Plasma burner for transferred electric arc

A plasma burner has an annular passage extending along an insulated tube defining the outer wall of the electrode lance and located between the electrode lance and the multi wall burner shell to deliver the auxiliary gas outwardly of the plasma gas. The nozzle forming end of the electrode lance has a coolant circulation as has the burner shell and the annular passage extends substantially to the coolant inlets and outlets of the burner. The construction avoids the formation of parasitic arcs.

FIELD OF THE INVENTION 
Our present invention relates to a plasma burner of the transferred 
electric arc type which can have a central electrode, a nozzle end piece 
concentric or coaxial therewith, an annular gap between the electrode and 
the nozzle end piece from which the plasma forming gas emerges and, 
coaxial therewith, a burner shell with outer, middle and inner walls such 
that between the nozzle end piece and the burner shell an annular passage 
is provided whose inner wall is at least partly formed by an electrically 
insulating tube separating the two parts. 
BACKGROUND OF THE INVENTION 
A significant problem in the operation of plasma burners with alternating 
current and three phase electric current is the development of parasitic 
electric arcs which burn in parallel to the main electric arc. These 
parasitic electric arcs are predominantly concentrated at the lower edge 
of the nozzle or burner shell and the outer regions of the nozzle endpiece 
and the end face of the burner. Parasitic electric arcs are not only 
detrimental to the stability of the arc column and hence the formation and 
stability of the plasma, but also affect the efficiency and economy of the 
plasma burner and any apparatus in which the plasma burner is parallel to 
a significant degree because of the excess utilization of energy. 
Parasitic arcs can also give rise to complete destruction of the plasma 
burner 
In German Patent Document DE-PS 33 28 777, a system for reducing parasitic 
arcs is described whereby the annular passage between the electrode and 
the nozzle along the inner surface of the nozzle is provided with an 
electrically insulating coating In practice this has been found to provide 
only partial protection since parasitic arcs can find current flow paths 
externally of the insulating coating. 
Another approach to the elimination of parasitic arcs is described in 
German Patent Document DE-PS 34 35 680. In this system, on the inner wall 
of a water cooled nozzle, an insulating part is provided to subdivide it 
into two separate wall portions. This insulating part extends over the 
entire cross section and electrically insulates the separated parts from 
one another. 
The drawback of this system is that it is difficult to find a fully 
satisfactory insulating material which can tolerate the extremely high 
temperatures of the furnace atmosphere in the apparatus in which the 
plasma burner is used. 
Other attempts have been made to eliminate plasma arcs by providing a 
groove in an end face of the nozzle and accommodating in this groove an 
insulating ring. The groove can be formed by the outer wall of a nozzle 
stub or end piece and can have a burner shell such that the burner shell 
at its end has a flange turned toward the axis of the burner forming a 
heat shield for the insulating member which is set back from it. 
When a burner of this type is used in an atmosphere containing electrically 
conductive particles, for example metal particles or molten plant or 
foundry dust, the electrically conductive dust can collect on the cooled 
insulating member and form an electrically conductive bridge 
shortcircuiting between the nozzle stub and the burner shell and 
sustaining parasitic arcs at least along the outer edge of the end face of 
the burner shell. 
In general, therefore, it can be said that prior techniques attempting to 
eliminate parasitic arcs from plasma burners have not been fully 
satisfactory. 
OBJECTS OF THE INVENTION 
It is, therefore, the principal object of the present invention to provide 
an improved plasma burner for a transferred-arc plasma generator which 
will obviate the drawbacks of earlier plasma burners, especially with 
respect to the formation of parasitic arcs. 
Still another object of the invention is to provide a plasma burner which 
can be operated more economically with greater main arc and plasma 
stability and with reduced wear by comparison with earlier plasma burners. 
Still another object of the invention is to provide a plasma burner in 
which the danger of the formation of parasitic arcs, especially when the 
system is operated in an alternating current mode, is greatly reduced or 
eliminated. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the invention, in a burner as initially 
described herein, wherein the annular passage between the nozzle end piece 
and the burner shell extends at least rearwardly from the plasma forming 
front end of the burner to the level of the coolant inlet and outlet of 
the burner shell and at its rearward end is connected with a source of a 
gaseous medium under pressure by a pipe connection. 
More specifically, a transferred arc plasma burner according to the 
invention can comprise: 
a central electrode; 
a nozzle end piece concentric with an end of the central electrode at a 
front of the burner and spaced from the central electrode by an annular 
gap for discharging a plasma gas from the burner; 
a burner shell surrounding the nozzle end piece at an end of the burner 
shell and extending axially from the end of the burner shell along the 
electrode, the burner shell being formed with a coolant inlet and a 
coolant outlet at a location axially spaced from and distal to the end of 
the burner shell, the burner shell defining an annular passage with the 
nozzle end piece communicating with a mouth spacedly surrounding the 
annular gap, the annular passage extending axially from the nozzle end 
piece distally at least to the location, an inner wall of the annular 
passage being formed by an electrically insulating tube separating the 
burner shell from the nozzle end piece; and 
means at a rear end of the annular passage remote from the nozzle end piece 
forming a pipe connection communicating with a source of a gaseous medium 
under pressure. 
The coaxial annular passage, by virtue of its significant length alone, 
provides a significant improvement in the operational reliability and life 
of the plasma burner and, by eliminating mechanical connections and 
surfaces on which electrically conductive vapors can condense or dust can 
deposit, reduces the tendency for conductive bridging between the nozzle 
stub and the burner shell. 
The pipe connection with a source of a gaseous medium under pressure 
permits the gaseous medium to flush through the annular passage and the 
flushing flow of gas through the annular passage provides a further step 
toward operational reliability and improved operating life of the plasma. 
For example, the mouth or outlet regions of the annular passage are 
additionally cooled by this gas. 
Any electrically conductive vapor, which might prove to be a problem on 
condensation, or dust which might collect, are effectively prevented from 
penetrating into or accumulating in the annular passage. 
Backfiring plasma arcs are precluded by the flushing flow and molten metal 
slag spatterings which might otherwise penetrate into the annular passage, 
are deflected away and cooled by this flow of gas. 
Furthermore, this gas flow through the annular passage forms a shielding 
gas envelope around the plasma arc and prevents oxidizing gas, for 
example, atmospheric air, which might be sucked into the plasma arc from 
the ambient atmosphere, from being drawn into the arc. Such oxidizing 
gases tend to erode high melting point metals and to be detrimental to 
those portions of the burner which are susceptible to oxidation even 
outside the plasma gas region and hence the gas flow can protect the 
burner. When the use of an oxidizing gas in the annular passage is not a 
problem, i.e. when the plasma needed not be shielded from the oxidizing 
atmosphere, the gas may be an oxidizing gas and is then effective to 
oxidize metal vapors which might be generated in the region of the annular 
passage to metal oxides of low metal conductivity and thereby provide 
additional assurance against conducted bridging and the formation of 
parasitic arcs. 
Because of the cooling effect of the gas traversing the annular passage, 
the coatings or materials used at the mouth of the annular passage can 
have increased useful life by comparison with earlier plasma burner 
systems. Furthermore, the gas enveloped formed by the shielding gas can 
reduce radiant energy loss from the plasma arc and thereby reduce 
detrimental effects on any containment. 
Because the annular passage extends substantially to the level of the 
coolant inlet and outlet of the burner shell, there is a sufficient length 
of the annular passage to insure a highly uniform distribution of the gas 
flow in this passage and hence a uniform discharge of the gas at the mouth 
of this passage. 
Since this additional gas is supplied independently of the feed of the main 
plasma gas, the plasma burner can be utilized in a highly advantageous way 
for the transport or entrainment of solid materials in pulverulent or 
granular form via the additional gas. By contrast to systems in which 
there is a local feed of solid materials by separate lances provided in 
addition to the plasma burner, the plasma burner can utilize the entire 
plasma circumference and a substantial part of the length of the plasma 
arc along its axis for the melting and evaporation of solid materials. 
According to a feature of the invention, the distribution of the additional 
or auxiliary gas flow in the annular passage is further improved by 
providing the pipe connection at the rear end of the annular passage so 
that it imparts a tangential flow component to the auxiliary gas which is 
there introduced. 
Instead of the single pipe fitting, of course, the pipe connection can 
include a plurality of pipe fittings connected to the source of auxiliary 
gas, angularly equispaced around the rear end of the annular passage and 
each opening tangentially into the annular passage to impart the 
tangential flow component to the gas. The at least partly tangential 
influx of the gas into the channel and the resulting uniform distribution 
of the auxiliary gas has been found to be especially effective when solids 
are to be entrained in the auxiliary gas for displacement thereby. 
Since the electrode and nozzle end piece require only a reduced coolant 
flow by comparison with earlier systems and based upon the cooling effect 
provided by the auxiliary gas, it has been found to be advantageous to 
provide the burner shell with its own coolant circulation so that the 
coolant throughput can be varied to suit the degree of thermal stress to 
which the burner shell may be subject. 
For securing the burner shell it has been found to be advantageous to 
provide the burner shell, according to the invention, with an outer wall, 
a middle wall and an inner wall axial with one another and forming a 
coolant circulation path communicating with the coolant inlet and the 
coolant outlet extending between the inner wall and the middle wall and 
between the middle wall and the outer wall. The outer wall can be 
connected to a housing part and preferably the middle wall and inner wall 
are connected to the outer wall with spacing from the coolant inlet and 
outlet, e.g. via the housing part. It has been found to be advantageous, 
therefore, to fix only the outer wall while the inner and middle walls can 
be permitted to slide relative to the housing part and sealingly engage 
the latter. This sliding action makes it possible for the parts of the 
burner shell to undergo thermal expansion and contraction without 
stressing the shell. 
Advantageously, the middle wall is connected with the outer wall by a 
connecting part formed with passages for the coolant and the connecting 
part has a front end carrying a nozzle-forming shell portion of the burner 
shell. The nozzle-forming shell portion can be connected to the connecting 
part by a simple screwthread connection. The term "simple screwthread" is 
used here to indicate that one part is simply threaded onto or into the 
other so that they can be separated by an unscrewing action of the two 
parts the nozzle-forming portion thus can be readily removed from the 
outer wall of the burner shell by simply unscrewing it. This enables ready 
replacement of the nozzle portion and also facilitates rapid replacement 
of the electrode, the nozzle end piece and any related elements, all of 
which may be mounted or dismounted by unscrewing. Of course, that means 
that the parts are assembled by screwthreads. 
It has been found to be advantageous, especially when the annular passage 
between the nozzle and burner shell is to be used to convey pulverulent or 
granular solids by the pressurized auxiliary gas to the electric arc, to 
form the annular passage in the region of its mouth so that it converges 
conically in the direction of the arc. This is advantageously achieved by 
forming the mouth between inner and outer conical surfaces. 
For stable construction of the burner and to avoid the formation of 
deposits, the electrically insulating tube which separates the nozzle end 
piece from the burner shell can lie along the outer surface of the nozzle 
end piece and can additionally lie along the conical inner surface of the 
annular passage in the region of the mouth. 
The surfaces forming the conical mouth region of the annular passage can be 
formed with coatings of an electrically insulating material, for example, 
a ceramic, which is refractory in nature so that it is possible, utilizing 
these surfaces, to feed even electrically conductive solids through the 
annular passage to the arc. 
In this case the insulating tube which surrounds the electrode lance 
extends to or includes the connecting piece of the feeder line of the 
rearward end of the annular passage. 
To impart to the insulating tube surrounding the electrode lance an 
additional heat protection, the lumen or clear diameter of the outer edge 
of the annular passage at its mouth is preferably smaller than the outer 
diameter of the inner wall of the annular passage upstream of the conical 
convergence. 
The nozzle end piece is advantageously mechanically connected via a 
ring-shaped body of electrically insulating material with the electrode 
and can form a unit therewith. This ring-shaped body is provided with 
passages parallel to the principal axis of the plasma burner and through 
which the main plasma gas passes to the annular gap between the electrode 
and the nozzle end shape. Additionally, for integration of the nozzle end 
piece with the electrode or the electrode lance, a common coolant 
circulation is provided for them. 
To influence the gas flow, a displacement body can be disposed in the 
annular passage to form a constriction. 
When the plasma burner has an extremely long shank and sharply inclined 
installation positions, the annular passage advantageously has spacer or 
supporting bodies included therein. These supporting bodies can have a 
streamlined shape, can be offset from one another and can be 
advantageously mounted on the insulating tube with which the electrode 
lance included nozzle end piece is surrounded. The support bodies can be 
formed as hollow or solid structures, can extend parallel to the axis or 
in a helical pattern and can effect the displacement and flow direction of 
additives through the annular passage. Hollow bodies extending parallel to 
the axis can be formed preferably as nozzle-shaped ceramic tubes which can 
be extended beyond the end face of the nozzle end shape so that powder can 
be locally and directionally fed to the plasma arc.

SPECIFIC DESCRIPTION 
The plasma burner of the invention shown in the drawing comprises an 
electrode lance and a burner shell. 
The electrode lance, in turn, basically is made up of an electrode 10, a 
nozzle stub or end piece 11 and the parts for holding these elements. 
The electrode 10 has a weakly conical end 10b at its front end and the 
nozzle end piece has a correspondingly conical surface 11a but a more 
steeply conical surface forming the inner surface of a mouth for the 
auxiliary gas as will be discussed in greater detail below. 
The electrode 10 has an outer wall connected via a subsequently 
sleeve-shaped connecting piece 12 with the current delivery tube 13 
connected to the main voltage source. Between the electrode 10 and the 
connecting piece 12 and between the connecting piece 12 and the current 
delivery tube 13, respective screwthreads 12a and 12b are provided 
The inner wall of the electrode is slidable on an inner tube 14 fixed at 
the rear of the plasma burner. Between the current supply tube 13 and the 
inner tube 14, an intermediate tube 15 of a plastic material is provided 
for separating the various partial circulations of the coolant traversing 
the electrode. At its lower region or toward the front of the burner, the 
central tube 15 also serves to deflect the coolant flow as represented by 
the arrows in FIG. 2. 
The inner wall of the nozzle end piece 11 is connected by a screwthread 11b 
with a ring-shaped body 16 of an electrically and thermally insulating 
material, for example, a ceramic and this body 16 is connected by a 
screwthread 12c with the connecting piece 12. 
The connecting piece 12 (see FIGS. 4 and 5) is provided along its periphery 
in angularly equispaced relationship with upper and lower radial passages 
17, 18 and, parallel to the burner axis, with axially extending passages 
19. 
Above the upper radial passages 17, the inner flange of a sleeve 21 of 
plastic material is connected by a screwthread 12d to the connecting piece 
12 while an inner flange of a central pipe 23 is connected by a further 
screwthread pairing 12e with this connecting body. 
On the lower sleeve-like projection, the connecting piece 16 is screwed and 
is threadedly engaged by the nozzle end piece 11. The ring-shaped body 16 
has passages 26 parallel to the axis of the burner and communicating with 
the corresponding passages 19 in the connecting piece 12 at their lower 
ends, the passages 26 open into an annular gap 27 between the electrode 10 
and the nozzle end piece 11. 
A plastic electrically insulating tube 28 surrounds the current-flow tube 
and has passages 29 parallel to the axis of the burner and opening at 
lower regions of the tube 28 into an annular compartment 31. The hollow 
compartment 31 communicates with the passages 19 of the connecting piece 
16. 
At its upper end, the pipe 28 is formed with a flange 32 which has radial 
passages 33 formed therein communicating with the axial passages 29. To 
increase the stability of the plastic pipe 28, the latter is surrounded by 
a steel tube 34 with a flange 35. The outer diameter of the steel tube 34 
corresponds to the outer diameter of the nozzle end piece. 
On the outer surface of the steel tube 34, an insulating tube 36 is 
provided and has, at its upper end, a flange 37. Between the cylindrical 
outer surface of the insulating tube 36 and the underside of the flange 
37, a transition surface 38 is provided. A further, lower ceramic tube 39 
has its inner surface abutting the outer surface of the steel tube 34. 
The ceramic tube 39 is connected releasably by a screwthread connection 39a 
with the upper insulating tube 36 at its lower end, the lower ceramic tube 
39 merges steplessly with the conical outer surface of the nozzle endpiece 
11, i.e. is flush therewith. 
Extending through the inner tube 14 of the electrode lance is an auxiliary 
electrode 42 which can be centered by spacers 42b in the tube 14 and can 
have a generally conical lower end 42a. At its upper end, the auxiliary 
electrode 42 is connected with a terminal 44 for the ignition current. 
The auxiliary electrode 42 at its upper end is separated by a plastic disc 
45 from the inner tube 14 and the remaining parts of the electrode lance. 
The plastic disc 45 has a radial inlet 46 through which an ignition gas is 
fed to an annular passage 42c between the auxiliary electrode 42 and the 
inner tube 14. 
The electrode 10 and the nozzle end piece form a common combined coolant 
circulation. The coolant flows from the coolant inlet 48 through the 
annular passage 49 between the inner tube 14 and the middle tube 15, is 
deflected about the lower end of the middle tube 15 and flows through the 
radial passages 18 to be deflected at 24 and then passes through the 
passages 17 and the annular passage 50 between the current supply tube 13 
and the middle tube 15 to the coolant outlet 51. All of the aforedescribed 
parts collectively form the electrode lance. 
A housing part 54, electrically insulated by a plastic washer or ring 53, 
is centered on the outer side of the flange 37 and is mechanically 
connected rigidly with the flange 32 of the plastic pipe 28. The housing 
part 54 has a pipe connection for the tangential inlet of a shielding or 
envelope gas as represented at 55, this pipe connection being connected to 
the compressed gas source 56. 
At its inner side, the housing part 54 is affixed to a so called pipe 
connector 58 which is formed with a coolant inlet 61 and a coolant outlet 
62. On the pipe connector 58, in addition, an outer wall 63 in the form of 
a tube, is connected by its flange 64. 
At the lower end of the tube 63 a connecting piece 65 is detachably 
connected. At an internal screwthread 65a of the connecting piece 65 (FIG. 
3), the upper tube 63 is connected, while at a lower screwthread 65b, the 
member 68 of the burner shell 73 is detachably connected. 
The connecting piece 65 also has an internal screwthread 65c at which an 
upper tube member 66 of the middle wall of the burner shell is attached, 
while a screwthread 65d connects the inner tube or wall to this connector 
as well. A screwthread 65e connects the lower tube member 67 of the middle 
tube to the connector as well. 
The connector 65 is provided with passages 71 and 71a through which the 
coolant can circulate in the axial direction. 
The upper central wall 66 has its upper end sealingly engaged with but 
slidable along an inner surface of the pipe connector 58 at 58a while the 
inner wall 72 is sealingly and slidably engaged with the pipe connector 58 
at 58b and with a cylindrical flange 57 of the housing 54. 
All of the elements from the housing 54 to the shell member 68 collectively 
form the burner shell 73 and form a unit therewith. 
The complete outer surface of the burner shell 73 is free from 
discontinuities like gaps and steps so that the conditions for effective 
sealing of the plasma burner in a wall of a vessel are met. The coolant 
path from the inlet 61 to the outlet 62 between the inner and middle walls 
and between the middle and outer walls is highly uniform and there are no 
formations or other structures which will allow parasitic arc to form. 
Between the insulating tube 36 which forms the outer wall of the electrode 
lance, and the burner shell 73, an annular passage 75 is formed which 
extends over the greater part of its length parallel to the principal axis 
of the burner and at its region adjacent the front end at the nozzle end 
piece 11, runs conically at 76. The conical portion 76 of the annular 
passage 75 forms a mouth which is defined by the outer surface of the 
nozzle end piece 11 and a conical inner surface of the nozzle wall of 
member 68. Both conical surfaces can have coatings 77 or 78 of ceramic. 
The clear diameter of the member 68 at its end face is less than the outer 
diameter of the nozzle end piece 11 so that the end face of the cooled 
member 68 can form a heat shield for the ceramic tube 39 against the heat 
developed around the plasma burner or arc and the hot atmosphere 
surrounding the same. 
Especially with large plasma burner lengths and/or inclined layers of the 
structure, the insulating tube 36 can be provided with spacing or support 
bodies 80 on which the inner tube 72 of the burner shell are braced. In 
addition, the insulating tube 36 can also have a displacement body 81 
(FIG. 3) therein to control the flow through the passage. The spacer 80 
can extend the full length of the annular passage if desired and can 
include measuring devices for measuring the flow of the auxiliary gas.