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Cupola

Cupola is the most common type of melting furnace used in foundry industry. The cupola is a vertical shaft cylindrical furnace charged from the top. Heat, released from combustion of coke in the bed, melts the metallic charge materials.

A schematic cross-section of the cupola, depicting the important zones, is shown in the figure.

Some advantages of the cupola are listed below.

Lower capital cost

Ease and flexibility of operation

Lower melting losses

Lower energy cost

Better metallurgical properties promoting machinability

Classification of cupolas

Cold blast operated cupola

Cold blast cupolas operate with ambient blast air. Cold blast is common in Indian foundries since it is easier to design and operate and entail lower capital investments compared to hot blast systems.

Hot blast operated cupola

Hot blast systems are designed to pre-heat the combustion air by heat exchange with the hot stack gases. Some advantages of hot blast operation are energy savings, lower sulphur pick-up and higher carbon pick-up. However, the benefits are substantially reduced if the blast air temperature is below 400oC. Second-hand imported hot bast cupolas are notorious for being difficult to rebuild and operate.

Continuous tapped cupola

In continuously tapped cupola, very little metal is held in the well. The slag and metal comes out from the same tap hole, and is separated in the siphon box provided at the launder. Since the metal is tapped immediately after melting, it is more an energy efficient operation compared to intermittent tapping of the cupola.

Intermittently tapped cupola

Also called bott and tap cupola, an intermittently tapped cupola stores the molten metal in the well before it is tapped in batches. Separate tap-holes are provided for iron and slag in this type of operation. Although an intermittently tapped cupola provides a more uniform composition of the molten metal, some energy is lost in the cupola well.

Conventional cupola

Conventional cupolas are designed to inject the blast air through a single row of tuyeres. Modified conventional cupola designs, injecting the blast air through two rows of tuyeres, connected to the same wind belt, are also common.

Divided blast cupola

The efficiency of a cupola can be improved by splitting the blast air, in the correct proportion, between two rows of tuyeres. The blast air is first divided between two wind-belts, before it is injected into the cupola through the tuyeres. Divided blast operation helps to reduce the coke consumption and increase the melting rate for a given cupola diameter.

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Important control parameters

Blast rate

The blast rate is one of the most important control parameters in a cupola. Apart from higher coke consumption, a higher blast rate creates an oxidising atmosphere, resulting in excess oxidation of iron and elements like silicon and manganese. Too little blast air does not generate enough heat for efficient combustion and leads to lower metal temperature, slower melting and higher coke consumption.

Although it is possible to calculate the blast rate from first principles, a rule of thumb for estimating the optimum blast rate based on the cupola cross-sectional area at the tuyeres is often used. The optimum blast rate has been found to be 375 ft³/min per square foot or 115 m³/min per square metre. The blower should be capable of delivering about 15%-20% more than the required blast rate, to account for air losses in the blast system.

Blast pressure

Proper blast pressure is required to penetrate the coke bed. Incorrect air penetration adversely affects the temperature, carbon pick-up and the melting rate of the cupola.

The blast pressure is a function of the cupola diameter. An empirical correlation to derived the blast pressure from cupola diameter is suggested below:

Blast Air Penetration

P = 0.005 D² - 0.0134 D + 39.45
Where,
P = Blast pressure, inch H₂0
D = Internal diameter at the melting zone, inches

Tuyere size

The tuyere size determines the velocity of the blast air in the bed. The specification of the tuyere differs between a cold blast and a hot blast system. For a cold blast system, the total area of the tuyeres is about 20% of the melting zone area. Size of each tuyere can be calculated by dividing the total tuyere area by the total number of tuyeres.

The recommended number of tuyeres per row for cupolas of various diameters is as follows.
Cupola internal diameter less than 30 inch: 4
Cupola internal diameter between 30 inch and 42 inch: 6
Cupola internal diameter between 42 inch and 60 inch: 8
Cupola internal diameter between 60 inch and 84 inch: 12

The shape of the tuyere can be either round (preferable) or rectangular.

Stack height

In the cupola, hot gases rising from the melting zone exchange heat with the descending charge materials. If the stack height is too short, inadequate charge pre-heating takes place and excess heat escapes in the top gases. A stack height between 16 ft to 22 ft is recommended for a cold blast cupola, depending upon its diameter.

Well depth

The well depth influences the carbon pickup and the metal tapping temperature. Research at BCIRA (BCIRA Journal, January 1978, Report #1289) has shown that increasing the well depth leads to higher carbon-up. On the flip side, increasing the well depth reduces the tapping temperature of the molten metal. As a rule of thumb, there is a drop of about a degree centigrade in the molten metal temperature for every additional inch increase in the well depth.

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Energy and environmental performance

In foundry parlance, the energy performance of a cupola is measured by the charged coke consumed (including boosters) per tonne of metallic charge material. Bed coke consumption is usually excluded while comparing the energy performance of cupolas. Apart from the design, energy performance of a cupola is affected by the operating practices, quality of coke, tapped melt temperature, composition of the metallic charged etc. Hence, the charge coke consumption in cupolas vary widely, typically ranging between 8% (coke: metal ratio of 1:12.5) to 13% (coke: metal ratio of 1:7.5) in most Indian foundry units.

Suspended particulate matter (SPM) is the major air pollutant from cupola stack. The quantity and composition of particulate emissions vary among cupolas, and even at intervals in the same cupola. It is necessary to clean the cupola stack gases in a pollution control system in order to comply with the local air emission standards.

The emission standards for cupola melting furnaces in India are given under environment regulations. Common pollution control systems used for cleaning cupola stack gases are outlined under air pollution control devices.

New developments

Coke-less cupola

Cupolas using natural gas or oil as fuel instead of coke were first developed in the 1950s in the United Kingdom, mainly driven by environmental pressures. Particulate emissions from a coke-less cupola are much lower compared to coke-fired cupolas. In a coke-less cupola, a water-cooled grate supports the charge material instead of a coke bed. High intensity gas or oil fired burners are used to generate the heat in the furnace. A carburiser is injected in the cupola to make-up the loss of carbon. However, high capital and operating costs and sophisticated control requirements have limited the adoption of coke-less cupolas to only a handful of larger foundry units in Europe. Even there, the coke-less cupolas are operated in duplexing mode with electric furnaces.

Oxygen enrichment

Oxygen helps to raise the melt temperature and increase the melting rate. It is usually introduced at the tuyere level after suitably modifying the tuyeres. The amount of oxygen in the blast air usually varies between 1% to 4%. Oxygen enrichment is common in United States and Europe, especially in large cupolas.

Cupola as a smelter and melter

Very large foundries have used the cupola for conversion of iron oxide to iron, a sort of mini-blast furnace, for smelting and melting together. A much larger cupola is needed in such cases because any carbon reduction of iron oxide takes time and additional coke, which inevitably slows down the melting rate.

Producing gray and ductile iron from the same cupola

It is possible to produce both gray and ductile iron from the same cupola by proper planning and control. A few changes such as reduced manganese and chromium in charge metallics and additional coke boosters are needed while switching from gray iron to ductile iron production. The melt chemistry needs to be closely monitored to detect a rapid drop in manganese and chromium, signalling a change to ductile iron. Increase in carbon content of the melt is achieved by lowering the air blast.

Long campaign cupola

Most recent cupola installations in Europe are of long campaign hot blast type. The operating period of the cupola could range from a few weeks to several months. The shell and tuyeres of long campaign cupolas are usually water-cooled and high quality refractories (alumina type) are used in the cupola well. Some advantages of a long campaign cupola are listed below.