X-Message-Number: 9971
From: 
Date: Fri, 3 Jul 1998 12:18:39 EDT
Subject: fiberglass cryostats

Thanks to Doug Skrecky for his many posts on topics of interest.

A couple of comments regarding his recent post (below):

First, Doug's wording seems to suggest a slight misunderstanding concerning
CI's cryostats. He seems to have the impression that CI units require pumping
because of permeability of the fiberglass. 

Actually--in our opinion--the tendency of the vacuum to soften is primarily
related to outgassing from the resins in the outer shell (the warm side). This
diminishes over time. For example, one unit (holds 6), our oldest rectangular
unit, is about 8 years old. At first we needed to run the pump about 8 hours a
day; now about 8 hours per month. 

Also, there is not much sensitivity of heat conductivity to vacuum changes, in
the range 1 micron to 10 microns. If the vacuum gradually softens from one
micron to ten microns, there is no appreciable increase in boiloff.   

I should look at the literature on tin foil embedded in fiberglass, but if its
only purpose is to reduce permeability, it does not seem important to us.
---------- Doug wrote:
Message #9967
Date: Thu, 2 Jul 1998 20:38:21 -0700 (PDT)
From: Doug Skrecky <>
Subject: gas permeation of fibre reinforced plastics

The following quote is from Cryogenics 38(1): 143- 147 1998. I understand
CI's fiberglass cryostats require periodic pumping to maintain a good
vacuum. There may be a way to avoid this, and at the same time increase R
value due to maintaining a more rarified vacuum - as follows.

Quote:

  "At room temperature, the gas permeability of fibre glass and carbon
fibre reinforced epoxy resins cannot be neglected; however, at cryogenic
temperatures it nearly disappears completely. In a temperature range down
to 77 K, the tested composite samples did not show formation of cracks
inside the fibre/matrix bond, which would lead to an increase of
permeability. Thermal and mechanical cycling did not make a large effect.
Fibre reinforced epoxy resins with tin foil embedded are materials suitable
for building storage and transportation vessels for liquefied gases; it is
resistant to thermal and mechanical fatique, and fully impermeable to gases
(at room temperature).

....However with aluminium foils delamination occurred under thermal and
mechanical cycling. Excellent results, by contrast, have been observed for
composites with tin foils. There is an excellent adhesion between tin and
epoxy matrix, which is resistant to fatigue loading. Thus, this material
seems to be a promising candidate for building vessels for liquefied
gases."

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