Patent GB-1508907-A (GB)
Title: Heating elements for furnaces
IPC: F27D11/02
Filed: 1975-06-06
Abstract: 1508907 Resistance heating element DAIDO STEEL CO Ltd and AGENCE NATIONALE DE VALORISATION DE LA RECHERCHE 6 June 1975 [10 June 1974 (2) 4 July 1974 (2)] 24441/75 Heading H5H In a resistance furnace having a generally cylindrical ceramic resistance heating element 52, the internal diameter of the element is maintained constant whilst the external diameter increases gradually from the middle section to the ends so that the central portion of the element 52 in which the object to be heated is located produces the highest temperature. Longitudinal slots 114 and 122 are provided in the enlarged ends 106, 108 of the element to allow for thermal expansion. Element 52 may be made of stabilized zirconia (ZrO 2  + CaO), lanthanum chromite or thorium oxide to which calcium oxide and yttrium oxide are added. The small wall thickness of the constant diameter centre section 104 ensures that the temperature difference between the inside and outside of the centre section 104 is minimal. In a further embodiment the centre section of the element comprises a cylindrical portion of enlarged diameter between two reduced diameter cylindrical sections which provide the hot zones. The cylindrical portion is longitudinally slotted and its ends taper down to the reduced diameter cylindrical sections. The centre section comprising the enlarged diameter portion and the two cylindncal sections is provided with tapered end portions 106, 108. Annular platinum foil electrodes 54, 56, preferably provided with at least three equispaced radial slits are secured to the ends 58, 60 of the element 52 by applying a paste of platinum powder and acetone or benzol to the machined and cleaned ends 58, 60, placing the electrodes on the coated ends, placing the element 52 in the furnace with a weight applied thereto, and passing current through the element 52 after preheating it with metal heating coil 78 to cause the platinum powder to fuse the electrodes 54, 46 to the element 52. Alternatively, molten lanthanum chromite may be sprayed on to ends 58, 60 and the coated ends then machined. Good contact ensues when the electrodes 54, 56 are merely placed in contact with the ends 58, 60 of the element. A vertically movable charge holder 98 of stabilized zirconia provided with a cavity 100 and an apertured heat sealing cap 66 define the heating chamber 102. The clearances between cap 66 and charge holder 98 and the inner wall of the element 52 are minimized to reduce heat loss. A refractory wall 38 is interposed between preheating element 78 and ceramic heating element 52 and is supported on a flange 32 bolted to the bottom 16 of the furnace casing 14. If it is desired to service the elements 52, 78, the flange 32 is unbolted to permit removal of the wall 38 when the charge holder 98 is lowered. Thermal insulation 26 surrounds the heating elements. A sighting device 24 enables the temperature of chamber 102 to be observed. A transverse sighting port 80 enables a temperature probe to be inserted to monitor the temperature of the preheater. The preheating temperature and the temperature of the heating chamber 102 are controlled automatically.