Effects of Alternating Currents in the Hearth of Submerged Arc Furnaces

Abstract

It is well known that alternating currents (AC) differ significantly from direct currents (DC) in large electrodes for Submerged Arc Furnaces (SAFs). The skin effect concentrates AC to the periphery of each electrode, while the proximity effect causes higher current concentration towards the leading electrode. In a presentation at INFACON XV it was further shown that there is also a significant proximity effect between electrode currents and induced currents in the furnace steel shell. Here, we will focus on differences and similarities between AC and DC below the electrodes. In many processes the current runs primarily vertically in a coke bed from each electrode to a metal bath, where it distributes to the other electrodes. In such coke beds we find no significant difference between AC and DC. In the highly conductive metal, there will be horizontal currents and a significant AC skin effect. Horizontal currents will also be present in the materials above the metal, depending on the electrical conductivity in this region. A simple model shows that there is a strong proximity effect between such adjacent currents, “pushing” the currents upwards within the slag/coke bed region. A simple estimate, based on furnace dimensions and assumed conductivities, will show whether this effect is significant, in which case DC computations are inadequate to estimate the current paths. The same proximity effect will significantly enhance induced currents in an electrically conductive lining. Finally, since the underlying Maxwell’s equations are linear, the fundamental current paths in 3-phase AC SAFs can be studied by computing two independent single-phase cases for a given geometry and distribution of electrical conductivities. Any other current distribution will be a linear combination of these two solutions.