PLASMA ARC TECHNOLOGY

PLASMA ARC TECHNOLOGY

                                                                                 

Plasma technology development began approximately 40 years ago as a means to provide an electrically generated energy source in the form of a high temperature gas. Plasma torches have been used for decades in many industrial applications. The technology is well established in metal manufacturing from metallurgical processing, such as material synthesis and surface coating, to the welding and cutting of metals. Plasma-arc furnaces have been used for waste destruction and disposal of hazardous, military, organic, and biological materials.

Plasma is often described as "the fourth state of matter." Electrical energy is applied to a gas (oxygen, argon, air, neon, etc.), which transforms the gas into plasma. Plasma is an ionized gas, which is composed of equal numbers of positive ions and electrons. Plasmas exhibit some properties of a gas but differ from a gas in that they are good conductors of electricity and magnetic fields. Natural occurrences of plasma include lightning and the Aurora Borealis. Plasma torches are devices that utilize plasma-produced heat for high­ temperature operations. Based on the concept of Joule heat, this is the conversion of electrical energy to heat energy (Mayne and Beaver, 1996).

Four sizes of plasma torches exist and are ranked by the electrical energy input, DC Power, applied to the torch: 100-kw, 240-kw, 1-Mw, and 6-Mw (for industrial purposes). The torch's basic components .

  

Air (gas), electricity, and water are supplied through the top of the torch. Electrodes are located at the rear and near the tip of the torch. Temperatures at the electrode exceed all melting points; therefore, copper electrodes are replaced after 500 hours and most alloys can last 1000 hours. The plasma torch configuration can easily be modified for various gases at a wide range of pressures (from 20 atm to a low vacuum). Only one-tenth of one percent (0.1%) of the gas is actually converted to plasma. The electrical arc traversing between electrodes through an induced partially ionized gas generates the plasma temperatures. Movement of the arc and water-cooling are essential for the life of the electrodes.

Plasma torches have two configurations. The most commonly used is the transferred arc; the non-transferred arc is the less common alternative. The transferred arc flow of energy moves from one electrode to another. Applications normally involve a closed furnace or melter. The non-transferred arc can be directed at specific targets since the arc travels between electrodes that are system-contained and do not require a separate base electrode. Arcs generated at the back electrode are passed out of the torch and return to the front electrode in a ')-shaped" orientation (Mayne, 2000). The difference between these two systems.