What is a cupola furnace and how does it work?

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In foundry shops, various types of melting furnaces are used to melt metal. Some of them are Blast Furnace, Cupola Furnace, Electric Furnace, Electric Arc Furnace, Induction Furnace, Crucible Furnace, Reverberatory Furnace, Open Hearth Furnace, Coreless Induction Furnace, Arc Type Furnace (Direct & Indirect), Rotary Furnace, Pot Furnace etc.

The above-mentioned furnaces are commonly used in foundries. Foundries used to decide the furnace type based on the factors like what type of metal being melted, efficiency, production capacity etc.

In this article, we will explore some new things in brief about Cupola furnace, its origin, construction, working principle, advantages & disadvantages, applications etc.

Cupola Furnace – Definition

Cupola Furnace is a type of melting furnace commonly used in foundries to make cast iron from pig iron charge which consists of pig iron, coke, limestone (flux) and scrap iron. The word “Cupola” means “small dome”. The name of this furnace resembles its shape and structure. Cupola furnace is a large vertical cylindrical furnace equipped with a tapping spout at its base. Other metal productions like nodular and malleable cast iron also can be done by cupola furnace. Even some copper alloys also can be melted by using cupola furnace.

The cupola furnace was originated from China during 403 – 223 BC and later it got evolved and widely used in European countries during industrial revolution. In 1720, the French scientist René Antoine Ferchault de Réaumur has made the modern cupola furnace.

It is widely used because it has a capability to operate continuously, have high melting rate, available in various sizes and capacity, easy to operate and maintain, and economical.

Parts of Cupola Furnace

Cupola furnace is constructed using many number parts and some of the major parts are listed below.

  • Spark arrester
  • Charging Door
  • Charging Floor
  • Air Blast Inlet Pipe
  • Wind Box
  • Tuyers & Peep Holes
  • Slag Spout
  • Tapping Spout
  • Drop Bottom
  • Prob
  • Supports
  • Shell

Let’s see about each part in a detailed way.

Spark Arrester

Spark arrester is conical in shape which is located at the top most of the furnace. It is made of sheet metal and steel structures which is attached to the cylindrical body of the furnace. The main function of spark arrester is to arrest sparks produced in the chamber. It has some opening at the top which helps smoke to escape from the furnace.

Charging Door

In cupola furnace, there is an opening with hinged door attached at suitable height where operator can feed metals, coke, scrap iron, pig iron and flux into the furnace with the help of charging cranes. These doors are made up of steels and lined with fire resistant material.

Charging Floor

Changing floor or charging stage is a platform made for operators where they can feed charges of metal, flux and coke inside the furnace via charging door either manually or by using charging cranes and buckets. It is always built around the furnace at suitable height (always below the charging door). Steel structures are always used to build and construct charging stage around the furnace.

Air Blast Inlet Pipe

Air blast inlet pipe or air supply duct is connected to blower and wind box at bottom of the furnace. The main function of air blast inlet pipe is to pass the air from blower to windbox of the furnace.

Wind Box

Wind box or wind belt is a circular steel jacket which connects the air blast inlet pipe from the motorised blower and the cylindrical shell of the furnace.

Tuyeres & Peep Holes

Tuyers are the small holes present in a circular pattern all around the cylindrical shell in the bottom region where wind box is present. The main function of the tuyers is to supply the air from wind box and blower to the combustion chamber. Peep holes are made radially on the wind box towards the tuyers holes which helps operators to see and examine the combustion process inside the chamber.

Slag Spout

Slag spout is a narrow opening which can be found at the bottom edge of furnace to discharge impurities in the form of slag from the combustion chamber.

Tapping Spout

Similar to slag spout, tapping spout is positioned at lower part of furnace which helps to discharge molten metal from the combustion chamber.

Drop Bottom

Drop Bottom in cupola is mostly used for cleaning, maintenance and repairing the furnace interiors. It is usually a hinged or drop bottom doors positioned at the bottom of the furnace which helps to discharge last molten metal in between the support legs.

Bottom Prop

Bottom prop or Prop bar is used as an additional support at the bottom of the furnace to prevent the opening of drop bottom doors. It is made of a sturdy material to withstand mechanical stress and high temperature that come from combustion zone.

Support legs & Foundation

Support legs and floor foundation plays a major role by supporting the entire furnace structure. Supports are often made up of durable and heat resistant material to withstand mechanical & thermal stress and to evenly distribute weight of the structure during its operation.

Cylindrical Shell

The cupola’s vertical structure is covered by a thin cylindrical steel about 6 to 12mm thickness. The inner side is lined with refractory bricks with is made up of silica oxide acid and alumina. Sometimes tamping clays are also used along with refractory bricks for lining the inner circumference.

Construction of Cupola Furnace

The exterior of cupola is covered by a thin cylindrical steel sheet about 6 to 12 mm thickness. The diameter of the cylindrical structure is about 1 to 2 meters, and the height varies from 4 to 5 times of its cylindrical diameter.

The cylindrical portion of the cupola is lined inside with refractory bricks made-up of alumina and silicon oxide. In Morden cupolas, the shell is mounted on the steel column base which comes with drop down doors at the bottom to discharge the debris, slag etc.

The base is rested at 0.85 to 1 meter above from the ground level. The shell is prepared with the molding sand which slopes towards the tapping spout. On the opposite side there is a slag spout where slag is discharged.

The air is always carried out through air blast inlet pipe from motorized blower, first to wind box and then to multiple tuyeres in the shell. Usually, the tuyeres are present at 400 to 500 mm height from the working bed of cupola and their size of tuyeres varies from 50 x 150mm to 100 x 300mm. The air pressure is always controlled by a valve present in the air blast pipe.

Above 3 to 6 meters from tuyeres there is a charging door which is used for charging coke, flux, metal etc. Charging platform is usually built around the cupola at 200 to 300mm below the charging door, so that operators can ergonomically use it charge the cupola.

The cupola is extended 4 to 5meters above charging door and has a conical cap like structure at the top called spark arrester. Sparks are arrested and only smoke escapes here. Some cupolas may have a precipitator, filter and collector fitted along with spark arrester to control the pollution. This is how the cupola is constructed from base to top.

Working principle:

The first operation is to clean up the cupola by opening the bottom drop doors to discharge the debris from the previous use. Then the furnace is ignited with the small fire and coke is added gradually. Coke should be added till the level of 0.5 meters above the tuyeres.

Then the cupola charge should be gradually added to the chamber through charging door. The cupola charge contains pig iron, scarps, cast iron casting rejections, coke and flux. Normally limestones are used as cupola flux at quantity of 2 to 4% of the weight of iron charge. Now tuyeres are opened, and a constant air volume is supplied to combustion chamber.

During this combustion stage, all available oxygen is consumed, and an exothermic reaction occurs in the combustion zone. Huge amount of heat at the temperature of $1550^{\circ } C$ to $1850^{\circ } C$ is liberated during this exothermic reaction.

$$C\> +\> O_{2} \longrightarrow \> CO_{2} \> +\> Heat$$

$$Si\> +\> O_{2} \longrightarrow \> SiO_{2} \> +\> Heat$$

$$2Mn\> +\> O_{2} \longrightarrow \> 2MnO \> +\> Heat$$

The temperature then drops to $1200^{\circ } C$ in the reduction zone. The reduction zone starts from the top of combustion zone and extends to the top of coke bed. In this zone the reduction of $CO_{2}$ occurs.

$$CO_{2}\> + \> C (Coke) \longrightarrow \> 2CO \> -\> Heat$$

Finally, the metal charge turns to molten state in the melting zone. At this zone the temperature is increased to $1600^{\circ } C$ to melt the metal charge. Then the molten metal flows down through the coke bed and reaches the well zone and then they are tapped out through tapping spout. The following reaction occurs during the melting zone.

$$3Fe\> + \> 2CO \longrightarrow \> Fe_{3} C\> +\> CO_{2} $$

In preheating zone, the alternate or mixed layers of coke, flux and metal are preheated by the upgoing gases that comes the from the melting zone. The temperature at this preheating zone is about $1100^{\circ } C$.

The hot gases produced within the furnaces are escaped through the stack zone. This is how the cupola furnace works.

Advantages of cupola furnace

 There are many advantages of having a cupola for industrial applications.

  • Simple and robust design.
  • Easy and economical to operate compare to other industrial furnaces.
  • Anyone can build their own cupola according to their needs and there are available in various sizes.
  • Wide range of metal alloys such as pig iron, nodular cast iron, white cast iron and copper-based alloys also can be melted.
  • Easy to discharge debris from the chamber due the presence of prob and drop bottom door.
  • Cupolas can be operated continuously with multiple production cycles.
  • Cupolas are highly efficient and gives high output.
  •   Depending upon their size, it requires less floor space compared to other industrial furnaces.
  • It can operate and withstand even at high temperatures.
  • Usually, cupolas have longer operational life that are properly maintained.

Disadvantages of cupola furnace

Though there are many advantages, they also have some limitations. They are as follows.

  • Since it is an industrial furnace, it emits lot of pollutants in the atmosphere which causes environmental concerns. Emission of pollutants can be controlled by fitting additional air filters, precipitators and collectors at the furnace top. Adding these additional equipments will end up with additional cost.
  • Though it has robust design, there is a limitation on controlling the furnace temperature over the melting cycle. The limited temperature control sometimes can impact the quality and consistency of molten metal.
  • Cupolas are not electrical. Coke is traditional used as a fuel for cupola. The production of coke mainly depends on coal.
  • It might be easy to operate but it is difficult to repair when there is a damage. A skilled technician is required in such situations.
  • Safety training should be given to operators to operate such industrial furnaces.

Applications of cupola furnace

The applications and uses of cupola are:

  • Used in foundries. At many places it used as a primary way to melt metals. It is necessary at foundries which makes various cast iron products by melting cast iron.
  • It is also been used in various artistic casting process. They use cupola to melt iron to make their metal sculptures and others.
  • Since it is economical, many small scale metal foundries use cupola as their primary melting furnace.
  • Industries also use cupola to melt copper-based alloys. Even steel, malleable cast iron and ductile cast iron can be prepared by combining two or three cupolas in single production cycle.

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