ELECTRICAL HEATING - PART - 10 - DIRECT AND INDIRECT AND SUBMERGED ARC FURNACES

ELECTRICAL HEATING - PART - 10 DIRECT AND INDIRECT AND SUBMERGED ARC FURNACES Full notes is available in http://zenmurali.blogspot.in/ FURNACE - A furnace is a device used for high-temperature heating. The name derives from Greek word fornax, which means oven. ARC When a high voltage is applied across the air gap, the air in the gap gets ionized under the influence of electrostatic forces and becomes a conducting medium. Current flows in the form of a continuous spark, called the arc. Very high voltage is required to establish an arc across an air gap but to maintain an arc, small voltage may be sufficient. PRINCIPLE OF ARC FURNACE - An Arc can also be produced by short circuiting the two electrodes momentarily and then withdrawing them back. In this method of striking an arc, high voltage is not required. Arc drawn between two electrodes produces heat and has a temperature between 1000 degree centigrade and 1500 degree centigrade depending on the material of the electrode used. TYPES OF ELECTRODE 1. Carbon electrodes are made of anthracite coal and coke, which are used with small furnaces for manufacture of ferro-alloys, Aluminium and Calcium carbide, Phosphorous etc. 2. Graphite electrodes are obtained by heating the carbon electrode to a very high temperature. 3. Self-baking electrodes are made of special paste, whose consumption depends upon the type of process for which they are used, contained in thin steel cylinder. The flow of current produces heat and the paste is baked and formed into an electrode which are employed in ferro alloys and electro-chemical furnaces and in electrolytic production of aluminum. PROPERTIES OF ELECTRODES Most commonly used electrodes are carbon and graphite electrodes with a size of diameter of 18 cm to 27 cm and are possessing the following properties. 1. Good electrical conductivity 2. Insolubility 3. Infusibility 4. Chemical inertness 5. Mechanical strength and 6. Resistance to thermal shock. [Thermal shock is a variation in temperature which causes tension in a material] COMPARISON OF CARBON AND GRAPHITE ELECTRODE 1.Carbon electrodes are amorphous (having no definite form or distinct shape). 2.Graphite electrodes are obtained by heating carbon electrodes to very high temperature, hence the impurities in the carbon electrodes are volatilized (chemistry- make volatile; cause to pass off in a vapor). 3.Specific resistance is lower in graphite electrodes than that of carbon, hence the size of the graphite electrode will be half of that of carbon for the same resistance, this leads to easy replacement and lighter control mechanism can be used compared to carbon. 4.Size of carbon electrode is higher than that of graphite electrode for same conductivity and therefore larger area of charge is in contact with the electrode this results in uniform distribution of heat. 5.The life of the refractory lining will be affected due to the arc being brought near to the side of the furnace, because of bigger size of carbon electrode. 6.When temperature exceeds 600 degree centigrade the oxidation of electrodes start and consumption of carbon electrodes begin. 7.Amount of graphite electrode consumed is about half that of carbon for the same work. 8.Carbon electrodes are cheap and cost less than one half as much for same weight as graphite electrodes. TYPES OF ARC FURNACES There are three types of arc furnaces namely (i) Direct arc furnace (ii) Indirect arc furnace and (iii) Submerged arc furnace.