US3869593A

US3869593A - Heating device

Info

Publication number: US3869593A
Application number: US308503A
Authority: US
Inventors: George William New; Alan Lawrence Hare
Original assignee: British Titan Ltd
Current assignee: British Titan Ltd
Priority date: 1971-12-09
Filing date: 1972-11-21
Publication date: 1975-03-04
Anticipated expiration: 1992-03-04
Also published as: GB1360659A; DE2255483C2; DE2255483A1

Abstract

A heating device including a pair of spaced electrodes one of which is hollow and has an outlet for a gas, an intermediate hollow electrode positioned between the principal electrodes, a control circuit including a control switch and a current sensing device connected between one of the principal electrodes and the intermediate electrode such that the current flow sensing device provides an indication of the current flowing between the principal electrodes and the intermediate electrode including a control switch and a current sensing device, an operating switch to render the control switch non-conducting and gas inlet means located between the principal electrodes and the intermediate electrode.

Description

United States Patent 1191 New et al.

[ Mar. 4, 1975 1 HEATING DEVICE [73] Assignee: British Titan Limited, Billingham,

Teesside, England 22 Filed: Nov. 21, 1972 21 Appl. No.: 308,503

[30] Foreign Application Priority Data Dec. 9. 1971 Great Britain 57155/71 [52] U.S.Cl 219/121 P,315/11l 1511 Int. CL... B23k 9/00 [58] Field of Search 219/121 P, 74, 75; 315/111; 313/731 [56] References Cited UNITED STATES PATENTS 2.922.869 1/1960 Giannini et a1 219/121 P 2.941.063 6/1960 Ducati et a1 219/121 P X 3.205.338 9/1965 Sunnen 219/121 P 3.297.899 1/1967 Pratt et a1. 313/231 3.309.550 3/1967 Wolf et a1 219/121 P X 3.344.256 9/1967 Anderson 219/121 P 3.375.392 3/1968 Brzozowski ct a1. 219/121 P X HEATED GAS OUT 3,538,378 11/1970 Kemeny et al 315/111 3,654,513 4/1972 Hammer 315/111 3.745.321 7/1973 Shapiro et a1. 219/121 P 3.760.145 9/1973 Wolf et a1 219/121 P 3.760.151 9/1973 Wolf et a1. 219/383 Primary Examiner.l. V. Truhe Assistant E.raminer-G. R. Peterson Attorney, Agent, or Firm-Schuyler. Birch, Swindle-r, McKie & Beckett [5 7] ABSTRACT A heating device including a pair of spaced electrodes one of which is hollow and has an outlet for a gas. an intermediate hollow electrode positioned between the principal electrodes, a control circuit including a control switch and a current sensing device connected between one of the principal electrodes and the intermediate electrode such that the current flow sensing device provides an indication of the current flowing between the principal electrodes and the intermediate electrode including a control switch and a current sensing device. an operating switch to render the control switch non-conducting and gas inlet means located between the principal electrodes and the intermediate electrode.

17 Claims, 3 Drawing Figures PATENTED 41975 3.869.593

' sum 1 or 2 HEATED GAS OUT GAS IN HEATED GAS OUT GAS IN HEATING DEVICE This invention relates to a heating device and particularly to an arc heating device.

According to the present invention a heating device comprises a pair of axially spaced principal electrodes, one at least of which is hollow and has an outlet for a gas, connectable to a source of high energy electric current; an intermediate hollow electrode located between said pair of principal electrodes and electrically insulated therefrom; a control circuit between one of said principal electrodes and said intermediate electrode comprising a control switch and a current flow sensing device; an operating switch to render the control switch electrically non-conducting; and gas inlet means located between each principal electrode and the intermediate electrode to introduce a gas into the hollow electrodes.

Preferably, the control switch is a silicon controlled rectifier more commonly referred to as a thyristor.

Heating devices having a pair of principal electrodes spaced apart by at least one intermediate electrode are advantageous when used for heating a gas in that a higher are voltage can be maintained than when the intermediate electrode is omitted and this results in an increased transference of energy to the gas for a constant gas flow rate and constant arc current. Whilst it is the object of maintaining the arc between the principal electrodes, it has been found difficult to do this throughout the heating operation. There is a tendency for the arc to wander between one of the principal electrodes and the intermediate electrode.

The heating device of the present invention includes a control circuit which permits the intermediate electrode to be switched out of circuit when the arc has been established between the principal electrodes. The control circuit includes a control switch which renders the control circuit non-conducting and the switch must be one which can be opened quickly and one in which any tendency to arcing across the switch is substantially eliminated. For this reason preferably the control switch is a silicon controlled rectifier commonly known as a thyristor.

To enable the control circuit to be broken the control switch, preferably a thyristor, must be opened or switched off and it is necessary for the heating device of the present invention to include a current flow sensing device in this circuit to detect the flow of current between the principal electrodes and the intermediate electrode. This current flow sensing device can be a suitably positioned shunt connected to an oscilloscope to give a visual indication when current flow through the intermediate electrode is ceasing. Alternatively, the current flow sensing device can be one which measures directly the flow of current through the thyristor from the intermediate electrode and initiate via appropriate circuitry, opening of an operating switch to switch off the thyristor trigger signal.

The operating switch usually is included in the trigger circuit of the thyristor and can be any suitable manual or automatic switch. For instance, the operating switch can be a liquid mercury switch operable by a solenoid activated either manually or automatically. For instance when the current flow sensing device includes an oscilloscope the manual opening of the operating switch can be affected when it is observed that current flow through the control circuit is falling. Usually, it is preferred to switch off the operating switch and hence the trigger circuit when there is a current fiow through the control circuit of less than 200 milliamps. When the current next falls to zero by the arc transferring completely from the intermediateelectrode, the thyristor will switch off.

When a thyristor is used then the control circuit should preferably also include a limiter device to prevent damage to the thyristor due to a too high a rate of current increase when the heating device is first switched on. Usually the limiter device will be a choke of suitable size for the electrical characteristics of the thyristor.

In addition, it has been found desirable to protect the thyristor against high voltage peaks by providing a voltage suppression circuit across the thyristor in the control circuit.

These high voltage peaks are generated by the heating device itself.

Heating devices according to the invention may be used in the welding or cutting of metals or in the production of heated gas flowing at high velocity and in the supply of heat to maintain chemical reactions. Typical chemical reactions are the oxidation of metal halides (including silicon) to the corresponding metal oxide or in the formation of metal carbides by reaction between a metal halide (including silicon) and a source of carbon usually in the vapour phase.

In heating devices according to the invention, particularly those to be used in chemical reactions, the gas (es) which is heated may be an inert gas and/or a reactant. Since some erosion of the electrodes occurs when used it is desirable in many applications that contamination of the gas by electrode material is minimised and often the effect of erosion can be minimised by selecting electrode compositions which are compatible, for example non-colouring, with the particular product of a chemical process.

Depending on the particular use of the arc heating device the principal electrodes and the intermediate electrode may be formed from aluminium, carbon, copper, gold, iron, platinum, silver, titanium, tungsten or zirconium, or from such alloys as aluminummagnesium, copper-gold, copper-platinum-silver, copper-silver, copper-zirconium or various alloy steels.

In the heating device according to the invention the hollow principal electrode usually is the anode but may be the cathode, if desired and is usually open at both ends. The other principal electrode axially spaced from the hollow principal electrode usually forms the cathode but, if desired can be the anode. The cathode may be a solid rod-like electrode but preferably is also hollow but has a closed distal end. The intermediate electrode is also hollow and open at both ends to permit the flow of gas and the arc to pass through the electrode. Usually the hollow electrode is at earth potential.

If desired the hollow principal electrode can have a bore of increasing internal diameter from the end adjacent the intermediate electrode. The bore can be said to be conical and may, have a cone angle of l to 30 preferably 2 to 20 It has been found desirable to position a field coil around the cathode to generate a magnetic field to rotate the are within the cathode to reduce erosion of the electrodes.

When the outer electrode has a conical bore then it has been found advantageous to apply a magnetic field to the are from a field coil around this electrode. An increase in gas enthalpy has been observed. This magnetic field is usually applied in addition to that applied to the other principal electrode. A suitable range of peak magnetic flux density is 0.0l to 4.0 tesla with a preferred range of 0.05 to 1.0 tesla.

Usually, it is necessary to provide cooling of the electrode during use and to enable this to be done the elctrodes will usually either be surrounded by cooling chambers or have channels in their walls to permit circulation of a cooling fluid. Preferably, the cooling fluid is water of a controlled quantity. Preferably the water has a heat flux capacity of at least 100 watts. The use of such cooling water is described and claimed in our co-pending patent application No. 50521/71. Preferably the water contains a nucleating agent for bubble formation and such agents are an alcohol, an organic acid or an inorganic acid salt of a metal such as aluminturn.

The heating device is provided with suitable inlets located between the intermediate electrode and the principal electrodes for a gas to be heated by passage through the device. If desired, these inlets can be so shaped and/or positioned to provide a tangential and/or helical gas flow through the device. If multiple gas injection points are employed then the gas must be introduced in the same manner and sense, e.g. tangential and clockwise between the electrodes.

The heating device can be operated by any suitable power source and the high energy electric current required to strike and maintain the arc can be high voltage-low current, low voltage-high current or high current and high voltage. For instance, the device can be operated in the range l volts to 20,000 volts and from amps. to 5,000 amps.

The heating device of the present invention is particularly useful in the manufacture of titanium dioxide by the vapour phase oxidation process. In this process, a titanium tetrahalide, e.g. the tetrachloride is oxidised in the vapour phase to produce pigmentary titanium dioxide. The oxidation process is carried out at an elevated temperature and it is necessary to preheat one or both of the reactants or an inert gas to a sufficiently high temperature to initiate the reaction. The heating device of the present invention is preferably used to heat oxygen or an inert gas in the process for the production of pigmentary titanium dioxide.

One form of heating device constructed in accordance with the invention will now be described by way of example only with reference to the accompanying drawings, in which FIG. 1 is a longitudinal sectional view through the device.

FIG. 2 is a diagrammatic view of the electric circuit.

FIG. 3 is a sectional view similar to FIG. 1 illustrating a modified form of the device.

As shown in FIG. 1 the device includes a hollow front electrode 1 which is the anode and axially spaced .therefrom is the cathode 2. An intermediate hollow electrode 3 is mounted between the anode 1 and cathode 2 and is electrically insulated therefrom by means of insulating

members

4 and 5.

The anode 1 is shaped to provide an annular channel 6 through which the gas to be heated is passed via

inlet pipes

7 and 8 and thence to the interior 9 of the hollow anode. The cathode 2 is also shaped to provide a similar annular channel 10 supplied by

inlet pipes

11, 12.

Each electrode 1 and 2 is surrounded by a chamber through which cooling water can be passed but this is not'shown on the drawings.

The control circuit for the device is shown in FIG. 2. The anode 1 and intermediate electrode are electrically connected via a thyristor 13 provided with a water cooled copper block 14 for cooling. Cooling water is circulated through the block 14 via inlet 15 and outlet 16. A choke 17 and current flow sensing device 18 incorporating an oscilloscope are connected in the control circuit. The oscilloscope is connected to give a vi- :sual indication of the current flowing through the control circuit. The choke 17 prevents too rapid an increase in current through the thyristor and thereby reduces the risk of inadvertent damage to the thyristor 13 choke 17 is connected by a lead 29 to intermediate electrode 3.

Connected across the thyristor 13 is a protection circuit 19 to protect the thyristor 13 against high voltage peaks and this protection circuit includes a resistor 20 in series with a resistor 21 and a condenser 22 connected in parallel.

The trigger circuit 23 for the thyristor 13 includes a a supply 24 of stabilised, smoothed direct current, a liquid mercury filled switch 25 and a resistor 26 to ensure that there is no potential difference across the trigger circuit 23 at the thyristor 13 when the switch 25 is open.

A source of high energy electric current is connected across the anode 1 and cathode 2 via leads 27 and 28 and the anode 1 is earthed.

In operation the trigger circuit 23 is energised by closing switch 25 and a high energy direct electric current is fed across the anode l and cathode 2 via leads 27 and 28. An arc is struck initially between the cathode 2 and the intermediate electrode 3 since the thyristor is conducting.

Gas is passed into the chamber 10 and into the interior of the intermediate electrode and the arc is transferred for a high proportion of the time on to the anode 1. Thistransference is effected by adjustment of the current and flow of gas. The current flow sensing device 18 indicates via the oscilloscope when the arc is transferred since current flow through the control circuit falls. When this current flow has fallen to, say, less than 200 milliamps then the switch 25 is opened to open the trigger circuit. When the trigger circuit is switched off the thyristor 13 is rendered potentially non-conducting and when the electrical flow to the intermediate electrode next falls to zero the arc is stabilised between the anode 1 and cathode 2.

Gas is then passed through

inlets

7 and 8 and this gas, together with that admitted through

inlets

11,12 is heated by the arc and passed out of the device through the anode 1.

FIG. 3 shows a device which is in many respect similar to that shown in FIG. 1 with like parts being numbered similarly. The hollow front electrode 30 in this device has an increasing internal diameter towards the outer end and is provided with a field coil 31 surrounding the electrode. The additional field coil 32 surrounds the cathode 2. The device is connected to a control circuit in a manner similar to that shown in FIG. 2.

In the heating device of the present invention, more than one intermediate electrode can be present and in such a device each intermediate electrode will form part of a control circuit so that progressive switching of the are from one intermediate electrode to the next and finally on to the principal electrodes takes place.

The invention is also illustrated in the folllowing Examples.

EXAMPLE l A heating device with a control circuit as show in FIGS. 1 and 2 was set up. The anode l was at earth potential and had an internal bore of 1.5 inches. The intermediate electrode 3 had an internal bore of 1.25 inches and was 6 inches long. A magnetic field of 0.25 tesla maximum was applied to the cathode.

The circuits were energised and gas passed through inlets l1 and 12. The are was struck and when the gas flow rate and current had been increased to transfer the arc for the majority of the time on to the anode, the switch 25 was opened and the arc was stabilised between the anode and cathode. Gas was then also passed into the device through

inlets

7 and 8. The gas was oxygen.

The total gas flow was 12,000 standard cubic feet per hour. The arc current was 300 amps. and the arc voltage was l,850 volts.

In a device without the intermediate electrode with the same gas flow and arc current, the arc voltage was 1,525 volts.

The device of the present invention clearly operated at a high voltage and there was a corresponding proportionate increase in average gas enthalpy.

EXAMPLE 2 The method of example 1 was repeated except that the outer electrode 1 was made the cathode and no magnetic field was applied to the cathode. An arc voltage of 1,900 volts was obtained on energizing the circuits but in a similar device without the intermediate electrode an arc voltage of only 1,600 volts was obtained.

EXAMPLE 3 The experiment described in Example 1 was repeated except that the anode l was provided with an outer field coil to apply a peak magnetic field of 0.15 tesla. On energizing the device as described in Example 1, with the intermediate electrode in position an arc voltage of 1,800 volts was obtained.

In a similar device without the intermediate electrode an arc voltage of 1,525 volts was obtained.

These results show that by applying a magnetic field to a non-conical electrode no significant effect is observed on the performance of the device.

EXAMPLE 4 In this example the apparatus described in Example 1 was modified so that the anode l was used at earth potential and had an internal diameter of 0.75 inch at its narrowest part and had a conical inner bore with a cone angle of 2.5. The intermediate electrode had an internal bore of 0.375 inches and was 5 inches long. A magnetic field of peak strength 0.25 tesla was applied to the cathode. Total gas flow was 750 standard cubic feet per hour.

The are current was 80 amps and an arc voltage of 600 volts was obtained when the circuits were energized.

When the intermediate electrode was omitted in a similar device it had an arc voltage of 525 volts.

EXAMPLE 5 Example 4 was repeated except that the conical outer electrode was provided with a magnetic field of peak strength 0.15 tesla. When so used the device produced an arc voltage of 800 volts which was appreciably greater than that when no magnetic field was applied to the anode.

EXAMPLE 6 The experiment described in Example 4 was repeated except that a magnetic field of peak strength 0.325 tesla was applied to the anode and the gas flow rate was 1,150 standard cubic feet per hour with an arc current of amps. On energizing the circuits an arc voltage of 990 volts was obtained when the intermediate electrode was in position.

In a similar device without an intermediate electrode an arc voltage of 700 volts was obtained.

What is claimed is:

1. A heating device comprising a pair of axially spaced principal electrodes, at least one of which is hollow and has an outlet for a gas; said principal electrodes being connected to a high energy electric current; an intermediate hollow electrode located between said pair of principal electrodes and electrically insulated therefrom; a control circuit comprising a control switch and a current flow sensing device connected between one of said principal electrodes and said intermediate electrode such that said current flow sensing device senses the current flowing between one of said principal electrodes and said intermediate electrode; an operating switch electrically connected to said control switch and operable to render said control switch nonconductive when the current between one of said principal electrodes and said intermediate electrode is sensed to have fallen below a predetermined value; gas inlet means disposed between each of said principal electrodes and said intermediate electrodes for introducing gas into said hollow intermediate and said hollow principal electrodes.

2. A heating device according to claim 1 in which the control switch is a silicon controlled rectifier.

3. A heating device according to claim 1 in which the operating switch is a liquid mercury switch operable by a solenoid.

4. A heating device according to claim 2 in which the control circuit includes a limiter device to prevent damage to the silicon controlled rectifier, as a result of too high a rate of current increase upon activation of the device.

' 5. A heating device according to claim 4 in which the limiter device is a choke of suitable size for the electrical characteristics of the silicon controlled rectifier.

6. A heating device according to claim 2 in which a voltage suppression circuit is provided across the silicon controlled rectifier in the control circuit.

7. A heating device according to claim I in which said hollow principal electrode is open at both ends.

8. A heating device according to claim 7 in which said hollow principal electrode has a bore of increasing internal diameter from the end adjacent the intermediate electrode.

9. A heating device according to claim 8 in which the inner surface of said hollow electrode is conical and has a cone angle of from 1 to 30.

10. A heating device according to claim 9 in which said cone angle is from 2 to 11. A heating device according to claim 8 in which a field coil is positioned around said hollow principal electrode to provide a peak magnetic flux density of from 0.01 to 4.0 tesla when in operation.

12. A heating device according to claim 11 in which said field coil is such as to provide a peak magnetic flux density of 0.05 to 1.0 tesla when in operation.

13. A heating device according to claim 1 in which the other principal electrode is hollow and has a closed distal end opposite the hollow intermediate electrode.

14. A heating device according to claim 13 in which said other principal electrode has a field coil around amps to 5,000 amps.

Claims

5. A heating device according to claim 4 in which the limiter device is a choke of suitable size for the electrical characteristics of the silicon controlled rectifier.

7. A heating device according to claim 1 in which said hollow principal electrode is open at both ends.

9. A heating device according to claim 8 in which the inner surface of said hollow electrode is conical and has a cone angle of from 1* to 30*.

10. A heating device according to claim 9 in which said cone angle is from 2* to 20*.

11. A heating device according to claim 8 in which a field coil is positioned around said hollow principal electrode to provide a peak magnetic flux density of from 0.01 to 4.0 tesla when in operation.

14. A heating device according to claim 13 in which said other principal electrode has a field coil around the electrode to generate a magnetic field within the electrode when in use.

15. A heating device according to claim 1 wherein means are provided to cool the electrodes when in use.

16. A heating device according to claim 1 in which the gas inlet means are shaped and/or positioned to provide a tangential or helical gas flow through the device when in use.

17. A heating device according to claim 1 having a power source sufficient to provide a high energy electric current of from 100 to 20,000 volts and from 10 amps to 5,000 amps.