EP0017201B1 - Direct current plasma torch - Google Patents

Description

The invention relates to a direct current plasma torch with a rod-shaped cathode and a concentrically arranged, rotationally symmetrical anode and an annular gap between the cathode and the anode for supplying the gas to be heated, the cathode having an annular edge at its free end and the free end the cathode is located in an axially parallel, temporally constant magnetic field, while in the region located downstream of the free end of the cathode there is a temporally constant magnetic field which diverges in the direction of flow, such that the plasma arc which forms between the cathode and the anode is set in rotation.

In various areas of process engineering, in plasma wind tunnel investigations, in plasma chemistry and for the operation of fluid dynamic lasers, highly heated gas flows are required, the temperature and pressure of which, depending on the purpose of use, have certain values in the range between a few 100 and approx. 20,000 K and pressures up to Must be 20 bar.

To achieve such values of temperature and pressure, gases can be heated in a direct or alternating current arc discharge. The main technological problems lie in the control or reduction of the electrode erosion through the arc attachments, directly related to the guarantee of a sufficient service life of the electrodes and the cleanliness of the arc plasma as well as in the realization of the properties of the highly heated medium required for the application. In certain applications, for example, spatial homogeneity and constant temperature can be required.

To achieve high thermal efficiency, i. that is, to achieve a large ratio of the energy contained in the hot gas to the electrical energy used, the aim is still to operate the discharge at the highest possible voltage. Since the electrophysical electrode losses are proportional to the current strength, the constant the current intensity, the lower the power loss at the electrodes, the higher the arc voltage.

The electrode erosion can be reduced by shortening the dwell time of the arc attachment on a specific surface element of the electrode. Devices are known which, for the purpose of moving the arc attachments, introduce the medium to be heated in its entirety or only partially tangentially into the space between the electrodes (German patents 1,564,333 and 2,236,487). In the case of an arc that is vortex-stabilized in this way, the arc attachment is driven in the circumferential direction, but because of the axial velocity component of the gas, which is caused by the flow through the hollow cylindrical electrodes, a longitudinal movement is also forced on it. The arc oscillates between different foot spots, causing its column length and thus voltage to take on temporally variable values. Naturally, a constant in time and spatially homogeneous hot gas or plasma cannot be achieved in this way.

Other devices take advantage of the physical fact that an arc can be moved by electromagnetic forces. Technical configurations are known in which the magnetic field required for this is generated by means of coils arranged in the vicinity of one or both electrodes (German patents 1 564 333 and 1 933 306). In principle, however, these explanations are such that a substantial part of the arch column and at least one arch extension must extend into an area with an increasing magnetic field. The momentum equation can be derived theoretically and practical experience shows that this effect not only leads to a shortening of the arch column and thus a reduction in tension, but also gives rise to instability, which has an adverse effect on the achievement of stationary plasma states.

While most high-pressure plasma generators use electrodes made of copper, the cathode can also be produced from tungsten or thoriated tungsten in apparatus operated with direct current (DE-OS 2027626, DE-PS 2 033 072). However, in practice it turns out that at pressures higher than about 2 bar and currents above about 250 to 300 A, the thermal load at the tip of a conical cathode, as is carried out in accordance with DE-OS 2 027 626, becomes so high that the material melts in the arch approach and gets into the plasma as an impurity. This effect is avoided in the technical design according to DE-PS 2 033 072, in that a certain amount of the arc gas flows into a hollow cathode and leads there to the arc attachment. However, since the bow does not find a preferred starting point, experience has shown it to perform an erratic movement, which in turn causes fluctuations in tension and ultimately temperature fluctuations.

From the German patent application 2609178 a plasma torch is also known, in which a rod-shaped cathode with a conical tip is provided. At this tip there is inevitably a localization of the plasma arc, so that here inevitably a very considerable electrode erosion must occur.

In a further known plasma torch, an outlet channel for the plasma gases is surrounded by a number of individual electrodes in the shape of a cone (British Patent 1 112 935). The entire arrangement is to be rotated so that the plasma arc begins successively on different individual electrodes. However, if the plasma arc begins at one of the many rod-shaped electrodes surrounding the gas outlet openings, it will maintain this starting point even during rotation, since the field distribution at the tip of this electrode is particularly favorable. The mechanical rotation thus swirls the plasma arc, which will only tear off after considerable swirling and will jump over to another electrode. This results in an extremely unstable plasma arc. In addition, severe erosion occurs at the tips of the individual electrodes, since the plasma arc emanates for a long time continuously from the same electrode tip.

The invention has for its object to improve a direct current plasma torch in such a way that it can be used to heat gases and gas mixtures at pressures of up to 20 bar to temperatures in the range from a few 100 degrees to approximately 20,000 K, with long service lives of the electrodes , Cleanliness of the plasma, high efficiency and stationarity and spatially uniform distribution of the gas properties are to be achieved.

This object is achieved according to the invention in a plasma torch of the type described in the introduction in that the cathode has a continuous inner bore through which part of the gas to be heated can be introduced into the plasma torch at such a flow rate that the cathode-side plasma arc attachment on the annular edge of the cathode is localized.

The advantages achieved by the invention are that the design of the cathode with an annular edge of the cathode approach and due to the axial gas flow and the anode approach of the arc are each fixed in a certain axial position, which results in a temporally constant arc length and arc voltage and finally Temperature results. On the other hand, the arc as such is set in rotation by electromagnetic forces, as a result of which its electrode attachments move rapidly in the circumferential direction, which leads to an extremely short dwell time on a specific surface element. As a result, the electrode material does not melt, and long service lives and clean plasmas can be achieved.

It is important that part of the supplied gas to be heated is supplied through the inner bore of the cathode, since this flow on the inside of the cathode shifts the starting point of the plasma arc away from the inner surface of the cathode onto the annular edge. Only when the plasma arc is attached to the annular edge does the arc approach parallel to the magnetic field, and only then is it ensured that the arc length and thus the arc voltage remain the same during operation of the plasma torch.

It is favorable if the cathode consists of thorium tungsten.

In a preferred embodiment of the invention it is provided that the annular edge has a cutting shape.

The anode can have the shape of a circular cylinder and have an internal width which is larger than the outside diameter of the cathode. The outside diameter of the cathode is preferably considerably smaller than the inside diameter of the anode, so that there is a relatively wide annular gap between the two. As a result, the gas supplied flows into the plasma torch at a relatively low speed, so that no strong negative pressure forms on the inside of the lower edge of the cathode. The formation of a negative pressure would be unfavorable, since the plasma arc tends to migrate into the area of lower pressure, so that with very rapid flow of the gas to be flowing in and strong negative pressure formation on the inside of the cathode, the starting point of the plasma arc migrates to the inside surface of the cathode would. Such a migration would be disadvantageous since the plasma arc approach would then no longer run parallel to the magnetic field lines. This non-parallel course leads to a rotational movement of the plasma arc attachment, in the opposite direction to that of the anode, so that overall a swirling of the plasma arc occurs which, as a result, can no longer have the required uniformity. By expanding the inlet gap for the inflowing gas, the formation of a negative pressure on the inside of the cathode is avoided, so that the plasma arc also has no tendency to migrate away from the edge into the inside of the cathode. In addition, the formation of a negative pressure on the inside of the cathode is also counteracted by the gas flowing through the inner bore.

In a further advantageous embodiment of the invention it can be provided that a magnetic coil or a permanent magnet are arranged concentrically to the cathode axis in order to generate the magnetic field in such a way that their plane of symmetry in a region between the annular edge of the cathode and a maximum of five diameters of the annular edge in Countercurrent direction shifted level.

The cathode and / or the element generating the magnetic field are preferably adjustable in the axial direction. With this arrangement, the arc approach is in an essentially axially parallel magnetic field. By the axial adjustment of the cathode and / or Magnetic field generating element, the effective length of the arc and thus its voltage can be adjusted, which in turn allows the temperature to be changed.

In a further advantageous embodiment of the invention it is provided that an equalization chamber with an outer housing and a jacket arranged therein connects to the combustion chamber and that an annular space extending essentially over the entire height of the equalization chamber is arranged between the housing and the jacket with a gas supply line and on the other hand via holes in the jacket with the interior of the compensation chamber in connection. The jacket is preferably made of a refractory material. It is also favorable if the housing is machined on the side facing the annular space in a mirror-like manner.

This configuration enables a gas to be introduced into the space between the jacket and the housing by a gas supply, which gas extracts heat from the jacket before it enters the compensation chamber through inlet openings in the jacket. At the same time, the energy flow from the jacket to the cooled wall of the housing is reduced to 30% of the value that occurs in known devices in which the energy flow from the hot gas to the cooled wall takes place through convective heat transfer, since the energy flow from the jacket to the cooled wall the compensation chamber according to the invention can only be carried out by radiation transport.

The combination of radiation cooling and convective cooling in the compensation chamber adjoining the discharge space thus reduces the heat loss to a minimum. This leads to high values of the overall efficiency of the facility. Finally, the blowing in of gases through the openings in the jacket additionally allows any temperatures to be achieved under optimal operating conditions of the burner even in those gas mixtures in which one component would have deleterious effects on the cathode if the arc discharge were heated directly.

Further advantageous embodiments are the subject of the dependent claims.

The following description of a preferred embodiment of the invention serves in conjunction with the drawing for a more detailed explanation. The drawing shows a preferred embodiment of a plasma torch in longitudinal section.

A cylindrical housing 1 has a central longitudinal bore 2 which is lined in its upper area by means of an insulating sleeve 3 and in its lower area by means of a circular cylindrical anode 4. A rod-shaped cathode 5 with a central longitudinal bore 6 projects into the longitudinal bore 2 from the open upper side. The cathode 5 is tapered in the region of its free end, so that a cutting-shaped ring edge 7 results at the outlet of the longitudinal bore 6. The outer diameter of the rod-shaped cathode 5 is smaller than the inside width of the insulating sleeve 3 and the anode 4, so that an annular gap 8 is formed between them and the cathode 5.

The cylindrical housing 1 is surrounded by a magnetic coil 9, which can be excited by a current source, not shown in the drawing. The magnet coil 9 is displaceable in the direction of the longitudinal axis of the housing. The rod-shaped cathode 5 can also be displaceable in the direction of the longitudinal axis of the housing.

Downstream of the housing 1 there is an equalizing chamber 10 with a likewise cylindrical housing 11 and a cavity 12 connected to the longitudinal bore 2. A circular cylindrical jacket 13 is inserted into the cavity 12 in such a way that in its end regions 14 and 15 it lies tightly against ring webs 16 and 17 at the upper and lower ends of the housing 11, while in the remaining region between the jacket 13 and the housing 11 an annular chamber 18 is formed. This annular space 18 is connected via a channel-shaped line 19 to a gas source (not shown in the drawing) and via openings 20 in the jacket 13 to the interior 21 of the compensation chamber 10. The side walls 22 of the annular space 18 on the housing side are machined in a mirror-like manner. The jacket 13 is preferably made of a refractory material.

In the operation of the plasma torch described, the working gas flows on the one hand through the annular gap 8 and on the other hand through the longitudinal bore 6 into the interior of the housing 1 and thereby flows essentially axially parallel to it. An arc burns between the anode 4 and the cathode 5, the cathode-side arc attachment being located on the cutting-shaped ring edge 7. The magnet coil 9 is excited and thereby generates a magnetic field which runs essentially axially parallel in the region of the ring edge 7, while diverging in a region located downstream. This magnetic field causes the arc to rotate around the longitudinal axis of the housing, so that the starting point of the arc on the cathode side travels along the cutting-shaped ring edge 7. The starting point runs in a radial plane, so that the length of the arc does not change, so that the voltage and temperature of the arc remain constant during this migration. It is essential that the magnetic field in the area of the ring edge runs essentially axially parallel. For this purpose, the magnet coil 9 is moved into a corresponding axial position, according to the invention it is arranged such that its plane of symmetry lies in a region between the annular edge 7 of the cathode 5 and a plane displaced in the countercurrent direction by a maximum of five diameters of the annular edge 7. The length of the arc and thus its temperature can be influenced by changing the axial position of the magnetic coil 9 and / or the rod-shaped cathode. It is therefore possible in a simple manner to set a desired temperature which, due to the constant length of the arc, also remains constant over time. Characterized in that the cathode-side arc approach moves along the ring edge 7, the dwell time of the approach in a certain surface element is extremely short, and there is practically no electrode erosion. The starting point on the anode side is also defined by the diverging magnetic field in the area of the anode in the axial direction, while a migration of the starting point is ensured in the circumferential direction.

Because of the migration of the starting point on the cathode side, cathodes made of conventional materials can be used even at high powers, but it is expedient to produce the cathode additionally from highly heat-resistant material, for example from thorium tungsten.

Favorable performance data result in particular when the anode has the shape of a circular cylinder and has a clear width which is larger than the outer diameter of the cathode 5.

Of course, the magnet coil 7 can be replaced by an appropriately magnetized permanent magnet.

The gases heated by the arc enter the compensation chamber 10 after passing through the arc. Here, the jacket 14, which preferably consists of a refractory material, together with the annular space 18 ensures good thermal insulation of the gases. Since the jacket 14 is only in direct thermal contact with the cooled housing 11 in a small area, heat can be transferred from the jacket 14 to the housing 11 essentially only by radiation. Radiation losses are additionally reduced by the reflective processing of the side walls 22 of the annular space 18 on the housing side. Overall, the jacket 14 thus acts as a heat shield.

A further gas or a gas mixture can be admixed to the heated gas in the interior 21 of the compensation chamber via the line 19, the annular space 18 and the openings 20. This gas entering through the annular space 18 is preheated in the latter, so that part of the heat losses can be compensated for in this way. The desired final temperature of the gas mixture emerging from the compensation chamber can be set by adding further gases. It is also favorable that gas components can be added to the superheated gas which cannot be heated to the high temperatures prevailing in the plasma torch itself, be it that they are decomposed at these temperatures or that they can cause harmful reactions at these temperatures with the components of the plasma torch.

With the combination of the described plasma torch and the compensation chamber, it is possible to heat gases even at high pressures (up to 20 bar) to extremely high temperatures (up to 20,000 K) and at the same time adjust the final temperature of the gas precisely, reproducibly and constantly, whereby the Wear of the plasma torch during operation is reduced to a minimum. The temperature control can be carried out in two ways, namely by moving the solenoid 9 and / or the cathode 5 and by admixing a gas in the compensation chamber. You get an extremely variable and simple combination.

Claims (12)

1. Direct current plasma torch comprising a bar-shaped cathode and an anode symmetrical with respect to rotation and disposed concentrically to said cathode and an annular clearance between cathode and anode for introducing the gas to be heated, the cathode having at its free end an annular edge and the free end of the cathode being located in an axially parallel magnetic field constant in time, while a magnetic field constant in time but diverging in the direction of flow exists in the area located downstream of the free end of the cathode such that the plasma arc forming between cathode and anode is caused to rotate, characterized in that the cathode (5) has a continuous inner bore (6), through which part of the gas to be heated is introduced into the plasma torch at such a velocity of flow that the point at which the plasma arc connects to the cathode is localized at the annular edge (7) of the cathode (5).

2. Plasma torch according to claim 1, characterized in that the cathode (5) consists of thoriated tungsten.

3. Plasma torch according to either of claims 1 or 2, characterized in that the annular edge (7) is wedge-shaped.

4. Plasma torch according to any of the preceding claims, characterized in that the anode (4) is circular-cylindrical in shape and has an inside diameter which is larger than the outside diameter of the cathode (5).

5. Plasma torch according to any of the preceding claims, characterized in that for generating the magnetic field a magnetic coil (9) or a permanent magnet are disposed concentrically to the cathode axis such that their plane of symmetry lies in the area between the annular edge (7) of the cathode (5) and a plane displaced in the direction of counterflow by a distance of maximum five times the diameter of the annular edge (7).

6. Plasma torch according to any of the preceding claims, characterized in that the cathode and/or the element (9) generating the magnetic field are displaceable in the axial direction.

7. Plasma torch according to any of the preceding claims, characterized in that the gas is introduced axially parallel.

8. Plasma torch according to any of the preceding claims, characterized in that an equalizing chamber (10) having an outer casing (11) and a jacket (14) disposed therein is connected to the combustion chamber and that an annular space (18) extending substantially through the entire length of the equalizing chamber (10) is disposed between casing (11) and jacket (14), said annular space (18) being connected to the inside space (21) of the equalizing chamber (10) on the one hand by a gas supply line (19) and on the other hand via holes (20) in the jacket (14).

9. Plasma torch according to claim 8, characterized in that the jacket (14) consists of a refractory material.

10. Plasma torch according to either of claims 8 or 9, characterized in that the casing (11) has a specular finish on the side facing the annular space (18).

11. Plasma torch according to any of claims 8, 9 or 10, characterized in that the equalizing chamber (10) has a cylindrical, semicylindrical, prismatic or spherical shape.

12. Plasma torch according to any of claims 8 to 11, characterized in that the main axis of the equalizing chamber (10) is disposed parallel or perpendicular to that of the plasma torch.

Images