There are three types of arc furnaces namely: 1. Direct Arc Furnace 2. Indirect Arc Furnace 3. Submerged Arc Furnace.
Design of Direct Arc Furnace:
Figure 3.16 shows a direct 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 this method.
As the current passes through the charge, it will produce automatic stirring action.
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The arc has a negative resistance characteristic i.e. resistance falls with 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.
Direct arc furnace is very commonly used for production of steel. It is costlier than cupola method.
There is a charging door from where the charge is supplied and also there is an outlet for molten metal. The usual size of furnace is 5 to 10 tonne capacity. For melting 1 tonne of steel 1000 units of energy are consumed.
Design of Indirect Arc Furnace:
Figure 3.17 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 these furnaces are suitable melting metals having lower melting points e.g. non-ferrous metals such as brass, copper, zinc etc.
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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 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 is similar to that used for direct arc furnace.
Design of Submerged Arc Furnace:
This furnace is used for manufacture of ferro-chrome and ferro-manganese. In this furnace the electrodes project down into the charge remain submerged in it as shown in Fig. 3.18. The power factor is about 0.8. The major part of 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 to the electrodes.
