US3456146A - Electric arc plasma burner - Google Patents

Description

July 15, 1969 Filed April 8, 1966 United States Patent 3,456,146 ELECTRIC ARC PLASMA BURNER Giinther Hess, Erlangen, Germany, assignor to Siemens Aktiengesellschaft, Erlangen, Germany, a corporation of Germany Filed Apr. 8, 1966, Ser. No. 541,223

Claims priority, application Germany, Apr. 12, 1965,

Int. Cl. HOlj 61/28 U.S. Cl. 313231 2 Claims ABSTRACT OF THE DISCLOSURE My invention relates to plasma burner for heating gases in an electric arc chamber wherein a burning are located between two electrodes is rotated in a magnetic field.

In plasma burners of this type, a continuous gas flow is heated in an electric arc so that the enthalpy of the gas is increased. With the heat energy absorbed by the gas, high flow velocities can be achieved with the aid of nozzles for use in wind tunnels and jet propulsion where such high fiow velocities are required, or for carrying out chemical reactions at high gas temperatures.

Plasma burners with a rotating are are particularly suitable for transforming a high power from the heated plasma to electricity because the load on the electrode caused by the electric arc is distributed over a large surface. The electrodes can be cooled continuously to such an extent that the foot or base of the electric arc does not burn into or cause vaporization of the respective electrode so that the material of that electrode is thereby protected against being consumed by such vaporization. Rotating arcs are primarily employable when electrodes of heavy metal such a tungsten are not installed in a plasma burner in which the working gas is highly aggressive, but rather metals of lower melting points such as copper are employed as the electrodes. An additional advantage of plasma burners having rotating arcs, besides that of being able to use electrodes of lower mel ing point, is that the arc passes through a large quantity of gas and, due to the turbulence produced, simultaneously heats up to some extent.

The rotation of the arc is carried out in an applied magnetic field whose field lines extend in a direction perpendicular to the are. In a heretofore known conventional plasma burner, the arc burns between a rod-shaped electrode and an annular electrode located coaxial to the rod-shaped electrode so that the arc extends in a radial direction between the electrodes. In this known plasma burner, the electrodes are centrally located in a cylindrical vessel on which a field coil is wound concentrical to the common axis of the electrodes. A disadvantage of this known plasma burner is that only a small part of the magnetic field contributes to the rotation of the arc. Moreover, because of the great difficulty in providing effective cooling, a yoke for concentrating the magnetic "ice field lines cannot be installed due to the lack of sufficient space as well as unsuitable geometry.

There has also been suggested heretofore that the are be produced in a magnetic cusp field between cylindrical electrodes of the same size located along a common axis. Symmetrical proportions and characteristics are thereby produced for cooling the electrodes. However, the disadvantage must then be taken into consideration that the strong central field of the coils cannot be utilized but rather only the portions of the weaker marginal field having radially extending field lines. The production of the magnetic field is consequently considerably more expensive.

In order to increase the heating effect of the plasma burners, it has also been suggested heretofore to construct an arc chamber wall of a stack of ring-shaped electrodes and insulating rings lying therebetween, which are provided with coolant and gas channels. Gas distribution channels are formed in the stacked cylindrical wall of the arc chamber for supplying the working gas between the electrodes. Several arcs can then be formed extending in the axial direction between pairs of ring-shaped electrodes. With this known arrangement, however, the disadvantages of the cusp field must also be taken into consideration.

It is accordingly an object of my invention to provide a plasma burner which avoids the disadvantages of the aforementioned known plasma burners, particularly with regard to the formation of the magnetic field, but without reducing the advantages derived from such known plasma burners.

It is, furthermore, an object of my invention to provide a plasma burner which utilizes the central or core field of the ring-shaped coils for producing the rotation of the arc and, however, supplying the magnetic field through pole shoes to the arc in a beam or narrow bundle.

It is also an additional object of my invention to provide a plasma burner having excitation windings and yoke that are easily cooled.

It is yet another object of my invention to provide a plasma burner wherein the length of the arc and the transformed power therewith can be adjusted to the gas throughput; in addition to widening or broadening the are for greater gas throughput, the length of the arc is controllable by adjusting or shifting the central electrode.

.With the foregoing and other objects in view, I provide a plasma burner having a ring-shaped electrode located between two annular cylindrical pole shoes having end faces extending forward conically with respect to the axis thereof, the pole shoes forming a cylindrical chamber wall carrying excitation windings which are located concentrically about the pole shoes for producing an axial magnetic field. In accordance with the invention, the plasma burner has a construction wherein an axially adjustable central electrode is located in a holder so that it is coaxial to a cylindrical chamber wall and a single ringshaped electrode, the gas which is to be heated being fed between the holder and thet cylindrical chamber wall.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

While the invention has been illustrated and described as plasma burner, it is not intended to be limited to the details shown since various modifications and structural changes may be made therein without departing from the spirit of the present invention and within the scope and range of equivalents of the claims.

The invention, however, together with additional objects and advantages thereof, will be best understood from the following description when read in connection with the accompanying single figure of the drawing showing in axial cross section an embodiment of the plasma burner constructed in accordance with the invention.

As shown in the figure, a ring-shaped electrode 1 is located between two annular cylindrical pole shoes 2 and 3. The electrode 1 and pole shoes 2 and 3 have a common axis 4. The ends of the pole shoes 2 and 3 adjacent one another extend conically in the direction of the axis 4 toward each other so as to form an annular, outwardly widening groove between each other. A central electrode 5 is located centrally to the pole shoes 2 and 3 and to the ring-shaped electrode 1. An electric are 6 is formed and extends between the inner periphery of the electrode 1 and the outer periphery of the central electrode 5 in a direction radial to the axis 4 inside the cylindrical arc chamber 7.

The pole shoe 2 is provided on the inner side of the arc chamber 7 with a sleeve 8 of material having a good heat conductivity such as copper, for example. This sleeve 8 is cylindrical and defines with the pole shoe 2 an annular slot 9 for coolant. An attached annular cover 10 and a thin-walled ring 11 of angular cross section complete the coolant channel 9. They define the distribution and collection channels or respectively the outlet and inlet manifolds for the coolant. As shown in the figure, the tubes 12 supply the coolant, such as water for example, and the tubes 13 remove the coolant.

The pole shoe 3 is essentially mirror-symmetrical to the pole shoe 2. A coolant channel is formed by an annular slot 9 between a cylindrical sleeve 8 and the pole shoe 3, as well as by a ring 11 of angular cross section and a ring-shaped member 14a. All connections are effected either by soldering or brazing. The coolant is supplied to the slot 9 between the sleeve 8 and pole shoe 3 by a tube 12 and is carried away by a tube 13. The ring 11 of angular cross section defines a coolant distribution channel with a side of the pole shoe which faces the gas flow. When it is sufiicient to cool the pole shoe 3, consisting of magnetically conductive material, only in the vicinity of the ring-shaped electrode 1, the coolant can then be supplied through the tube 13 and removed through the tube 12, both of which are shown directly connected to the ring-shaped electrode 1 in the figure.

The magnetic field in the annular slot between the electrodes is concentrated to angles of inclination of about 60 between the ends of the pole shoes and the cylindrical axis due to the conical shape of the pole shoe ends. For the particular ararngement and construction of the pole shoes, reference can be had, moreover, to the experimental knowledge obtained in the field of electronic or magnetic lenses. It is worthy of note that in the immediate surroundings or vicinity of the conical tips of the pole shoes, the field strength is at its greatest, so that a stabilizing effect is produced for the rotational plane of the arc.

The ring-shaped electrode 1 is radially inwardly, stepwise reduced in thickness and fitted between the shoulders of the ring 11 of angular cross section. The ring-shaped electrode 1 is irremovably gripped thereby. The ringshaped electrode 1 is formed of a non-magnetic material, such as copper, which is a good conductor of heat and electric current. It consists of an outer casing 14 having an inner peripheral surface 15 facing the arc chamber 7 on which the path of the foot or base of an arc is located. An intermediate member 16, for example an annular disc with a shoulder 17 extends into the casing of the ring-shaped electrode 1 so as to form a narrowed cross section of the coolant channel between the inner peripheral surface 15 and shoulder 17 through which the coolant at the ring surface 15 flows with great velocity. The ringshaped surface 15 is thus effectively cooled. As aforementioned, the connecting tubes for supplying and removing coolant are indicated by the reference numerals 12 and 13. If these tubes 12 and 13 are formed of a material such as copper, they can also serve simultaneously as electrical connecting leads. The structural components with the pole shoes 2 and 3 can be insulated against the ringshaped electrode 1 by insulating rings such as of ceramic, for example. Instead of employing rings 11 of angular cross section, the conical ends of the pole shoes can also be provided with a surrounding casing which conforms to the illustrated conical shape. The ring-shaped electrode can also be suitably formed with a reduced portion at the middle of the ring. Such a construction has the advantage that the electrodes between the pole shoe structural units remain fixed centrally and immovably. The portion of the coolant channel having a greater cross section which is located at the outer edge of the ring-shaped electrode, acts simultaneously as coolant distributor or coolant collector channel.

The structural units with the pole shoes 2 or 3 and the ring-shaped electrode 1 form part of the cylindrical chamber wall of the arc chamber 7. With an aligned arrangement of these units a streamlined suitably smooth channel wall can be obtained. A smooth channel wall is preferable because projecting edges are difficult to cool and, furthermore, remove a relatively great amount of heat energy from the plasma. The plasma burner constructed in accordance with the invention has an advantage attributable to the central electrode that the magnetic field is strongest in the arc foot or base having the widest path. Annular excitation coils 18 and 19, as Well as 20 and 21, located concentrically to the pole shoes 2 and 3 respectively, serve to produce the axial magnetic field. To achieve an axial magnetic field with coils wound in the same direction, all the annular coils must be connected in the same way. The excitation windings can be wound of copper tubes through which the coolant is conducted. Th electrical sources for exciting the excitation coils are shown schematically by the batteries 22. In the embodiment illustrated in the figure, the excitation coils 18 and 19, as well as 20 and 21, are shown as being wound mirror-symmetrical to one another so that they therefore are to be connected in the manner shown in the drawing. The excitation windings of the pole shoes are so distributed that coil units are formed which can be readily cooled.

The magnetic field is conducted through yokes of material having a good magnetic conductivity in the magnetic serially arranged pole shoes 2 and 3. The excitation windings 18 to 21 can accordingly be mounted within an annular casing 23 which is provided with a slot at the back thereof (shown at the top of the figure) providing access for the connecting tubes 12 and 13, and enclosed within the casing 23 by the annular discs 24 and 25. The particular advantage of permitting a coil core field in a beam or bundle in the pole shoes to react on the arc, can be recognized from the following comparison: If one assumes that the inner diameter of the arc chamber is mm. and the air gap between the pole shoes is 20 mm. wide, a power loss of only about 6 kilowatts has to be anticipated with the magnetic coils for a magnetic field of 15,000 gauss, measured at the axis. If the same field were produced with air core coils alone without any iron core, about kilowatt power loss of the coils would have to be reckoned with. In contrast thereto, for a plasma burner wherein an arc extends axially in a cusp field between cylindrical electrodesthe pole shoes 2 and 3 being considered to be electrodes--a coil power loss of 3000 kilowatts for a radial field strength of 15,000 gauss has to be produced in the absence of an iron core. This disadvantage of the cusp field is due to the fact that only the weaker radial portion of the field lines acts on the arc. With cusp fields it is also much more diflicult to provide yokes for conducting the field.

A neck 26 is mounted on the ring 1411 of angular cross section on one side of the arc chamber 7 and serves for guiding the axially adjustable central electrode 5. An electrically insulating ring providing a gastight seal is located between the ring 14a and the neck 26. The insulating ring can be formed, for example, of a plastic material such as polytetrafiuorethylene. The ring-shaped member 14a of angular cross section and the neck 26 define an annular space in which the working gas is supplied in a tangential direction through bores 27'. The gas which is to be heated then flows between the chamber wall and a holder 27 of the central electrode with an axial and a radial component. Thus, the working gas can be supplied in a tangential direction so that the fiow acts opposite to the rotation of the arc 6. The gas will then be conducted in a longer path through the arc and heated especially strongly.

The central electrode 5 is made up of a plate-shaped head portion 28 having an annular periphery and of the rod-shaped holder 27 which is encased in a plurality of tubes. A dish-shaped part of the head portion 28 presenting a concave face to the gas flow maintains the arc foot or base on an annular path. An outer casing 28 can be formed from glass and serves as a slide tube in the neck 26, which can also be formed of glass. A casing 29 which can be constructed of a mica base, for example, or other suitable non-conducting material, serves as electrical insulator. The holder 27 can consist of coaxial tubes which serve as coolant supply tubes. The coolant is conducted in the dish-shaped electrode around an annular core 30. Due to the narrow cooling gap adjacent the electrode ring periphery, an efiective coolant action is able to be achieved. The coolant can be supplied to the central electrode 5 through a head piece 31 with a distributor channel by means of a tangential bore 32. The supply voltage for the are 6 is produced by the schematically shown source 33, the central electrode 5 being connected as the cathode and the ring-shaped electrode 1 as anode. In order to rigidly secure the central electrode 5 after its adjustment in the axial direction, as indicated by the double-headed arrow 33', a set screw can be suitably employed; however, the set screw has been omitted from the drawing in the interest of clarity.

The entire plasma burner of the illustrated embodiment can be held together or connected with a few screws indicated by the dot-dash lines 34. If the screws 34 at the annular discs 24 and 25 are loosened, on the other hand, the structural units containing the pole shoes can be disassembled and separated from one another, and the ring-shaped electrode 1 can be very easily inspected and replaced, if desired.

Insofar as the electrical insulation is exposed to the arc radiation or the heat rays of the surrounding gas, it can be protected by suitable labyrinthine construction. Accordingly, the slots in which the insulating rings are inserted can be covered from view by suitable meandershaped construction.

If it is desired to provide high flow velocities of the working gas, which can consist of air in the simplest case, a Laval nozzle can be connected to the open end of the arc chamber. For particular applications, the excitation windings 18 to 21 can be dispensed with and the pole shoes 2 and 3 can be formed of permanent magnets. By means of a tangential gas supply tube extending in the direction of the arc rotation, the magnetic field can be supported or enhanced.

I claim:

1. Plasma burner for heating a gas to plasma-forming temperature, comprising a pair of coaxially aligned annular cylindrical pole shoes having conical end faces extending in the axial direction toward one another, electrode means including a ring-shaped electrode located between said conical end faces, said annular pole shoes and ring-shaped electrode forming a wall defining a cylindrical chamber, said electrode means also including another electrode located in said chamber, and excitation coils carried by said chamber wall and located concentrically on said pole shoes, said coils being adapted to produce an axial magnetic field when electrically energized, said cylindrical wall defining an arc chamber, and said other electrode being axially adjustable and being located centrally in said chamber with respect to said cylindrical wall, and means for supplying gas to be heated between said central electrode and said cylindrical wall.

2. Plasma burner according to claim 1, wherein said central electrode has a dish-shaped face located inside an annular rim thereof, and is carried by an axially adjustable, centrally guided holder member.

References Cited UNITED STATES PATENTS 2,217,187 10/1940 Smith 313161 3,201,560 8/1965 Mayo et al 3l3231 X 3,343,019 9/1967 Wolf et a1 313231 X JAMES W. LAWRENCE, Primary Examiner R. F. HOSSFELD, Assistant Examiner -U.S. Cl. X.R.

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