Photo: Robert Wiezorek, www.film-manufaktur.de.
Modern high-temperature applications have to meet the highest requirements and the toughest conditions.
Depending on the process conditions, it can happen, that some distinctive features can arise in the furnace lining. We present you here some of these phenomena as well as solutions for their prevention. The goal is a reliable and long-lasting furnace insulation.
Figure 1: Cracks and discoloration of a furnace insulation. Photo: SCHUPP® Ceramics.
1. Cracks in the insulation
The insulation boards can have larger or smaller cracks after some sinter/firing cycles (see figure 1). Cracks occur due to thermal and/or mechanical stress.
Temperature inhomogeneity or different thermal expansion coefficients of the installed materials lead to the thermal stress in the furnace lining and as a consequence to the deformations or even cracks. Small cracks affect only visually, whereby bigger cracks may cause the heat loss in the furnace and a risk that parts of insulation fall into the furnace chamber and onto the treated products. To avoid the cracks in the insulation, it is not recommended to install the whole boards in the furnace. The furnace lining should consist of smaller, cut slices (see figure 2).
Figure 2: Construction of the furnace lining from cutted parts. Photo: SCHUPP® Ceramics.
Due to the thermal expansion of the furnace lining, the metal case causes also an additional mechanical stress on the insulation. In order to avoid the stress, the insulation should not be installed too tightly in the lining of the furnace. A few millimetres of free space can already play a major role.
2. Shrinkage of insulation boards
Depending on the firing/sintering cycles, the furnace is quickly heated up to required temperature. The temperature change affects the furnace lining which can shrink or expand. Minimal changes do not ainfluence the efficiency of the furnace. In case of e.g. stronger shrinkage, gaps can occur between the insulation boards (see figure 1) and heat escapes from the furnace.
To avoid the strong shrinkage of the insulation material, we recommend to pre-fire the boards first, before they are installed in the furnace. One of the advantages of the pre-firing is burn-out of the organic binder, thereby the insulation material becomes harder and more stable.
3. Spalling on the surface of the insulation
Heat treatment of your products in the furnace can cause various chemical processes, e.g. gas emissions what negatively influences the surface of furnace lining and finally leads to the surface spalling or delamination. To avoid the spalling, it is advisable to have a good ventilation during the process inside the furnace. This can be done for e.g. with a chimney.
If the insulation is already damaged, it should be replaced. We offer you insulation boards made of polycrystalline mullite/alumina wool, short PCW. The parts are a high-quality alternative for insulations of aluminosilicate wool (ASW), also known as refractory ceramic fibre (RCF). The PCW material is chemically more resistant, in comparison to ceramic fibers, and has a minimal shrinkage as well as high form stability. We will be glad to advice you.
4. Discoloration of the insulation board
Discoloration of insulation occurs often through the chemical processes caused by the fired/sintered product. The outgassing vapours lead to a chemical attack of the insulation boards and sometimes to a colour change (see figure 1). A slight discoloration usually does not affect the insulating function. Nevertheless, the lining is already contaminated and can thereby influence the product’s purity in the further treatments. If this phenomenon occurs and in case of strong evaporations, we recommend to have a good ventilation during the process inside the furnace, e.g. with a chimney, or to replace the furnace lining.