Patent GB-835205-A (GB)
Title: Improvements relating to methods of and furnaces for melting and refining glass electrically
Inventor: GELL PHILIP ANTHONY MAUNSELL
IPC: C03B5/027
Filed: 1955-02-22
Abstract: 835,205. Electric resistance furnaces. ELEMELT Ltd. April 12, 1956 [Feb. 22, 1955], No. 2053/59. Divided out of 835,202. Class 39(3) Also in Group XXXV] Molten glass e.g. flowing unidirectionally along a path 12 from a melting or refining chamber 10 to a delivery tank 11 is heated by passing A.C. therethrough, and the distribution of current over a transverse cross-section of the current path along which it passes is controlled by measuring or detecting an electrical quantity dependent on the electric resistance of the glass along subdivisions of the transverse cross-section of the current path and varying the magnitude of alternating voltage applied along the subdivisions to bring about the required current distribution within the cross-section of the current path, preferably so that the glass is heated to a uniform temperature. Thus glass in the chamber 10 is heated by A.C. between opposed electrodes 13, 14, and the path 12, which may be a submerged conduit, is arranged transversely to the direction of current between the electrodes 13, 14. The tank 11 contains one or more electrodes 15 for supplying A.C. passing through the glass in the duct 12 in its direction of flow. The phase angle between the voltage applied to the electrode 15 with respect to either of the electrodes 13, 14 may be varied to ensure equal magnitudes of current at the sides 12<SP>1</SP>, 12<SP>2</SP> of the path 12 of the glass to prevent glass from cooling in the path on one side and thereby rendering this glass less conductive and flow in the duct uneven. Current is fed to the main electrodes 13, 14 from a single phase transformer T1 through leads 21, 22 carrying current transformers CT1, CT2 the outputs of which are connected to a differential ammeter A1. The electrodes 15 are connected to one pole 23 of a further transformer T2, the other pole 24 of which is connected to a slider 25 of an impedance, e.g. inductance L1 connected across the leads 21, 22 in parallel with the electrodes 13, 14. The transformer T2 is energized from a further phase of an A.C. supply, the voltage of which is in quadrature with that supplied to the transformer T1. The voltage in each transverse subdivision of the current path 12 is varied in magnitude by applying the combination of two alternating voltages which are varied relatively to each other. The central tapping point of the inductance L1 is the electrical centre between the electrodes 13, 14, and when the ammeter A1 indicates asymmetrical heating, the slider 25 is adjusted along tapping points 27 of the inductance Ll to correct this either manually or automatically. The latter is effected by apparatus shown in Fig. 4, in which the current transformers CT1, CT2 are connected to a comparison circuit 55 containing two transformers having the same step up ratio connected respectively to the transformers CT1, CT2 and having their secondary windings connected to rectifying circuits connected to feed the rectified voltages in opposition across a resistor. Opposite ends of this resistor are connected by lines 56, 57 to a relay 58 having an armature movable in opposite directions from a central or unoperated condition according as to whether the line 56 is positive or negative with respect to the line 57. The relay 58 has three contacts to which lines 59, 60, 61 are connected. Lines 59, 61 are connected to a reversible regulator motor 62 connected by a line 63 to one of the mains terminals 64. The other terminal 64 is connected to the line 60. The motor is connected mechanically at 65 to the slider 25, which is thus moved to restore current in lines 21, 22 towards equality. In order to prevent hunting, a time delay switch may be connected in series with lines 56, 57 so that it is opened when the slider moves between successive contacts 27. Instead of correcting inequalities in the magnitudes of current flowing to the electrodes 13, 14 a phase shifting network for varying the phase applied to the electrodes 15 may be used. The tapped inductance L1 may thus be omitted and the terminal 26 of the transformer T2 may be supplied through a phase shifting transformer. Specification 835,201 is referred to.