical refrigeration (if it really 
> is maintenance free) could save $9000 a year in operating costs.  This 
> in itself would justify an extra capital cost of up to $100,000 
> amortized over 15 years.  Then there are the additional advantages of 
> electricity instead of LN2 which have already been discussed.  
> Substituting capital cost for operating cost is also an excellent hedge 
> against inflation.

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!

Other problem I see: -133C (min operating temperature listed above) is
three degrees(!) shy of the freezing point of Ethyl Chloride at -136C
(see more below).

>         One of the big capital costs in a non-LN2 system will be the 
> thermal ballast material.  There appear to be two possibilities (thanks 
> to Steve Harris for locating them):
>  
>                 Ethyl Chloride          Ethyl Bromide
>                 --------------          -------------
> Formula         C2H5Cl                  C2H5Br
> Mol. wt.        64.52                   108.98
> Melting Point   -136'C                  -119'C
> Boiling Point     12'C                    38'C
> Heat of Fusion    69 J/g                probably similar
>  
> Because the heat of fusion is only one third as much per liter as the 
> heat of vaporization (and warming to -130'C) as LN2, it takes a lot of 
> Ethyl Chloride/Bromide to get a decent safety factor in a Cold Room.  
> 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. In fact, taking up that
much volume is a GOOD thing in some ways. The larger the percentage of
the vault taken up with solids and thermal mass and the less taken up
by gas, the smaller the amount of gas that can be displaced when the
vault door is opened and the greater the temperature stability under
those circumstances.  Especially if we have "passive" ballast (see
below) simply filling up containers throughout the room, we might be
able to fill all sorts of nooks and crannies with the stuff. Prehaps
the patients can be surrounded by a form fitting container of the
stuff in slurry, stabilizing the patient's temperature, providing a
place to put some of the ballast, and keeping the patient safe from
transient temperature fluctuations in the room.

Again, of course, these figures all depend on the figures you are
using for size of vault, heat loss, etc.

>         Steve Harris and others have advocated Ethyl Chloride as the 
> ballast material of choice.  I suggest that Ethyl Bromide is better if 
> we can afford it.  My reasoning is as follows:
>  
>         A Cold Room can operate either above or below the freezing point 
> of the ballast material.

We can also operate exactly AT the freezing point of the balast and
gain temperature stability from it that way.

> Operating above the freezing point is tougher 
> and less flexible because the ballast must be insulated, and the room 
> operating temperature is essentially fixed by the thickness of the 
> ballast insulation.  Also the refrigerant must be circulated through 
> each and every ballast location.  (This would not even be doable in a 
> hybrid thermoelectric system such as Steve has recently suggested.)  
> This means complicated plumbing and limited distribution of heat sink 
> locations.  By contrast, consider a Cold Room operating at or below the 
> freezing point of the ballast.  You could stick uninsulated ballast 
> material everywhere (between the walls, under the floor, on top of the 
> patients--- everywhere and anywhere).

My suggestion would be that by storing at exactly the freezing point
of the balast (-136 for ethyl chloride, which seems like a better
temperature), we can use a passive system with the balast simply
placed in sealed containers throughout the vault as you advocate and
gain temperature stability because the room will have physics reasons
for trying to stay at the freezing point (good old Le Chateliers
principle saves the day in a slurry at equilibrium between melting and
freezing: if the slurry is chilled, it will release heat in partially
freezing, tending to keep the room from dropping in temperature -- if
the slurry is warmed, it will absorb heat in partially melting,
tending to keep the room from rising in temperature). Although I have
no evidence for it, I suspect that keeping the temperature of the
patients highly stable is a desideratum -- it will minimize thermal
stresses, for one thing.

>         Finally there is the boiling point issue.  Ethyl Chloride boils 
> at 12'C which would make it very difficult to handle during the 
> construction phase.  This stuff probably ships packed in ice or 
> something, and would have to be transferred to numerous small 
> pressurized containers for distribution throughout the room.  Numerous 
> pressurized containers filled with volatile organics that badly want to 
> boil away make me nervous.  Ethyl Bromide by contrast boils at 38'C.  
> Although still very volatile and odorous (about equivalent to diethyl 
> ether) it could be poured at room temperature into unpressurized 
> containers.

I don't think you would want to handle either of those without special
conditions. I vaguely recall that Ethyl Bromide is a highly toxic
substance used in fumigation, and I would guess that Ethyl Chloride
likely is toxic, too. I suspect it is time to search the CRC handbook
for as many candidates for balast as possible.

Perry

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