The top is open or fitted with a cap
to escapes gases or rain entering. The cupola may be fitted with a cap to
control emission of gases and to Pull the gases into the device to cool the
gases and remove all the Particulate Matter.
The cupola shell is made of steel and
has a refractory brick and plastic refractory Patching material lining. The
clay and sand mixture is used as a bottom line and the lining is temporary. The
coal can be mixed with the clay lining so when it heats up the coal decomposes
and the bond becomes friable. This makes opening up of the two holes easy. The
bottom of the cupola lining is compressed against the bottom doors. Cooling
jackets are also fitted with some of the cupola’s to keep the sides cool and
with oxygen injection to make the coke fire burn hotter.
Cupola Furnace:
Principal:
The cupola
furnace works on a simple principal that combustion of coke generates carbon
dioxide and heat and this causes the iron to melt. The iron drains downward
when get melted.
Afterwards,
the carbon dioxide is reduced partly, reduced again by consuming energy and
coke with carbon monoxide, carbon dioxide and supplied coke is present in the
reaction equilibrium so it is possible to show a defined combustion ratio for
the utilization of thermal energy for the coke combustion.
Finally, high
concentration of carbon monoxide is present in the exhaust gas and it can be
extracted from the furnace.
Construction:
Cupola
furnace constructed in the form of a hollow cylindrical vertical
steel shell and it is lined from inside with
a refractory material. This furnace
is generally supported on four cast iron lags mounted on a
concrete base.
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The bottom of the furnace is closed
by two cast iron doors hinged to the bed plate of the furnace. A wind box cast iron encircles to the
outside of the furnace bottom.This box is connected to the furnace blower pipe
known as the blast pipe. Air which supplies the oxygen necessary to burn the
fuels forced through the cupola by a blower.The top of the furnace is shielded
by a mesh screen and topped with a cone-shaped spark arrester, which permits
the free vent of the waste gas and deflects spark and dust back into the
furnace.
Working
Basically, the operation of cupola
furnace consists of following steps:
After building the cupola make sure
it is dried completely before getting it to fire. Any slag around the tuyeres
from previous runs needs to be cleaned properly.
Also, A broken part is repaired with
the mixture of the silica sand and fire clay. Over the Brunt area, a layer of
refractory material is applied To about thickness 6 inches or more is rammed on
the bottom sloping toward the tap hole to ensure better flow of molten metal.
A hole opening of about 30 mm diameter
and a tap hole of about 25 mm diameter is being provided there.
a fire of wood is ignited. When the
wood burns well
coke is dumped on the bed well from the top. Make sure that the coke gets burned too. A
bed of coke about 40 inches is placed next to the sand.
Firstly The
air blast is turned on At a lower blowing rate than as normal for provoking the
coke. A measuring rod is also used which indicates the height of the coke bed. For about 3 hours firing is done
before the molten metal required.
Now the
charge is fed into the cupola. Many factors like charge composition, affect the
final structure of the gray cast iron obtained. It composed of 10% steel,50%
grey cast iron scrap, and 3% limestone as a flux.
Alternate layers are formed by these
constituents. Besides limestone, fluorspar and soda ash are also used as flux
material. The main function of flux is to remove the impurities in the iron and
protect the iron from oxidation.
after
the fully charged furnace, it is allowed to remain as such for about 1 hour. As
this process goes in charge slowly gets heated up as the air blast is kept shut
this time and because of this, the iron gets soaked up.
At the end of the soaking period, the
air blast is opened. The
topmost opening is kept closed till the metal melts. The sufficient amount of
metal is collected. The contents of the charge move downwards as the melting
proceeds.
The rate of charging is equal to the
rate of melting. The furnace is kept full throughout the heat.
Closing feeding of charge and air
blast is stopped when no more melting is required. The bottom plate swings to
open when the prop is removed. The deposited slag is being removed.The cupola
runs continuously and The
melting period does not exceed 4 hours in most of the time. But can be operated
for more than 10 hours.
Advantages:
- Low cost of construction.
- Low cost of maintenance.
- Low cost of operation.
- Very skilled operators are not required.
- Simple in construction
- Simple in operations.
- Melting composition can be controlled.
- Small floor area is required.
Disadvantages:
With a long
list of advantages, cupola furnace also comes with few of limitations or
disadvantages and they are listed below:
- It is sometimes difficult to maintain the temperature in a cupola furnace.
- Metal elements are converted to their oxides which are not suitable for casting.
Application:
Cupola
furnace is a used widely as a melting unit for cast iron. Some of the
characteristic that makes the cupola furnace a primary method used in melting
irons in the foundries. Some of them are :
- Cupola furnace is the only method which is continuous while operations.
- The melting rate of cupola furnace is high.
- It is easy to operate.Operating costs in use of cupola furnace is comparability very low to other methods for this purpose.
Although
having such great factors, the use of cupola furnace is declining and people
are opting for electric induction melting.
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