Dewars heat load

Dewar heat load calculations involve a special geometry the outer surface of the reservoir and the inner surface of the vacuum container are so close together that each surface increment sees the other surface through a hemisphere. The ideal case is two concentric spheres, but two concentric cylinders or two parallel plane surfaces would behave similarly as long as their separation is small compared to their dimensions. The shape factor F.. is unity if the inner surface can see only the outer one. 

One criterion for adequate vacuum must be the ability of the vacuum to prevent gaseous heat conduction from the warm dewar walls to the cryogen. The material in this section will allow us to predict the gaseous component of the dewar heat load as a function of dewar pressure. 

In the case of the photometer the complete instrument is cooled below 2K by liquid helium. The heat loads and cooled volumes have been found to be similar enough in the three cases however to consider development of a single dewar design compatible with these and presumably other Instruments. Preliminary sizing of the dewar is compatible with at least two of the model instruments being flown on any one mission. The masses, thermal control, data handling and control of these instruments present no major problems. 

To reduce the conducted heat load, the mechanical supports are made as small as possible, and special materials are used. Handle the dewars gently they are often fragile. 

TABLE 12.6 Heat Load Calculations for Helium Dewar Design... 

The importance of these effects is illustrated by comparing the heat load components for two hypothetical dewar designs (Table 12.6). The dewars and the calculations are clearly simplified, and some features are exaggerated Dewar A is clearly... 

At high pressures, the mean free path varies inversely with the pressure, so the thermal conduction is independent of pressure. The heat flow formula is the same as used for conduction by solids, with a thermal conductivity K for each gas. Conductivity values are given in Table 13.1 for common gases. Typical heat loads in this regime are not adequate for insulating most dewars. 

At low pressures, the molecular path length is limited by the dewar dimensions, so the thermal conduction is proportional to the pressure. Figure 13.8 shows the heat conducted by the gas in the vacuum space of two hypothetical dewars as a function of pressure. Pressures below 10 or 10 " torr would be needed to make the gaseous conduction negligible compared to other sources of heat loads. 

The double-split-cone loading mechanism worked well. The cone was designed to match the circumferential stiffness of the test specimen. To achieve minimum heat loss into the dewar, the required axial load and the actuator rod cross... 

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. 

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