X-Message-Number: 1999 Date: Sat, 20 Mar 93 17:45:00 CST From: Brian Wowk <> Subject: CRYONICS No moving parts! Yesterday I outlined how a -130'C storage room can be made failsafe by replacing the LN2 reservoir dewar with a lightly insulated steel drum. Today I will outline how to make the room even more failsafe by dispensing with the need for mechanical devices (thermostat solenoids, recirculators, etc.) entirely. One potentially serious problem with vapor cooling systems is the problem of temperature non-uniformity, or stratification. Warm air is less dense than cold air. Therefore the air inside any sealed room will tend to stratify, with colder air settling at the bottom of the room and warmer air at the top. Mike Darwin has proposed electric stirring fans to prevent this. There are problems with electric fans. A 200 square foot operating cost-minimized room (one with 2 meters of foam insulation) will boil 70 liters of LN2 per day. This corresponds to a heat flow into the room of about 150 watts. This means that the power dissipated by a mere 50 watt fan will increase LN2 consumption by 30% ! As an alternative, consider reducing the two meter insulation thickness underneath the room to one meter. (This will make for easier access anyway.) LN2 consumption will increase by about 10%, and the warm floor (compared to the ceiling) will cause convective air circulation. This effect might be enhanced by covering the floor and ceiling with metal. Note that for the price of increased LN2 consumption, you can make the convection as vigorous as you want by continuing to reduce the thickness of the foam under the room. You can even recover some of your LN2 efficiency by moving that foam above the room (giving 3 or more meters of foam thickness over the room). So don't use fans! There is no need for them. What else can be done to passively combat stratification? An insulated pipe should carry nitrogen vapor from the LN2 resevoir to a network of pipes on the ceiling of the room. The boiling LN2 in the 200 square foot room (Should I now call this the Standard Room?) will produce about 1 liter of gas per second. This gas should be vented uniformly over the whole ceiling through tiny holes in the pipes. By venting cold gas at the ceiling we further enhance convection. Mike Darwin suggested using the nitrogen gas flow from the boiling LN2 to power fans to stir the room air. There is a physics flaw in this idea. The flowing gas already has its own kinetic energy that will stir the room air as it is vented. By reducing flow energy in the vented gas to power fans which in turn increase air flow elsewhere you gain nothing. Having dispensed with the need for fans, what about the mechanical complexities of the temperature control system? I suggest the following: Begin with an insulated barrel of LN2 (unpressurized) as I have already discussed. The only way for gas to get out of this barrel is through an insulated pipe leading to the ceiling vents outlined above. The insulation around the barrel is adjusted so that the instrinsic boiloff rate will keep the room at -110'C (failsafe mode). At the bottom of the barrel, immersed in the LN2, you have a electric heating element. An electronic thermostat (copper-constantin thermocouple) controls the current to the heating element. Applying current to the element increases the LN2 boiloff above the intrinsic level, causing the room to cool to the desired level of -130'C (or whatever). How does electrically boiling LN2 cause cooling? Well, the heat of vaporization is supplied entirely by the electricity, so you get no cooling from that. However, the extra boiling vents cold -196'C gas into the room, and *that* is where you get your cooling. The system may seem inefficient, but it is not. Remember that you are actively controlling only about a few percent extra boiloff beyond the intrinsic amount, so the inefficiencies in this few percent do not significantly affect overall efficiency of the room. Voila! A -130'C storage room with air circulation, active temperature control, and not a single moving part! --- Brian Wowk Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=1999