Technological Challenges in Thermal Plasma Production

Abstract

Thermal plasmas, generated by electric arc discharges, are used in a variety of industrial applications. The electric arc is a constricted electrical discharge with a high temperature in the range 6000-25,000 K. These characteristics are useful in plasma cutting, spraying, welding and specific areas of material processing. The thermal plasma technology is an enabling process technology and its status in the market depends upon its advantages over competing technologies. A few technological challenges to enhance the status of plasma technology are to improve the utilisation of the unique characteristics of the electric arc and to provide enhanced control of the process. In particular, new solutions are required for increasing the plasma-material interaction, controlling the electrode roots and controlling the thermal power generated by the arcing process. IntroductionA thermal plasma is often considered as a plasma in which local thermodynamic equilibrium (LTE). conditions prevail. Under LTE conditions, the heavy particles in the plasma have approximately the same temperature as the electrons and hence the plasma has a high temperature. This allows the plasma to be used in a number of industrial processes requiring process heat at a high temperature (Pfender 1988). Typical applications of thermal plasmas in industry include joining of metals, plasma cutting, plasma spraying, arc wire spraying, plasma surfacing and material processing.Thermal plasma technology is essentially a process technology that enables the development of an industrial process to manufacture a product that meets market demands. The success of an application of plasma technology in industry will depend upon the technical suitability and advantages the process offers and the cost of the final product.Thermal plasma technology is based on the physical phenomenon of electrical discharges. The basic properties and generic features of the phenomenon create a unique position for the industrial applicability of the process. On the other hand, the limitations and the constraints imposed by the phenomenon on the application process have a strong bearing on the total cost and hence on the acceptance of the process by industry. In this paper, the advantages of plasma technology, its phenomenal constraints and future challenges for technology developments are highlighted.

Influence of gas composition on plasma arc cutting of mild steel

“…Taking an arc diameter of 2 mm, the plasma generated for cutting by a transferred arc has a length-to-diameter ratio of ∼8. The proportion of the electrical energy input that is converted to high-enthalpy plasma in such a cutting arc is expected to be higher than the typical value of ∼50% for non-transferred plasma torches used for plasma spraying (Ramakrishnan 1995). In transferred, free-burning arcs of similar lengthto-diameter ratio, approximately 70% of the electrical power input is converted into plasma power in the hot core (Jones and Fang 1980).…”

“…Previous work (Ramakrishnan and on the properties of air plasma for cutting shows that the arc plasma jet emanating from the nozzle of the plasma torch shown in figure 1 is an under-expanded supersonic jet. The arc column expands with distance from the cathode inside the plasma torch, and through the nozzle to the workpiece.…”

“…However, in the downstream region of the nozzle chamber, the entrainment of gas into the arc column is small and the expansion of the arc column with distance is also small. Most of the electrical power that goes into unit length of the developed downstream region of the arc column is lost from the arc column to the surroundings (Ramakrishnan 1995). It is to be noted that the region of cut front is part of the surroundings in the developed region of the arc column.…”

Maximum plasma power for a cylindrical bore torch

“…Plasma devices using spatially stabilized electrical arcs are known variously as plasma torches, plasma guns, plasmatrons and plasma arc heaters and were first developed in the early 1950s [1]. Since then the torch design has undergone significant development based on application requirements [2,3]. There are two basic torch configurations.…”