Dc & Ac Electric Arc Furnace Working.
► Direct Electric Arc Furnace Working
The diagram 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 the 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 current limiting device is required in the circuit to prevent short circuits. It may be in the form of a reactor. The direct 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 the direct electric arc furnace is 5 to 10 tons capacity. For melting 1 ton of steel, 1000 units of energy are consumed.
► Indirect Arc Furnace Working
The figure shows an indirect arc furnace. In this furnace, the arc is formed between the two electrodes. And heat so produced is transmitted to the charge by radiation. The temperature is lower than the 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, a single phase. Since during the process of heating, the electrodes are consumed, so the feeding of electrodes to the indirect arc furnace is automatic.
The furnace is cylindrical. Since the arc does not come in contact with the charge, so the automatic stirring action, which is present in the 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.
Graphite electrode specifications, arc furnace model matching
► Ac steelmaking arc furnace
Specifications of graphite electrode used ac steel-making arc furnace, 10 ~ 30 t arc furnace using graphite electrode diameter from 300 ~ 400 mm, 30 ~ 50 t arc furnace using graphite electrode diameter of 450 mm, 60 ~ 80 t arc furnace using graphite electrode diameter of 500 mm, 100 ~ 170 t arc furnace using graphite electrode diameter from 550 ~ 600 mm, 200 t arc furnace using uhp graphite electrode diameter from 600 ~ 700 mm, 250 ~ 300 t eaf with 700 mm diameter uhp graphite electrodes.
► Dc steelmaking arc furnace
Dc steelmaking arc furnace, 30T arc furnace using 450mm diameter graphite electrode, 60T arc furnace using 500mm diameter graphite electrode, 70-80t arc furnace using 600mm diameter uhp electrode, 100-130t arc furnace using 700mm diameter uhp electrode, 150T arc furnace using 750mm diameter uhp electrode.