What is the optimum density for ceramic fiber products?
The bulk density of refractory ceramic fibers refers to the ratio of the weight of the ceramic fibers to the overall material. The bulk density is an index that determines whether the selection of the furnace lining is reasonable. It is an important factor affecting the thermal conductivity:
a. The thermal conductivity of ceramic fiber products decreases with the increase of bulk density, but the decreasing range gradually decreases, so that when the density exceeds a certain range, the thermal conductivity does not decrease, but tends to increase.
b. There is a minimum thermal conductivity and a corresponding minimum bulk density at different temperatures, and the bulk density corresponding to the minimum thermal conductivity increases with the increase of temperature.
Correct understanding and application of the above laws are of great significance to the application of ceramic fibers. The thermal insulation performance of ceramic fibers is mainly based on the thermal insulation effect of airtight air in the pores of the product. When the specific gravity of solid fibers is constant, the larger the porosity, the smaller the bulk density.
When the bulk density is less than 96Kg/m³, due to the enhanced oscillating convection and radiation heat transfer of the gas in the mixed structure, the thermal conductivity decreases with the bulk density and increases exponentially.
When the bulk density is greater than 96Kg/m³, as the bulk density increases, the pores distributed in the fiber are closed, the proportion of micropores increases, the air flow in the pores is restricted, the heat transfer in the fiber decreases (thermal resistance increases), and at the same time leads to The radiation heat transfer through the hole walls is also reduced accordingly, thereby reducing the thermal conductivity.
When the bulk density increases to a certain range of 240~320Kg/m³, the solid fiber contact points increase, so that the fiber itself forms a mark, and the heat transfer weight increases through the bridge. The damping effect is weakened, so that the thermal conductivity no longer decreases and tends to increase. Therefore, the porous fiber material has an optimal bulk density with the smallest thermal conductivity. The foreign layered ceramic fiber wall lining, the hot surface ceramic fiber bulk density is specified as 130~160Kg/m³, and the flat laminated composite furnace lining hot surface ceramic fiber volume The density is specified as 200~240Kg/m³.
After the hot surface of the furnace wall, the bulk density of each layer of fiber products decreases successively, because the thermal conductivity of ceramic fibers decreases with the increase of bulk density at high temperature, which is more significant than that at low temperature. The use of fiber lining structure with different layers in the thickness direction according to the density can keep the thermal resistance uniform in the thickness direction of the furnace lining. Under the same thermal resistance conditions, the amount of fiber can be saved and the cost of furnace construction can be reduced.
