- By striking the arc between the charge and electrode: In this method, the heat is directly conducted and taken by the charge. The furnaces operating on this principle are known as direct arc furnace. These furnaces are used for production and refining of various grade of steel.
- By striking the arc between two electrodes: In this method, the heat is transferred to the charge by radiations. The furnaces operating on this principle are known as indirect arc furnaces. These types of furnaces are used for melting of non-ferrous metals such as brass, copper and zinc.
There are two types of arc furnaces namely:
- Direct Electric Arc Furnace
- Indirect Electric Arc Furnace
Direct Electric Arc Furnace Working Principle
The figure shows a direct electric arc furnace. The chamber of the furnace is lined with refractory material. The arc is struck between the electrode and the charge. Three electrodes made of carbon or graphite are projected from the top of the furnace and three phase supply is given.
The current passes through them via the charge. Since the arc is in direct contact with the charge so it is possible to produce highest temperatures by direct electric arc furnace. As the arc passes through the charge, it will produce automatic stirring action.
The arc has a negative resistance characteristic i.e. resistance falls with the increase in temperature. Thus some sort of current limiting device is required in the circuit to prevent short circuits. This may be in the form of reactor. The direct electric arc furnace is very commonly used for the production of steel.
There is a charging door from where the charge is supplied and also there is an outlet for molten metal. The usual size of direct electric arc furnace is 5 to 10 tons capacity. For melting 1 ton of steel 1000 units of energy are consumed.
Indirect Electric Arc Furnace Working Principle
The figure shows indirect arc furnace. In this furnace, the arc is formed between the two electrodes and heat produced is transmitted to the charge by radiation. The temperature is lower than direct arc furnace. So indirect arc furnaces are suitable for melting metals having lower melting points e.g. non-ferrous metals such as brass, copper, zinc etc.
The arc is struck between the electrodes so only two electrodes are required. The supply is, therefore, single phase. Since during the process of heating the electrodes are consumed, so the feeding of electrodes to the indirect electric arc furnace is automatic.
The furnace is cylindrical in shape. Since the arc does not come in contact with the charge so the automatic stirring action which is present in direct arc furnace is absent. The furnace may be equipped with automatic rocking equipment. The power factor varies from 0.7 to 0.8. The electrode material and electrical equipment of indirect-arc furnace are similar to that used for direct arc furnace.
Submerged Electric Arc Furnace Working Principle
This furnace is used for the manufacture of ferrochrome and ferromanganese. In submerged arc furnace the electrodes project down into the charge remain submerged in it as shown in the figure. The power factor is about 0.8.
The major part of the heat is produced due to the resistance offered by the charge to the flow of current. Moreover, power is controlled by varying the distance between electrodes or by varying applied voltage to the electrodes.
Power Supply for Electric Arc Furnace
The power consumption of the arc furnaces is very high. The arc voltage is low i.e. between 50-150 V but the current required is of the order of several hundred amperes. As the heating effect is proportional to the square of the current, therefore, the higher current is essential to achieve the higher temperature.
For obtaining the required power supply a 3-phase arc furnace transformer is used. When the electrodes are short-circuited, the total input to the furnace is almost zero. On the other hand, when the electrodes are far away arc is extinguished and there is no power drawn from the supply mains.