X-Message-Number: 2140
Date: 21 Apr 93 11:13:35 EDT
From: "Steven B. Harris" <>
Subject: Bifrost? Asgard? Valhalla?

Dear Cryonet:

   I hate to be a wet blanket, but I want to return to a big
potential problem with our fancy -135 C cold-room that has only
been briefly touched on before in this thread: at -135 C it's
going to be difficult if not impossible to design our room to be
"frost free."   Ice just has too little vapor pressure until you
get to temperatures much higher than we want to operate at.  Thus
we are inevitably doomed to accumulate frost on everything in the
cold-room, from moisture present in ambient air introduced in
operations, until everything is totally choked and encased in it
like the end state of the old ice box freezers of days gone by,
when they weren't cleaned (anybody remember them?).  Being
careful with air humidity and room seals and access times will
delay this problem, but will not prevent it from eventually
occurring in full and bothersome messiness.

   Anybody see any way out of this?  Other than the obvious (and
really nasty) solution of designing the thing in sections such
that patients can be moved out of one section to others while it
is warmed up and defrosted?  

   Here's the best I can do at the moment: to minimize frost
formation on ballast and surfaces in contact with the cool
airflow, we need to change the design somewhat from what we've
been looking at: in particular it may be necessary to have all
ballast containers stored in one or more larger hermetically
sealed bays to minimize surface area on which frost will deposit
(in fact, we may have to go with ballast only in one layer on the
false floor, and skip any ballast between patients).  Also, there
should be no air access at all from the over-room into the
general air coolant flow of the cold room:  patients should go
into pre-positioned aluminum tubes instead of directly into the
coolant air flow, with air access from the room above only into
the interior of these storage tubes (we can defrost the interiors
of each of these one at a time by moving patients).  All this
will cut way down on frost deposition in the "interior" of the
cold-room (i.e, on surfaces in contact with the cold air flow),
but not stop it.  To deal with eventual frost buildup inside the
room we'll either have to blow in hot air (as I think Mike D sug-
gested for an early walk-in cold-room model), and/or have an
electric grid on all inner cold-room surfaces (another reason to
minimize areas and sequester ballast behind a plate), which, with
the coolant air flow off, could be heated rapidly enough to melt
frost, but not transfer significant heat to patient pods or
ballast (we'd have some insulation under the grid, but little
over it).  

   Darn if I know in either case how we'll get the melted water out.

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