Question

explain the construction and working of indirect arc furnace with sketch

Answer 1

Solution :

Indirect Arc Furnace:

In this case arc is formed between two electrodes above the charge, as shown in Fig. 5.5 and heat is transmitted to the charge solely by radiation. In this case the temperature of the charge is, therefore, lower than that in case of direct arc furnace. Since in this furnace current does not flow through the charge, there is no stirring action and the furnace is required to be rocked mechanically. That is why the furnace is made of cylindrical shape, with the electrodes projecting through the chamber from each end and along the horizontal axis.

By rocking action there is thorough mixing of the charge. The life of the refractory lining also increases since the molten metal comes in contact with the lining and takes away some of its heat thus preventing it from attaining excessive tem­perature. The efficiency is increased because the charge is heated not only by radiation from the arc but by conduction from the heated refractory during rocking action.

Its con­struction limits the number of electrodes to two, so single- phase supply is required. The size of the furnace is thus limited by the amount of single-phase load, which can be taken from one point. The arc is produced by bringing the electrodes into solid contact and then withdrawing them. Power input is regulated by adjusting the arc length by moving the electrodes.

An electric motor is employed for operating suitable grinders and rollers to provide rocking action to the furnace. At start, the rocking action is carried through an angle of 15 to 20° and as the melt proceeds the angle is increased to about 200° at a frequency of about 2 cycles per minute.

Such furnaces have got following advantages:

(i) Low Metal Losses:

Since furnace chamber is closed and a reducing atmosphere above the metal is produced due to carbon arc, therefore, metal losses due to oxidation and volatilisation are quite low.

(ii) Economical:

Melting is rapid and takes place in a completely closed chamber resulting in small heat losses and low power consumption. Higher production rate gives low labour costs resulting in lower overall production cost per tonne of molten metal.

(iii) Sound Castings:

The agitation produced by the rocking action of the furnace and the absence of combustion gases ensure products free from blow holes, inclusion and segregations. Higher pouring temperature may readily be attained. Thus sound castings in thin and intricate designs can be produced.

(iv) Flexible:

Single furnace is capable of handling small or large heat of widely differing analysis. Interchangeable furnace shells can be used for different alloys and can be substituted in a few minutes.

Due to indirect heating the furnace is suitable for comparatively low melting point. So these furnaces are used for melting non-ferrous metals. These are also used to a considerable extent in iron foundries where relatively small quantity of metal is required intermittently. The capacity of furnace varies from 0.25 tonne to 3 tonnes. The power factor is about 0.85. The energy consumption is about 275 kWh/tonne for brass; 325 kWh/ tonne for bronze and 400 kWh per tonne for copper.

Even though Ajax Wyatt induction furnace is its chief rival in the field of the non-ferrous metals, owing to its higher efficiency, it is more suitable in situations where the charge has to be varied frequently or where heating is intermittent.

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