Example of a Cryogenic Dewar

High performance cryogenic dewars are prime candidates for thermal modeling. A computer model is a quick inexpensive was to iterate design options. These options might include support strap cross-section, number of shields, or electrical wire counts. Sensitivity studies on insulation effectiveness, power dissipation, and vacuum shell temperatures are typical of parameters that might be examined. The basic telescope configuration is shown below.(Note: The process shown here can be applied to any thermal system.)

This is an example thermal analysis of an early (mid 80s) version of SIRTF (Space Infrared Telescope Facility) telescope concept. (The configuration shown here is a very early "cold launch concept.) SIRTF is similar to IRAS and COBE which were orbiting infrared telescopes. It utilizes a Super Fluid Helium dewar to cool a compliment of infrared detectors. It operates at 1.7 Kelvin. The basic thermal control technique is:

A vacuum shell to eliminate gaseous conduction and convection while on earth
Multilayer Insulation (MLI) composed of 0.25 mil mylar aluminized on both sides to minimize radiation transfer
Fiberglass tension suport straps to minimize conductions from the Outer vacuum shell and the helium tank
Small diameter, low thermal conductivity wires to minimize conduction
Vapor cooling to intercept parasitic heat leaks. These VCSs form nested shells around the cryogen tank; the innermost VCS is the coldest and the outer VCS the warmest. Each VCS buffers the next, intercepting parasitic heat leaks. The normal helium boil-off gas provides the cooling capacity of the VCS. The gas vent line is routed over each VCS on its way out of the dewar. As the gas leaves the system, it cools each shield. The bigger the heat leak to the dewar, the more gas is boiled off, leading to more cooling by the VCS.

This dewar model can be used to examine the basic configuration and perform top level trades prior to detail design and analysis. After a baseline design approach is chosen this dewar model should be continually upgraded so as to evaluated each design detail as it is made.

The thermal characteristic of a dewar make it relatively simple to model. This model will be composed of just fifteen nodes. It will require less than two seconds to run. A plethora of sensitivity trades can be made in less than a day with this simple model. The effect of outer vacuum shell temperature on dewar life will be examined in this example.

In the schematic the nodes are shown as heavy horizontal lines. Conduction paths are shown as thin solid lines and radiation paths are shown as dotted line. The vapor cooling line is shown as a gray line. Nodes are generaly (near) isothermal "lumps" for which we want to predict temperatures. Conductors are the heat flow paths that connect the nodes. The users defines nodes and calculates the values of conductors. If a transient thermal analysis is needed, the thermal capacitance of the nodes also needs to be calulated. Another wrinkle in this analysis is that material properties undergo drastic changes as a function of temperature. Therefore, these properties can be put in the model as "array look-up" tables.

For expediency TAK 2000 will not use the LOGIC BLOCK OPTION in this example. Logic BLOCKS permit the analyst to insert Fortran codes to manipulate the model during solution or write out special report after a solution. A typical use would be to be to calculate and printout dewar life in "days" at the end of steady state solution. When the LOGIC BLOCK OPTION is chosen, the the Fortran code must be compiled and linked. In our case this would turn a two second computer run into a one minute computer run. While LOGIC BLOCKS can be extremely useful to the "power user", they are not always appropriate and always exact a solution time penalty.

Examine the thermal model file. It is explained in detail. This is the ascii file used as input to the TAK 2000 analyzer. It can be built with a text editor, word-processor, or from the TAK WorkBench.

Also examine the raw output file. This file contains all the details from the simulation run. This include a recap of the input as well as predicted temperatures and heat flows. There are many options available to the user to mask out detail. As much or as little detail is preserved as the user decides.

Parametric or sensitivity trades studies are one of the key reasons thermal models are built. This example study examines the effect of outer shell temperature on cryogen heat load and dewar component temperatures.

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