X-Message-Number: 2253 Date: 15 May 1993 12:33:40 -0700 (MST) From: Subject: CRYONICS dewars in cold room Dear Brian: After mulling over your comments in Cryomsg #2244 about the possible dangers of putting our present dewars in a future cold room, I have to say that I don't think the problem you mentioned is real. You constructed a scenario wherein the dewar, inside the room, lost vacuum and therefore started to boil off its LN2 at a high rate, sucking heat from its surroundings, and therefore causing a drop in temperature in the rest of the room. This process, if allowed to proceed unchecked to thermal equilibrium, would indeed lower the core temperature of the patients below -135C, with a resulting devitrification of those patients. But both the time scale of these events and the magnitude of the resulting temperature drop you claim will occur are not supported by your reasoning or the evidence you present in the Cryomsg. *"If a Bigfoot failed inside the room, you [the patient caretaker] would have all of five minutes to lift a three ton -196C metal object out of the room before adjacent patients were cooled dangerously." Firstly, removal of the dewar is not necessary to halt the sudden cooling process--only removal of the rst of the LN2. This can be done by sucking the LN2 out of the dewar with a pipe dipping into the dewar; such transfer plumbing is needed in any case to add LN2 to the dewar and to remove LN2 on a regular basis to keep the concentration of LOX at acceptably low levels, if that is deemed to be necessary. Futhermore, the time scale you mention (five minutes) is unjustifiably short in my opinion. Air is not a good heat transfer medium: try cooling a bottle of wine by putting it in your home refrigerator's freezer compartment. (The temperature difference between the room temperature bottle at 25C and the inside of the freezer at 0C is similar to the difference between the pateints at -135C and the cold nitrogen vapor at -196C.) The time to cool the bottle is measured in hours, not minutes, provided you don't bury the bottle in ice or frost! In addition, our patients are stored inside double sleeping bags, lowering the termal conductivity between their bodies and the surrounding air of the cold room. So a mad dash to lift a 6000lb dewar out of the room in less than five minutes is uncalled for. Appropriate action would instead be to pump the LN2 out of the failed dewar sometime within an hour of failure. And if things really got out of hand, a little warm air from outside the room could be brought in. All of these actions could be taken by a single competent caretaker. It is true that removing the dewar from the room, and transfer of the patients within it, would be the work of a several people, but this repair work would not be done in a crisis setting, since (unlike the situation with a dewar outside the room) the patients in the dewar would not be in danger of thawing--they could get no warmer than the -135 temperature of the cold room. *"Putting fragile dewars in the coldroom, with the associated requisite of constant human monitoring, is not in keeping with the dsign philosophy of a room that would otherwise be failsafe." --- "The question of human supervision is almost acdemic." I do not consider the proposed cold room to be "failsafe". The room must be held at cryogenic temperatures by active means: there is no way around this requirement, since it is in a nonequilibrium state with the surrounding environment. The machinery to maintain the cold temperatures whether onsite in the form of a refrigeration system or offsite in the form of an LN2 plant, is subject to breakdowns and therefore must be watched over at all times by a human being. If your design goal is to eliminate the need for a human caretaker, you will have a tough time convincing me as an Alcor Director of the feasibility of that goal. *"Putting dewars......inside the room, while conceptually elegant, is in practice too complicated and dangerous to consider." Since we have no practical experience at putting dewars inside a cold room, the statement that "in practice [this is] too complicated" is unsupportable. Therefore the only way to evaluate the proposal is in fact to consider it. *"A vacuum-failed dewar would suck heat from the room at the ferocious rate of 10kW." This is a nice round number. Do you have any calculations or data to back it up? *"Vaporizing LN2 in pipes under the room would be an explosion hazard." As pointed out above, the transfer pipes are needed whether the dewar is stored at 25C or -135C. LN2 is not an explosion hazard unless it is confined to an enclosed volume while allowed to boil. Pipes are not enclosed volumes: they have a hole at each end. Finally, in my opinion, words such as "ferocious" and "failsafe" do not belong in an engineering vocabulary. They play more to the reader's emotions than to his or her reason. Should Alcor decide to keep its pre-coldroom patients at -196C even after building a coldroom, I think that we should consider putting their dewars into that room, for reasons of safety to those patients and lowered LN2 costs. No arguments have been presented yet to convince me otherwise. Thank you, Brian, for your work so far on this concept. Sincerely, Mark Voelker Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2253