X-Message-Number: 2080
Date: Thu, 8 Apr 93 12:01:07 CDT
From: Brian Wowk <>
Subject: CRYONICS Mechanical Mania
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).
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. By contrast, cooling our cold room with LN2 would burn 67500
liters annually. At 20 cents per liter for bulk purchased LN2 this is
an annual operating cost of $13,500.
What I really need to nail down these numbers firmly is the cost
of electricity and liquid nitrogen in Arizona and Nevada. Could any net
readers in these states kindly post the per-kilowatt-hour rate from
their last electric bill on the net? Is there much difference between
business and residential rates? Also, could someone please find out how
much liquid nitrogen would cost in major cities in these states if we
had 1000 liters delivered once a week?
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.
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.
Ethyl Chloride is used in substantial quantities industrially to
make tetraalkyl leads (paint, I presume). Ethyl Bromide is more exotic,
but perhaps we could get a batch custom-cooked for us since we will be
buying it by the ton. Maybe Steve could look into this.
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. 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). Your refrigerant or
thermoelectric modules need only make contact with one central heat
exchanger located near the air circulation fans. If the fans fail, or
heat exchanger is removed for servicing, then the temperature slowly
rises (with great uniformity) to the melting point of the ballast and no
higher. Finally, if operating below the ballast freezing point, your
room temperature can be easily adjusted. (Suppose, for example, that
cryobiologists discover five years from now that it is better to store
at -128'C instead of -133'C.)
It is my impression that the yet-to-be-decided optimum storage
temperature lies between -140'C and -120'C. Ethyl Bromide would allow
us to cover this entire range, while Ethyl Chloride cannot be used above
-135'C for the reasons above.
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.
--- Brian Wowk
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