US3543084A - Plasma arc gas heater - Google Patents

Description

Filed Jan. 22, 1968 Nov. 24, 1970 J MlCHAELls 3,543,084

PLASMA ARC GAS HEATER 2 Sheets-Sheet 1 47 GAS FLOWIN GAS FLOW OUT INVENTOR JOHN L. MICHAEL/5 BY flvgw g ATTORNEYJ Nov. 24, 1970 J. MlCHAELlS 3,543,034

PLASMA ARC GAS HEATER Filed Jan. 22, 1968 2 Sheets-Sheet 2 //vv/vr0/? JOHN L. MICHAEL/5 ATTORNEYS United States Patent O US. Cl. 315-111 13 Claims ABSTRACT OF THE DISCLOSURE A plasma arc heater having a plurality of hollow electrodes forming a central axial passage for material being heated is disclosed. Each electrode is in the form of continuous coiled metal tubing, having at least three turns. A first turn of larger diameter is secured in electrically conductive relationship to a cylindrical mounting ring, the additional turns being of a smaller diameter than the first turn, preferably by at least twice the diameter of the tubing itself so that the smaller diameter turns will fit within the larger diameter turns. The tubing of each electrode is coiled, for example, generally in the form of a helix or spiral, the smaller turns being within the larger turn and successive smaller turns extending axially beyond said mounting ring. The mounting ring has openings through which the ends of the tubing pass for introducing and withdrawing a cooling fluid. The complete arc heater assembly is a stack of essentially duplicate electrode subassemblies, adjacent subassemblies being separated by an electrically insulating ring. The insulating ring is of a larger inside diameter than the mounting ring, thereby providing a space between the outside of the smaller coils and the interior surface of the insulating ring, and shielding the in sulation. Also provided are means for reversing the polarity of adjacent electrodes in the heater by external circuits to cause two electrodes to alternately perform the same function, e.g., as the cathode for an arc. An arc rotating coil surrounds the heater and is connected electrically in series with the electrodes.

BACKGROUND OF THE INVENTION The present invention relates to a plasma arc heater, particularly for gases.

A well known design for such heaters comprises a series of hollow electrodes forming an axial passage for the material being heated and having arc rotating means, such as an electromagnet, which applies a tangential component of force to the arc and causes the arc to rotate. Commonly the hollow electrodes are cooled by a fluid such as water with suitable insulating means being provided between the source of cooling water and the apparatus. As is well known, de-ionized water is a poor conductor of electricity and a suitable length of plastic tubing between the heater and the source of cool water provides insulation. Previously proposed cooling means involve expensive machined or cast hollow electrode elements, having relatively thick walls to contain the Water flow at high velocity at high pressure. In addition to being expensive and diflicult to fabricate, the thickness of the walls is a serious factor in providing adequate cooling. Since the input of energy and the consequent heating of the gas passing through the apparatus depends upon the cooling capability and efiiciency of the cooling apparatus, the importance of the efficiency of the cooling means will become apparent.

Another difliculty encountered is related to the fact that the cathode deteriorates at a rate many times that of the anode, requiring more frequent replacement of the cathode.

SUMMARY OF THE INVENTION One of the objects of the present invention is to over- 3,543,084 Patented Nov. 24, 1970 come the foregoing and other difliculties and problems encountered in the art.

Another object of the invention is to provide electrodes of simple manufacture and of an economical material which accomplishes the foregoing object.

Another object of the invention is to provide means for controlling erosion of the electrodes, and particularly the cathode.

Briefly, the present invention is directed to a plasma arc heater having a plurality of hollow electrodes which are aligned to form an axial passage for the material being heated. The electrodes each consist of continuous coiled metal tubing of substantially uniform diameter, the coil having at least three turns, including a first turn of larger diameter. The tubing for this turn is secured within a cylindrical mounting ring and is in electrically conductive relationship therewith. The additional turns of tubing are of a diameter smaller than the first turn and thus are closer to the axis of the heater. These additional turns are generally helical, are located within the large turn, and extend axially of the heater so as to provide an axial passage. The mounting ring has two openings through which the tubing passes for introducing and withdrawing a cool ing fluid. The axial length of each electrode is greater than the axial length of its corresponding mounting ring, by reason of the helical arrangement of the smaller windings. When the heater is assembled, adjacent electrode subassemblies are, therefore, spaced apart by suitable spacer rings and by cylindrical insulating rings which also serve to electrically insulate adjacent electrodes. These insulating rings have a greater inner diameter than the exterior diameter of the electrode turns, whereby when the arc heater is assembled and the elements are axially aligned, a space will remain between the electrode and the insulating cylindrical ring. This space protects the insulation from the arc, and may also be used to provide further cooling of the heater by including therein suitable cooling coils adapted to carry a cooling fluid. The heater electrodes are interconnected so that a pair of adjacent electrodes may serve as alternate cathodes for a single anodic electrode, thereby reducing the deterioration of the heater cathode. This is accomplished by providing a voltage on first one and then the other of the two cathodes which will serve to draw the are from the anode first to one cathode and then to the other.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and additional objects, features and advantages of the present invention will become apparent to those skilled in the art from a consideration of the following description of a preferred embodiment thereof, taken with the accompanying drawings, in which:

FIG. 1, in cross-section, illustrates an embodiment of the invention involving the use of subassemblies of mounting rings and coiled metal tubes to provide a series of hollow electrodes providing a passage for the arc and for the gas being heated.

FIG. '2 in perspective, and partially in section, illustrates one embodiment of the coiled metal tube electrode of FIG. 1.

FIG. 3 illustrates a suitable embodiment of a supporting frame or mounting ring having passages through which the coiled metal tube leads to introduce and withdraw cooling fluid.

FIG. 4 shows a suitable electrical circuit for providing a change in polarity of a given electrode.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring more specifically to the apparatus illustrated in FIGS. 1-3, the apparatus comprises electrodes 1a, 1b, and 10, each of which comprises a continuous metal tube, as of copper for example, coiled to form the electrode. Each coil has a turn 50 of larger diameter than the turns, with the larger coil being within, and secured in electrically conductive relationship to a cylindrical mounting ring 2, preferably constructed of a conductive material. One end of the larger turn, as is shown more particularly in FIG. 2, passes through an opening 6 in the mounting ring for connection to a source of coolant. The other end of the coil leads through a passage 7 in the mounting ring also for connection to the coolant source. These passages 6, 7, are more particularly illustrated in FIG. 3. Each coiled tube electrode comprises a tube of uniform diameter, the coil having, in addition to turn 50, at least two turns 51, 52, 53, having a generally spiral or helical form coaxial with its corresponding mounting ring, but of a diameter smaller than the first turn 50. Preferably the helical turns 51, 52, 53 are smaller than turn 50 by an amount at least equal to twice the diameter of the tubing whereby, turn 51, as shown in FIG. 1, fits snugly within the larger turn 50, and turns 52 and 53 form a helix extending axially along the heater. Each mounting ring and its corresponding coiled electrode forms a subassembly, and a plurality of these subassemblies may be secured together to form the plasma arc heater of the present invention. In the embodiment shown the mounting rings 2 are assembled with adjacent rings separated by cylindrical insulating rings 11. These insulating rings are coaxial with the mounting ring and, as shown, have a larger interior diameter than said mounting rings. The fact that the smaller turns 51, 52, 53, of the coiled electrode are of the dimension specified provides a space between the electrode and the insulating ring. As is shown also, the fact that the smaller turns of the electrode coil are of the specified dimension arrangement causes the arc to jump between smaller or inner adjacent coils 52, by providing the proper spacing.

In the embodiment illustrated in FIG. 1 cylindrical rings 12 are interposed between the insulating rings 11 and the next adjacent supporting frames or mounting rings 2. These spacer rings 12, as shown are of greater internal diameter than both the mounting rings 2 and the insulating rings 11. Another continuous cooling coil 8 is secured interiorly of and adjacent the interior surface of each spacer ring 12, with openings 9, 10, being provided in the spacer ring 12 for the ingress and egress of a cooling fluid. Alternatively, the spacer ring cooling coil may be located exteriorly of the rings. It will thus be seen that when the subassemblies of electrodes and cooling coils are stacked with the insulating and spacing rings to form an arc heater, the smaller turns of the electrode coils 1a, 1b, and 10 will be radially spaced from the spacer ring 12 and the spacer ring cooling coils 8. In this way the smaller turns of the electrode coils shield the insulating rings 11 and, to a lesser extent, shield the spacer rings and their associated cooling coils from the heat of the arc. Since the cooling coils are of tubular metals such as copper, silver, or nickel and may be easily fabricated on a mandrel, the economies will be apparent. The cooling coil tubing is from about 0.25 to 0.75 inch, outside diameter, and may have a thin wall, for example 0.028 inch to 0.1 inch in thickness. Also, the electrodes are fabricated of standard tubing, which is available with various strengths and alloy compositions, and, if desired, may have a bi-metallic type of construction. It is preferred that the circulation of the water be in the turbulent range, for example linear feet per second or greater, to assure maximum heat transfer and to assure no dead spots in any portion of the tubes. Further, turbulent flow assures a high heat transfer rate. In assembling the tubing it may be coiled on a mandrel and then dipped in silver solder or a brazing alloy for the particular metal type of the tube. Although the physical bonding of the tubular element is not essential, it is preferred that this be done.

The plasma arc heater shown in FIG. 1 includes a coil 45 encircling the generally cylindrical plasma heater. This coil is connected electrically in series with the power supply to the heater to create a magnetic field within the are heater. The coil comprises a continuous wire 46which at one end is connected to a suitable source of electricity, shown for illustrative purposes as direct current batteries 47, 48. The batteries or other source of direct current are arranged to apply the proper current and voltage to the heater, with the internal impedance thereof being sufficient to provide a high degree of regulation. The negative terminals of the sources 47, 48, of electricity are connected to one end of the continuous wire 46 forming the coil, while the positive terminal of one of the sources 48 is connected by a conductor 44 to the lower electrode 1c. The positive terminal of the other source 47 of electricity is connected by a conductor 43 to the upper electrode assembly 1a. The other end of the continuous wire 46 forming the coil is connected by another conductor 49 to the intermediate electrode subassembly 1b. Accordingly, the top and bottom electrodes will be positive and the intermediate electrode will be negative thus providing for an are from one electrode to the next. The coil 45 includes suitable electrical insulation 50, surrounding the continuous wire 46 and the magnetic field it produces within the heater exerts a generally tangential force on the are which causes the arc to continuously move, or rotate, around the circumference of the electrodes. If desired more than one electromagnetic coil may be utilized. For purposes of clarity the complete structure of the gas heater is not shown in FIG. 1. The apparatus includes means for feeding the stream of gas being treated, with the direction of flow being indicated in the drawing by arrows. Further, the apparatus may include a plurality of inlets for gas or several types of gas, preferably in the form of tangential conduits which provide a swirl to the gas. These gas feeding means may be at the inlet end as well as spaced along the length of the gas heater to provide for feeding cool and/ or hot gases as may be desired.

FIG. 4 illustrates means whereby the polarity of a given hollow electrode may be reversed to minimize wear. With the rotating arc, erosion is greatly minimized, as has been known for nearly 50 years. Such rotation, however, does not solve the problem since an arc in rotating, moves in a series of pinpoint steps around the circumference of the electrode. For example, as has been shown by experiments, arc rotation may be in the neighborhood of 70,000 revolutions per minute in some specific designs. If it is assumed that the arc changes position around the circumference three times to make one revolution, at the given rate of rotation this is 210,000 pinpoint contacts per minute or 3,500 pinpoint contacts per second on a given electrode. Accordingly, the circuit of FIG. 4 illustrates a new and improved technique to alternately use two mechanically separate electrodes, and by external electrical circuits cause them each to function alternately as an electrode of a given polarity. The electrodes 1b and 1c are such mechanically separate electrodes, and may be the corresponding electrodes of FIG. 1. A suitable source of direct current potential is applied across the conductors 35, 41, one of which is connected to an upper electrode such as subassembly 1a shown diagrammatically. The other conductor is connected to a center tap of a coil 32, the terminals of which are connected across a capacitor 33 to form a conventional LC tuned circuit 34. Two conductors lead from suitable intermediate taps on the coil 32 to diodes 30 and 31, respectively, which in turn are connected by means of conductors 37 and 39, respectively, to corresponding electrode assemblies 1b and 1c. When the tuned circuit 34 oscillates, an alternating current of a suitable voltage is produced'which, when fed through diodes 30 and 31, provides periodic reversal of the polarity of electrodes 1b and 1c with respect to each other, first one and then the other becoming more negative. An arc originating at electrode 1a, as anode, will, therefore, strike electrodes 1b and 1c alternately as one becomes more negative than the other, and thus these electrodes will alternately operate as cathodes.

The oscillations in the tuned LC circuit 34 may be excited by, for example, a source of alternating current energy. An AC generator 60 which may operate at approximately 1750 cycles per second may be a suitable source of such energy. The resulting A.C. energy in the coil 32 of the tuned circuit cooperating with rectifier diodes 30 and 31 alternately adds to the main power supply and causes the arc to strike cathode 1b and 1c alternately at a rate of 3,500 separate pinpoint contacts per second. The alternating use of these electrodes 1b and 10 as the cathode for the are from anode 1a enormously prolongs the useful service life of this pair of elec trodes. Although an external source of alternating current such as the generator 60 may not be required in all applications to provide the reversing current in coil 32, nevertheless some suitable means for alternating the DC polarity of electrodes 1b and 10 with respect to each other is contemplated. Coil 32 and coil 48' are magnetically wound on one core to serve as a transformer, or coil 32 may operate separately as an inductance only.

In each of the embodiments of FIGS. 1 and 4, magnetic field coil 45 may be supplied with D.C. energy from a power supply separate and independent of the power supply of the arc, while in the embodiment of FIG. 4, this coil is supplied from a DC source separate from the exciter source 60.

By the alternate use of the adjacent cathodes 1b, 1c in the arrangement shown in FIG. 4, the are simulta neously rotates by reason of the magnetic field created by coil 45 and in addition alternates between the adjacent cathodes. This provides much more rapid stepping or location changes at which the arc strikes. Further, it permits the use of a weaker magnetic field, thus minimizing danger of blowing out the arc, which may occur with a strong magnetic field.

While the preferred embodiment involves the use of a direct current arc, it will be apparent that the invention is also useful with an alternating current arc.

Conventional arc starting methods and means, not illustrated, are useful with the apparatus of the invention. One common technique is to provide ionized gas in the desired arc path prior to starting the are. This may be done either by introducing pro-ionized gas into the gas heater, or by soldering fuse wires to the electrodes, which vaporize upon having current passed therethrough to provide the ionized atmosphere. Another method is to apply high voltage, high frequency pulses across the electrodes.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected without departing from the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A plasma arc heater comprising a plurality of axially aligned hollow electrode subassemblies forming an axial passage for material to be heated, each electrode subassembly including an electrode formed from continuous metal tubing coiled in at least three turns and a conducting cylindrical mounting ring, each electrode including a first turn secured within and electrically connected to its corresponding mounting ring, each of said electrodes further including second and third turns forming a helix within said first turn to provide said axial passage, said helix having a greater axial extent than said mounting ring; an electrically insulating ring separating adjacent ones of said plurality of electrode subassemblies; and circuit means including a DC source of power for causing a DC are to flow between the helical turns of said electrodes,

said helical turns serving to shield said insulating rings from said arc.

2. The arc heater of claim 1, wherein said metal tubing for each said electrode is of substantially uniform diameter.

3. The arc heater of claim 1, wherein said second and third turns are of a diameter smaller than the diameter of said first turn, whereby said second and third turns are closer to the axis of said heater than said first turn.

4. The are heater of claim 1, wherein the ends of the coiled electrode tubing for each said subassembly extends through its corresponding mounting ring and is adapted to receive cooling fluid.

5. The arc heater om claim 1, wherein the internal di ameter of said insulating rings is greater than the diameter of said helical turns, whereby said turns are spaced inwardly from said insulating rings.

6. The arc heater of claim 1, further including arc rotating means connected to said DC source of power.

7. The are heater of claim 1, wherein said heater includes first, second and third electrode subassemblies in sequence, said first electrode constituting an anode for said arc, and said second and third electrodes constituting adjacent, mechanically separate cathodes for said arc, said circuit means varying the DC voltage on said cathode electrodes whereby said are will flow from said anode to alternate ones of said cathodes.

8. The arc heater of claim 7, wherein said circuit means includes a source of alternating current for applying a varying voltage to said cathodes.

9. The arc heater of claim 7, wherein said circuit includes tuned circuit means connected across said second and third electrodes.

10. The are heater of claim 7, further including spacer rings between each said insulating ring and one of the adjacent mounting rings, the internal diameter of said spacer rings and of said insulating rings being greater than the diameter of said helical turns, whereby said turns are spaced inwardly from said insulating rings.

11. The are heater of claim 10, further including cooling coils, adjacent said spacer rings.

12. The are heater of claim 11, wherein the ends of the coiled electrode tubing for each said subassembly extends through its corresponding mounting ring and is adapted to receive cooling fluid.

13. In a method of operating a plasma arc heater comprising a direct current source and at least a first, second and third generally cylindrical electrodes axially spaced in sequence, the improvement of applying a DC voltage to said first electrode to form an anode, applying a DC voltage to said second and third electrodes to form a cathode, and varying said voltage on said cathode electrodes whereby an arc flowing from said anode to said cathode will strike said cathode electrodes alternately.

References Cited UNITED STATES PATENTS 2,964,679 12/ 1960 Schneider et al. 315-111 3,140,421 7/1964 Spongberg 315111 3,360,682 12/1967 Moore 315-111 3,402,366 9/ 1968 Williams 331-94.5 3,43 6,679 4/1969 Fenner 331--94.5

RAYMOND F. HOSSFELD, Primary Examiner US. Cl. X.R.

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