In this article we will discuss about Cupola Furnace, Parts of Cupola, Cupola furnace diagram, Zones in Cupola, Cupola operation.
The primary objective in cupola is to produce iron of desire composition, temperature and properties at the required rate in the most economical manner. Besides, this furnace has many distinct advantage over the other types, e.g., simplicity of operation, continuity of production and increased output coupled with a high degree of efficiency.
Various types of melting furnaces are used in different foundry shops depending upon the quantity of metal to be melted at a time, and the nature of work that is carried out in the shop. Only cupola furnace used in foundries for melting an refining pig iron along with scrap is described below.
Parts of Cupola furnace
The cupola furnace consists of a vertical, cylindrical steel sheet, 6 to 12 mm thick, and lined inside with acid refractory bricks or acid. tamping clay. The refractory bricks or the tamping clay used consist of silicon oxide acid (SiO2) and alumina (Al2O3). The lining is generally thicker in the lower region where the temperatures encountered are higher than in the upper region.
The shell is mounted either on a brick work foundation or on steel columns. In a steel column arrangement, used on most modern cupolas, the bottom of the shell is provided with drop-bottom doors through which debris, consisting of coke, slag, etc. can be discharged at the end of a melt. In drop–bottom cupolas, the working bottom is built up with moulding sand which covers the drop-doors.
This bottom slopes towards the metal tapping hole situated at the lowest point at the front of the cupola. Opposite this tap hole, and somewhat above it, is another hole, called the slag hole, which enables the slag to be taken out.
A constant volume of air for combustion is obtained from a motorised blower. The air is carried from the blower through a pipe called wind pipe (air blast inlet), first to a circular jacket around the shell called windbox and then into the furnace through a number of openings called tuyeres which are provided at a height of between 450 to 500 mm above the working bottom or bed of the cupola.
These tuyeres are generally 4, 6, or 8 in numbers depending on the size of the cupola and they may be fitted in one or more number of rows. The total area of the tuyeres should be about one-fifth to one-sixth of the cross-sectional area of the cupola inside the lining at tuyere level. Usually tuyeres have a size of 50×150 mm or 100×300 mm. Auxiliary tuyeres are sometimes provided to raise melting efficiency.
A valve is provided in the blast pipe to control the supply of air. Depending on the size of the cupola furnace, the type of iron melted, and the compactness of the charge the pressure of air may very from 250 mm to 400 mm of water for small and medium-sized furnaces and from 400 mm to 850 mm for large-sized furnace.
A volume meter is sometimes installed to know the volume of air passing. The amount of air, required to melt one ton of iron depends upon the quality of coke and the coke-iron ratio. Long practice proves that it takes about 800 to 900 cu m of air to melt one tonne of iron in a cupola furnace, assuming that a 10 to 1 ratio of iron to coke is used. For lower ratio, higher volumes of air will be needed.
A charging door is provided through which metal, coke and flux are fed into the furnace, and this is situated 3 to 6 m above the tuyeres, according to the size of the cupola. A large platform or stage usually surrounds the cupola at the level of about 300 mm below the bottom of the charging door.
The shell is usually continued for 4.5 to 6 m, above the charging door to form a chimney. At the top of the furnace a conical cap called the spark arrested, prevents the spark from emerging to the outside. The spark arrested cools down the sparks and allows only smoke to escape from the opening. Sometimes, a cupola furnace may be fitted with a collector, fitter and precipitation to minimize atmospheric pollution.
Zones in Cupola Furnace
On the basis of combustion reactions, the entire shaft of the cupola may be divided as under:
Crucible zone :- It is between top of the sand bed and bottom of the tuyeres. The molten iron is accumulated here. This is also called the well or hearth.
Combustion or oxidizing zone :- It is situated normally 150 to 300 mm above the top of the tuyeres. All the oxygen in the air blast is consumed here owing to the actual combustion taking place in this zone. Thus a lot of heat is liberated and this is supplied from here to other zones .
Heat is also evolved due to the oxidation of silicon and manganese. Due to this high heat, the temperature being 1550° to 1850°C, molten drops of cast iron pour into the hearth. The chemical reactions which occur in this zone are :
C + O2 —–>CO2 + Heat
Si + O2 —–>SiO2 + Heat
2 Mn + O2 —–> 2 MnO2 + Heat
Reducing zone :- It extends from the top of the combustion zone to the top of the coke bed. In this zone, the reduction of CO2 to CO occur and the temperature drops to about 1200°C at the coke bed. Due to the reducing atmosphere, the charge is protected from any oxidizing influence. The reaction taking place in this zone is :
CO2 + C (coke) —-> 2 CO-Heat
Melting zone :- It starts from the first layer of metal charge above the coke bed and extend up to a height of 900 mm. Highest temperature is developed in this zone for complete combustion of the coke and iron is thus melted here. The temperature in this zone is around 1600°C. A considerable carbon pick-up by the molten metal also occurs in this zone according to the following reactions :
3 Fe + 2 CO—–>Fe3C + CO2
Preheating zone or charging zone :- It starts form above the melting zone and extends up to the bottom of the charging door. Preheating zone contains cupola charge as alternate layers of coke, flux and metal and they are preheated there at a temperature of about 1100°C before coming to the melting zone.
Stack zone :- Stack zone extends from above the preheating zone to the top of the cupola. It carries the gases generated within the furnace to the atmosphere.
Capacity of Cupola Furnace
The output of a cupola Furnace is defined as the tonnes of molten metal obtained per hour of the heat. Cupola capacities (sizes) vary from 1 to 15 tonnes (or even more) of melted iron per hour. The size depends not only upon the cross-sectional area of the cupola, but upon the intensity of coke consumption as well.
But the intensity of coke consumption is meant the tonnes of coke burned per sq. m of the cross sectional area of the cupola in unit time. It has been observed that 14 cm of cupola plan area burns about 1 kg of coke per hour. The diameter of cupola varies from 1 to 2 m with a height of from 3 to 5 times the diameter.
Cupola Furnace Working
The different steps involved in cupola furnace operation are :
1. Preparation of cupola
The first operation in preparing a cupola furnace is to clean out the slag and refuse on the lining and around the tuyeres from the previous run. Any bad spots or broken bricks are repaired with a daubing mixture of fire clay and silica sand or ganister. The preparation of the sand bottom in the cupola is begun as soon as the patching of the lining has been completed.
The bottom doors are raised and held in this position by metal props. The bottom sand is introduced through the charging door and is rammed well around the lining and across the intersection of the bottom doors. This layer of sand is built up to a height of 100 to 200 mm above the cast iron door.
The surface of the sand bottom is sloped from all directions towards the tapping hole so that the molten metal can be drained completely from the cupola at any time. An opening about 35 mm in diameter is provided for the removal of the slag, and a tap hole is formed around a wooden pattern about 20 mm in diameter. The cupola should be thoroughly dried before firing.
2. Firing the cupola
In firing a cupola furnace, a fire of kindling wood is ignited on the sand bottom. This should be done 2.5 to 3 hours before the molten metal is required. On the top of the kindled wood, a bed of coke is built. When the wood is burning well coke is dumped into the well from above in several portions making sure that the coke begins to burn too.
The coke is added to a level slightly above the tuyeres and the air blast is turned on at a lower than normal blowing rate to ignite the coke. As soon as red spots begin to show over the top of the fuel bed, additional coke is introduced into the cupola to reach a height of 700 to 800 mm above the upper row of tuyeres.
The coke bed must be thoroughly hot before it is finished off to its final height. The height of the coke bed is determined by using a measuring rod which has been prepared to indicate the distance from the sill of the charging door to the top of the coke bed. The layer of coke resting on the sand bottom before beginning the heat is called bed charge. The amount of coke in the bed is dependent upon the pressure of the air supplied to the cupola.
The height of the bed charge or coke-bed is very important to the cupola operation ; it affects the temperature, melting rate, and chemical composition. Other things being equal, a low bed will yield cooler metal than one which than one which is high.
3. Charging the cupola
As soon as the coke bed is built up to the correct height and ignited uniformly throughout, alternate layers of pig iron, coke and flux (limestone) are charged from the charging door until the cupola is full. Suitable scrap is also added along with the pig iron, to control the chemical com-position of the iron produced. The proportion of this scrap is ordinarily from 25 to 50 per cent of the total weight of the metal poured.
When considerable steel scrap is used along with pig iron., a small amount, say from 2 to 4 per cent of ferro-manganese is used as a deoxidizer. The weight of the metal charge should be from 10 to 15 per cent of the hourly out-put of the cupola. The object of adding flux is to remove impurities in the iron, and to protect the iron from oxidation, to reduce the melting point of the slag, and to increase its fluidity for easy disposal. Besides limestone, fluorspar and soda ash are also sometimes used as fluxing material.
The quantity of limestone required may be 30 to 40 kg per tonne of iron melted or 25 per cent by weight of the coke charged. The ratio between the metal melted and coke charged depends on a great number of factors. So it is not possible to give definite recommendations for this ratio which can be achieved on different classes of work. Table 11.11 is given only as a guide and shows good average practice in the industry. More commonly it is kept 10: 1. This means that 1 tonne of coke is required to melt 10 tonnes of iron.
4. Soaking of iron
After the cupola is fully charged upto the charging door, the charge should soak in the heat for about 45 minutes. The charge gets slowly heated since the air blast is kept at a lower than normal blowing rate (practically kept shut) during this time. This causes the iron to get soaked.
5. Air blast
At the end of the soaking period, full blast is turned on. Before turning on the blast, the tuyere openings and the tapping hole are kept closed. After the blast has been on for a few minutes, say about 10 minutes, molten metal starts accumulating in the hearth. When the metal in the cupola starts melting, the rate of charging should be equal to the rate of melting, so that the furnace is kept full throughout the heat. At the end of the melt the charging is stopped but the blast is kept on until all the metal has melted.
6. Tapping and slagging
The first tapping can be made 40 to 50 minutes after the full air blast is turned on. During this period, sufficient metal is collected in the hearth above the sand bed. When slag accumulates in the well, the slag hole is opened and the slag is run off, preferably into a bogie for easy removal. Molten metal is collected in ladles and is carried to the moulds for pouring. The same procedure is repeated until all the metal is melted and the operation is over.
7. Closing the cupola
When the operation is over, the blast is shut off and the prop under the bottom door is knocked down so that the bottom plates swing open. This enables the cupola remains to drop on to the floor or into a bucket. They are then quenched and removed from underneath the cupola.
Generally, cupolas are run continuously as are blast furnaces, but are worked only for such periods as may be required. At many foundries the melting period does not exceed 4 hours, but cupolas may be operated continuously for 10 hours or more.
Efficiency of Cupola furnace
Thermal or melting efficiency of a cupola in per cent is expressed as:
(Heat utilised in preheating, melting and superheating ) / (Potential heat in coke + heat from oxidation of Fe, Si, Mn + heat in the air blast ) x 100
The efficiency of a cupola furnace varies from 30 to 50 per cent depending on
1. Coke rate or coke ratio expressed as the inverse of the metal-fuel ratio in percentage,
2. Blast rate, and
3. Mean coke size.
Air requirements for Cupola Furnace
For complete combustion of the fuel in the cupola furnace, about 8.4 cu m of air is required per kg of coke at normal atmospheric pressure and temperature. If the ratio of meal to coke charged 10:1, which is considered a satisfactory figure, the coke required per tonne of iron will be 1000/10 kg, i.e. 100 kg. Thus, the volume of air required per tonne of melted is >
8.4 x 100 = 840 cu m.
To allow for leakage, etc., the air supplied is generally a little in excess i.e. about 900 cu m per ton of iron.
Dimensions of Cupola Furnace
The principal dimensions of a cupola are selected on the basis of empirical data. Thus, the cross-sectional area A. of a cupola depends upon the designed hourly output and is determined from the formula
A = π d^2/4 = Q/Q1 m^2
Where d = cupola diameter in the clear , m, Q = designed cupola output, tonnes per hour, Q2 = specific output per sq. m of cross-sectional area, tonnes per hour. As a rule, Q1 = 6 to 8 tonnes per hour.
The useful height of a cupola (distance from the axis of the main tuyeres to the lower edge of the charging hole) depends upon the diameter and is designed according to the ratio H: d = from 3 to 5.
The cupola furnace height directly affects the melting rate, fuel consumption and the temperature and quality of molten metal. If it is too high the coke may be crushed as the charge drops ; if it is too low the metal is not heated to a sufficient degree, the draught is reduced and the cupola output is decreased.
The inside diameter of the cupola determines the amount of coke consumed and the amount of iron melted per unit of time. It has been found that 14 cm? of cupola plan area burns about 1 kg of coke per hour. Thus, a cupola having a capacity of 3 tonnes per hour will require (3×100)or 300 kg of coke per hour, assuming a metal-fuel ratio of 10 :1. The cupola area will therefore be equal to (14×300) or 4200 cm². The internal diameter will then be
Square roots of (4200×4 )/ π
= 73cm (approx)
Cupola Furnace Charges
If products of uniform quality are desired, a careful consideration must be given to the cupola charge. Usually, several grades of pig iron and scrap are available to the foundry man. To achieve a desired composition of the cast metal, these grades need to be adjusted and controlled. Since the various elements in metal undergo chemical changes during the re-melting operation, allowances have to be given for their loss or gain while making up the charge. The loss or gain of various elements is as follows.
1. Carbon : Molten metal picks up carbon as it passes through the incandescent coke forming the bed. With properly controlled melting conditions, a gain of 0.15 per cent may be expected. While the carbon content of the metal increases because of carbon absorption from the coke, the same suffers a little loss due to oxidation.
2. Silicon : Silicon suffers some loss due to oxidation as the drops of the molten iron trickle past the tuyeres. The loss may be 10 per cent of the silicon present in the charge.
3. Manganese : Manganese also has a tendency to get lost along with silicon during melting. The loss may be about 15 to 20 per cent of the manganese present in the charge.
4. Sulphur : Sulphur is picked up from coke, scrap and flux, etc. Generally, the gain in sulphur content is assumed to be about 0.03 to 0.05 per cent.
5. Phosphorus : There is practically no loss or gain in the phosphorus content.
6. Iron : Iron itself also tends to get oxidized and lost, but the loss which is quite small, may be assumed to be about 3 to 4 per cent.
This was all about the cupola furnace, it’s zones, operations and parts. Hope you liked the article. Please give your feedback in the comments below.
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