Introduction to Basic Manufacturing Processes and ... - always yours

Melting Furnaces 109
operations. In duplexing melting operation two furnaces are used, and triplexing operations,
three furnaces are employed
6.4.4 Jamming of Cupola Furnace
Jamming of cupola furnace is a very common problem for which sufficient care should be
taken to prevent it. It occurs frequently due to the negligence of the furnace operator. The
cupola furnace may be either jammed temporarily or permanently and the same are discussed
as under.
6.4.5 Temporary Jamming of Cupola Furnace
Temporary jamming of cupola furnace means the temporary suspension of air supply in
the tuyere zone due to choking of the tuyere mouths. This is caused mainly due to the low
temperatures at the tuyere openings resulting in the solidification of iron and slag around
these openings. The air passage is hence chocked and the supply of air is temporarily stopped.
It results in incomplete combustion of the coke/fuel inside the furnace and hence leads to
rapid reduction of the temperature in the furnace. Therefore this temporary jamming must
be prevented by frequent poking of this solidified material by means of a poking bar through
the tuyeres. The operator of cupola should always keep himself alert enough for not allowing
such solidification for continue melting in furnace for a longer period.
6.4.6 Permanent Jamming of Cupola Furnace
Permanent jamming of cupola furnace means a complete cut-off of the air supply to the cupola
due to the permanent choking of the air passage. This occurs on account of the overflow of
the slag into the wind chamber. As the metal melts and starts collecting in the well of the
furnace, its level rises gradually and the slag being lighter, always floats on the top surface
of the molten metal. If, the molten metal is not tapped out in time due to the negligence of
the operator, the level of molten metal in furnace will start rising in the well and a stage will
come when this level approaches to the tuyere level. Any slightest delay further, in tapping
by the cupola operator will cause the slag to flow through the tuyere openings into the wind
belt. Since the slag comes in contact with the air at low temperature it quickly solidifies in
the wind belt as well as in the tuyers and therefore chock the passages of air flow permanently
in the furnace. Hence the cupola furnace reaches in such undesirable condition on account
of the above occurrence. Therefore, to avoid such undesired situation, the operator of cupola
furnace should always be vigilant enough to tap the molten metal out from well of the cupola
furnace before the level of molten metal rises up to the tuyere level. Hence the cupola
furnace may continue melting for longer period.
6.4.7 Thermal Efficiency of Cupola Furnace
Thermal efficiency of cupola furnace is the ratio of heat actually utilized in melting and
superheating the metal to the heat evolved in it through various means. The total heat
evolved involves the heat due burning of coke, heat evolved due to oxidation of iron, Si and
Mn and heat supplied by the air blast. During melting it is observed that approximately 48-
70 % of the evolved heat is going as waste.
6.4.8 Precautions for Safety of Cupola Furnace
For operating the cupola successfully, the following safety precautions must always be kept
into account.