MadSci Network: Engineering
Query:

Re: What is the best thermal ceramic insulator to keep heat in a vessel?

Date: Mon Jan 3 00:33:15 2000
Posted By: Martin Thomas, Post-doc/Fellow, Phyiscal Chemistry, Quantachrome Corporation
Area of science: Engineering
ID: 945364782.Eg
Message:

Dear Dave,

There are a number of materials available, some being more appropriate than others depending on the exact application. For example, must the vessel's insulation be able to withstand mechanical shock, must it be waterproof, or have inherent strength? Most ceramic insulators rely upon their intrinsic low thermal conductivity ( compared to metals that is), combined with some manufacturing process which imparts low overall (bulk) density. By including air pockets, by virtue of a fibrous structure like a wool blanket, or by enclosing cells or pores (like a foam rubber or plastic) the thermal conductivity is further lowered (air having a lower conductivity than the solid).

Alumina (aluminum oxide, Al2O3) and silica (silicon dioxide, SiO2 are probably the widest used ceramic insulation material, but there are others. Visit these commercial websites to discover more plus their applications:

If your vessel needs to withstand the heat and stresses of (or at least similar to)reentry into the earth's atmosphere as does the Space Shuttle, then you might consider the material that the famous tiles (properly known as High-Temperature Reusable Surface Insulation Tiles, or HSRI Tiles) are made of. Early HRSI tiles were 99.8% amorphous silica fiber. Surface heat dissipates so quickly that an uncoated tile can be held by its edges with an ungloved hand seconds after removal from a 2300oF oven while its interior still glows red. Newer Fibrous Refractory Composite Insulation Tiles (or FRCI tiles) have a less dense construction utilizing slightly different materials.

But at $ 3000 per 6" x 6" x 2" thick tile, you might find the cost prohibitive! Read the overview of the Space Shuttle's thermal protection system (TPS) for more background.

If you are looking for purely the lowest thermal conductivity, then look no further than aerogels, open-cell ceramic foams with remarkable thermal properties but poor mechanical ones. For an illustrated background to microporous insulators, visit Microtherm.

For your "vessel" application, then a flexible ceramic fiber product such as moldable Kaowool, Kaowool strip or Kaocrete might be appropriate. You might be able to get away with simple furnace cement. If your device does not operate under extremely high temperatures, simple "foam-in-a-can" polymeric insulation from your local home improvement store might suffice.

I hope that I have provided you with a little insight and that you can select an appropriate material for your heat-exchanger.

Thanks for using MadSci Network.


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