[0001] This invention relates to a high enthalpy plasma torch.
[0002] Plasmas have been produced using variations of three basic plasma generating devices:
r.f. or induction torches, transferred arcs, and d.c. torches. The r.f. torch uses
no electrodes and the energy is transferred from a high frequency electromagnetic
source to the plasma by induction. However, both the transferred arc and the d.c.
torch use electrodes to pass current through a gas thus generating the plasma. The
geometry and composition of the electrodes are critical in determining the torch operation
and utility. Since this innovation relates primarily to a new electrode configuration
for a d.c. torch, a more elaborate discussion of conventional electrode technology
is justified.
[0003] Two types of d.c. torches are commonly used. The first type uses a conical thoriated
tungsten rod as the cathode and a copper tube as the anode. The gas is introduced
behind the cathode tangentially, creating a vortex past the cathode and through the
anode, which is located in the front of the torch. The arc is attached on one end
at the tip of the cathode and is rotated at the other end along the inside surface
of the anode. The momentum of the plasmagas vortex, the plasmagas composition, the
diameter of the anode and the arc current can be used to control the length of the
arc. The anode attachment determines the arc length since the cathode attachment is
fixed. These torches, also known as FCC or fluid convective cathode torches, are most
commonly used in low power applications, such as plasma spraying, cutting and laboratory
investigations. Typical operating characteristics at atmospheric pressure and using
nitrogen plasmagas may be: Plasmagas Flowrate = 50 - 100 L/min, Arc Current = 200
- 600 Amperes, Arc Voltage = 70 - 110 Volts, and Plasmagas Enthalpy 1-3 kJ/kg. The
fixed cathode attachment prevents the torch from operating at very high currents and
the use of thoriated tungsten limits the possible plasmagas compositions to a few
inert and reducing gases (e.g. Ar, Ar/H₂ mixtures, N₂, He). Neither oxygen nor halides
can be used as plasmagas. FCC torches are currently being marketed by a wide variety
of companies.
[0004] The second type of d.c. torch uses two coaxial tubes as the electrodes. The plasmagas
is introduced by a vortex generating ring tangentially between the two electrodes
creating two vortices in opposite direction. Each vortex pushes an arc attachment
away from the vortex generating ring. Thus, the arc elongates and tubular torches
offer significantly higher voltages than FCC torches. Tubular torches can employ a
variety of electrode compositions with copper being the most common. Thoriated tungsten
is not being used as a cathode since it is not fabricated in the required large size
tube. An exemption is the small (6 mm I.D., 16 mm O.D) tubular thoriated tungsten
cathode used by Nippon Steel Corp. However, that electrode was used in a transferred
arc system with the plasma operating between the lip of the tube and an anode located
outside the torch.
[0005] Tubular torches have been used mostly for melting and as heaters for high temperature
reactors. Unfortunately, they need extremely high gas flowrate to stabilize the arc
and prevent electrode destruction. Typical operating characteristics for a one MW
tubular torch at atmospheric pressure and using nitrogen plasmagas may be: Plasmagas
Flowrate = 3000 - 10000 L/min, Arc Current = 500 - 800 Amperes, Arc Voltage = 700
- 2000 Volts, and Plasmagas Enthalpy = 0.5 - 1.5 kJ/kg. Companies such as Plasma Energy
Corp. and Aerospatiale are marketing tubular torches. Their most effective role is
as air heaters. However, they encounter serious electrode erosion problems at arc
currents above 800 A. Conventional tubular torches can not be used whenever low plasmagas
flowrate is required as is the case in plasma spraying or in reactors using an inert
plasmagas. Furthermore, the tremendous gas flow required prevents them from being
economical in many applications. Finally, they can not use any halide plasmagas because
the halides would both destroy conventional electrodes and condense in the gas feeding
tubes.
[0006] It is the object of the present invention to provide a d.c. plasma torch offering
higher enthalpy than conventional torches, very low electrode erosion rate, extremely
stable operation, high voltage, low plasmagas flowrate, and capable of operating with
a metal halide plasmagas.
[0007] The plasma torch, in accordance with the present invention comprises a torch housing,
rear and front tubular electrodes coaxially mounted within the housing with a gap
therebetween, both electrodes being fabricated from copper having tubular inserts
of refractory material, a vortex generator for introducing a tangential flow of gas
in opposite direction in the tubular electrodes through the gap between the two electrodes,
and a cooling system for cooling the tubular electrodes.
[0008] A plasmagas feed system is mounted in the housing and includes thermally insulating
tubes for preventing condensation of plasmagas onto the cooled electrodes.
[0009] The front electrode includes a cup shaped exit portion comprising an expansion followed
by a constriction both to create a plasmagas back pressure for improving rotation
of the arc inside the electrodes of the torch and thus minimize electrode erosion
and to prevent materials from the surrounding atmosphere from entering the electrode
region.
[0010] The refractory electrode material may be thoriated tungsten or a tantalum carbide
composite including tantalum carbide infiltrated with aluminum or copper. Other refractory
electrode materials may also be used.
[0011] The cooling system comprises a water guide surrounding the rear electrode, a brass
cooling jacket surrounding the front electrode, and annular passages in between the
water guide and the rear electrode and between the cooling jacket and the front electrode
for circulating a cooling liquid in serial relationship around the rear electrode
and then around the front electrode.
[0012] The invention will now be disclosed, by way of example, with reference to the accompanying
drawings in which:
Figure 1 is a sectional view through the plasma torch in accordance with the present
invention; and
Figure 2 is a view taken along lines 2-2 of Figure 1.
[0013] Referring to the drawings, there is shown a plasma torch comprising generally a rear
electrode (anode) 10 and a front electrode (cathode) 12 which are coaxially mounted
within a stainless steel housing made of a rear section 14 and a front section 16
assembled together by bolts 18.
[0014] The rear electrode comprises a tubular metal member 20 made of copper which is threadedly
mounted to one end of a metal electrode holder 22. The rear electrode holder 22 also
serves as a fluid conduit for the torch cooling system and for this purpose the rear
end of the holder includes a bore 24 which communicates with radial apertures 26 for
the passage of a cooling fluid, such as water. A water guide 28 in the form of a thin
walled metal tube is threadedly mounted on the electrode holder and surrounds the
rear electrode to form an annular water passage 30 which is part of a fluid cooling
system for cooling the rear electrode.
[0015] The front electrode 12 is mounted in a brass annular member 32 which is itself threadedly
mounted to a stainless steel tubular electrode holder 34 having a flange 36 which
is clamped between the rear and front sections 14 and 16 of the housing. The front
electrode holder is electrically insulated from the housing by means of an insulating
annular member 38 made of a high temperature chemically resistant plastic material.
[0016] The front and rear electrodes are electrically insulated from each other by means
of an annular insulating member 40 made of a high temperature chemically resistant
plastic material which extends rearwardly between the housing portion 14 and the water
guide 28. The upper part of the insulating member 40 has an extension made of electrically
insulating plastic material 41 which is secured to the housing portion 14 by means
of a threaded insulating member 42 also made of electrically insulating plastic material.
A narrow annular water passage 43 is provided in the annular insulating member 40
behind the water guide for a purpose to be disclosed later. A plurality of holes 44
communicating with channels 46 are spaced around the annular member 40 and communicate
with annular water passage 42 forming part of the fluid cooling system. The brass
annular member 32 is also provided with a narrow annular water passage 48 which is
part of the cathode cooling system. A plurality of radial holes 50 are provided in
the rear end of the brass member 32 for communicating the channels 46 to the annular
water passage 48. A plurality of radial holes 52 are also provided for communicating
the front end of the water passage 48 with an annular passage 54 formed between the
anode holder 34 and the housing 16 to direct the cooling water to an outlet 56.
[0017] In accordance with the main feature of the present invention, the copper electrodes
10 and 12 are provided with inserts 58 and 59, respectively, which are attached by
high temperature soldering. This make it possible to use a much wider range of electrode
materials. Indeed the torch can operate using all suitable refractory electrode materials
including both thoriated tungsten or a composite material including tantalum carbide
infiltrated with aluminum or copper as disclosed in Canadian Patent Application No.
2,025,619 filed September 18, 1990, and suitable for operation with metal halide plasmagas.
The rear end of the refractory insert 58 is insulated from the electrode holder 22
by ceramic electrical insulator 60. Similarly, the rear end of the refractory insert
59 is separated from the plastic insulating material by a ceramic electrical insulating
ring 61.
[0018] A conventional vortex generating ring 62 is mounted between the rear and front electrodes.
The vortex generating ring is provided with tangential holes 64 for creating two gas
vortices A and B in opposite directions in the center of the annular anodes and cathodes.
Each vortex pushes an arc attachment away from the vortex generating ring 62. Thus
the arc elongates and such tubular torches offer significantly higher voltages than
the FCC torches.
[0019] In accordance with another feature of the present invention, gas is delivered to
the vortex generating ring through thermally insulating tubes 66, such as quartz,
which prevent condensation of the plasmagas into the torch body. The plasmagas gas
is fed from inlet port 68 through opening 70 in insulating ring 38, tubes 66 and annular
passage 72 around the vortex generating ring and into the tangential holes of the
vortex generating ring 62.
[0020] The front end of the front electrode (cathode) includes an expansion 73 followed
by a constriction 74 near to the exit. This design creates a plasmagas back pressure
which significantly improves the rotation of the arc inside the electrodes of the
torch thus minimizing electrode erosion. It provides a stable arc attachment zone
thus minimizing fluctuation in power output. It also confines the arc jet within the
expansion thus offering a long and symmetric tail flame ideally suited for cutting,
welding and spray-forming operations. Finally, it prevents materials from the surrounding
environment from entering the electrode region where they can destroy the electrodes.
[0021] The plasma torch cooling system permits to circulate a cooling liquid, such as water,
in serial heat exchange relationship with the rear electrode 10 and the front electrode
12. The cooling water enters the torch through the bore 24 in the electrode holder
22. The water than passes through the radial apertures 26 and flows into the annular
passage 30 between the outside surface of the rear electrode and the water guide 28
to cool the rear electrode (anode). The water then flows back behind the water guide
and into holes 44 in annular insulating member 40 and through channels 46. It is to
be noted that holes 44 are located toward the rear portion of the annular insulating
member 40 to avoid any possibility of electrical short circuit between the electrodes
through the cooling water. The cooling water then passes through holes 50 in bronze
cooling jacket 32 and annular passage 48 around the front electrode 12 to cool the
front electrode (anode). The water then returns to the water outlet 56 through holes
52 in the front end of the cooling jacket and annular space 54 behind the cooling
jacket.
[0022] Although the invention has been disclosed, by way of example, with reference to a
preferred embodiment, it is to be understood that it is not limited to such embodiment
and that other alternative are envisaged within the scope of the following claims.
1. A plasma torch comprising
a) a torch housing,
b) rear and front tubular electrodes coaxially mounted within said housing with a
gap therebetween, both electrodes being fabricated from copper having tubular inserts
of refractory material;
c) a vortex generator for introducing a tangential flow of gas in opposite direction
into said tubular electrodes through the gap between the two electrodes; and
d) a cooling system for cooling the tubular electrodes.
2. A plasma torch as defined in claim 1, further comprising a plasmagas feed system mounted
in said housing and including thermally insulating tubes for preventing condensation
of plasmagas onto the cooled electrodes.
3. A plasma torch as defined in claim 1, wherein the front electrode includes a cup shaped
exit portion comprising an expansion followed by a constriction.
4. A plasma torch as defined in claims 1, 2 or 3 wherein said refractory electrode material
is thoriated tungsten.
5. A plasma torch as defined in claims 1, 2 or 3 wherein said refractory electrode material
is a tantalum carbide composite including tantalum carbide infiltrated with aluminum
or copper.
6. A plasma torch as defined in claim 1, 2 or 3, wherein said cooling system comprises
a water guide surrounding said rear electrode, a brass cooling jacket surrounding
said front electrode and annular passages in between said water guide and said rear
electrode and between said cooling jacket and said front electrode for circulating
a cooling liquid in heat exchange relationship around the rear electrode and then
around the front electrode.