US2314956A - Electric furnace - Google Patents

Description

March 30, 1943. G, SLAYTER ETAL 2,314,956

ELECTRIC FURNACE 3 Sheets-Sheet 2 Filed April 5, 1941 March 30, 1943- G. SLAYTER ErAL 2,314,956

' ELECTRIC FURNACE Filed April 5, 1941 3 Sheet's-Sheet 5 vuvvvvvv ATTORNEY Patented Mar. 30, 1943 ELECTRIC FURNACE Games Slayter, Flavius W. Atkinson, Ed Fletcher,

and Harry V. Smith, Newark, Ohio, assignors to l Owens-Corning Fiberglas Corporation, Toledo, Ohio, a corporation of Delaware Application April 5, 1941, Serial N0. 387,052

3 Claims.

Our invention relates to electric furnaces and methods for melting and refining materials including glasses or solutions of metal oxides in each other, silicates, vitreous materials and the like.

An object of the invention is to provide a novel type of furnace which may be used successfully for melting and refining glasses or materials having different formulae 'and in which the melting temperatures and other characteristics may vary through a wide range.

A further object of the invention is to provide an electric melting furnace and electrical controlling means therefor arranged and adapted to give stability, constancy and uniformity of operation which have not been heretofore obtained. In the use of electric furnaces for glass melting operations, great difficulties have heretofore been encountered in the attempts to control the melting conditions particularly as the glass while at a temperature at which it will conduct current. has a negative coeilicient of resistance characteristic such that its electrical resistance rapidly decreases as its temperature rises. Electric current flowing through the hot glass generates heat therein, causing a decrease in the resistance as the temperature rises. This permits an increased flow of current, supplying additional heat at a rapidly increasing rate and also further augmenting the current flow so that without very close supervision, the temperature will qulitckly rise beyond control with destructive resu s.

An aim of the present invention is to overcome such difilculty by the provision of v automatic means for electrically controlling the current supply, preventing the current from rising above a predetermined limit and stabilizing the melting and refining operations. Such electrical controlling means operates further to increase the current supply if the temperature of the glass is lowered, thereby maintaining a stabilized uniform melting and refining operation.

The invention in its preferred form provides an electric melting and refining furnace of the type indicated in which more than two electrodes -are provided and arranged in such manner that the preliminary heating and melting of the batch may be carried on between two electrodes and the continued operation by which the materials are more completely melted or brought into solution and refined, are carried on between other electrodes. The melting of the batch may be effected in an electric arc circuit in which the arc is passed through the batch material between a,

pair of electrodes or between one electrode and the body of molten glass in which the second electrode is immersed, while a further heatin and renning is taking place by passing electric current through the glass between the second and a third electrode, the glass serving as a resistor in which the heat is generated.

A further object of the invention is to provide a furnace and electrical control means constructed and arranged to give adequate control of the temperatures and the rate at which the glass flows, thereby permitting a single furnace to be satisfactorily used for supplying glass at rates varying through a wide range and also serving for use with different batch formulae having various melting and temperature-resistivity characteristics.

A further object of the invention is to provide an electric melting furnace of the character in'- dicated which gives a high thermal efficiency.

Other objects of the invention will appear herenafter.

Referring to the accompanying drawings:

Fig. 1 is a part-sectional elevation of an electric melting and refining furnace constructed in accordance with the principles of our invention.

Fig. 2 is a diagrammatic view of the furnace and electrical heating and control circuits therefor.

Fig. 3 is a wiring diagram of the electrical control system.

Fig. 4 is a diagrammatic view illustrating a modified form of electrical controlling apparatus.

The present application discloses a form of electric furnace and electrical heating and controlling apparatus also disclosed in our copending application, Serial Number 326,714, filed March 29, 1940, Electric furnaces and melting methods, and is a continuation-in-part of said copending application which has now matured into Patent No. 2,280,101.

The electric furnace as shown in Fig. 1 comprises walls of refractory material. The side walls include refractory blocks I5 defining a lower compartment herein referred to' as the refining compartment, and blocks I6 providing an upper compartment serving as a melting compartment. -The floor Il of the furnace is formed with a well I8 providing an outlet opening extending through the floor. The refractory walls are supported by an outer metal frame structure including angle bars I9 and 20 surrounding the lower end of the furnace, angle bars 2l surrounding the upper end of the furnace, vertical tie rods 22 and straps 23 surrounding the furnace at intermediate points.

The furnace is provided with three electrodes, namely, a 4bottom electrode 24, an intermediate electrode 25 and an upper electrode 26. The bottom electrode 24, which may consist of platinum, a platinum iridium alloy or the like, is shown in the form of sheet metal overlying the furnace floor and formed with an integral sheet metal bushing 21 lining the walls of the well i6. The bushing extends below the furnace floor and is formed with an outlet opening or openings 28 through which the molten glass is discharged.

The intermediate electrode 25 may comprise a series of rods, preferably parallel and uniformly spaced and distributed throughout the transverse area of the furnace chamber. 'I'he ends of the rods are anchored in blocks 29 made of electrical conducting material such as copper and embedded in the furnace walls. 'Ihe rods may be made of tungsten or other metal or alloy capable of withstanding the high temperature and physically and chemically resistant to the action of the molten material.

Baiiie plates 25B are disposed around the interior of the furnace walls in proximity to the metal electrode 25. These plates may be made of molybdenum, tungsten or other suitable metal and serve to confine the flow of glass to the central portion of the furnace as it passes the electrode 25, thereby minimizing the attack of hot glass upon the terminals 29 which support the rods 25. The upper electrode 26 as shown is an arc electrode and consists of a carbon rod extending downwardly into the furnace centrally thereof. Any suitable means may be provided for adjusting this electrode up and down, either manually or automatically.

The platinum bottom electrode 24 may be used as an electrical resistance heating element for supplying heat to the batch in order to start the furnace when cold. This electrode is further used as a heating unit to control the rate of flow of the molten glass from the furnace. For this latter purpose electric current may be supplied through a transformer 3U (Fig. 2) whose voltage is variable, the secondary circuit 3| of which transformer is connected to terminal blocks 32 on the bushing 21. The part 24 further serves as one of the electrodes in an electrical heating and refining circuit including the molten glass contained between the electrodes 24 and 25. The current in suchl circuit generates heat in the body of glass itself by the resistance of the glass to the current flow.

The tungsten rods 25 are also used as resistance heating elements as well as serving as an electrode to aid in bringing the cold furnace up to a temperature at which the current may be passed through the glass itself. Usually a temperature of not less than roughly 1400" F. isl necessary to allow sulcient current to pass through the glass so that the melting and refning operations may be continued with the electrodes 24 and 25 functioning as such rather than as resistors.

The raw batch materials and/or cullet, which may be in granular, comminuted, powdered or other form, are fed into the top of the furnace through a hopper 33. In starting the operation this material may be heated by the electrodes 24 and/or 25 operating as resistors in4 the manner above described until the batch is brought to a temperature at which it will conduct the electric current, permitting the electrodes to function as such. A heating circuit may then be established through the upper electrode 26 and current passed therethrough. An arc is drawn between the carbon electrode and the upper surface of the molten material. The batch material, which is being fed downward through the upper or melting compartment of the furnace surrounds the carbon electrode and is sintered and melted more or less completely by the electric arc.

The material as it is thus melted is fed downward continuously from the melting compartment above the electrode 25 into the refining compartment comprised between the electrode 25 and the bottom electrode 24. In this lower compartment the molten glass may be heated to a higher temperature and brought to a highly fluent condition in which the batch materials are completely dissolved. The gases or seeds which occur in the molten glass are permitted to escape and the glass is thereby refined to a high degree before being discharged through the outlet 26. The upper and lower compartments are herein referred to as the melting and refining compartments respectively, as the melting operation takes place mainly in the upper compartment and the refining action is carried to completion in the lower compartment, although refining may take place to a certain extent above the intermediate electrode 25.

The batch fed into the hopper 33 surrounds the carbon electrode 26 and moves downward therealong, thus serving to keep the electrode cool and also keeping the heat which is being generated confined mainly to a small space in the central portion of the furnace. This results in giving a very high thermal elciency since the hottest portion of the furnace is at or near its geometrical center and none of the heat which is generated can go to waste without rst passing through the material which is being melted. This heat is to a large extent absorbed by such material. That the heat is conserved in this manner has been demonstrated by us in the use of the present furnace and is shown by the fact that we have, for example, discharged glass from the bottom of the furnace at the center at 2780 F. while the inside of the side walls registered a maximum of 2300 F. and a minimum of 1000 F.

Referring to Fig. 2, the intermediate electrode 25 when used as a resistor may be heated by current supplied from a variable voltage transformer 34, the secondary of which is connected in a circuit 35 including the resistor 2l. 'I'he intermediate and bottom electrodes 25 and 24 are connected in the secondary circuit of an adjustable voltage transformer 31 through conductors 38 and 39. The circuit through the electrodes 25 and 26 includes the conductor 3l and a conductor 4I connected to the opposite terminals of the secondary coil 42 of a variable voltage transformer 43.

In an electrical furnace of the type herein shown comprising an arc, there is a substantial amount of resistance in the arc between the carbon tip and the molten glass 45 and considerable resistance in the molten glass as well as some resistance in the external circuit. We have found it necessary in a furnace employing such a circuit to have a proper balance between the amount of heat generated in the arc and the amount of heat generated in the body of glass. If too much heat is liberated in the arc, the batch is sintered and trode 25 and the carbon electrode 26. ing circuit comprises the body of molten glass reduced to a semi-molten mass containing gas and some incompletely melted materials faster than the body of glass below can assimilate them and convert them into a homogeneous mass. Ii' too small a proportion of the heat is developed in the arc, then the glass in the pool will be heated to a higher temperature than necessary, resulting in a waste of power.

It is possible to design a furnace of proper dimensions so that a circuit from the bottom electrode 24 to the top of the carbon electrode will give the proper balance providing the furnace is used for only one specic glass. Such particular design of furnace would not serve satisfactorily for glasses of different formulae because such glasses of different compositions have different electrical characteristics, and the volt ampere characteristics suitable for one glass may not be suitable for another. In accordance with the present invention the furnace as herein disclosedV is designed to melt any and all glasses or similar refractories and the electrical heating circuits have accordingly been made very flexible.

Itwill be observed that, as shown in Fig. 2, there are four separate heating circuits, each controllable separately from the others. Two of these circuits supply power from the transformers I and 34 respectively for heating the electrodes 24 and 25 as resistors and are maintained primarily for starting the melting operations and for controlling the glass issuing from the furnace. but

may be used further to supply additional heat when and where needed during the continued melting and refining operations. vThe current transmitted from the transformer through the feeder bushing 21 supplies localized heat to the walls of the outlet 28 for regulating the temperature and fluency of the issuing glass at the outlet and thereby also effectively controlling the rate of flow. f

The third circuit extends between the electrodes 24 and 25 and impresses a suitable voltage upon the glass between these electrodes in the refining compartment and is designed to give the proper amount of current flow through the glass. Since the electrical resistance of the glass decreases with an increase in temperature, we employ a variable tap type of transformer 31 to control the amount of heat generated in this portion of the furnace. A reactor is also included in this heating circuit, and there is provided automatic means controlled by the current flow within the circult'to adjust and control the reactance and thereby automatically regulate and control the current flow and the amount of heat supplied to the glass, all as hereinafter more fully set forth.

The fourth circuit is that comprising the elec- This heatabove the electrode 25, the arc between the glass and the carbon electrode and a reactor in the circuit externally of the furnace. This circuit also has associated therewith automatic means for controlling the volume of current flowing, substantially as above noted in connection with the lower circuit including the electrodes 24 and 25.

With an electric furnace of the type herein shown, designed for melting and fining glass, it is found necessary to provide automatic means for controlling the volume of current flow in order to obtain satisfactory operation. The electrical resistance of the glass depends on the temperature and as the temperature of the glass rises, the

resistance rapidly decreases. Owing to this negative. characteristic, any rise in temperature, by decreasing the resistance, tends to increase the volume of current now, resulting in a further rise in temperature and further increase of current. Without adequate means of control. the temperature is thus quickly raised to a point which not only overheats the glass but is destructive of the furnace itself. In order to counteract this tendency and to stabilize the current flow, we provide in the external circuit means for producing a certain amount of reactance, such means including one or more reactors such as will be described presently. Any rise in the temperature of the glass causing an increase of current flow, operates opposing such current increase. In this manner a substantially constant current flow may be maintained automatically within a working temperature range, regardlcsspf variations in the temperature of the glass within said range. In such a circuit, under normal working conditions and with a normal current flow, the reactance or the impedance externally of the furnace should be at least equal to or greater than the resistance of the internal circuit through the arc and molten glass in order to stabilize the current. If the reactance is less than the said internal resistance. such reactance is inadequate to fully compensate for the lowering of the internal resistance when the temperature of the glass rises. As a result the current increases and the temperature continues to rise at an accelerating rate, causing the furnace to go out of control. On the other hand. if the reactance is greatly in excess of the normal resistance, within the furnace, the operation is inefficient and the loss of power is excessive.' It can be demonstrated that the maximum output which can be obtained in a circuit of this character in which there is sufficient reactance to stabilize the current, is attained when the reaciance is equal to the resistance within the furnace. The power factor for such circuit is then at a maximum, being substantially .707.

In practicing our invention we provide in the external circuit a reactor or reactors which, with normal working conditions and current flow, provides a voltage drop through the reactance somewhat greater than the voltage drop through the glass and the arc, in order to assure stability. The reactance may be provided by means of an air core reactor, by the use of which a substantially constant current flow may be obtained where a constant voltage is impressed on the circuit notwithstanding fiuctuations in the ohmic resistance within the furnace. However, in order to obtain a more adequate control, reduce the liability of an excessive current flow and of the furnace going out of control, and to obtain a more effective control through a comparatively wide variation of temperature conditions, we provide in the external circuit an iron core reactor together with an automatic control system for controlling the magnetic flux in the reactor, in response to variations in the current flow, in a manner to maintain a substantially constant heating current. Such control system will now be described.

Referring to Figs. 2 and 3, the secondary 42 of the transformer 43 impresses an alternating voltage of constant value on the upper circuit through the furnace, including the carbon electrode 26 and electrode 25, and herein termed a heating circuit. Assuming the glass has been heated as by means of the transformer 34 and a current flow established through said heating circuit, the current induces a potential in a current transformer 53 in the control circuit. This causes a voltage drop across a variable resistor I in the control circuit. 'I'he resistor 5| may be adjusted to variably adjust the voltage drop in accordance with the particular volume of current flow which it is desired to maintain within the heating circuit. The control circuit comprises a series of vacuum tubes including amplifier tubes I and 3 and rectifier tubes 2, 4, 5 and 6. 'I'he voltage supplied by the current transformer 50 is impressed on the grid circuit comprising the grid 52 of the amplifier tube I. The output of the tube I passes through a rectifier tube circuit including condensers 53, 54, a resistor 55 and rectifier tube 2. The rectified alternating current from the tube 2 passes through a resistor 56.

The amplifier tube 3 has its control grid 51 connected to one terminal of a battery 58 which provides a reference voltage of a value and polarity tending normally to maintain the current flow through the tube 3 at a minimum for the particular tube employed. The resistor 56 is in series with the battery 58 and is arranged to supply a voltage drop of opposite polarity to the reference voltage provided by the battery so that an increase in the voltage drop across the resistor greater than the reference voltage will cause a current flow through the amplifier tube 3 corresponding to the amount that said voltage drop exceeds the reference voltage.

The rectier tubes 4 and 5 are connected to provide a full wave rectification. For this purpose they have their plate circuits connected by conductors 6I, 60' through transformers 59 and 60 to a source of of alternating current. Control grids 68 and 69 for the tubes 4 and 5 respectively are connected to a battery 69' which provides a voltage having a value and polarity tending normally to maintain the flow of rectified current through the tubes 4 and 5 at a maximum for the particular tubes employed.

' The output current from the amplifier tube 3 passes through resistors 10 and 1I and causes a voltage drop which is in series with but of opposite polarity to the voltage of the battery 69'. Any increase of the Voltage drop across the resistors 1U, 1I, of a value exceeding the reference voltage supplied by the battery 69' causes a decrease in the current through the grid controlled rectier tubes 4 and 5 corresponding to the amount the voltage drop exceeds the reference voltage.

The rectiiied controlled output from the tubes 4 and 5 passes through conductors 63, 66 shunted by a condenser 64 and a resistor 65 and to a direct current winding 'I2 on a saturable iron core reactor 13 in the heating and melting circuit which extends through the electrodes 26 and 25. In addition to the A. C. coils 14 and 15 of the iron core reactor, there may be provided also a reactance coil 16 in the heating circuit. This latter may be an air core reactor of greater or less capacity to supplement the iron core reactor. A circuit indicated generally at 'l1 and including the full wave rectifier tube 6, is provided to supply the plate and screen grid circuits of the amplifier tubes I and 3.

Recapitulating the operation of the control system, the voltage drop induced in the circuit of the resistor 5I, by the current in the heating circuit, is amplified by the tube I and rectified by the tube 2. When the voltage drop through resistor 5I exceeds a predetermined value de- 78 pending upon the adjustment of said resistor, such drop exceeds the voltage provided by the battery 58 and causes an increase in current now through the amplifier tube 3. The increased voltage drop through the resistors 13 and 1I due to such increased flow, causes a decrease of the output from the tubes 4 and 5 and thereby de creases the current flowing through the Winding 12 ofthe iron core reactor. Thus it will be seen that an increase in the current through the heating circuit above the predetermined value, causes a decrease in the current owing through the winding 12. This decreases the D. C. flux in the core of the reactor to decrease the degree of saturation thereof in effect and increase the effective reactance of the iron core reactor and thereby reduce the flow of current through the furnace heating circuit. In this manner a substantially constant flow of current through the heating circuit may be maintained with only slight fluctuations through a comparatively wide range of variations in the resistance of the heating circuit.

The direct current through the coil 12 of the iron core reactor may be sumcient to substantially saturate the core when the current flow through the heating circuit is normal so that any decrease in the current through said coil 'l2 will decrease the D. C. magnetic flux through the reactor, permitting the reactance of the coils 14, 15, to be correspondingly increased, thereby effectively reducing the current in the heating circuit.

The current flow through the lower heating circuit, including the electrodes 25 and 24 may Abe controlled and stabilized in the same manner as above described in connection with the upper heating circuit, and by means of a substantially similar controlling system. For this purpose the lower heating circuit includes an iron core reactor 13a corresponding to the reactor 13. Current flow through the direct current coil 12'* of -the reactor is induced by a current transformer lill*1L operating through a vacuum tube control system such as above described.

The modified control system illustrated in Fig. 4, and which may be used as a substitute for the system disclosed in Fig. 3, will now be described. The current supplied by the current transformer 50 and variably regulated and controlled by the variable resistance 5I, is impressed on the solenoid of a balancing relay 8|. The resistor 5I' is so adjusted that when a current of normal volume is flowing through the heating circuit the relay 8| is balanced and the contacts 83 and 34 thereof are held open. The contacts 83 and 34 are in the circuits respectively of the magnet coil 85 of a relay 86 and the magnet coil 81 of a relay 88. Said magnet coils are connected in circuit with the mains 89 and 90 of an alternating current supply circuit.

If the current flow through the heating circuit for the furnace should increase above the normal flow determined by the setting of the resistor 5|, the coil 80 of the balancing relay will operate to close the contacts 83 and thereby complete a circuit through the magnet coil 85. Such circuit extends from the main 89 through coil 85, conductor 8|, contacts 83 and conductor 92 to main 9D. The coil 85 being energized closes the relay 86 and thereby establishes a circuit for a rheostat motor 93, which circuit may be traced from the main 90 through relay 86, conductor 84, field coil of the motor 93 and through the motor and wire 96 to the main 89. The motor 33 being thus energized is rotated in one direction and operates through a connection 91 to rotate the movable element 98 of a rheostat 99 in a clockwise direction.

The rheostat is connected in a circuit including the coil 'l2 of the iron core reactor and a direct current generator |00. This circuit may be l. plier, means for amplifying the current output traced from one brush of the generator through a conductor rheostat 99, conductor |02, coil 'l2 and conductors |03 and |04 to the other brush of the generator. The latter runs continuously and supplies a current through the coil 12 of a value depending on the amount of resistance in the rheostat. When the balancing relay 8| has operated in the manner above described to cause operation of the motor 93, the rheostat 99 introduces additional resistance into the circuit of the generator |00, thereby reducing the current flow through the coil l2. This results in decreasing the D. C. magnetic flux in the iron core thereby increasing the reactance of the iron core reactor I3 with a consequent reduction of the current flow in the heating circuit.

If the heating current should fall below normal, the coil 80 operates the balancing relay to close contacts 84, thereby energizing the magnet coil 81 and closing the relay 88. This establishes a circuit through a field winding |05 of the rheostat motor S3 and operates it in the direction opposite to that above described, thereby cutting out resistance from the rheostat 99. This increases the current output from the generator |00 so that the coil 12 operates to increase the D. C. magnetic flux through the iron core reactor 13. This in turn reduces the reactance in the heating circuit, permitting an increased heating current flow. i

The control system disclosed in Fig. 4 may be used either with the heating circuit comprising the electrodes 25 and 26 or the lower heating circuit comprising electrodes 25 and 24, or with both said circuits, or may be used in conjunction with the control system shown in Fig. 3.

Modiiications may be resorted to within the spirit and scope of our invention.

We claim:

1. A glass heating and melting electric circuit including a body of molten glass therein, means for impressing on the circuit a periodic electromotive force and causing an electric current to pass through said glass, stabilizing means for maintaining the current flow in said circuit at substantially constant amperage, said stabilizing means including an iron core reactor in said circuit, an electrical control system including a current transformer arranged to be energized by the current in said circuit, a direct current winding on the core of said reactor, a vacuum tube amplifier connected to receive its input from the transformer, a vacuum tube rectiiier connected to receive the current output from the said amfrom said rectiiier, means for increasingand decreasing saidampliiled output in response to increases and decreases in the current flow of the load circuit, and means for supplying current through said direct current winding variable directly with and in response to variations in said amplified output, and thereby varying the reactance in said reactor in response to variations in the amperage of said load circuit.

2. A glass heating and melting electric circuit including a body of molten glass therein, means for impressing on the circuit a periodic electromotive force and causing an electric current to pass through said glass, stabilizing means for maintaining the current flow in said circuit at substantially constant amperage, said stabilizing means including an iron core reactor in said circuit, a direct current winding on the core oi' said reactor, an electrical contro1 system including a current transformer arranged to be energized by the current in said circuit, a full-wave vacuum tube rectifier connected at its input side toasourceof alternating current and at its output side to said direct current winding, and a control grid in connection with said vacuum tube rectifier connected through a rectifier to said current transformer, whereby variations in the heating circuit current act to vary the output of said vacuum tube rectier inversely and thereby change the reactance of said reactor in direct relation to the change of heating circuit current to stabilize said current.

3. A glass heating and melting electric circuit including a body of molten glass therein, means for impressing on the circuit a periodic electromotive force and causing an electric current to pass through said glass, stabilizing means for maintaining the current flow in said circuit at substantially constant amperage, said stabilizing means including an iron core reactor in said circuit, a direct current winding on the core of said reactor, an electrical control system including a current transformer arranged to be energized by the current in said circuit, a full-wave vacuum tube rectifier connected at its input side toasource of alternatingcurrent and at its output side to said direct current Winding, and control grids for said vacuum tube rectifier in circuit with said current transformer, and a vacuum tube rectifler and a vacuum tube amplifier in said lastnamed circuit, whereby variations in the heating circuit current act to vary the output of said full-wave vacuum tube rectifier inversely and thereby change the reactance of said reactor in direct relation to the change of heating circuit current to stabilize said current.

- JAMES SLAYTER.

FLAVIUS W. ATKINSON. ED FLETCHER.

HARRY V. SMITH.

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