Long Arc Column Plasma Generator And Method

Abstract

A long arc column plasma generator and method provide a means and method for initiating and sustaining an exceptionally long transferred arc. The improved vortex strength, mass flow rate and radial pressure gradient are obtained by a new relationship between the nozzle length, the nozzle diameter, and the vortex chamber width.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatus for obtaining arc plasma and particularly to the relationships of the geometric configuration of the nozzle length, bore and vortex chamber width.

2. Description of Prior Art

Methods and apparatus for generating high voltage arc plasma and for incorporating material to be treated by such plasma as part of the electrical circuit which generates the plasma has been known for some time. It has also been known to operate the arc in a transferred mode in order to deliver the maximum heat energy to the workpiece.

In what is perhaps the closest prior art to the present invention there is disclosed in U.S. Pat. No. 3,194,941 to Robert J. Baird a high voltage arc plasma generator. In the Baird patent recognition is given to the importance of the relation between the nozzle length and the nozzle diameter and which relation is shown to have an effect both on heat and energy transfer in the transferred mode and also in minimizing electrode erosion.

While the Baird disclosure represents an advance in the art such disclosure was primarily directed to the relationship of the nozzle length and nozzle diameter but did not take into account the equally important dimension of the vortex chamber width. For example, in the Baird disclosure it is stipulated that the ratio of the nozzle length to the nozzle diameter must be greater than 1.2 and less than 3 with 2 being the recommended value. When this teaching is followed the chamber width is necessarily too wide for stable operation with long arcs, greater than 12 inches. Furthermore, a plasma generator made according to the Baird disclosure calls for a relatively high gas flow rate for any desired power input level to the generator. This is necessary in order to minimize the material erosion at the electrode and to prevent undesirable internal arcing between electrode and nozzle. Furthermore, a generator according to the Baird disclosure requires a substantially high mass flow rate of gas in order to maintain a stable operation. Also, in order to develop a proper radial pressure gradient it is necessary that the nozzle length be relatively long, at least 1.2 times the nozzle diameter.

SUMMARY OF THE INVENTION

The apparatus and method of the present invention are directed to an arc plasma generator in the same general sense of the previously referred to Baird U.S. Pat. No. 3,194,941. In the present invention, there is employed an apparatus having a cylindrical electrode, a gas directing nozzle spaced from the electrode and a chamber surrounding the space between the electrode and the nozzle and which includes means for introducing an arc gas into the chamber to produce a vortical flow in the chamber and in the gas directing nozzle.

The apparatus of the invention basically includes cylindrical electrode and a constricting nozzle which are separated by an insulator and these three major components are positioned in axial alignment with each other. The electrode and constricting nozzle are spaced axially a distance designated A. The electrode-nozzle spacing A has been found to be critical and should be one-fourth to one-third of the actual length, designated B, of the nozzle. The ratio of the nozzle axial length, designated B, to the nozzle diameter, designated C, should be greater than 0.2 and while there is no upper limit to this ratio the non-transferred mode takes place when the ratio of the nozzle length to the nozzle diameter is approximately equal to 4. A ratio value of 0.75 is recommended and used in the transferred mode for which the invention is primarily intended.

In addition to discovering new operating characteristics which can be obtained by previously undiscovered relations between the nozzle length and nozzle diameter, it has also been discovered that the chamber width A should preferably be sized according to the plasma generator current rating as later discussed in more detail.

In operation, gas is injected tangentially in the chamber space defined by chamber width A to form a vortex with a radial gradient toward the axis of the generator. An electric arc is initiated between the cylindrical electrode and an external element in the arc circuit. The vortical flow of gas in the chamber spacing A stabilizes the arc on the axis of the generator and prevents any undesirable current conduction through the nozzle. When the dimensional conditions of the invention are satisfied a minimum gas flow rate is required through the spacing A to prevent undesirable internal arcing between the electrode and nozzle.

The method and apparatus of the invention, in general, satisfy the long established requirement for initiating and sustaining an extra long arc. In this regard, it is recognized that a long arc column is particularly useful in plasma melting of steel scrap because of the potential for eliminating complex equipment necessary for raising the lowering heavy graphite electrodes as presently used in electric arc furnaces. The complex equipment for raising and lowering the electrodes is necessary because of the short arc length possible with graphite electrodes. In the same regard, it may be noted that the initial capital investment for electric furnaces can be reduced significantly by eliminating the electrode follower equipment. The long arc column is also desirable because it makes possible a high power level operation at relatively low current. As a result, the lower current operation increases the life of the plasma generator.

The apparatus and method of the invention can be adapted for the disintegration of both electrically conducting materials as well as insulator materials. When used for the melting of high conductivity materials like metallics, the workpiece itself serves as the external circuit element. When used for the disintegration of insulators such as rocks or glass and the like, the apparatus and method depend upon employment of an additional electrode as the external element in the arc circuit. In all instances, the apparatus and method of the invention are concerned with a transferred mode of operation and with relatively high voltage and low current operation. It can thus be said that for plasma generators having electrodes of the same internal diameter a generator made according to the present invention will, for a given gas flow rate, strike a substantially longer stable arc than is possible with a Baird type generator.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic section view taken through a plasma generator made according to the invention and shown in a typical application wherein the workpiece is of a conducting nature.

FIG. 2 is similar to FIG. 1 but showing in a schematic diagram an application of the invention with a workpiece of a non-conducting nature.

FIG. 3 is a somewhat schematic diagram illustrating a prior art construction.

FIG. 4 is similar to FIG. 3 but illustrating, schematically, for comparison the same dimensions according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A plasma generator made according to the invention incorporates three basic elements, namely: a gas system, an electrical power system, and a cooling system, and physical structure is provided for each system. In the drawings there is shown a stainless steel shell 10 secured by means of bolts 11 to a copper nozzle 12 which provides a passageway 13 for the reception of cooling water as indicated at 14 and for the discharge of the cooling water through an exit 15. Nozzle 12 receives through a coarsely threaded connection indicated at 20, an insulator 21 which may be made of "Synthane" insulation or of a similar insulation material. Insulator 21, in turn, is mounted on a stainless steel jacket 22 which is spaced from a concentrically mounted cylindrical copper tube 23 which acts as the main electrode of the generator. Stainless steel jacket 22 is supported within the shell 10 by a plurality of fin members 25 having a sliding fit as indicated at 26 and 27. The notch arrangement also indicated at 27 prevents relative lateral motion between the stainless steel jacket 22 and fin members 25. An annular ring 28 secured by bolts 29 is fitted to the fin members 25. Fin members 25 are preferably made of an insulating material, e.g. nylon, and ring 28 may be of metal, e.g. aluminum. Stainless steel jacket 22 mates with an electrode manifold 30 and the two pieces are secured together by a metal clamp nut 31 having a threaded connection with manifold 30 as indicated at 32. Manifold 30 is generally cylindrical and provides a path for the reception of an appropriate pressurized arc gas, e.g. air, argon, carbon monoxide, oxygen, et cetera, which follows the path generally indicated in dashed lines as at 35, 36, 37 and 38. Typical gas pressures are generally several atmospheres and may, for example, be from 2 to 10 atmospheres. Manifold 30 is connected to a high voltage power supply which may vary from, say, 100 volts AC for a small-length arc to 1,000 volts AC for a long-length arc. Connection is made through a plurality of lines 40 secured by bolts 41.

At the back end of the torch there is provided a water manifold structure generally represented at 50 and which provides means (a) for connecting a water supply as at 51, (b) for distributing that water supply by electrically non-conducting flexible hose, not shown, to appropriate input points in the nozzle such as previously mentioned at 14 and (c) for receiving the heated water back through appropriate electrically non-conducting flexible hose, not shown. The heated water is, in turn, discharged through portions of manifold 50 which are kept separate for hot water discharge distinct from cold water intake. Since the invention is primarily concerned with the front or nozzle end of the generator and since the specific cooling arrangements are generally known in the plasma generator art and may vary substantially, no further details are deemed necessary concerning the manifold structure.

Prior to discussing the operation of generator, brief mention will be made of the physical assembly prior to operation. Stainless steel shell 10 and manifold 50 are integrally formed and are detachably secured to nozzle 12 by bolts 11. This then represents a nozzle sub-assembly. A separate sub-assembly consisting of insulator 21 is screwed to stainless steel jacket 22. Next, electrode 23 is screwed to manifold 30 by an appropriate threaded connection as indicated at 60. Clamp nut 31 is then tightened on mainfold 30. Centering guides 25 are next assembled on jacket 22 and are secured to retaining ring 28. A flexible electrically non-conducting gas hose, as indicated at 61, is connected and electrical power lines 40 are appropriately connected by bolts 41. These last mentioned components, as a separate subassembly, are then slid into the interior of stainless steel shell 10 with appropriate notches, not shown, being provided in the manifold as at 62 to allow passage of the centering guides 25. Upon insulator 21 making contact with nozzle 12 the two subassemblies are "mated" and are threadably connected by the previously mentioned threaded connection 20. To replace the electrode 23 it will be noted that all that is required is to unscrew the threaded connection 20, remove the electrode sub-assembly, install a new electrode 23 and replace the sub-assembly as previously explained.

The description next proceeds to a description of a typical operation and later will deal with a more specific comparison of those dimensions and their relationships both in the prior art as well as in the present invention which are deemed critical. At the outset it may be noted that the generator of this invention while operable in a non-transferred arc mode is primarily intended to operate in a transferred arc mode. Furthermore, the workpiece or material against which the plasma is directed may be either of electrically conducting or of an electrically non-conducting material. Both modes will be subsequently discussed.

In FIG. 1 a typical electrically conducting external electrode or workpiece is indicated at 70. This might be, for example, a material to be subjected to extreme heat for research purposes. In another instance, it might be a charge of scrap metal to be melted and recycled into finished steel.

For starting the plasma generator, water flow is initiated to cool the nozzle 12. Next air flow is established through flexible electrically non-conducting gas hose 61 to cool electrode 23 and provide the necessary gas constricting vortex. A potential is applied between electrode 23 and nozzle 12. A small pilot arc of relatively small current between electrode 23 and nozzle 12 is established by means of inserting a suitable electrically conducting, easily ionizable material, e.g. a metal scouring pad. This pilot arc initiates the conduction column for the high current main arc between 23 and workpiece 70. The pilot arc egresses from the nozzle to establish electrical contact with the workpiece. The electric arc so formed will protrude through nozzle 12. A potential is next applied between electrode 23 and workpiece 70. The generator next is positioned near the workpiece 70 to establish the main arc column between electrode 23 and workpiece 70. The nozzle 12 is then disconnected from the electrical circuit. The gas vortex centers the arc column axially inside the plasma generator thus preventing an undesirable internal arc between nozzle 12 and electrode 23.

As previously mentioned the generator of the invention is applicable to heating or melting both electrically conducting as well as electrically non-conducting workpieces. The electrically conducting workpiece application has been explained in connection with FIG. 1. In FIG. 2 there is a diagrammatic view of the generator applied to a workpiece of a non-conducting nature and in which the electrode is schematically indicated at 80, the nozzle at 81, an external forwardly placed, axially aligned electrode at 82 and an electrically insulating workpiece, e.g. glass, at 83. In this case, the plasma is formed between electrode 80 and the external electrode 82, e.g. a water cooled ring. The plasma so formed is the medium which melts the workpiece 83.

Those skilled in the art will readily appreciate the tremendous potential for using the generator in the configuration of FIG. 2 for rock excavation, rough drilling, soils treatment and the like. Also, as will be seen from a later description which compares the present invention to the prior art, the generator of the present invention is vastly superior for this kind of application than is the generator of the prior art. More specifically, with the present invention in use for this kind of application there is a substantially higher efficiency and substantially less consumption of gas required.

The description next refers to FIGS. 3 and 4. FIG. 3 in diagrammatic form shows the prior art construction of FIG. 4 in diagrammatic form shows the construction of the present invention. Both figures show the dimension A representing the gas chamber width, dimension B representing the nozzle length and dimension C representing the nozzle bore diameter. In the previously referred to Baird Patent the following relation is stipulated: 1.2 < B/C < 3, with B/C = 2 recommended and used in the preferred embodiment of the Baird Patent. While the Baird Patent was admittedly a recognizable advance in the art the stipulated relationship has been found by reason of the present invention to have restricted further advances in the art. More specifically, the discovery of the present invention is, effectively, that not only must the nozzle length dimension B and nozzle diameter dimension C be considered for purposes of efficiency, stable operation and arc length but also there must be taken into account the chamber width dimension A and its relation to the nozzle length dimension B and nozzle diameter dimension C. The Baird Patent has called for the chamber width dimension A to be substantially too wide. In order to obtain the required vortex strength, the gas flow rate must be high and/or the nozzle dimension B must be exceptionally long in order to develop the proper radial pressure gradient.

In the present invention the chamber width dimension A was intentionally sized by the method later explained to achieve the required vortex strength independent of the nozzle length dimension B. This change in concept lead to the important discovery that the gas flow rate normally required for stable operation could be substantially reduced in half. In particular, according to the present invention the relation of nozzle length dimension B to nozzle diameter dimension c for the transferred mode is stipulated as: B/C > 0.2 with B/C = 0.75 recommended and used, and has no upper limit. The non-transferred mode, according to the invention, takes place according to the relation: B/C .gtoreq. 4.

In addition to providing a new structure, the invention also provides a method of fabricating a long arc plasma generator. This method is essentially directed to the steps followed to determine the dimension A, B and C. As a first step, B is reduced to its most minimum thickness, say in the order of one-eighth inch, with B at minimum thickness and C at some fixed value equal to the electrode internal diameter the value A is diminished until there is created a vortex strength of at least 0.25 Mach. When the vortex strength is obtained at 0.25 Mach and with minimum width A, the dimension B is then increased with respect to C within the relationship B/C > .2 for the transferred mode and within the relationship B/C .gtoreq. 4 for the non-transferred mode.

Claims

What is claimed is:

1. An apparatus adapted to generate a long arc high-temperature plasma between the apparatus and an electrical conductor in an arc circuit comprising, in combination:

a. a cylindrical shaped electrode;

b. a gas-directing nozzle axially aligned with, forwardly spaced and insulated from said electrode and having an internal diameter designated C and a length designated B and with said electrode providing a vortex forming gas chamber of a width designated A, said dimension A being selected as the minimum width at which a vortex strength of 0.25 Mach is obtained when B is of minimum arc sustaining width and C is equal to the internal diameter of said electrode, and B and C having the relationship B/C > 0.2 for the transferred mode and B/C .gtoreq. 4 for the non-transferred mode; and

c. gas supply means for introducing an arc gas into said chamber to produce a vortical flow in said chamber and nozzle.

2. An apparatus as claimed in claim 1 including means to direct said plasma against a workpiece comprising an electrical conductor and which is in the arc circuit.

3. An apparatus as claimed in claim 1 including electrical conducting means spaced forward of and axially aligned with said nozzle and effective to direct the plasma against a workpiece comprising an electrical non-conductor and said electrical conducting means is in said arc circuit.

4. A method of constructing and operating a long arc high-temperature plasma generator of the type having a cylindrical shaped electrode, a gas directing nozzle, a vortex forming gas chamber and gas supply means, said method being with respect to the vortex chamber width, the nozzle internal diameter and nozzle length comprising the steps:

a. while maintaining the nozzle internal diameter at a dimension designated C which is equal to the internal diameter of the electrode and maintaining the nozzle length designated B at a minimum arc sustaining width, determining the minimum vortex chamber width designated A at which a vortex strength of Mach 0.25 is obtained; and

b. then while maintaining said vortex chamber width A as so determined, operating said generator in the transferred mode with said dimensions B and C having the relationship B/C > 0.2 and in the non-transferred mode with said dimensions B and C having the relationship B/C .gtoreq. 4.