X-Message-Number: 2142
Date: Thu, 22 Apr 93 00:23:52 CDT
From: Brian Wowk <>
Subject: CRYONICS Frost Control

        Frost, as Steve Harris points out, is not a problem that can 
be dismissed cavalierly (at least not as cavalierly as I once 
thought).  Unless we handle this problem very comprehensively, we will 
be in big trouble.  Consider, for example, the consequences of the 
inaccessible under-floor air circulation space clogging with frost!!
 
        The first thing to realize is that frost does not accumulate 
equally well on all surfaces.  High conductivity, high heat capacity 
surfaces like metal accumulate frost like crazy.  In contrast, low-
conductitivity surfaces like wood or foam hardly gather frost at all.  
This is because these surfaces quickly warm to the temperature of air 
that touches them, without grabbing a lot of heat (and thereby 
condensing a lot of water) in the process.
 
        I therefore suggest that every exposed surface in the room 
(walls, ballast tops, patient packaging) *except* the heat exchangers 
be covered with one to two inches of foam.  This will allow the 
cryocoolers to perform the function they are normally sold for: 
cryopumping of water.  Any water vapor in the room will preferentially 
condense on the heat exchangers instead of anywhere else.  Since there 
are two of them, the heat exchangers can be periodically removed and 
defrosted.
 
        Having all surfaces except the heat exchangers mildy insulated 
also means that the entire room can be literally defrosted from time 
to time.  With your refrigerators and fans going full blast, you 
activate heating elements downstream of the heat exchangers that raise 
the air temp to 0'C or more (ouch, that's hot!).  Frost inside the 
room rapidly sublimes and gets collected on your cold heat exchangers.  
With two inch thick foam on everything, you could do this for a full 
hour with only a 1'C rise in the ballast and patients.
 
        I still don't have a good quantitative feel for this problem.  
What I need to do is calculate an upper-bound frost accumulation rate, 
and a lower-bound on how fast we could remove it.  I will of course 
keep our faithful net readers abreast of my progress in this area.  In 
the meantime, I think the general strategy is sound.
 
                                        --- Brian Wowk

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