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.
The primary objective in cupola is to produce iron of desired composition, temperature and properties at the required rate in the most economical manner. Besides, this furnace has many distinct advantages over the other types, e.g., simplicity of operation, continuity of production, and increased output coupled with a high degree of efficiency.
Description Of a Cupola :
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 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 molding sand which covers the drop-doors. This bottom slopes towards the metal tapping hole situated at 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 motorized 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 wind box 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, the type of iron melted, and the compactness of the charge the pressure of air may vary from 250mm to 400mm 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 tonne of iron depends upon the quantity 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, 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 may be fitted with a collector, fitter, and precipitator to minimize atmospheric pollution.
Zones in a Cupola :
On the Basis of combustion reactions, the entire shaft of the cupola may be divided as under :
Crucible zone 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 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 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*C to 1850*C, molten drops of cast iron pour in to the hearth. The chemical reactions which occurs in this zone are :
C + O2 ----------> CO2 + Heat
Si + O2 -----------> SiO2 + Heat
2 Mn + O2 ---------> 2 MnO2 + Heat
Reducing zone 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) -------> 2CO - Heat
Melting Zone 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 starts from 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 extends from above the preheating zone to the top the cupola. It carries the gases generated within the furnace to the atmosphere.
Capacity Of a Cupola :
The Output of a cupola is defined as the tons of molten metal obtained per hour of the heat. Cupola capacities (sizes) vary from 1 to 15 tons (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 tons of coke burned per sq. m of the cross-sectional area of the cupola in unit time. It has been observed that 14 cm2 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.
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