X-Message-Number: 13852
Date: Tue, 6 Jun 2000 14:13:55 EDT
Subject: cryostats

Jeff Davis (#13844) had suggestions as to cryostats and economies of scale. 
However, his analysis had some errors and oversights. The following may have 
some small utility in informing relative newcomers.

First of all, in ordinary commercial cryogenics, there are (or were last time 
I looked) three standards. For small units or for transport, the standard is 
high vacuum with multiple radiation barriers, the so-called super-insulation, 
such as made by MVE and used by Alcor. For somewhat larger units, the 
standard is perlite (a mineral powder) with moderate vacuum, used by Cryonics 
Institute. For very large units, the standard is perlite in air, or else foam 
insulation like styrofoam. 

As the size gets larger, the conductivity of the insulation becomes less 
important. Eventually, as Jeff says, economies of scale will reduce liquid 
nitrogen cost per patient, regardless of the system used. The question is 
what can be done in the near and intermediate term to save money, in addition 
to keeping large temporary storage units on site rather than constantly 
buying small batches.

One tantalizing prospect is the advent of rigid open-cell foam, with 
sufficiently small cells, allowing evacuated load-bearing insulation as good 
as evacuated perlite. There have been several claims over the years that this 
was achieved or imminent, but every one we tried was defective.

As for making somewhat larger cryostats, if you are talking about cylinders 
of the MVE type, prospects are very limited. Alcor's "Big-Foot" holds four 
whole bodies, I believe; Trans Time's "King Kong" held 10, but was defective. 
Beyond four, one might go to seven (draw yourself a sketch) as the next 
logical step, but what would the savings be? For a cylinder, disregarding the 
ends, the lateral surface area is 2piRH and the volume is HpiR^2. If we 
assume the height remains constant, and going from one patient to seven means 
tripling the radius, then the lateral heat leak is multiplied by 3 while the 
payload is multiplied by 7, so the lateral heat leak per patient is 3/7 of 
the original, a nice saving. However, if you try to take another step up you 
run into a lot of problems, and it probably isn't worth it.

Jeff also misspoke himself or erred in a couple of places. For one, he said

>with two cryostats, one ten times as large as the other, the boil-off per 
patient-->assuming that the number of patients inside the larger cryostat is 
>larger: ten times as many--the cost per patient for liquid nitrogen
>replenishment would be reduced by a factor of ten.

The lateral heat leak is proportional to the radius, and the volume is 
proportional to the square of the radius. To multiply the gross volume by 10 
you multiply the radius by the square root of 10 = 3.16, increasing the 
lateral heat leak by a factor of 3.16, and improving the ratio of payload to 
lateral heat leak by a factor of 3.16--nice again, but not 10, and this is 
idealized, disregarding the end caps and supports. 

Also, Jeff said you don't need stainless steel for the inner cylinder. 
Actually, you do need either stainless steel or aluminum, if you want to stay 
with the MVE type unit, because most other metals--most steels, for 
example--cannot be used in contact with liquid nitrogen.

CI, as noted, uses perlite with moderate vacuum, of fiberglass construction, 
fabricated on site, very rugged, and we have used both cylindrical and 
rectangular units. Our latest and largest is rectangular and holds 14 
patients. We can make rectangular units as large as we please, without 
running into the limitations of MVE cylinders, although the economies of 
scaling up are not a simple proportionality.

Robert Ettinger
Cryonics Institute
Immortalist Society

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