TH30230A - How electric furnaces and melting furnaces work. - Google Patents

Abstract

This invention concerns a furnace for extremely high operating temperatures, and relates to the method of operation of such a furnace. These high temperatures, exceeding 1800 °C, are achieved by the use of stabilized zirconium dioxide resistive elements. The electric hot furnace according to the invention includes an inner furnace chamber supplied with a stabilized zirconium dioxide resistive element, and an outer furnace chamber in which a resistive element capable of operating at temperatures exceeding 18000 °C in an oxygen atmosphere is supplied. The outer chamber is of a readily available molybdenum silicide type, for example, an element marketed under the designation KANTHAL Super. The walls enclosing the inner furnace chamber are composed of zirconium dioxide or some other suitable material with low specific thermal conductivity and capable of withstanding the high operating temperatures and apparent temperature swings. The outer chamber, which completely encloses the inner chamber, is conventionally insulated by walls, such as ceramic fibers and/or high-temperature resistant bricks.

Claims (21)

1. A method of operating an electrically heated furnace with an inner chamber provided with an inner resistance element of stabilized zirconium dioxide, and an outer chamber with an outer resistance element made of another material. A special characteristic is that the walls of the outer chamber (5, 8, 9, 12, 14), which are closer to the environment, have a higher thermal conductivity than the walls of the inner chamber (1, 19), that is, closer to the inner chamber (15) of the furnace, and in which, for the intention of maintaining a specified operating temperature in the inner chamber of the furnace, the resistance element (17) in the outer furnace chamber is supplied with sufficient electrical power to maintain the desired temperature in the outer furnace chamber (13), where the previously specified electrical power is fed to the resistance element (2) in the inner furnace chamber (15), and thus maintain thermal balance between the inner chamber (15), the outer chamber (13), and the environment. 2. According to claim 1, a special characteristic is that at least one portion of the outer wall (14, 27) closest to the surrounding area has a thermal conductivity greater than that of the remaining portion (5, 8, 9) of such wall, and in the resistive component (17) is provided at least at the same temperature as the inside of such portion of the outer wall. 3. The method under claim 1 or claim 2 is of a special nature in which the thermal conductivity of the outer chamber walls (5, 8, 9, 12, 14), which are close to the environment, is very high compared to the thermal conductivity of the inner chamber walls (1, 19), which are close to the inner chamber (15), in which case at the predetermined operating temperature in the inner chamber, the resistor elements in the outer chamber are operated at least 10% of the maximum power required to maintain the predetermined temperature in the outer chamber (13). 4. Methods according to claims 1, 2 or 3, special characteristics in which the predetermined operating temperature in the inner chamber (15) is maintained in the outer chamber (13) at a temperature which is at least 50% of the prevailing 75% of the temperature in the inner chamber (15) measured in degrees Celsius. 5. Methods according to claims 1, 2, 3 or 4 in which the temperature of the outer chamber is measured by the thermoduple method (18), by the method of measuring the temperature of the inner chamber (15), by the method of a pyrometer connected to the inner chamber (15), by the method of a fiber optic cable (21). 6. Methods according to claims 1, 2, 3, 4 or 5, special characteristics, in which the outer resistance element (17) is set on the first two opposite sides (22, 23) of the inner chamber wall and in which the second remaining opposite sides (24, 25) of the inner chamber wall are devoid of the outer resistance element, in which the thermal conductivity of the outer walls closest to the environment is chosen so that the thermal conductivity of the two opposite outer walls (26, 27) placed outside the first such side (22, 23) of the inner chamber is higher than the thermal conductivity of the two opposite outer walls (28, 29) placed outside the second such side (24, 25) of the inner furnace chamber. 7. The method according to claims 1, 2, 3, 4, 5 or 6 is of special nature in which the power supply to the inner resistor element (2) is controlled and determined by the temperature of the inner chamber (15). 8. The method under claims 1, 2, 3, 4, 5, 6 or 7 is of a special nature in which the power supply to the outer resistor element (17) is controlled and determined by the temperature of the outer chamber (13). 9. Method according to claim 8, special characteristics by measuring the temperature in the outer chamber (13) at the point between the outer resistor element (17) and the inner chamber wall (23). 10. One of the preceding claims shall, by means of a special feature, control the power supply to the inner and outer resistor elements (17), respectively, in relation to the temperature display of the inner chamber (15) and outer chamber (13), at least over time. 11. Method according to claim 10, characterized by the transfer of power to the outer resistor element (17) when the furnace is started; the transfer of power to the inner resistor element (2) when the predetermined temperature has been reached in the inner chamber (15); reducing the power supply to the outer element (17) to a level corresponding to less than half of the power previously supplied, when the temperature in the two furnace chambers has reached approximately the same temperature level during the heating process. 12. An electric furnace with an inner furnace chamber containing an inner resistor element of stabilized zirconium dioxide and an outer furnace chamber containing an outer resistor element of another material, characterized by the outer chamber (13) walls (5, 8, 9, 12, 14) closer to the environment having higher thermal conductivity than the inner chamber walls (1, 19) closer to the inner furnace chamber (15); in which the control device acts to activate the resistor element (17) in the outer chamber (13) at the point of power delivery to the resistor element (2) in the inner chamber (15), the resistor element is supplied with sufficient power to maintain the required temperature in the outer furnace chamber and thus maintain the required temperature in the inner furnace chamber and thus maintain thermal equilibrium between the inner chamber (15), the outer chamber (13) and the environment. 13. The electric furnace under claim 1 is of a special characteristic in which at least one part of the outer chamber wall (14,27) adjacent to the surrounding area has a higher thermal conductivity than the thermal conductivity of the remaining parts (5, 8, 9) of such wall, and in which the resistor element (17) is provided at least at and inside such part of the outer chamber wall. 14. The electric furnace under claim 12 or claim 13 shall, in which the thermal conductivity of the outer chamber walls (5, 8, 9, 12, 14) adjacent to the ambient area shall be higher than the thermal conductivity of the outer chamber walls (1, 19) adjacent to the inner chamber (15) and in the control pathway shall be provided for the resistor element (17) in the outer chamber (13) with at least 10% of the maximum power at the predetermined operating temperature in the inner chamber (15). 15. The electric furnace under claims 12, 13 or 14 shall have a specific function in which the predetermined operating temperature in the inner chamber (15) shall be maintained in the outer chamber (13) at least 50%, preferably 75% of the temperature in the inner chamber, measured in degrees Celsius. 16. Electric furnaces under claims 12, 13, 14 or 15 in which the temperature of the outer chamber is measured by means of a thermocouple (18), a special characteristic in which the temperature of the inner chamber (15) is measured by a pyrometer connected to the inner chamber by means of a fiber optic cable (21). 17. The electric furnace under claims 12, 13, 14, 15 or 16 is of a special nature in which such an outer resistive element (17) is provided on the first two opposite sides (22, 23) of the inner chamber walls; in which the second, remaining opposite sides (24, 25) of the inner chamber walls are devoid of such resistive elements; in which the outer chamber walls closest to the environment are modified so that the thermal conductivity of such two opposite outer chamber walls (26, 27) placed outside such first inner chamber walls (22, 23) is higher than the thermal conductivity of such two opposite outer chamber walls (28, 29) placed outside such second inner chamber walls (24,25). 18. The electric furnace according to any of the claims 12-17 has a special characteristic in which the outer resistor element (17) is of the molybdenum disiliside type. 19. The electric furnace pursuant to any of the claims 12-18 shall be of a special nature in which the walls (1,19) dividing the inner chamber shall be composed of one of the materials of the stabilized zirconium dioxide group, hafnium dioxide, thorium dioxide or yttrium oxide or a mixture of them. 20. An electric furnace according to any of the claims 12-19, of a special nature in which the walls (5, 8, 9, 12, 14) separating the outer chamber (13) from the surrounding area are made of one of the materials: aluminum oxide lumps and aluminum oxide fibers. 21. An electric furnace according to one of the claims 12-20, of a special nature in which the function of the control of the power supply to the inner (2) and outer (17) resistor elements depends on the dominant temperature in both the inner (15) and outer (13) furnace chambers for at least a period of time.