Dewar vessel insulation

Critical heat production rates (i.e., heat production rates that still do not lead to a runaway), are often determined by small scale experiments. However, the effect of scale-up on these rates, as discussed in [161], must be taken into account. An indication of the effect of scaling in an unstirred system is shown in Figure 3.2. In this figure, the heat production rate (logarithmic scale) is shown as a function of the reciprocal temperature. Point A in the figure represents critical conditions (equivalent heat generation and heat removal) obtained in a 200 cm3 Dewar vessel set-up. It can be calculated from the Frank-Kamenetskii theory on heat accumulation [157, 162] that the critical conditions are lowered by a factor of about 12 for a 200 liter insulated drum. These conditions are represented by... 

A practical container for holding coolants is the cylindrical Dewar vessel (fig. 9, diameter -15 cm, height -20 cm). For reactions in flasks with a volume greater than 1 1, the cooling liquid may be placed in a pan which is insulated by straw or cotton wool contained in a larger pan (fig. 10). 

In this testing procedure to propellant sample (about 50 g) is heated in a insulating Dewar vessel, and the rise in temperature produced by the heat of decomposition of the powder is determined. The powder sample is heated to 80 °C (176 °F) the time is determined in which the powder reaches 82 °C (180 °F) by its own heat development on decomposition. 

A thermos bottle (Dewar vessel) has an evacuated space between its inner and outer walls to diminish the rate of transfer of thermal energy to or from the bottle s contents. For good insulation, the mean free path of the residual gas (air average molecular mass = 29) should be at least 10 times the distance between the inner and outer walls, which is about 1.0 cm. What should be the maximum residual gas pressure in the evacuated space if T = 300 K Take an average diameter of d = 3.1 X 10 °m for the molecules in the air. 

The thermal resistance R is supposed to increase from the isothermal to the isoperibol and then to the adiabatic type of calorimeter. It would probably be more correct and general to base the distinction between the adiabatic and the isoperibol calorimeters on the heat transfer (involving simultaneously the thermal conductance and the temperature difference) rather than on the value of the thermal resistance. For instance, a simple Dewar vessel calorimeter provides a very high thermal resistance between the central system and the surroundings, though it is simply an isoperibol calorimeter (called quasi-adiabatic in section 4.2.), whereas Swietoslawski s adiabatic calorimeters, which do not use any vacuum insulation, certainly provide a much lower thermal resistance [15]. 

It is interesting to consider an insulation of this type as a support for the inner shell of a dewar vessel, heat transport due to support members thus being reduced to zero. With this problem in mind we measured the stress—strain curve of a stack of 160 fiber glass papers and 160 aluminum foil reflectors. The result is shown in Fig. 5. This result does not mean very much unless we know how the conductivity varies with the applied stress, because there is probably an optimum density for a minimum conductivity. At the present time we cannot measure this effect in our apparatus. However, the curve does show that at loads below about 5 psi the sample is still fairly loosely packed. 

Also from Table II it is evident that in direct contrast to dewar vessels which require the best possible polished finish of the warm and cold walls and careful selection of surfacepropertiestoreduce theemissivity to the technical minimum, the finish ofboundarywalls in a vessel insulated with high-performance insulation has very little effect on the over-all heat transfer. This is evident from the calculations which were made for 25 effective radiation shields at 0.04 emissivity and boundary walls with emissivities as high as 0.7. The temperature profile in Fig. 2... 

Dewar Vacuum-insulated vessel, of a type commonly used for cryostats or as containers for liquefied gases. 

Dewar vessel (vacuum technology) A vacuum-insulated container commonly used to contain liquefied gases. 

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