The volume density of refractory ceramic fibers refers to the ratio of the weight of ceramic fibers to the overall material. The size of the volume density is an indicator that determines whether the selection of furnace lining is reasonable, and it is an important factor affecting the thermal conductivity:
The thermal conductivity of ceramic fiber products decreases with the increase of bulk density, but the extent of the decrease gradually decreases, so that when the density exceeds a certain range, the thermal conductivity no longer decreases, but instead tends to increase.
There is a minimum thermal conductivity and corresponding minimum volume density at different temperatures, and the volume density corresponding to the minimum thermal conductivity increases with increasing temperature.
The correct understanding and application of the above laws are of great significance for the application of ceramic fibers. The insulation performance of ceramic fibers mainly utilizes the insulation effect of sealed air in the pores of the product. When the specific gravity of solid fibers is constant, the larger the porosity, the smaller the volume density.
Volume density<96Kg/m ³ Due to the enhanced oscillatory convection and radiation heat transfer of gases in the mixed structure, the thermal conductivity decreases with volume density and shows an exponential increasing trend.
Volume density>96Kg/m ³ As the volume density increases, the pores distributed within the fibers become closed, and the proportion of micropores increases. The airflow in the pores is restricted, resulting in a decrease in heat transfer within the fibers (increased thermal resistance). At the same time, it also leads to a corresponding decrease in radiation heat transfer through the pore walls, resulting in a decrease in thermal conductivity.
When the bulk density increases to a certain range of 240~320Kg/m ³ The increase in contact points of solid fibers forms a standard for the fibers themselves, which increases the heat transfer weight through the bridge. Secondly, the increase in contact points of solid fibers weakens the damping effect of pores on heat transfer, resulting in a decrease in thermal conductivity and an increasing trend. Therefore, there is an optimal volume density for porous fiber materials with the lowest thermal conductivity. For foreign layered ceramic fiber wall liners, the volume density of hot surface ceramic fibers is specified as 130-160Kg/m ³， The volume density of ceramic fibers on the hot surface of the flat stacked composite furnace lining is specified as 200-240Kg/m ³。
After the hot surface of the furnace wall, the volume density of various layers of fiber products gradually decreases, because at high temperatures, the thermal conductivity of ceramic fibers decreases more significantly with increasing volume density than at low temperatures.
The use of fiber lining structures with different layers in the thickness direction and density distribution can maintain uniform thermal resistance in the thickness direction of the furnace lining. Under the same thermal resistance conditions, the amount of fiber used can be saved and the cost of furnace construction can be reduced.