Electric heating – Metal heating – By arc
Patent
1991-05-07
1993-07-13
Paschall, Mark H.
Electric heating
Metal heating
By arc
21912152, 21912157, 21912148, B23K 900
Patent
active
052276031
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with the generation of electric arcs and is particularly although not exclusively concerned with plasma torches for spraying, arc heaters and arc furnaces.
In the context of this specification, an electric arc is to be understood as an electric discharge in a gaseous medium sustained between spaced electrodes by the passage of relatively large currents and characterised by low voltage drops at the cathode. The properties of the electric arc are influenced by a number of parameters such as the arc current, the fluid dynamics, the containment, the electrode material temperature and shape, the external magnetic fields (if used), and the gas in which the arc burns.
The gas in an electric arc attains very high temperatures (6,000-30,000 K) and for that reason electric arcs have been proposed for use in a variety of industrial processes and applications which require very high temperatures. In many applications, an electric arc at a current of hundreds of amperes is allowed to burn between two electrodes within a chamber which may be identified as a plasma torch, or an arc heater, or an arc reactor. Gas of suitable composition is forced to flow through the arc region of the heater so that the thermal energy liberated by the arc is transferred to the gas to produce a high-temperature gas stream at the exit of the arc heater. This high-temperature gas produced by the arc heater can be used for the treatment of materials at high temperatures or the treatment of surfaces. U.S. Pat. No. 3,832,519 (Westinghouse) is directed to an electric arc reactor which has been considered useful in the destruction of hazardous waste at high temperatures. The APG ("NOVA") advanced plasma gun developed by Metco and which is the subject of U.S. Pat. No. 4,780,591, is another example of an arc heater used for melting and spraying of powders.
In some applications related to material treatment using an electric arc, the material to be treated is injected into the region of the electric arc within an arc reactor to increase the resident time of the material in a high-temperature environment. Patent application PCT/AU89/00216 entitled "Electric Arc Reactor" describes a method of injecting material into the core of an electric arc.
Effective and broad ranging control of arc power is important in devices such as plasma spraying torches, arc heaters and arc reactors for material treatment to attain high process efficiency and quality. The ability to select any of a variety of methods and location of material feed is also important, but the majority of prior devices only provide for injection of material near the exit of the device and therefore away from a location at which direct interaction with the arc would be possible. If material is fed in such a way that interaction of the material with the electric arc takes place, then a control of the power distribution within the arc region is important. That will also apply when the material is fed further downstream into the arc flame.
In prior devices, the main method of control of arc power is achieved by operating the arc at different levels of arc current and/or by changing the composition and the flow rate of the gas in which the arc burns. A consequence of these variations may result in a change of the arc length in a few devices, but such a change is generally small. In a device such as the APG plasma gun of Metco, the main control parameter is the arc length which is effected by moving mechanically one of the electrodes of the arc with respect to the other while maintaining the arc current to be the same. Arc heaters have also been proposed in which arc lengthening is achieved by the use of electrical switches during the start-up of the heater, and an example of such a heater is the well known Tioxide torch. Systems with multiple arcs operated from different supplies have been proposed with the main intention of distributing the arc activity over a larger volume for material injection into the arc. Apart from the APG plasma gun of Metco, all prior devices do n
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Doolette Ashley G.
Oppenlander Walter T.
Ramakrishnan Subramania
Commonwealth Scientific & Industrial Research Organisation
Irons Edward S.
Paschall Mark H.
Siddons Ramset Limited
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