X-Message-Number: 1970
Date: Tue, 16 Mar 93 14:03:54 CST
From: Brian Wowk <>
Subject: CRYONICS Dewar Construction

Darran Edmundson:
 
> I have a few questions.  Firstly, do LN2 costs make up a significant
> fraction of Alcor's operating expenses?
 
        LN2 costs are only a couple of percent of Alcor's operating 
 expenses. They are, however, almost half of the patient care expenses.  
 It is important to understand that Alcor operations (membership 
 administration, public relations, magazine production, etc.) are 
 financially segregated from patient care.  So when you hear about Alcor 
 being in a "cash crunch," this is in operations, not patient care.  
 Patient care is extremely well funded.  Nevertheless, any improvements 
 we can make in this area will improve security even further and help 
 hold down funding minimums for new members coming in.
 
> If so, does the 3 litre figure represent a high degree of thermal
> isolation?  (For comparison, what is the boiloff
> rate of LN2 from a typical styrofoam coffee cup scaled to the dewar
> volume size?)  Are Alcor's dewars constructed like large thermoses?
> That is, two concentric shells with the inner one thermally
> isolated from the outer protective shell by means of a partial vacuum.
 
        A styrofoam coffee cup scaled up to the size of a Bigfoot dewar 
would have styrofoam (actually expanded polystyrene) walls 5 centimeters 
thick, would and boiloff 700 liters of LN2 per day compared to the 
Bigfoot's 13.  So as you can see, Alcor's dewars are pretty efficient.
 
        Partial vacuum, by the way, is a useless insulator since 
(amazingly) the thermal conductivity of gases is independent of density.  
Not until the mean free path of the gas molecules becomes comparable to 
the wall separation does the conductivity suddenly drop.  Alcor's 
cryogenic dewars are two concentric stainless steel shells with a *high 
vacuum* in between.  In fact, the vacuum is so high that even trace 
amounts of helium in the atmosphere can diffuse through the steel walls 
and ruin the vacuum enough to increase the LN2 boiloff rate.  (Please 
don't bring helium balloons into a cryonics facility!)  In addition, the 
vacuum space is also filled with "superinsulation."  Superinsulation 
consists of multiple layers of reflective metal foil that reduces 
radiative heat transfer to near zero.  The net result is a ~1cm thick 
wall that insulates as efficiently as about 3 meters of foam.
 
                                                --- Brian Wowk   

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=1970