How do choose suitable ceramic fiber products for industrial furnaces?

Ceramic fiber products are used under different conditions and their long-term use temperature is different. For example, the operating system of industrial kilns (continuous or intermittent kilns), fuel types, furnace atmosphere, and other conditions are all factors that affect the temperature and service life of ceramic fibers.

At present, there is no ideal method for measuring the heat resistance index of ceramic fiber. Generally, the ceramic fiber product is heated to a certain temperature, and the heat resistance of the ceramic fiber product is evaluated according to the linear shrinkage change and crystallinity of the sample.

The influence of temperature on the performance of ceramic fiber

From a thermodynamic point of view, the glassy ceramic fiber is in a metastable state. Therefore, as long as it is heated under a certain temperature, there will be a rearrangement of particles inside the fiber, then the glassy state will be transformed into a crystalline state, and the fiber will crystallize.

Table 1 Crystallization changes of ceramic fibers at different temperatures

When the crystal grain size grows to be close to the fiber diameter size, the bonding force inside the fiber will change from the intermolecular chemical bonding to the grain bonding between the crystal grains. Because the grain bonding force is relatively fragile, it will lead to further brittleness of the fiber, and the fiber is easily damaged under external force, and finally loses the fiber characteristics.

The influence of the ambient atmosphere on the performance of ceramic fiber

In a reducing atmosphere, the SiO2 in fiber easily reacts with CO and H2 as follows:

SiO2+CO→SiO↑+CO2

SiO2+H2→SiO ↑+H2O

As SiO2 is reduced to volatile substances, the fiber structure gradually changes and the surface gradually becomes rough. When mullite grains are formed inside the fiber, the fiber is easily broken, which accelerates the pulverization of the fiber.

The influence of impurities on the performance of ceramic fiber

Some impurities in ceramic fiber, such as Fe2O3, Na2O, K2O, etc., will react with other components in ceramic fiber to form eutectics at a lower temperature. The existence of eutectics destroys the network structure of the fiber. Viscosity inside the fiber is reduced, the crystallization activation energy needed to overcome the rearrangement of ion clusters during crystallization is reduced, and the crystallization temperature is reduced. At the same time, the existence of eutectics accelerates the growth of crystal grains and promotes the pulverization of fiber.

High temperature oxidation corrosion of furnace skin steel plates and metal anchors

Fiber shrinkage in volume is manifested in the enlarged contact gap between the products. The flame enters from the gap, causing serious oxidation and burning of the anchors and the furnace roof steel plate. Since the high-temperature working surface is heated for a long time, the contact seam splits into a cone shape and the flame begins to contact the fibers on both sides of the seam so that the fibers on both sides become the working surface again. The shrinkage direction of the fiber is always perpendicular to the flame direction and inward so that the shrinkage gap becomes larger and larger.

1〉When flame touches the steel plate, the steel plate burns red and deforms. The stress caused by the deformation of steel plate is enough to cause fatal damage to the fiber structure;

2）When the flame touches the inner fiber if it is a low-temperature fiber (glass fiber), the fiber will pulverize from the middle layer, and the gravity of the lower half of the fiber is enough to cause the fiber to fall off;

3） When the flame touches the buried metal anchors, deformation and oxidation of the anchors are enough to destroy the fiber structure, as shown in the figure below:

Service temperature of ceramic fiber

The maximum use temperature of ceramic fiber refers to the limit temperature that the ceramic fiber can withstand in a short time, which is used to characterize the heat resistance of ceramic fiber products. The classification temperature is not the safe use temperature of fiber. The use temperature in the oxidizing atmosphere furnace should be lower than the grade temperature by about 200℃; in the reducing atmosphere furnace, it should be lowered by 300-400℃.

It is international custom to divide ceramic fiber products into 4 grades according to temperature, namely 1000 ℃ type, 1260 ℃ type, 1400 ℃ type, and 1600 ℃ type. Domestically, it has been subdivided according to the ceramic fiber composition and the maximum use temperature to meet the different grades of thermal insulation selection requirements.

Ceramic fiber classification temperature and use temperature table

Conclusion： When designing ceramic fiber lining, the designer first considers the safe use temperature of the selected material. Before selecting this material, the designer needs to leave a certain safety factor to deal with abnormal working conditions such as instantaneous high temperature in the furnace.