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 electrochemical
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.
DIRECT
ARC FURNACE
As the arc in direct contact with the
charge, and heat is produced by the current flowing through the charge itself,
the direct arc furnace gives high temperature.
CONSTRUCTION
1.
This furnace consists of circular steel casting lined inside with refractory
material
2.
The cover is removable and a spare is usually kept for rapid replacement.
3.
The holes are provided on the cover through which the electrodes are inserted.
4.
The electrodes may be of carbon or graphite.
5.
Automatic regulators are used to maintain the desired length to be inserted in
the furnace.
6.
The arc chamber consists of a suitable acid (ground ganister) or basic
(magnesite mix) refactory lining supported on a metal frame work.
7.
The acid process is mainly for making steel castings
8.
This furnace is suitable for making alloy steels such as stainless and
high-speed steel (High-speed steel (HSS or HS) is a subset of tool steels,
commonly used in tool bits and cutting tools)
9.Ttwo
electrodes are sufficient for single phase and d.c supply with this furnace
operation.
10.
The voltage between steel and electrodes may be of 40-145V.
11.
Three phase supply given to the electrodes spaced at the corners of an equilateral
triangle; the charge forms the star point.
12.
In three phase furnace the voltage applied varies between 6.6 kV – 20kV.
13.
The arc is controlled by varying the input voltage or by varying the arc length
or by arc resistance.
14.
Its operating power factor is 0.8 lagging and for one ton furnace, power
required is 200 kW and the energy consumed is 1 MWh per ton.
15.
The capacity of this furnaces is between 5 – 10 tons.
ADVANTAGE
Since
the arc current flows through the charge, the stirring action is inherent due
to the electromagnetic force setup by the current which is the salient feature
of this furnace. This results in uniform heating of the charge.
DISADVANTAGES
Cost of this furnace is very high, due to this it is restricted to refining
than melting.
APPLICATION
The
common application of this type of furnace is to produce high speed steel.
INDIRECT
ARC FURNACE
It is generally in cylindrical shape and
arc formed between the electrodes above the charge and heat is transmitted to
the charge solely by radiation.
CONSTRUCTION
1.
The arc is produced by bringing the electrodes into solid contact and then
withdrawing them.
2.
Heat developed is lower than that of direct arc furnace.
3.
The arc is struck between the charge and the electrodes which are projected
from the top of the furnace.
4.
The electrodes are projecting through the chamber from each end and along the
horizontal axis.
5.
The heat from the arc and the hot refractory lining is transferred to the top
of the layer of the charge by radiation.
6.
In this furnace current does not flow through the charge, so there is no
stirring action.
7.
This furnace requires racking mechanism that is the main reason why it is in
cylindrical shape.
8.
Racking mechanism must be used for thorough mixing of the charge.
9.
While racking the molten metal in contact with the refractory lining will take
some of its heat thus preventing it from high temperature.
10.
The charge in this furnace is not heated only by radiation from the arc between
electrode tips but also by conduction from the heated refractory, during
racking action; so the efficiency of this furnace is high. Sometimes it also
called as racking furnace.
In
this furnace input power is regulated by adjusting the arc length by moving the
electrodes.
SUPPLY
1. Suitable
for single and three phase supply system.
2. Capacity
of this furnace varies from 0.25 ton to 3 tons.
3. The
operating power factor is 0.85 lagging.
ADVANTAGES
1.
Flexible in operation.
2.
Melting is rapid and takes place in a complete closed chamber resulting in
small heat losses and low power consumption.
3.
Metal losses due to oxidation and volatilization are quite low.
4.
Overall cost of production of molten metal per ton is low.
5.
Sound casting in thin and intricate (having many complexly arranged elements;
elaborate) designs can be produced.
DISADVANTAGES
1.
Rocking mechanism is required and
2. Costly than direct arc furnace
APPLICATIONS
1.
This furnace is employed in iron foundries where relatively small quantity of
metal is required intermittently.
2.
Mainly used for melting non-ferrous metals
SUBMERGED ARC FURNACE
In this furnace arc is formed in between electrodes and hearth
electrodes.
CONSTRUCTION
1.
It is cylindrical in shape and carbon electrodes are commonly used.
2.
In this furnace hearth is lined of magnesite which becomes comparatively good
electrical conductor when hot.
3.
Sometimes conduction hearth is also used as an electrode in this furnace.
4.
The number of electrodes depends upon the supply i.e. single or three phase
supply in which bottom conductor is connected to neutral.
5.
The holes are provided on the cover through which the electrodes are inserted.
6.
The arc current from the top electrode passes through the arc to the charge and
returns through electrodes at the bottom of the charge.
7.
By varying the distance between the electrodes or supply voltage, power can be
controlled.
8.
Charge between the electrodes acts as a resistance providing better heat
distribution and better mixing of charge is naturally obtained in this furnace.
9.
Under short-circuited condition the current is limited by the charge.
10.
Its operating power factor is 0.8 lagging.
APPLICATIONS
It
is suitable for manufacturing of Ferro-alloy like Ferro chrome and Ferro-
manganese.
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