US3194941A

US3194941A - High voltage arc plasma generator

Info

Publication number: US3194941A
Application number: US223484A
Authority: US
Inventors: Robert J Baird
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1962-09-13
Filing date: 1962-09-13
Publication date: 1965-07-13
Anticipated expiration: 1982-07-13
Also published as: NO115114B; NL134809C; DE1916912U; NL297831A; GB1007429A

Description

SEARCH RM July 13, 1965 R.J.BA1RD 3,194,941

HIGH VOLTAGE ARC PLASMA GENERATOR Filed sept. 1s, 1962 \O` N 'mw U1 Q i Il TQ I N l "1 Q"`` Q z "`-`N l I N *N 1- N r """OQ OO J\` Q I \f 1 INVENTOR ROBERT- J. RnB-MRD BY 'l United States Patent 3,194,941 HIGH VOLTAGE ARC PLASMA GENERATOR Robert J. Baird, Indianapolis, Ind., assignor to Union Carbide Corporation, a corporation of New York Filed Sept. 13, 1962, Ser. No. 223,484 4 Claims. (Cl. 219-121) This invention relates to an improved method and apparatus for ,obtaining a high voltage arc plasma and more particularly to such a high Voltage arc plasma established in a system wherein material to be treated by such arc plasma is part of the electrcial circuit.

Electric arcs have been used for many years for cutting, for plating and welding processes and of recent years for metal melting furnaces, metal scarling, and metal cutting. In applications such as these, it is oftentimes preferable to operate the arc in a transferred mode; that is, having the work in the arc circuit. This mode of operation has the advantage of more fully utilizing the heat energy -of the arc in the workpiece rather than having a portion of it dissipated to a separate electrode.

Further, in such applications, it is of prime importance to get the maximum amount of the power being supplied to the arc-generator device to be transferred to the gas and the work. It has been found that if higher power to the arc device is achieved solely through current increases, such additional power is used up primarily in heating the electrode and the cooling fluid streams. On the other hand, higher power obtained by voltage increases is substantially transmitted as higher heat to the arc gas and the work.

I-t is the main object of the invention to provide a method 4and apparatus for producing a high voltage arc plasma for use in electric arc working of materials.

It is a further object to provide a novel high voltage device capable of operating in a transferred mode.

It is still another object to provide a novel high voltage device which is preferably operated with alternating current.

These and other objects will either be pointed out or become apparent from the accompanying description and drawings wherein the sole ligure is a cross-section View of the device of the invention.

In -a general way, the objects of the invention are accomplished by providing an -apparatus having a cup shaped electrode, a gas directing nozzle having an L/ LD. of at least about 1.2 spaced from the cup shaped electrode, and a chamber surrounding the space between the cup electrode and the nozzle and having means for introducing `an arc gas into the chamber to produce a vortical ilow in the chamber and in the gas directing nozzle. For purposes of this disclosure, the length (L) of the nozzle is measured from the nozzle exit to end of the chamber nearest the exit. The inside diameter (LD.) is the minimum inside diameter of `the nozzle.

Further, the invention provides a novel process for generating .a high voltage, high temperature arc plasma for working materials. In this process, a cup shaped electrode and the material to be worked are connected in circuit relation. A high voltage arc is established between the cup-electrode and the material to be worked. A vortical ilow of arc gas is provided in the region of the arc to form .an arc plasma. The arc plasma is then passed through a gas directing nozzle having an L/I.D. of at least 'about 1.2. The vortical flow of arc gas is maintained inside the gas directing nozzle to fcollimate `and direct the arc plasma. Then the so-collimated and directed arc palsma is applied to the material to be treated.

The method and apparatus of the invention fulfills a long felt need for -a high power high voltage electric arc plasma generator for use in a transferred mode and having minimum electrode erosion.

Referring now to the drawings, the torch T primarily consi-sts of a cup shaped electrode 14 that is in axial alignment with a gas directing nozzle 16 and which is separated therefrom by an arc chamber 10. Chamber 10 is electrically insulated from the electrode 14 by insulator 11. Arc gas is introduced to torch T through inlet 13. From inlet 13 the arc gas passes through passage 18 down to a plurality of tangential apertures 12, through such apertures into the chamber 10. The torch T is cooled by passing -a cooling fluid from coolant inlet 20 through passage 22 into a second passage 24 formed between the nozzle 16 and member 26. From passage 24 the coolant passes from the torch through chamber 28 and passage 30- and outlet 32.

The electrode 14 is also cooled by introducing a coolant through inlet 34 down passage 36 -through passage 38 down passage 39 and back up passage 41 through cross-passage 43 and out passage 40. The outer tubular member 42 which deiines the passage 36 also carries means for connecting a power supply to the torch T. Therefore such member 42 is insulated from the remainder of torch T by insulator 44.

In actual operation, a quantity of arc gas is introduced into chamber 10 through apertures 12 so as lto impart a swirling or vortex motion to the gas. The general configuration of the torch permits part of the g-as to liow into and out of the rear electrode 14 and -then all of the gas through the nozzle 16 so long as the ga-s is introduced at a suflicient velocity. That is, if the inlet velocity is greater than 0.25 Mach, then there will be a sufficient pressure drop within the chamber 10 -between its outer wall 60 and the area near the torch axis to force a substantial portion of the gas into ele-ctrode 14. This results in -a longer arc which increases the arc voltage. This pressure drop also aids in maintaining a vortex flow of gas.

An appropriate power supply (not shown) is connected to the cup electrode 14 Iand to a workpiece. The workpiece will, of course, vary according to the application for which the torch is being used. For example, it may be .a metal plate that is being scarfed, or it may be the charge of a metal melting furnace. Whatever it may be, the combination of the high voltage arc operating in a transferred manner renders this torch extremely useful for such applications.

The arc is initiated by .any .suitable means, such as for example a high-frequency start, capacitor discharge or by inserting a conducting rod into the torch through the nozzle. The quantity of gas is then increased as desired. The arc that is finally established will, because of the flow of gas in the torch, ex-tend from Van area appreciably along the length of the cup electrode, through nozzle 16, to an electrode workpiece.

In order to successfully operate the torch of the invention, the configuration of Ithe nozzle 16, which constricts and directs the arc and aids lin increasing the arc voltage, is critical. It has been'found that if the .ratio of the length (L) of the nozzle -to the inside diameter (LD.) becomes too small, -there will be an insuicient radial pressure gradient within the nozzle to center lthe arc. Consequently, the -arc will strike to the nozzle and then to the work; this phenomenon is known as double arcing. It has been found that if the L/I.D. is less .than 1.2, the low pressure region toward the center of the nozzle will be decreased, thus causing double arcing and severe nozzle erosion. On t-he other hand, an L/LD. much greater than 1.2 makes it more difficult to transfer the `arc and reduces the heat efficiency of the arc eiliuent. It is desirable, therefore, that the L/I.D. of

the nozzle be between about 1.2 and 3.0 and preferably about 2.

While direct current with straight or reverse polarity connection may be used with this device, it is preferred not only from the standpoint of achieving higher power levels at lower costs, but also from the standpoint of longer life for the cup electrode to operate the torch with single phase alternating current. For example, when operated with direct current, the arc tends to localize over a particular area of the cup-shaped electrode, whereas, with alternating current the arc will run along substantially the entire length of the electrode. More specically, the arc termination will alternate between an area somewhere near the closed end of the cup-shaped electrode and its mouth on every half cycle of the alternating current. It should be emphasized that such alternating or traveling of the arc has been made possible to a great extent by the particular configuration of the torch which, because of the intermediate chamber, forces a portion of the gas entering the device to travel into and out of the cup-shaped electrode. This substantially reduces erosion of the cup electrode. Such an electrode not only reduces material usage, but also prevents contamination of the work for which the torch is being used.

As an alternate means of providing for longer electrode life through the prevention of electrode erosion, a watercooled copper field coil could be placed around the cup electrode so as to produce a magnetic field, thus causing the arc to rotate. Such coil will not only cause the arc to rotate, but it will also spread out the arc on the electrode so as to permit greater overall currents with workable current densities.

Various electrode materials have been found to be useful with various types of gases. Thus, copper, silver, aluminum, zirconium, and molybdenum are useful materials when the device is operated with reactive gases such as air, oxygen, carbon dioxide, and carbon monoxide. Such material is useful to minimize electrode damage in the presence of oxidizing atmospheres. The material is also useful for mixtures of an inert gas with air, oxygen, or carbon monoxide. When inert gases such as hydrogen, argon, helium, and nitrogen are being used, tungsten, tungsten containing emissive material such as thoria, and carbon is the preferred electrode material.

Suitable materials for the insulator have been found to be phenol-aldehyde condensation resins and nylon. However, materials exhibiting like properties could also be used.

The following examples indicate the utility of this transferred, high voltage torch. of the general type depicted in the drawing was used.

EXAMPLE 1 Oxidation scarfing In this example, the cup electrode had a length of 91/2 in. and an I.D. of 1% in. The nozzle had a length of 2.75 in. and an I.D. of 0.951 in. The workpiece, that is the other electrode, consisted of a type 304 stainless steel bar approximately 4 in. wide, 2 ft. long, and 3%: in. thick. Oxygen was supplied to the device at the rate of 1000 c.f.h. so as to cause oxidation of the metal, thus, permitting the defects of the metal to be blown away. A tield coil having a magnetomotive force of 16.5 kilo-ampere turns was placed around the rear electrode, the direction of the field being toward the bottom of the cup electrode. With the workpiece acting as the cathode, 665 amperes (D.C.) were supplied to the device. The arc voltage was 285 volts. The total power to the torch was 190 k.w. of which approximately 152 kw. went to the gas and to the workpiece yielding an efficiency of approximately 80 percent.

The torch was set at an angle of 55 degs., the nozzle being directed opposite the direction of travel of the workpiece. The stando@ distance of the torch from the In these examples, apparatus workpiece was approximately ll/z in. The workpiece had a travel speed of approximately 250 in./min.

Under these conditions, a scarf having a width of approximately 0.94 in. and a depth of 0.09 in. was made. The scarfed area had a surface quality judged rollable based on past experience with this problem.

EXAMPLE 2 Fusion scarfing The dimensions of the torch used in this example were the same as those of Example 1. Likewise, the torch angle and the standoff distance were the same. The travel speed of the workpiece was 50 i.p.m. The workpiece electrode was again a 304 stainless steel bar of approximately the same dimensions. Again a field coil was used, the magnetomotive force this time being 14.3 kiloampere turns, the direction of the r'ield again being toward the bottom of the rear electrode. However, in this example, argon was supplied to the torch at the rate of 2000 c.f.h. so as to melt the metal to remove the surface defects. In this case the metal is not blown away, rather it solidities.

With the workpiece acting as the anode, 550 amperes (D.C.) were supplied to the device. The arc voltage was 210 volts. The total power to the torch was 115 kw. of which approximately 92 kw. went to the gas and to the workpiece yielding a torch efficiency of approximately percent.

Under these conditions, a scarf having a melt width of approximately 0.63 in. and a depth of 0.04 in. was made. The surface quality was good.

EXAMPLE 3 Metal Cutting In this example, the cup electrode had a length of 61A in. and an I.D. of 1/2 in. The nozzle had a constriction having a length of 1/2 in. and an I.D. of M1, in. Oxygen was supplied to the device at the rate of 750 c.f.h. The electrode workpiece was an 8 in. stainless steel plate. With the workpiece acting as the cathode, 300 amperes were supplied to the device. The arc voltage was 650 volts. The total power to the torch was 195 kw. of which approximately 154 kw. went to the gas and to the workpiece yielding a torch eiciency of approximately 79 percent. The torch was directed at the workpiece at an angle of degs. from a standoff distance of 1/2 in.

Under these conditions a cut 5 in. in length was made through the plate at the rate of 3 i.p.m. The qualtity of the cut was judged to be good.

EXAMPLE 4 Metal melting In this example, the cup electrode had a length of 10 in. and an LD. of 1% in. The nozzle had a length of 21/2 in. and an I.D. of 1% in. The workpiece electrode consisted of 1500 lbs. of carbon steel scrap. Air was supplied to the torch at a rate of 600 c.f.h. Using single phase alternating current, 1100 amperes were supplied to the torch. The arc voltage was 400 volts. The total power to the device was 440 kw. of which 362 kw. went to the gas and to the workpiece during the initial run yielding an etliciency of approximately 82 percent. As the environment surrounding the torch became hot (1600 degs. C.), the torch etliciency dropped to 40 percent due to inadequate external thermal insulation of the torch.

Under these conditions the scrap was converted to a molten bath in 21/2 to 3 hours.

The ability of the torch to operate on alternating current in air to render the cup electrode substantially nonconsumable can be shown.

EXAMPLE 5 A Ifernatng current operation in air In this example the cup electrode had a length of 9% in. and an I.D. of 1% in. The nozzle had a length of 2% in. and an I.D. of 1% in. The electrode workpiece was a plate of carbon steel. Air was supplied to the torch at the rate of 500 c.f.h. Using single phase, alternating current 1200 amperes were supplied to the device. The arc voltage was 320 volts. The chamber pressure was approximately 1 atmosphere. The total power to the torch was 375 kw. of which 300 kw. went to the gas and to the workpiece yielding an eiciency of 80 percent.

Under these conditions, the torch operated for a period of about 10 minutes without any appreciable signs of electrode erosion.

What is claimed is:

1. Apparatus for producing a high-Voltage high-temperature arc plasma between such apparatus and a metal connected in arc circuit relation therewith which comprises a cup-shaped electrode, a gas-directing nozzle having a L/I.D. of at least about 1.2, said nozzle being in axial alignment with but spaced from said cup-shaped electrode, a chamber surrounding the space between said cup-shaped electrode and nozzle and having means for introducing an arc gas into such chamber to produce a vortical ow in such chamber and said gas-directing nozzle.

2. Apparatus according to claim 1 wherein said gasdirecting nozzle has a L/I.D. of between about 1.2 and 3.0.

3. Apparatus according to claim 1 wherein said gasdirecting nozzle has a L/ I D. of about 2.

4. Apparatus for producing a high-voltage high-temperature arc plasma between such apparatus and a metal connected in arc circuit relation therewith which comprises a cup-shaped electrode, a gas-directing nozzle having a L/I.D. of at least about 1.2, said nozzle being in axial alignment with but spaced from said cup-shaped electrode, a chamber surrounding the space between said cup-shaped electrode and nozzle and being provided with means positioned between the cup electrode and the nozzle for introducing arc gas into such chamber directly into the space between said cup-shaped electrode and nozzle to produce a vortical ow in such chamber and said gas directing nozzle.

References Cited bythe Examiner UNITED STATES PATENTS 3,118,046 1/ 64 Harrington 219-75 3,131,288 4/64 Browning.

RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, Examiner.

Claims

1. APPARATUS FOR PRODUCING A HIGH-VOLTAGE HIGH-TEMPERATURE ARC PLASMA BETWEEN SUCH APPARATUS AND A METAL CONNECTED IN ARC CIRCUIT RELATION THEREWITH WHICH COMPRISES A CUP-SHAPED ELECTRODE, A GAS-DIRECTING NOZZLE HAVING A L/I.D. OF AT LEAST ABOUT 1.2, SAID NOZZLE BEING IN AXIAL ALIGNMENT WITH BUT SPACED FROM SAID CUP-SHAPED ELECTRODE, A CHAMBER SURROUNDING THE SPACE BETWEEN SAID CUP-SHAPED ELECTRODE AND NOZZLE AND HAVING MEANS FOR INTRODUCING AN ARC GAS INTO SUCH CHAMBER TO PRODUCE A VORTICAL FLOW IN SUCH CHAMBER AND SAID GAS-DIRECTING NOZZLE.