GB2329981A - Electrical control system for a glass furnace - Google Patents

Abstract

A glass furnace or forehearth 1 has a plurality of electrodes 2a supplied with AC by a regulating transformer 3. In order to reduce the formation of bubbles by residual DC currents, the DC voltages at the electrodes are maintained at desired levels by injecting appropriate DC currents at selected locations. The voltage at electrode 2a is sensed and filtered 8 to form a DC component VE which is compared 9 with a desired voltage VR. The resulting difference controls a current source 10 which injects a current I into the electrode 2a. This procedure is carried out at a number of locations throughout the furnace. Each of the sensed voltages VE and injected currents I is transmitted in digital form to a microcontroller 12 which calculates appropriate values for the desired voltages VR, and also minimises the total injected current.

Description

ELECTRICAL CONTROL SYSTEMS The present invention relates to electrical control systems and finds particular application to electrically heated furnaces and forehearths, and especially glass furnaces and forehearths.

In the production of glass articles a refractory structure is used to contain the molten glass. This structure is designed to retain the glass for a sufficient time to enable melting and refining of the raw materials.

A refractory channel is attached to the melting structure to enable the flow of the glass to be controlled as it is delivered to processing apparatus. Typical refractory materials used for this purpose contain high concentrations of zirconia.

Such systems use the heating effect of electrical current, known as the Joule effect. Electrodes are immersed in molten glass within the furnace. Since glass acts as an electrically resistive load, electrical power can be delivered to the furnaces to melt and control the temperature of the glass by placing electrodes at strategic points within the furnace. Alternating current is conventionally used, since direct current would give rise to polarisation of the electrodes.

However, even when the electrodes are supplied with alternating current, small residual direct currents occur, which are typically orders of magnitude lower than the main alternating currents. The ac currents are typically 10 to 100 amps supplied at 50 to 400 volts, whereas the dc voltages are of the order of a few volts which give rise to dc currents of a few tens of milliamps. These direct currents give rise to electrolysis, which causes both small bubbles ("seeds") and large bubbles ("blisters") to form within and on the glass. In addition, the electrodes are subject to excessive wear.

An attempt has been made to overcome these problems, as described in published UK Patent Application No. GB-A2091006 which describes a forehearth heating system. In this system, the dc voltage level of the electrodes is sensed, and, in response thereto, direct current is injected into the glass by an additional electrode, in an amount stated to be sufficient to maintain the dc voltage at the electrodes at zero volts.

However, residual direct currents are caused not only by the electrodes presenting a dc voltage other than ground, but also by dc voltages appearing between the electrodes and other electrically conductive objects in contact with the glass, such as the surface of the furnace refractory lining material. Such dc voltages may be generated through a variety of electro-chemical and electro-thermal processes.

In addition, such dc voltages can vary with time.

Furthermore, dc voltages can arise between two parts of a furnace neither of which is at zero volts, so that maintaining one part of the furnace at zero volts would not prevent direct currents from arising. Therefore, simply controlling the voltage level of the electrodes to remain at zero volts has proved insufficient to prevent the formation of bubbles in the glass. Furthermore, in one embodiment, the system requires an additional electrode to be inserted in the glass.

In accordance with the present invention, there is provided an electrical control system for controlling direct currents flowing in a furnace which is heated electrically using alternating current, the system comprising means enabling a user to select a desired reference potential and means for controlling the dc voltage level at a location within the furnace by measuring the dc voltage level at a position within the furnace and, in dependence on the measurement, supplying sufficient current to maintain the dc voltage level at said location at the selected reference potential.

The self-adjusting nature of the invention permits correct operation of the control system, with minimal manual intervention. A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a diagrammatic representation of a glass forehearth and an electrical power supply connected to electrodes within the forehearth; and Figure 2 is a diagrammatic representation of the control circuitry in accordance with the preferred embodiment of the present invention.

With reference to Figure 1, a glass forehearth 1 is heated using immersed electrodes 2a - 2d which are supplied with alternating current sufficient to melt the glass and maintain it in its molten state. Tin oxide electrodes may be used, these being suitable for melting lead glasses.

Power control to the furnace is achieved by using a regulating transformer 3, the secondary windings 4 of which are connected to the heating electrodes 2a - 2d. Power control to the forehearth heating electrodes is achieved by using semi-conductor thyristor devices 5 connected in series with the primary winding 6 of the power transformer 3.

Thus, an ac power source is connected to the primary winding 6 of the transformer 3 via the thyristor devices 5. Each secondary winding 4 of the power transformer 3 is connected to one or more pairs of heating electrodes.

With reference to Figure 2, the electrode 2a is additionally connected to an electrical control system which is arranged for controlling the dc voltage level on the electrode so as to maintain it at a value VR This is achieved by the following.

The voltage at electrode 2a comprises a large ac component and a relatively small dc component. Ths electrode 2a is connected to a filter circuit 8 whizz attenuates the ac component thereby to isolate and extract the dc component. The filter circuit is formed from c suitable resistor-capacitor network and connected to c common reference point CR having the potential with respect to which VR is sensed. The dc component VE extracted by the filter circuit 8 is supplied to a first input of c- comparator 9. A signal at the desired voltage level VR iE supplied to the second input of the comparator 9. Thz comparator 9 generates an output signal which is a functior of the difference between VE and VR, which is supplied to e current source/sink circuit 10, which is itself connected ir a feedback loop to the electrode 2a. The current source sink circuit 10 is powered by a power supply unit 11 which supplies the current source/sink circuit 10 with power us inc negative and positive voltage rails having the same voltage magnitude relative to the common reference potential CR.

The current source/sink 10 causes currents to be inject into or withdrawn from the electrode 2a thereby to maintair the dc voltage level of the electrode 2a at the desire level VR. A common reference point at the common reference potential CR serves as the point of return for the inject or withdrawn current.

The dc voltage is sensed relative to the commor reference potential CR, which may be that of a further electrode immersed in the glass, earth or some other point, provided that conductive paths exist between that point an each of the points at which the dc voltage level is sensed.

To prevent the relatively large ac current component from interfering with the current source/sink circuit 10, this circuit incorporates a high-frequency switch.

Alternatively, a choke could be used to isolate the current source/sink circuit 10 from the ac power supply.

It will be appreciated that a major advantage of the control system of the present invention is that it can be retrofitted onto furnaces without the need for additional electrodes or replacement of existing heating systems.

Although Figure 2 indicates that the dc voltage level is sensed at, and dc current injected into or withdrawn from, one of the ac power electrodes, it is possible to perform the voltage sensing and current injection at any suitable location within the glass furnace or forehearth.

However, it is desirable that the current is injected at the same location as that at which the dc voltage level is sensed.

In practice, different pairs of electrodes, which are connected to different secondary windings of the transformer 3, will be controlled by separate respective electrical control systems.

Within each electrical control system, signals indicative of the injected current and the sensed dc voltage level VE are transmitted to the micro-controller circuit 12 via a common digital communications bus. This serves additionally to transmit signals indicative of the desired voltage levels VR to the respective comparator 9. The analogue voltage level is then generated using a digitalto-analogue converter 13. Similarly, analogue-to-digital converters 14 and 15 are provided within each electrical control system to convert the current and dc voltage levels to digital representations which are then transmitted on the digital communications bus to the micro-controller circuit 12.

Typically, the respective amounts of current that will need to be injected or withdrawn to maintain the desired dc voltage levels thereat will differ. However, the possible combinations of these currents, consistent with maintaining the desired voltage levels, is potentially large, because of the multiplicity of electrical conduction paths existing between the electrodes, through the glass, external power cabling, transformer windings and ancillary equipment.

Therefore, to conserve power within the system and to maintain overall system stability, the dc currents are controlled so as to be minimised. This is effected using a micro-controller circuit 12 which is supplied periodically with signals indicative of the current I supplied to or withdrawn from each electrode pair, together with the dc voltage level VE at the electrode pairs. The microcontroller 12 is programmed to effect this current minimisation by way of an algorithm which operates in the following manner. Suppose it is desired that the voltage levels VE(1)...VE(N) are to be maintained at dc voltage levels VT(1)...VT(N) respectively. VR(1)...VR(N) are selectively incremented or decremented (as appropriate) by a small step. Following this the resulting magnitude and rate of change in the corresponding dc currents I(1)...I(N) are noted. The process is repeated indefinitely, on each cycle selecting to increment or decrement only those of VR(1)...VR(N) which result in the lowest relative magnitudes and rates of change of 1(1).. .1(N) respectively, while causing the averages of VE(l).. .VE(N) to converge towards VT(l)...VT(N)

Claims (8)

1. CLAIMS: 1. An electrical control system for controlling direct currents flowing in a furnace which is heated electrically using alternating current, the system comprising means enabling a user to select a desired reference potential and means for controlling the dc voltage level at a location within the furnace by measuring the dc voltage level at a position within the furnace and, in dependence on the measurement, supplying sufficient current to maintain the dc voltage level at said location at substantially the selected reference potential.
2. 2. A system as claimed in Claim 1, wherein said location and said position are the same.
3. 3. A system as claimed in Claim 1 or Claim 2, arranged for controlling the dc voltage level at each of a plurality of locations within the furnace.
4. 4. A system as claimed in Claim 3, wherein said enabling means is arranged to permit a user to select different reference potentials for the different locations.
5. 5. A system as claimed in Claim 3 or Claim 4, further comprising means for selecting the desired values for the dc voltage levels at each location such that the total current supplied is minimised.
6. 6. A system as claimed in any one of Claims 3 to 5, wherein at least one of said locations is an electrode which supplies the alternating current.
7. 7. A system as claimed in any one of Claims 3 to 5, for use with a system wherein alternating current is supplied by electrodes connected to the secondary windings of a transformer, said locations comprising said electrodes, wherein the locations of the electrodes connected to each secondary winding are maintained at the same dc voltage level.
8. 8. An electrical control system substantially as hereinbefore described with reference to the accompanying drawings.