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 Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2142