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 
                                                --- Brian Wowk

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