GB2091006A - Forehearth heating system using immersed electrodes - Google Patents

Abstract

A forehearth heating system comprises two sets of electrodes (12a, 12b) extending into a moving mass of glass to be heated. The two sets of electrodes (12a, 12b) are connected respectively to the ends of the secondary winding (16) of a transformer (18). In order to eliminate D.C. voltages which develop on the electrodes (12a, 12b), the secondary winding (16) has an earthed centre- tap and there is provided a control means (26, 28, 30) which senses the magnitude and polarity of the D.C. voltages, generates an electrical signal of opposite polarity, and injects same at a location and at a magnitude such as to reduce said sensed voltages to zero. In one embodiment (Fig. 4), the signal is injected into the glass by an additional electrode (12e) to cause currents to flow to earth via the two sets of electrodes and the associated transformer winding halves, the magnitude of these two currents being such as to reduce the D.C. voltage at the electrodes to zero. In another embodiment (Fig. 5) the signal is introduced into the earth line of the transformer (18). <IMAGE>

Description

SPECIFICATION Forehearth heating system using immersed electrodes The present invention is concerned with the direct heating of materials in the liquid or molten state by the passage of an electric current therethrough.

Although the description which follows is all related to the heating of glass, it may however be applied to the heating of other electrically conductive materials in the molten state where problems of the type explained below are met. The following may also be applied to any form of immersed electrode heating system whether it be for melting, conditioning or holding.

In the production of glass articles, molten glass is conveyed from a glass forming furnace to, for example, a forming machine by means of a forehearth which is essentially a channel extending from the furnace to the forming machine. During passage along the forehearth, the glass has to be heated to maintain it in its correct molten state prior to being discharged, in the form of one or more gobs, into the forming machine.

Several techniques have been conventionally used for heating forehearths. One such conventional technique employs a plurality of gas burners disposed above the surface of the molten glass. However, because the heating is above the glass surface, gas heated forehearths have a very poor thermal efficiency since only a small proportion of the energy goes to heat the glass, the majority being lost in the waste gases which exhaust at very high temperatures. Also, burners must be kept alight even when the associated cooling system is in operation so that gas is burned over a considerable forehearth length resulting only in making the cooling less efficient.

In order to reduce fuel costs, attention has recently been directed to other, more direct forms of heating which have a much higher thermal efficiency. One such method uses a plurality of electrodes immersed directly in the molten glass to pass an A.C. current through the glass and so obtain resistive heating. This technique has the advantage that only a relatively small amount of energy is necessary, compared with the gas burner method, in order to achieve the same heating effect.

A serious problem encountered in practice with immersed electrode heating of glass, however, has been the unwanted formation of seed (small bubbles) and blister within and on the glass, particularly in the production of flint glass.

It is an object of the present invention to substantially eliminate the formation of such seed and blister.

Experimentation has suggested that the primary causes of the formation of these unwanted effects are as follows. Due to the fact that two disimilar materials are in contact, i.e. the electrode material (e.g. molybdenum) and the glass, a partial semi-conductor is formed at each interface. The presence of such partial semiconductors causes partial rectification of the A.C.

current passing through the interface which results in a D.C. voltage appearing across the glass. This in itself is likely to be of no consequence as long as there is no return path for the D.C. voltage to create a D.C. current through the glass. However, in all forehearths at the temperatures involved, the refractory material along which the glass is flowing exhibits a low electrical conductivity. Furthermore, the refractory material is usually contained in a steel casing which in turn is in contact with earth. Hence, there exists a path along which the D.C. current can flow. In addition to the latter path, further paths can be formed at points where earth electrodes (used for screening proposes) enter the forehearth.

A further cause of D.C. voltages appearing on the A.C. electrodes is the thermally generated emfs inevitably generated along metallic electrodes under these conditions.

The presence of the aforegoing self-generated D.C. voltages results in partial electrolysis of the glass in the regions of the electrodes, the result of which is the introduction into the glass of bubbles of gas (mainly SO3) and, although these tend to rise towards the surface, as a result of the viscosity of the glass there is usually insufficient time for them to escape from the glass before the glass is expelled from the forehearth to the forming machine. This therefore leads to unsightly bubbles in the finished product, and in the case of glass, to a reduction in the overall strength of the finished product.

In accordance with the present invention, the formation of seed and blister is reduced by the use of a forehearth heating system which substantially eliminates the self-generated D.C. voltages appearing on the immersed electrodes.

A forehearth heating system in accordance with the present invention comprises at least two spaced sets of electrodes which, in use, are immersed directly in molten glass, the two sets of electrodes being connected respectively to the ends of the secondary of a transformer whereby to pass an AC current through the glass and so obtain resistive heating, and wherein, in order to eliminate self-generated D.C. voltages appearing at the electrodes, the secondary of the power transformer has an earthed centre-tap and there is provided a control means which senses the magnitude and polarity of said D.C. voltages and which generates an electrical signal of opposite polarity to said sensed voltages, and injects same at a location and at a magnitude such as to reduce said sensed voltages to zero.

In one embodiment, said electrical signal can be arranged to be injected into the molten glass by way of an additional electrode, whereby to cause respective currents to flow to earth via the two sets of electrodes and the earthed winding halfs connected thereto, the magnitudes of these two currents being arranged to be such as to reduce said D.C. voltages at the two sets of electrodes to zero.

In another embodiment, the injected electrical signal can comprise a current introduced into the line connecting the transformer secondary centretap to earth the magnitude and polarity of said current being respectively equal and opposite to the sum of the currents in the two halves of the transformer secondary resulting from the presence of said D.C. voltages at the electrodes, whereby the latter currents and hence said D.C. voltages, are cancelled out.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic plan view of a typical forehearth utilising electrode heating; Fig. 2 is a side view of the forehearth of Fig. 1; Fig. 3 is a basic circuit diagram of a conventional electric supply system for a forehearth using immersed electrode heating; Fig. 4 is a basic circuit diagram of part of a first embodiment of a forehearth heating system in accordance with the present invention for such a forehearth; and Fig. 5 is a basic circuit diagram of part of a second embodiment of a forehearth heating system in accordance with the present invention.

The forehearth illustrated schematically in Figs.

1 and 2 comprises basically an elongate channel 10 constructed of heat resistant material. Heating electrodes 1 2a, 1 2b extend into the channel 10 transversely of the longitudinal axis of the channel from opposite sides, respectively. The heating electrodes 1 2a, 1 2b are connected together in groups having a plurality of electrodes, e.g. 6, in each group. Each group of electrodes 1 2a, 1 2b is individually energised to heat a respective one of a plurality of zones into which the channel 10 can be considered to be divided. For the present purposes, only one zone need be considered but it must be borne in mind that a similar arrangement would normally be provided for each zone, respectively.

With reference to Fig. 3, the group of six electrodes 1 2a on one side of the zone under consideration (zone A) are connected by a cable 14 to one end of the secondary winding 16 of a transformer 1 8, the other end of the winding 1 6 being commonly connected to the opposing six electrodes 1 2b in this zone by way of a cable 20.

Opposed end electrodes 1 2c, 1 2d are earthed for screening purposes. The primary winding 22 of the transformer 1 8 is energised by an A.C.

supply, e.g. the mains, via a power regulator 24.

As explained in the introduction hereto, with this known arrangement D.C. voltages are generated at the electrodes 12a, 12b which give rise to D.C. currents through the glass to earth and result in the unwanted production of bubbles.

Referring now to Fig. 4, which shows one embodiment of a control system in accordance with the present invention, identical parts are identified by the same reference numerals as in Fig. 3. The first difference is that the centre-tap of the transformer secondary winding 1 6 is firmly earthed. This has the result of effectively earthing the power electrodes 1 2a 1 2b as far as D.C. is concerned and this eliminates most of the D.C.

currents previously passing through the glass since such currents now have a preferential low resistance path to earth.

However, due to small but significant resistance to D.C. of the electrodes, the cables 1 4, 20 and the transformer secondary 1 6, small but significant D.C. voltages remain present on the electrodes. These remaining D.C. voltages must be eliminated.

This is achieved in the arrangement of Fig. 4 as follows, this figure showing the additional circuitry to be associated with the electrodes 12b. Identical circuitry may also, but not necessarily, be associated with the electrodes 12a. The voltage appearing at the electrodes 1 2b (and/or 12a) is an A.C. voltage with a relatively small D.C.

component, the latter voltage being that which is to be eliminated. The A.C. component is removed by an A.C. filter 26, which can be in the form of a "Q" notch filter and a capacitor/resistor network.

the D.C. component being passed via a buffer amplifier 28 of variable gain to the input of a PID controller 30. The latter controller 30, of which the differential function is probably not necessary, provides two outputs, one of which (line 32) is energised when a positive D.C. voltage is sensed and the other (line 34) of which is energised when a negative D.C. voltage is sensed. The outputs on lines 32 and 34 drive respective amplifiers 36 and 38 whose outputs are commonly coupled to a line 40 which leads, via a choke 42, to an additional electrode 1 2e which is disposed intermediate the electrodes 1 2b or in the region of these electrodes. (The electrode 1 2e could equally well be in the region of the electrodes 1 2a or at a position intermedite the electrodes 12a, 12b, i.e.

its position is not critical provided it lies within the zone A). The outputs on lines 32 and 34 are of such a nature as to create a voltage on the common output line 40 which is of opposite polarity to the voltage sensed on the electrodes 1 2b and of magnitude such as to drive sufficient current through the glass and the line 20 (and through the line 14) as to bring the voltage at the electrodes 1 2b (and 12a) to zero. Thus, in this instance the described system serves to eliminate the D.C. voltages at both sets of electrodes 1 2a and 1 2b and a second control monitoring the electrodes 1 2a is not necessary.

The choke 42 in the output line 40 serves as an impedance buffer against reflected A.C. voltages.

A similar system would normally be associated with each other group of electrodes 1 2 in the forehearth.

By virtue of the aforegoing provisions, the D.C.

voltages and hence the unwanted seed and blisters, have been found to be substantially eliminated.

A second embodiment for achieving the same result is illustrated diagrammaticaily in Fig. 5.

Similar reference numerals are used to denote parts identical to those appearing in Fig. 4.

This embodiment again derives a control signal on one of the two outputs 32, 34 of the PID controller 30 (in dependence upon the magnitude and polarity of the D.C. voltage sensed at the electrodes 12b) but in this case the latter signal is used to drive a current source (or drain) 44 disposed in the earthed line 46 leading to the centre tap of the transformer secondary winding 1 6. As a result of the unwanted D.C. voltages at the electrodes 1 2a and 12b, the leads 14, 20 connecting these electrodes to the ends of the transformer secondary 16 carry currents Ii and i2, respectively. The signal on the line 32 or 34 is arranged to control the current source 44 so as to introduce onto the line 46 a current equal and opposite to the sum (11 + 12) of the currents It, 12 in the lines 14, 20 whereby to bring the D.C. voltage at the electrode 1 2b (and at the electrode 12a) to zero.

Claims (7)

1. A forehearth heating system comprising at least two spaced sets of electrodes which, in use, are immersed directly in molten glass, the two sets of electrodes being connected respectively to the ends of the secondary of a transformer whereby to pass an AC current through the glass and so obtain resistive heating, and wherein, in order to eliminate self-generated D.C. voltages appearing at the electrodes, the secondary of the power transformer has an earthed centre-tap and there is provided a control means which senses the magnitude and polarity of said D.C. voltages and which generates an electrical signal of opposite polarity to said sensed voltages and injects same at a location and at a magnitude such as to reduce said sensed voltages to zero.

2. A system as claimed in claim 1, wherein said electrical signal is arranged to be injected into the molten glass by way of an additional electrode whereby to cause respective currents to flow to earth via the two sets of electrodes and the earthed winding halfs connected thereto, the magnitudes of these two currents being arranged to be such as to reduce said D.C. voltages at the two sets of electrodes to zero.

3. A system n as claimed in claim 1, wherein said injected electrical signal comprises a current introduced into the line connecting the transformer secondary centre-tap to earth the magnitude and polarity of said current being respectively equal and opposite to the sum of the currents in the two halves of the transformer secondary resulting from the presence of said D.C.

voltages at the electrodes, whereby the latter currents and hence said D.C. voltages, are cancelled out.

4. A system as claimed in claim 2, in which the control means includes an AC filter connected to one of said sets of electrodes, and PID controller which responds to the DC output of the AC filter to provide a control signal on one of two outputs in dependence upon the polarity of the sensed D.C.

voltage at said one set of electrodes, said two outputs of the PID controller respectively controlling the output signals of an inverting and a non-inverting amplifier coupled commonly to said additional electrode.

5. A system as claimed in claim 3, in which the control means includes an AC filter connected to one of said sets of electrodes, and a PID controller which responds to the D.C. output of the AC filter to provide a control signal on one of two outputs in dependence upon the polarity of the sensed D.C. voltage at said one set of electrodes, said two output signals of the PID controller controlling the polarity of the current provided by a current generator (or drain) disposed in said line connecting the transformer secondary centre-tap to earth.

6. A forehearth heating system substantially as hereinbefore described with reference to and as illustrated in Fig. 4 of the accompanying drawings.

7. A forehearth heating system substantially as hereinbefore described with reference to and as illustrated in Fig. 5 of the accompanying drawings.