Like all materials, refractories expand and contract when heated and cooled. However, given the fact that refractories are almost always placed into extremely hot environments (that’s their job!), thermal expansion can be a significant factor in the overall performance of a lining and must be carefully considered.
But “thermal expansion” is only one of two quantitative measurements that must be considered when designing a monolithic refractory lining. The other, often overlooked, measurement is the Permanent Linear Change (PLC). A material’s thermal expansion is calculated as a rate of growth per inch, per deg of temperature increase. A material’s PLC is expressed in a percentage and is usually negative.
The reversible thermal expansion of a product is a characteristic of that material’s components. It will be virtually the same on all ensuing heat-ups unless the material is changed somehow due to service conditions.
Permanent linear change in a monolithic refractory is defined as a percentage change in length that happens during the initial heat-up. It only happens once as a refractory is heated up to its operating temperature, and is a result of moisture being released from the material and mineralogical changes that may occur within the material’s ingredients. PLC is typically negative, meaning there is shrinkage happening during the first heat-up.
So for the complete picture in thermal expansion and lining design, one must consider both the true reversible thermal expansion as well as the permanent linear change. The chart below shows how they work together: