Electric heating – Metal heating – By arc
Reexamination Certificate
2001-10-12
2003-02-04
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S075000, C219S121500, C219S121510, C313S231410
Reexamination Certificate
active
06515252
ABSTRACT:
FIELD OF THE INVENTION
The invention is located in the field of plasma torches.
PRIOR ART
Arc plasmas belong to the family of thermic plasmas. They are partially ionised gaseous media, conductive of electricity but by and large electrically neutral, at pressures in the region of atmospheric pressure. They are generated by means of a plasma torch, by passing one or more plasmagene gases through an electric arc which is maintained between two electrodes.
To bring gases to a high temperature and high specific enthalpy, blown arc torches are used. This means that the arc is confined to the inside of the torch containing the two electrodes and it is the high speed jet of high temperature gas (the plasma) which is used in the process.
FIG. 1
shows in a very diagrammatic way the principle of such a torch. A torch of this kind includes two electrodes, an anode
1
and a cathode
3
, concentric with each other and providing between them a gas circulation channel
7
.
The two electrodes
1
,
3
are connected to a high voltage, high frequency (HV-HF) generator and to a direct current generator. They must of necessity be energy cooled (by water circulation) to prevent their fusion.
Initially and by means of the HV-HF generator, an electric arc
8
flashes between the two electrodes (cathode and anode) ionising the gas introduced and making the inter electrode space conductive. The direct current generator may then issue into this space and maintain the arc.
The power supplied to the torch is equal to the product of the strength (which can be regulated) of the voltage established between the anode and the cathode. This voltage is dependent on several parameters such as the type and flow of gas used, but also, to a not insignificant degree, on the wear and tear on the electrodes. The power of the plasma
9
is equal to the power supplied to the torch minus the losses in the cooling water. Wear and tear on the electrodes puts them therefore at a serious disadvantage. It depends on their geometry, their cooling efficiency, their coaxiality, and on the type and purity of the gases.
Equipment allowing an arc
8
plasma
9
to be generated is used for thermal spraying (surface treatment), gas heating or chemical synthesis. The energy supplied to the gas(es) by the electric arc allows them to be heated to temperatures above 10,000 K.
The choice of plasmagene gas or gases is almost unlimited. It is a function of the demands of the process (oxidation, nitridation, high temperature in a reduction medium, etc.). The power range is very extensive, running from a few kilowatts to several megawatts. Very often, the potential operational range is dictated by the type and flow of the plasmagene gases selected.
A torch is therefore often designed for a given application since its technology must be compatible with the choice of plasmagene gas and the desired work power.
Its size, its form and its simplicity can also become important if it is required to work in a cramped or hostile environment.
Torches currently in existence are complex units, including at least about ten parts (excluding seals, screws and fluid connectors). The coaxiality of the electrodes depends on the stack of manufactured parts with acceptable tolerances for the seals.
Replacing one or both electrodes is an operation which has to be performed regularly (in most cases after some ten hours in operation). This operation always requires sub-units to be dismantled/re-assembled and the seals to be changed.
To illustrate this, three examples of known plasma torches will now be briefly described.
A first known torch operates with an air/argon or oxygen/argon mix, its power is about 100 kW. It consists of 15 manufactured parts, 21 seals, 22 screws and 6 fluid connectors. The parts subject to regular wear are the cathode and the anode, an insulation bush and an injection nozzle. Minimum maintenance (changing the anode) is required at less than 100 hours of operation in the best conditions of use.
A second known torch has been developed for the hydropyrolysis of heavy hydrocarbons. The plasmagene gases are argon and hydrogen, which are mixed with methane at the torch output. This torch is similar to a thermal spray gun. It has, excluding the fluid feed connectors and the screws, 10 manufactured parts and 7 O-rings.
As a third example may be cited one of the simplest torches, marketed by the company SULZER METCO. This is the thermal spray gun F4-MB. This type of torch operates conventionally with argon, helium and nitrogen singly or in a mixture. Hydrogen is often added to gain power (increase in peak arc voltage). There are nevertheless 8 manufactured parts, 14 O-rings, 12 screw components and 3 fluid connectors.
Japanese patent application JP 04-249 096 describes a plasma torch wherein, in order to reduce the probability of creating an arc between the anode and the cathode, the plasmagene gases follow a path which allows them to eddy. To this end, a centring device
10
a
, which is a part placed between the anode and the cathode, has an opening
106
, which goes from the top face of the centring device to a lateral face. Another conduit
102
located between the anode and the centring device part
10
allows the gases coming from the conduit
106
to be guided to the bottom of the anode.
Conduits
107
join the outside of the centring device
10
to a central cavity
105
of the latter. This particularity allows an eddying jet of plasmagene gas to be created. A more even wear of the cathode is thus obtained.
BRIEF DESCRIPTION OF THE INVENTION
The purpose of the torch, according to the invention, is as far as possible to simplify the assembly of the torch itself and, on the other hand, the replacement from time to time of worn out parts. It has been developed in particular for a gas heating application in a pyrolysis gas postcombustion reactor for chlorinated radioactive waste, heavily contaminated by alpha emitters. This reactor is intended to operate in a glove box.
In a hostile environment (radioactive, being compelled to work in a glove box or in the remote manipulator), the work must be simplified as far as possible. The standard exchange of sub-units is often preferable to the dismantling and to the re-assembly of isolated parts in a complex unit. The intervention time is shorter, the reliability of a new and inspected sub-unit is much better than that of a dismantled and re-assembled complex unit.
To this end, the plasma torch according to the invention is designed in two parts, a disposable interchangeable cartridge constituting a plasma generator intended to be inserted into a cartridge connecting and holding structure. The purpose of this cartridge connecting and holding structure is to connect the cartridge to its supplies of plasmagene gas, cooling fluid and electrical currents. This structure comprises to this end first cartridge connection means.
These first means serve as intermediaries for the supplies of electrical currents, water and gas. These supplies are therefore completely dissociated from the plasma cartridge.
The structure comprises second means engaging or not engaging with cartridge fixing means so as to keep the cartridge mechanically connected to the first means of supplying electrical currents, water and gas.
The invention relates to a cartridge generating plasma for a plasma torch, having, centered on an axis AA′, an annular anode comprising a central cavity receiving a cathode centered on AA′, the anode and the cathode providing between them an annular space for producing an arc, plasmagene gas distribution means, the distributed gas circulating in the annular space between the cathode and the anode, anode cooling means, comprising particularly conduits for an anode cooling fluid, these conduits having an inlet and an outlet, assembly means, a cartridge characterized in that the plasmagene gas distribution means in the annular cavity formed between the cathode and the anode comprise conduits formed in a central ring of the anode surrounding the central cavity of the anode, a first end of these condu
Commissariat a l'Energie Atomique
Krebs Robert E.
Paschall Mark
Thelen Reid & Priest LLP
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