Porous Ceramics Thermal Insulation Application
For most of people, ceramics are synonymous with hard, density ceramic tableware or wall tile, but modern technologies enable people to produce highly porous foam ceramics, intended for use as energy-efficient thermal insulation in blast furnaces. The new developed porous ceramic material possesses a unique combination of thermophysical and thermomechanical properties, as a result of high porosity, demonstrating outstanding operating characteristics with working temperatures of up to 1800 °C.
Heat transfer in porous ceramics is a complicated process which comprises conduction, convection and radiation, however in many cases, the latter two items are usually much smaller than conduction so they can be neglected.
Heat flux across the porous ceramic layer “q” (W/m-2) is related to the temperature difference between two sides of the layer “T2 – T1” (K), the thickness of the layer “L” (m) and the thermal conductivity of the porous ceramic layer “k” (W/(mK)). The thermal conductivity “k” is a parameter to estimate the heat transfer capability of a material: if k is larger, we say the material is a good thermal conductor; when k is small on the other hand, the material is then regarded as thermal insulator.
Copper is a typical thermal conductor: kcopper = 401 (W/(mK)) at 25 0C. Air is usually treated as thermal insulated: kair = 0.024 (W/(mK)) at 25 0C. The thermal conductivities of several common density ceramic material are list in the following Table.
Research shows that the thermal conductivity of porous ceramics can be approximately expressed as a function of the porosity of the material:
k0 is the thermal conductivity of density ceramics whose porosity is zero; φ is the porosity of porous ceramic material. According to the formula, the thermal conductivities of several ceramics at various porosity can be plotted as Figure.
The figure shows even the thermal conductivities of density ceramics have larger difference, they become almost similar when their porosities are bigger than 75%. This provides boarder choices of material when designing thermal insulation. Benefits of porous ceramics for use as thermal insulation include:
- longer life
- more uniform in firing atmosphere
- less shrinkage in insulation layer
- chemical inertness
- cost effective
- ceramic fibre free