Furnaces in casting:
Melting
furnaces used in the foundry industry are of many diverse configuration
The selection of the melting unit is one of the most important
decisions foundries must make with due consideration to several
important factors including:
· The temperature required to melt the alloy
· The melting rate and quantity of molten metal required
· The economy of installation and operation
· Environmental and waste disposal requirements
Several types of furnaces are most commonly used in foundries:
· Cupolas
· Direct fuel-fired furnaces
· Crucible furnaces
· Electric-arc furnaces
· Induction furnaces
Selection
of the most appropriate furnace type depends on factors such as the
casting alloy; its melting and pouring temperatures; capacity
requirements of the furnace; costs of investment, operation and
maintenance and environmental pollution considerations.
Cupolas
A
cupola is a vertical cylindrical furnace equipped with a tapping spout
neat its base. Cupolas are used only for melting cast irons, and
although other furnaces are also used the largest tonnage of cast iron is melted in cupolas.
It
consists of a large shell of steel plate lined with refractory. The
charge, consisting or iron, Coke, flux and possible alloying elements,
is loaded through a charging door located less than halfway up the
height of the cupola.
The iron is usually a
mixture of pig iron and scrap (including risers, runners, and sprues
left over from previous castings). Coke is the fuel used to heat the
furnace. Forced air is introduced through openings near the bottom of
the shell for combustion of the coke.
The flux is a
basic compound such as limestone that reacts with coke ash and other
impurities to form slag. The slag serves to cover the melt, protecting
it from reaction with the environment inside the cupola and reducing
heat loss. As the mixture is heated and melting of the iron occurs, the
furnace is periodically tapped to provide liquid metal for the pour.
Cupola zones
Combustion or Oxidizing zone
It
is the zone where combustion takes place. It extends from the top of
the tuyeres to a surface boundary below which all the Oxygen of air is
consumed by combustion, chemical reaction that takes place in the zone
is
C(coke) + 02 (from air) -> C02 + Heat
The temperature in this zone is about 1800°C.
Reducing zone
It
extends from the top of the combustion zone to the top of the initial
coke bed. The CO2 produced in the combustion zone moves up and is
reduced to CO. The temperature also drops to 1650°C.
C02 + C2 -» CO – Heat
Melting zone
It
includes the first layer of pig iron above the initial coke bed. In
this zone, the pig iron is melted. The following reaction takes place.
3 Fe + 2 CO -» Fe3C + C02
Preheating zone
It
includes all the layers of cupola charges placed above the melting zone
to the top of the last charge. The layers of charges are heated by the
out-going gases. The temperature in the zone may be up to 1050°C.
Stack:
It is the zone beyond the pre-heating zone, through which the hot gases go to the atmosphere.
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