The bulk density of refractory ceramic fiber refers to the ratio of the fiber's weight to its total volume. This parameter is critical for selecting appropriate furnace linings and is a key factor influencing thermal conductivity.
A. The thermal conductivity of ceramic fiber products generally decreases as bulk density increases, but the rate of reduction gradually slows. Beyond a certain density threshold, thermal conductivity may cease to decrease and can even begin to rise.
B. At different temperature ranges, there exists a minimum thermal conductivity value, each corresponding to an optimal bulk density. This optimal density tends to increase as the service temperature rises.
Understanding these principles is essential for the effective application of ceramic fiber. The insulation performance primarily relies on the trapped air within the product's pores. When the density of the solid fiber itself is constant, a higher porosity results in a lower bulk density.
When Bulk Density < 96 kg/m³: Excessively low density strengthens convective heat transfer and radiation within the pore structure. Consequently, thermal conductivity increases exponentially as density decreases.
When Bulk Density > 96 kg/m³: As density increases, the pores within the fiber structure become more enclosed and micro-sized. This restricts air movement, reducing heat transfer (increasing thermal resistance) and simultaneously decreasing radiant heat transfer through the pore walls, leading to lower thermal conductivity.
When Bulk Density Reaches 240–320 kg/m³: Beyond this range, increased contact points between solid fibers create more conductive "heat bridges," which can increase heat transfer. Additionally, the damping effect of pores on heat transfer weakens. As a result, thermal conductivity stops decreasing and may start to increase.
Therefore, porous ceramic fiber materials have an optimal bulk density that delivers the lowest possible thermal conductivity. Internationally, for layered ceramic fiber wall linings, the hot face density is typically specified at 130–160 kg/m³, while for composite linings with flat-laid and stacked modules, the recommended hot face density is 200–240 kg/m³.
In multi-layer linings, the bulk density of successive layers behind the hot face can be progressively reduced. This is because the effect of higher density in reducing thermal conductivity is more pronounced at elevated temperatures. A layered lining structure, graded by density, ensures uniform thermal resistance across the lining thickness. This approach saves material and reduces construction costs while maintaining the same overall thermal resistance.
