Temperature effects influencing the spectral and total emissivity of refractories

The spectral reflectivity of some refractories has been measured. This has been done to obtain specific (instead of general) data that will be used in the modeling of radiative heat transfer in furnaces. The measurements were performed using an integrating sphere method. Because of the lack of a diffuse reference sample for longer wavelengths, a procedure was developed for the use of a specular reference sample. Although the individual reflectivities follow the general trend, two characteristics features can be reported. Refractory material that has not been heated yet has a higher reflectivity than heated material in the spectral range 2.5–4 μm. This is attributed to water absorption. Refractory material that came from the inner side of a combustion chamber of a glass melting furnace showed different reflectivity behavior for wavelengths smaller than about 5 μm than that of material of the furnace wall originating from a position about 20 cm away from the inner side. The hemispherical emissivity as a function of temperature is lowered by the first effect and increased by the second. The magnitude of the effects can be up to 20%.