Plasma torch for noncooled injection of plasmagene gas

The present invention concerns a plasma torch of the type including: PA1 two tubular and axially spaced coaxial electrodes (5 and 6), each electrode being arranged in a support (3 and 4) in which a cooling circuit (8 and 9) of the corresponding electrode is fitted; and PA1 a passageway for injecting a plasmagene gas between said electrodes, said passageway including a revolution piece (17) coaxial with said electrodes and defining with the latter and their supports a chamber (18) into which the plasmagene gas is injected via transverse orifices (17B) provided in the piece. According to the invention, the revolution piece (17) is not provided with internal cooling means.

FIELD OF THE INVENTION 
The present invention concerns electric arc plasma torches implementing the 
injection of a plasmagene gas into an internal chamber which is provided 
in the torch and is traversed by an electric arc generated between two 
electrodes. The temperatures reached by the plasma at the torch outlet may 
exceed 10,000.degree. C. 
BACKGROUND OF THE INVENTION 
In normal plasma torch embodiments, the two electrodes are tubular and 
coaxial with one in prolongation of the other and are each arranged in a 
support. It is necessary to provide a cooling circuit between each 
electrode and the support surrounding it owing to the temperatures 
reached. Furthermore, in order to produce the electric arc between the 
electrodes, means are provided to initiate said arc, these means possibly 
being of the type with electric discharge produced between the two 
electrodes or of the short-circuit type by means, for example, of the use 
of an auxiliary starting electrode. Torches most frequently include at 
least one electromagnetic coil disposed around one of the electrode 
supports so as to allow for moving of the catching feet of the electric 
arc and thus avoid any premature wear of the internal surfaces of the 
tubular electrodes. 
As regards the means for injecting the plasmagene gas, such as air, into 
the internal chamber of the torch, they generally include a revolution 
piece coaxial to said electrodes and defining with the latter and their 
supports said injection chamber. 
Transversal orifices are provided in the piece so as to authorize injection 
of the plasmagene gas derived from a feed circuit into the chamber. As the 
piece is directly exposed to the thermic radiation generated by the 
electric arc and the chemical reaction which ensues with the plasmagene 
gas, said piece is made of a metallic material and further comprises a 
cooling circuit. In order to do this, longitudinal passages for 
circulation of the cooling fluid are provided in the revolution piece. For 
example, these passages communicate on one side with an external annular 
groove provided in the piece into which the cooling fluid arrives, and on 
the other side these passages are placed in communication with the cooling 
circuit of the downstream electrode (with respect to circulation of the 
plasmagene gas). By means of this disposition, the same cooling fluid 
travels over the cooling circuits of the injection piece and the 
downstream electrode. 
However, as the injection piece is metallic and accordingly electrically 
conducting, it is essential to provide an electrically nonconducting 
device so as to guarantee maximum insulation between the two electrodes. 
To this effect, nonconducting devices are provided between the injection 
piece and the upstream electrode which, in addition, may act as a thermic 
screen for the upstream or rear section of the torch. 
Thus, one can readily understand the drawbacks generated by these plasma 
torches and mainly concerning, owing to the temperatures reached, the 
complex embodiment of the injection piece of the plasmagene gas provided 
with an internal cooling circuit and also the need to add, for those 
reasons mentioned earlier, nonconducting devices requiring an increase of 
the spatial requirement of plasma torches and the cost of these torches. 
The Applicant has thus sought to overcome these drawbacks by carrying out 
on a plasma torch of the type described above various tests on the 
injection piece so as to study its behaviour according to the temperatures 
encountered. 
The results of these tests have shown that the injection piece did not 
undergo temperatures as high as one would have imagined. These results 
have proved that the temperature of the cooling fluid at the outlet of the 
longitudinal passages was only slightly different from that recorded at 
the inlet of said passages. The Applicant thus deduced from this that the 
fresh plasmagene gas injected continuously through the orifices in the 
direction of the chamber constituted an effective thermically protective 
layer for the internal wall of the injection piece in relation to the 
temperature existing in the middle of the chamber, that is at the level of 
the electric arc. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention concerns a plasma torch which, by taking 
account of the unexpected results revealed by the various tests conducted, 
possesses a considerably simplified embodiment whilst guaranteeing 
performances similar to plasma torches of the prior art. 
To this effect, the plasma torch of the type including: 
two tubular and coaxial electrodes with one in prolongation of the other, 
each electrode being arranged in a support in which a cooling circuit of 
the corresponding electrode is provided; 
means to produce the initiating of an electric arc between the two 
electrodes, and 
means to inject a plasmagene gas between said electrodes, said means 
including a revolution piece coaxial to said electrodes and defining with 
the latter and their supports a chamber into which the plasmagene gas is 
injected via transversal orifices made in the piece, 
is notable according to the invention in that said revolution piece is 
without internal cooling means. 
Thus, by virtue of the unexpected results of these tests, the revolution 
piece, usually complex, is produced from a much easier embodiment, 
injection orifices solely being effected by piercing said piece. 
Advantageously, the revolution piece is made of an electrically 
nonconducting non-metallic material. 
In fact, since the injection piece is not subjected to high temperatures, 
it is not necessary for this piece to be made of metal. Now, as the 
injection piece is also nonconducting, it is also no longer necessary to 
provide the nonconducting and thermic screen devices previously disposed 
between the two electrodes and which required an additional spatial 
requirement for the torch. 
Thus, it can be seen from the foregoing that the embodiment of the torch is 
considerably simplified. 
The revolution piece may then be made of a plastic material, such as a 
polytetrafluorethylene. 
The revolution piece may structurally have a crownshaped section. 
Preferably, the injection orifices of the plasmagene gas are regularly 
distributed around said piece. 
Furthermore, so as to provide the plasmagene gas injected into the chamber 
with a vortex effect, the geometrical axes of the transversal injection 
orifices contained in planes perpendicular to the longitudinal axis of the 
torch, instead of converging towards the latter, are slightly offset with 
respect to their position for which they would converge towards said 
longitudinal axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1, the plasma torch 1 comprises a body 2 including 
in particular two cylindrical supports 3 and 4. An upstream electrode or 
cathode 5 is housed inside the support 3, whereas a downstream electrode 
or anode 6 is housed inside the support 4. These electrodes 5 and 6 have a 
tubular shape and are disposed coaxially to a longitudinal axis 7 by being 
spaced from each other along said axis. These electrodes are connected to 
electric power sources (not shown). 
In addition, between each support and its corresponding electrode, a 
cooling circuit, respectively 8 and 9 is provided in which a cooling fluid 
circulates. Only the inlet, respectively 8A and 9A, of these cooling 
circuits has been shown. The structure of these cooling circuits is of a 
known type and shall not be described in further detail, these circuits 
being connected to a cooling fluid feeding point. 
So as to initiate the electric arc 11 between the two electrodes 5 and 6, 
an auxiliary starting electrode is provided in this embodiment. In 
addition, an electromagnetic coil 14 is disposed around the support 3 of 
the upstream electrode 5 so as to make it possible, under the action of 
the axial magnetic field it generates, to move the feet of the electric 
arc 11 respectively around the internal surfaces 5A and 6A of the 
electrodes 5 and 6, thus avoiding any premature wear of these electrodes. 
The plasma torch 1 also includes means 16 to inject a plasmagene gas, such 
as air, between the electrodes 5 and 6 as soon as the electric arc 11 is 
produced. These means 16 include a revolution piece 17 having a 
crown-shaped section and surrounding the opposite ends 5B and 6B 
respectively of the electrodes 5 and 6. Thus, in this embodiment, the 
internal wall 17A of the piece 17, the ends 5B and 6B of the electrodes 
and the front face 3A of the support 3 define an internal chamber 18 into 
which the plasmagene gas is injected via transversal orifices 17B provided 
in the injection piece 17. 
The plasmagene gas is derived from a feeding point (not shown) and arrives 
at 19 in an annular space 20 delimited between an external casing 21 of 
the body 2 of the torch and the external wall 17C of the injection piece 
17. 
For those reasons mentioned earlier, the injection piece 17 of the 
invention is without internal cooling means. In fact, the cold plasmagene 
gas injected into the chamber 18 constitutes close to the internal wall 
17A of the injection piece a thermic barrier for protection against the 
high temperatures generated by the electric arc 11 inside the chamber 18. 
It thus ensues that the embodiment of the injection piece 17, as shown 
more particularly on FIG. 2, is considerably simplified. In fact, the 
piercing of the injection orifices 17B regularly distributed around the 
piece 17 does not raise any difficulties. 
It shall be observed that on FIG. 2 the geometrical axes 17D of the 
injection orifices 17B contained in planes perpendicular to the 
longitudinal axis of the piece 17 corresponding to the longitudinal axis 7 
of the torch are slightly offset with respect to the position for which 
they would converge towards the latter. This offset orientation of the 
injection orifices 17B makes it possible to advantageously provide the 
plasmagene gas injected into the chamber 18 with a vortex effect. 
As the injection piece is not subjected to high temperatures, it may be 
made of a plastic material, such as a polytetrafluorethylene, and 
preferably be electrically nonconducting. This plastic piece may also act 
as an electric nonconductor between the two electrodes 5 and 6 so that it 
is no longer necessary to provide thermic screen and nonconducting devices 
usually equipping the plasma torches of the prior art. 
FIG. 1 makes it possible to illustrate the small spatial requirement of the 
plasma torch obtained according to the invention.