Operating problems
Air heaters - Cowpers devices

A series of incidents occurred on the first high-temperature cowpers shortly after commissioning: the sheet metal cracked. It took several years to confirm that these disorders were due to stress corrosion initiated in the weld seam areas by the presence of alkali nitrates. Heating the air to high temperature leads to the synthesis of NO_x, which combine with water and condense on the wall, forming alkali nitrates with the insulation.

Figure 3 shows the NO_x levels recorded as a function of temperature. Several palliatives have been proposed. For the cowpers to be built, it seems that limiting the cupola temperature and running on pure blast furnace gas is the best protection. However,...