Study of heat transfer in tubular-panel and spray cooling systems applied to the electric arc furnace walls
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This project consists in a heat transfer study in the electric arc furnace (EAF) walls, focused in the tubular-panel and spray cooling systems for the EAF located at Ternium-Guerrero plant, in the northeastern region of Mexico. The tubular-panel system is the one currently used to keep the walls cooled, composed of a total of 14 tubular panels. More dangerous accidents in the EAF operation, are the water steam explosions, which occur due to water leaks in the piping system inside the furnace. Spray cooling is given from the outside of the EAF, reducing the possibility of water directly impacting molten steel. The main purpose of this research is to know the operational conditions of both systems, verifying if the spray cooling system could be as good as the tubular system for the removal of heat on walls, efficiency and keeping the walls at low temperatures. The following procedures were used to estimate the water flow distribution in the cooling systems and the heat transfer in the walls. Piping network configurations are proposed for both systems. Models that consider surface-energy balances between different layers of the EAF’s walls and the heat radiated onto the walls by the electric arc and the molten-slag surface are developed herein. Conventional correlations were used for determining the heat transfer coefficients inside the tubular panels (Internal convection) and alternate correlations for determining the heat transfer coefficients for the external convection (spray cooling). Additional scenarios were done trying to improve the operational conditions and heat removal of each system. Water flow regulation by valves in each panel in tubular system and jet nozzles are used instead of spray nozzles in the spray system to verify the effectiveness of the spray cooling. The study was conducted via a parametric analysis in which the principal factors governing the process—the arc coverage and slag-layer thickness adhering to the walls—were varied. The results of the tubular-panel system were compared with experimental measurements of the outlet water temperature in each panel, showing a good approximation; allowing us to predict the operational conditions of the furnace. For both systems the optimal operating condition of the EAF, is when the arc is completely covered and the maximal thickness of the slag-layer that can be reached is around to 4.5 cm, it does that energy losses to decrease significantly and to keep walls at low temperatures. The minimal temperature difference between the inlet and final flow is around to 3 K. The spray cooling system operates with a lower heat removal capacity and pressure than the tubular-panel, causing that inner wall surface temperature to be approximately 20 degrees higher than when using the tubular system for critical operating conditions. Under optimal operating conditions each nozzle removes approximately 8.5 kW of thermal power. It is concluded that each cooling system has different temperatures and heat-removal capacity, which are highly dependent on the water flow within them. It is proved that slag-layer thickness and arc-coverage factors significantly affect the safe operation of the EAF, as well as its energy efficiency. This is a semi-analytical study; the equations of models were obtained analytically, and an equation-solver program is necessary to treat the non-linear equations obtained. Relatively few computational resources are required for this method.
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