Cupola furnace

BY:

SHUBHAM KHARWAR

TOPIC:CUPOLA FURNACE

SR.NO. CONTENTS

1. INTRODUCTION

2. HISTORY

3. ILLUSTRATED FIGURE

4. CONSTRUCTION OF CUPOLA FURNACE

5. ZONES OF CUPOLA FURNACE

6. WELL, COMBUSTION, REDUCTION, MELTING,

PREHEATING,STACK

7. CHARGING OF CUPOLA FURNACE

8. WOKING OF CUPOLA FURNACE

9. JAMMING (TEMPORARY, PERMANENT)

10. THERMAL EFFICIENCY OF CUPOLA FURNACE

11. ADVANTAGES OF CUPOLA FURNACE

12. LIMITATIONS OF CUPOLA FURNACE

13. APPLICATIONS OF CUPOLA

14. PRECAUTIONS FOR SAFETY OF CUPOLA

15. REFERENCE

CUPOLA FURNACE

IntroductionA cupola furnace is a melting device used in foundries that can be

used to melt cast iron, Ni resist iron and some bronzes.

For many years, the cupola was the primary method of melting used

in iron foundries.

The cupola furnace has several unique characteristics which are

responsible for its widespread use as a melting unit for cast iron.

Cupola furnace is employed for melting scrap metal or pig iron for

production of various cast irons.

It is also used for production of nodular and malleable cast iron. It is

available in good varying sizes.

The main considerations in selection of cupolas are melting capacity,

diameter of shell without lining or with lining, spark arrester

HISTORY OF CUPOLA FURNACE

Cupola furnaces were first built in china as early as the Warring

States period (403-221 BC).

A Cupola furnace was made by RENE-ANTOINE FERCHAULT

DE RESUMUR around in 1720.

During the Han Dynasty (202 BC – 220 AD), most, if not all, iron

smelted in the blast furnace was remelted in a cupola furnace; it was

designed so that a cold blast injected at the bottom traveled through

tuyere pipes across the top where the charge (i.e. of charcoal and

scrap or pig iron) was dumped, the air becoming a hot blast before

reaching the bottom of the furnace where the iron was melted and

then drained into appropriate molds for casting..

Fig 1.2 CUPOLA FURNACE

Construction of cupola furnace

Shape A typical cupola melting furnace consists of a

water-cooled vertical cylinder which is lined with

refractory material.

The construction of a conventional cupola consists

of a vertical steel shell which is lined with a

refractory brick.

The charge is introduced into the furnace body by

means of an opening approximately half way up

the vertical shaft.

The charge consists of alternate layers of the

metal to be melted, coke fuel and limestone flux.

The fuel is burnt in air which is introduced

through tuyeres positioned above the hearth. The

hot gases generated in the lower part of the shaft

ascend and preheat the descending charge.

WELL

The space between the bottom of the tuyers and the sand bed inside the cylindrical shell of the cupola is called as well of the cupola.

OBJECTIVE OF THE WELL :

As the melting occurs, the molten metal is get collected in this portion before tapping out.

COMBUSTION ZONE

The combustion zone is also known as oxidizing zone.

It is located between the upper of tuyers & a theoretical level above it.

The total height of this zone is from 15cm to 30cm.

The combustion takes place due to free oxygen completely from the air

blast &generate tremendous heat

The heat is further evolved also due to oxidation of silicon & manganese.

The temperature achieved is about 1540 ˚C to 1870˚C.

Few exothermic reactions takes place in this zone are represented as:

C + O2 CO2 + Heat

Si + O2 SiO2 + Heat

2Mn + O2 2MnO2 + Heat

REDUCING ZONE

Reducing zone of Cupola is also known as protective zone.

It is located between the upper level of combustion zone & the upper level of

coke bed.

In this zone CO2 changes to CO through an endothermic reaction.

Due to which temperature falls from combustion zone to 1200˚C at the top of

this zone.

The important chemical reaction takes place in this zone is :

CO2 + C (coke) 2CO + Heat

MELTING ZONE

The lower layer of metal charge above the lower layer of coke bed.

The metal charge starts melting in this zone & trickles down through coke bed and

collected in the well.

Sufficient carbon content picked up by the molten metal in this zone is represented

by the chemical reaction given as :

3Fe + 2CO Fe3C + CO2

PRE HEATING ZONE

Pre heating zone starts from the upper end of the melting zone & continues upto the bottom

level of charging door.

This zone contains a number of alternate layer of coke bed, flux & metal charge.

The main objective of this zone is to preheat the charges from room temperature to about

1090˚C before entering to the melting zone.

The pre heating takes place in this zone due to the upward movement of hot gases.

During the pre heating process, the metal charge in solid form picks up some sulphur content in

this zone.

STACK

The empty portion of cupola above the pre heating zone is called as stack

OBJECTIVE OF THE STACK :

It provides the passage to hot gases to go to atmosphere from the cupola furnace.

CHARGING OF CUPOLA FURNACE

Before blower is started, the furnace is pre heated and the metal & coke charges, lying in

alternate layers, are sufficiently heated up.

The height of coke charge in the cupola in each layer varies generally from 10 to 15 cms.

The requirement of flux to metal charge depend upon the quality of the charged metal &

scrap, the composition of the coke and the amount of ash content present in the coke.

About 40 Kg to 50 Kg of limestone, in the form of flux, per metric ton of the metal is used.

The excess of amount of flux affects the acid lining of cupola.

Less amount of the flux results in the loss of molten metal.

First charge received of molten metal is allowed to drain out or used for rough castings.

For having desired composition of casting, it is essential to control the proportion of its

various constituents at the stage of raw material requirement for melting.

During the process of melting ,number of losses &gains of different constituents take place

inside the cupola.

The losses and gains in composition during melting as identified are given as under :

1. Iron - Loss of about 4%

2. Carbon - Gain of about 0.1 to 0.15%.

3. Silicon - Loss of about 10%

4. Manganese - Loss of about 15% to 20%

5. Phosphorus - Practically no change.

6. Sulphur - gain of about 0.03 to 0.05%

WORKING OF CUPOLA FURNACE

The charge, consisting of metal, alloying ingredients, limestone, and coal coke for

fuel and carbonization (8-16% of the metal charge), is fed in alternating layers

through an opening in the cylinder.

Air enters the bottom through tuyeres extending a short distance into the interior of

the cylinder. The air inflow often contains enhanced oxygen levels.

Coke is consumed. The hot exhaust gases rise up through the charge, preheating it.

This increases the energy efficiency of the furnace. The charge drops and is melted.

Although air is fed into the furnace, the environment is a reducing one. Burning of

coke under reducing conditions raises the carbon content of the metal charge to the

casting specifications.

As the material is consumed, additional charges can be added to the furnace.

A continuous flow of iron emerges from the bottom of the furnace.

Depending on the size of the furnace, the flow rate can be as high as 100 tones per

hour. At the metal melts it is refined to some extent, which removes contaminants.

This makes this process more suitable than electric furnaces for dirty charges.

A hole higher than the tap allows slag to be drawn off.

The exhaust gases emerge from the top of the cupola. Emission control technology is

used to treat the emissions to meet environmental standards.

Hinged doors at the bottom allow the furnace to be emptied when not in use.

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.Ja

mm

ing Temporary

Jamming

Permanent jamming

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 mustbe 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.

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.

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.

ADVANTAGES

It is simple and economical to operate. A cupola is capable of accepting a wide range of materials without reducing melt quality. Dirty, oily

scrap can be melted as well as a wide range of steel and iron. They therefore play an important role in the metal recycling industry

Cupolas can refine the metal charge, removing impurities out of the slag. From a life-cycle perspective, cupolas are more efficient and less harmful to the environment than

electric furnaces. This is because they derive energy directly from coke rather than from electricity that first has to be generated.

The continuous rather than batch process suits the demands of a repetition foundry. Cupolas can be used to reuse foundry by-products and to destroy other pollutants such as VOC

from the core-making area. High melt rates Ease of operation Adequate temperature control Chemical composition control Efficiency of cupola varies from 30 to 50%. Less floor space requirements comparing with those furnaces with same capacity

LIMITATIONS

Since molten iron and coke are in contact with each other, certain elements like Si, Mn are

lost and others like sulphur are picked up. This changes the final analysis of molten metal.

Close temperature control is difficult to maintain

Applications of Cupola

Cupola is most widely used for melting practices for production of grey cast

iron, nodular cast iron, malleable cast iron and alloy cast iron.

It can be used for melting some copper-base alloys, and in duplexing and

triplexing operations for making of steel, malleable cast iron and ductile cast

iron.

Steel can be also prepared in cupola by employing duplexing and triplexing

operations.

In duplexing melting operation two furnaces are used, and triplexing operations,

three furnaces are employed.

Precautions for Safety of Cupola Furnace

1. For safety of Cupola, better quality refractory lining must be used for preparing or repairing furnace so that it can

withstand high temperature as produced inside the furnace during melting, otherwise it will fuse and mix with

molten metal to form slag.

2. The furnace operator during firing the furnace should always make an effort to

place the metal charge in the centre. He must ensure that the coke charge is well distributed all around and towards

the firebrick lining to ensure uniform and thorough melting of the metal.

3. As the air passes through the tuyeres, the temperature near the tuyere openings will therefore be comparatively

lower and consequently the molten iron and slag will have a tendency to solidify near these openings and block

them. This should be prevented by frequent poking and removal of these materials by means of a poking rod through

the tuyeres.

4 Amount of air supply should be properly controlled. An excess amount of air will

always result in waste of fuel and lowering of temperature inside and a lesser

amount, than required, will cause incomplete combustion of fuel which is undesirable.

5. Tap hole must be properly closed by means of a well suitable plugging means. Clay mixed with an equal amount

of coal dust forms a very suitable mixture for plugging up the tap hole.

6 In closing the tap-hole, precaution must be taken to press the plug downwards in the hole so that the splash of the

molten metal, during plugging does not fall on the hands of the furnace operator.

7 Molten metal should always be tapped out well in time before its level rises too high in the well of the cupola

furnace. Any delay of tapping molten metal, the slag

floating on the surface of the molten metal, will start flowing into the wind belt

through the tuyeres and air passage will be choked and it will result in severe

problem of the jamming of cupola furnace.

BOOKS:

INTRODUCTION TO BASIC MANUFACTURING PROCESSES AND WORKSHOP

TECHNOLOGY BY,

RAJENDRA SINGH

MANUFACTURING TECHNOLOGY BY,

HAZRA CHOUDHARY

&

GOOGLE.COM

REFERENCE