X-Message-Number: 2251 Date: Sat, 15 May 93 10:37:48 CDT From: Brian Wowk <> Subject: CRYONICS Dewar Insulation! In my last message I wrote that a vacuum-failed Bigfoot dewar in a -130'C coldroom would draw heat at a rate of about 10kW. While we're on the subject, let's discuss the problem of vacuum failure *outside* the coldroom in a typical room-temperature cryonics facility. Because of the greater temperature gradient, we can expect a failed Bigfoot dewar in Alcor's patient care bay to draw heat at 30 kilowatts instead of 10. This is very, very serious. The outside of the dewar would quickly cool to something near -190'C, creating an extreme burn hazard and "cold zone" near the dewar because of thermal radiation disequilibrium. LN2 in the dewar would boil at about 10 liters *per minute*, generating 10,000 gaseous liters of nitrogen per minute-- enough to pose an asphyxiation risk to personnel. In fact the entire patient care bay (if sealed) would fill with nitrogen in under an hour. During this time the dewar LN2 level would drop by one meter, and begin to expose patients. One hour is thus the maximum time available to transfer four Bigfoot patients to other dewars following a vacuum failure. Consider that the most likely time for a dewar failure is during an earthquake. Can you imagine a worse time to be faced with an emergency patient transfer than after your facility (and possibly staff) have been trashed by an earthquake? What can be done about this? Well, all the above heat flow calculations assume free convection around the outside steel wall of the dewar. It turns out that adding as little as 1 inch of foam insulation around the dewar will reduce heat flow during a vacuum failure by a factor of *TEN*. This means one tenth the hazard, and ten hours to transfer patients instead of one hour. ***>> I urgently recommend that any cryonics organizations storing patients in vacuum dewars cover the sides and undersides of dewars with at least one inch of foam insulation (or several inches of fiberglass). The shiny steel dewars will no longer look as pretty, but they will be much safer in an emergency. The insulation will also provide modest mechanical protection against shocks and projectiles, thereby reducing the risk of a vacuum failure in the first place. <<** Getting back to the coldroom, it is now apparent that putting at least neuro dewars in the coldroom may be feasible after all. Since even modest amounts of foam insulation reduce vacuum-failure heat flows dramatically, these dewars need no longer pose a risk to coldroom patients. An XLC-1520 neuro dewar with one inch foam insulation around it would draw less than 100 watts from a coldroom if its vacuum failed. If the dewar was situated so that the room airstream circulated around it, a vacuum failure would simply put the room into de-facto LN2 backup mode. The room thermostat would automatically reduce the mechanical refrigerators from pumping 500 watts down to 400 watts to compensate. With LN2 boiloff going at about 50 liters per day (ten times normal room temp boiloff), staff would have up to several days to remove the dewar and transfer patients. The LN2 savings of coldroom-stored dewars are potentially dramatic. Conductive losses would be reduced by 75% due to the smaller temperature difference and reduced mobility of residual gas molecules in the dewar vacuum. Radiative losses (proportional to the 4th power of ambient temperature) would be reduced by 95%. Depending on the relative contribution of radiative losses, we can expect at least a 75% LN2 saving by storing dewars in the coldroom. For two neuro dewars, this adds up to savings of $1000 per year, which would pay 20% of the coldroom electric bill. There would also be a floor space saving because the bulky concrete neuro vaults would no longer be needed (the coldroom itself is a vault). My only remaining concern is the added design complications, which may negate the $1000 annual savings. We shall see. --- Brian Wowk Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2251