Cupola Furnace -Construction, Working and Applications [Complete Guide ]

Cupola Furnace is a melting device which used to melt cast iron, Bronze and other alloying elements are melted. It is mainly used to convert pig iron to cast iron.
Cupola Furnace was first built in China in the Warring States Period ( 403 -221 BC).
Cupola furnace is cylindrical in shape and the equipment of this furnace is vertically fitted inside this cylindrical shell with doors.

For many years Cupola Furnace was used to melt iron in iron foundries because it produces good Cast iron from Pig Iron.

The top of the cupola furnace is sometimes fitted with a cap to prevent the escape of harmful gases to the environment and this cap also protect it from rainwater.

The cupola shell is made of steel and has a lining of refractory brick and plastic refractory patching material. The bottom of this shell is lined with clay and sand mixture and it is a temporary lining. Sometimes coal is mixed with clay lining so that when coal heats up it decomposes and the bond becomes friable.
In some cupola lining cooling jackets are also fitted to keep the sides cool. Sometimes oxygen injection are also provided to make the coke fire burn hotter.

Parts of Cupola Furnace :-

1 Cylindrical Shell:
It is the outermost part of the Cupola Furnace. It is made up of steel sheet and other parts of this furnace are present inside this shell.

1 Legs:
At the bottom of the Cupon Frunace legs are provided to support this furnace.

2 Cast Iron Door:
This cast iron is present at the bottom of the furnace above the legs which is closed by the support of the legs.

3. Sand Bed:
Above the cast iron door sand bed is present. It is in taperred form so that the melted iron can flow out easily from its top.

4. Slag Hole:
It is present at the opposite side of the hole from which melted iron comes out.It is present near the elevated part of sand bed. This slag hole is used to remove slag formed on melted iron due to impurities.

5. Air Pipes and Tuyers:
The air pipe is provided to allow the air to reach inside the furnace. Inside the furnace wind belt is present. The air entering from the air pipes reaches each part of the wind belt and in the wind belt there are holes which are called tuyers. Air reaches the furnace through this tuyers and will help in combustion.

6. Spark Arrester Or Cap Of Furnace :
It is present at the top of the furnace. When gases are released out of the furnace, some burning particles are present in it which can harm the environment. So this cap or spark arrester is used to capture the burning particles and only allow the gas to pass to the environment.

7. Charging Door :
It is present near the top of the furnace. It is used to supply charge to the furnace. The charges in this furnace are Pig Iron, Coke and Lime Stone. Coke is used for combustion, pig iron is the material that is to be melt and lime stone is used as a flux. This flux mix with impurities to form slag and this slag comes out of the slag hole.

8. Well:
The part of the furnace from the send bed to lower part of tuyers is known as Well. It is named as well as in this part molten iron is stored and then the molten iron comes out of the tapping hole.

9. Tuyers Zone:
The part of the furnace in which the wind belt and tuyers are present is known as Tuyers Zone.

10. Combustion Zone:
In this zone combustion takes place. The air coming from the tuyers contains oxygen and this oxygen reacts with carbon to form carbon dioxide. It is also known as oxidixing zone as carbon oxidizes in this zone to form carbon dioxide and liberate heat. Apart from carbon other impurities like manganese,silicon also oxidizes in this zone to form their oxides and liberate heat.

11. Reducing Zone:
It is present above the combustion or oixdizing zone. It this zone carbon reacts with carbon dioxide to form carbon monoxide. In this zone temperature is reduced by a small amount, so this zone is known as reducing zone.

12. Melting Zone:
In this zone iron melts and this molten iron comes out of the tap hole. The temperature of this zone is very high nearly 1600 degree Celcius.

13 Peheating Zone:
In this zone the metal to be melted is preheated, in this zone metal is heated to about 1090 degree celcius.

14 Stack Zone:
Gases formed in the furnace after burning passes to the environment through this zone. In this zone, spark arrester is present which prevent burning particles to reach the environment.

Construction Of Cupola Furnace :

The outermost part of cupola furnace is cylindrical steel shell. The diameter of this shell ranges from 1.5 to 13 feet depending upon the size of the furnace. The inner side of the furnace is lined with
refractory brick and plastic refractory patching material.

This furnace is supported on Cast iron legs mounted on concrete base. At the bottom of the furnace, two cast iron doors are hinged with the bed plate of the furnace. Near the bottom, it has send bed above which the melted iron flow. This sand bed is tapered. Near the elevated side of the tapered sand bed, slag hole is present through which slag formed from impurities comes out. Near the downside of the down bed, the tap hole is present through which molten iron comes out.
Above the send bed, tuyers are present through which air reaches the furnace and helps in combustion.
At the top of the furnace spark arrester or cap is present that traps the burning particles and only allow the gases to release to the environment.
Near the top of the furnace, charging door is present through which metal, coke and lime stone are fed into the furnace.

Working of Cupola Furnace :

At first wood is ignited above the sand bed. When the wood starts burning properly, coke is dumped on the well from the top to a predetermined height of nearly 40 inches. This forms a 40 inch coke bed.
Then the combustion starts in the coke bed using the fire from the burning wood and using the air from the tuyers. At this time, the air blast is turned out at a lower blowing rate than normal to provoke the coke.
After nearly 3 hours of burning when the coke starts burning properly, alternate layers of limestone, pig iron and coke is charged until it reaches the level of charging door is reached. At this time the air blast is tuned on to normal blowing rate and the combustion occurs more rapidly in the coke bed.
All oxygen from the air blast is consumed by the combustion in the combustion zone.The chemical reaction which takes place is,
C + O₂ -> CO₂ + Heat
This is an exothermic reaction and in the combustion zone the temperature varies from 1150 to 1850 degree Celcius.
The portion of the coke bed above the combustion zone is reducing zone. This zone prevent the oxidation of metal charge above and while dropping through it.As the ho carbon dioxide moves up through this zone, some of it is reduced by the following reaction,
CO₂ +C -> 2CO
The layer of iron above reducing zone is melting zone where the solid iron is converted into molten iron.This melted iron trickles down through the coke bed and is collected in the well. Sufficient carbon comtent is picked up by the molten metal in this zone and is represented by the chemical reaction given as :-
3 Fe + 2 CO —> Fe₃ C + CO₂
Above the melting zone, there is preheating zone where the charge is preheated by the outgoing gases and the temperature of this zone is about 1900 degree Celcius.

Apart from limestone, fluorspar and soda ash are also used as flux material. Main function of flux is to remove impurities from iron and protect iron from oxidation.
Within 5 to 10 minutes of starting of air blast to normal blowing rate , the first molten iron appears at the tap hole.
The charging door is closed till the metal melts. The contents of the charge move down as the melting proceeds. The rate of charging i.e the rate of adding layers of charge is equal to the rate of melting. The furnace is kept full throughout the process.
When the melting process is finished and no more molten irons is required then the feed of charge is stopped and the air blast is also stopped. The bottom plate swings to open when the prop is removed and the slag is removed.
Normally copula furnace is not used more than 4 hours but can be used for 10 hrs of continuous operation.

Advantages of Cupola Furnace :-

• Simple in Construction.
• Wide range of material can be melted.
• Less floor space is reuired.
• Very skilled operators are not required. Can be easily operated by low skill person.
• Low cost of operation.
• Low cost of maintenance.
• Low cost of construction.

Disadvantages of Cupola Furnace:-

• The main disadvantage is that sometimes it is very hard to control the temperature in this furnace.
• Metal elements are converted to their oxide which are not suitable for casting.

Application of Cupola Furnace:-

• It is mainly used to convert pig iron to molten iron.
• More types of cast iron are produced from this furnace like malleable and grey cast iron.
• The cooper base alloy is also manufactured by this device.