2. FURNACE
• A furnace is a device used for heating. The
name derives from Latin fornax, oven.
• It is essentially a thermal enclosure and is
employed to process raw materials at high
temperatures both in solid state and liquid
state.
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3. The principle objectives are:
a) To utilize heat efficiently so that losses are
minimum, and
b) To handle the different phases (solid, liquid or
gaseous) moving at different velocities for
different times and temperatures such that
erosion and corrosion of the refractory are
minimum.
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4. • Furnaces are usually made of either
insulating firebrick or firebrick covered with
refractory material. The charge, or inlet
material, is introduced by chutes, conveyors
or pipes. The furnace can run in batch mode,
or in continuous mode. The charge moves
through the furnace on skids or rolls, or by
gravity, rotation, slope, or mechanical
pushers such as screws.
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• Furnaces can be direct fired, over fired, under
fired, or side fired. In direct fired furnaces, the
heat is produced on the inside of the furnace
chamber. In over, under, and side fired
furnaces, heat is produced in a chamber in
the respective area and flows throughout the
furnace.
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Table 1 Classification of Combustion Furnaces
Classification Method Types and Examples
Type of fuel used
Combustion Type
Oil-fired
Gas-fired
Coal-fired
Mode of charging materials Intermittent / Batch
Periodical
Forging
Re-rolling (batch/pusher)
Pot
Continuous
Pusher
Walking beam
Walking hearth
Continuous recirculating bogie furnaces
Rotary hearth furnaces
Mode of heat transfer Radiation (open fire place)
Convection (heated through medium)
Mode of waste heat recovery Recuperative
Regenerative
8. TYPICAL FURNACE SYSTEM
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i. Forging Furnaces
The forging furnace is used for preheating billets and
ingots to attain a ‘forge’ temperature. The furnace
temperature is maintained at around 1200 to 1250°C.
ii. Rerolling Mill Furnace
a. Batch type. A box type furnace is employed for
batch type rerolling mill. The furnace basically used
for heating up scrap, small ingots and billets
weighing 2 to 20 kg. for rerolling.
b. Continuous Pusher Type. The process flow and
operating cycles of a continuous pusher type is the
same as that of the batch furnace.
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iii. Continuous Steel Reheating Furnaces
The main function of a reheating furnace is to
raise the temperature of a piece of steel,
typically to between 900°C and 1250°C, until it
is plastic enough to be pressed or rolled to the
desired section, size or shape.
10. ELECTRIC FURNACE
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• A chamber heated by electric current is
known as Electric Furnace. Electric furnaces
are cheaper than oil fired furnaces and gas
fired furnaces.
11. ELECTRIC FURNACE
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• Operating Principle
Steel scrap and a small amount of carbon and
limestone are dropped into the electric furnace
through the open roof. The roof is then closed, and
the electrodes are lowered. Power is turned on,
and, within a period of about two hours, the metal
melts. The current is then shut off, the electrodes
are raised, the furnace is tilted, and the molten
metal is poured into a ladle, which is a receptacle
used for transferring and pouring molten metal.
12. ELECTRIC FURNACE
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Fig. 1 Schematic illustration of types of Electric
Furnaces: (a) direct arc, (b) indirect arc , and (c)
induction.
14. APPLICATION AND USES
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• High temperatures soften, melt, and anneal
the metals. Heating can also cause the
absorption of carbon. Furnaces are used in
various stages of heat treatment, as shown in
the table below for steel treatment.
Treatment
heat-treating
annealing
hardening
heating
reheating
Temperature Range
up to 1200°F
1200-1600°F
1500-1600°F
up to 2300°F
up to 2300°F
15. APPLICATION AND USES
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• Furnaces are also used to melt
glass, coke coal, distill zinc, and many other
processes. Hearth furnaces can be used to
remove hazardous waste. They are also used
in the microelectronics industry in
semiconductor wafer production.
16. KILNS
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• A kiln is a thermally insulated chamber, a
type of oven that produces temperatures
sufficient to complete some process, such as
hardening, drying, or chemical changes.
• Kilns are furnaces used for ceramic or brick
heating, causing moisture and organic
materials to evaporate. Kilns are heated by
horizontal space burners with gas, liquid, or
solid fuel.
17. TYPES / CLASSIFICATION / CLASSES
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1. In using an intermittent kiln, the ware to be
fired is loaded into the kiln. The kiln is closed,
and the internal temperature increased
according to a schedule. After the firing is
completed, both the kiln and the ware are
cooled.
Kilns in this type include:
• Clamp kiln
• Skove kiln
• Scotch kiln
• Down-Draft kiln
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2. A continuous kiln, sometimes called a tunnel
kiln, is a long structure in which only the central
portion is directly heated. From the cool
entrance, ware is slowly transported through
the kiln, and its temperature is increased
steadily as it approaches the central, hottest
part of the kiln. From there, its transportation
continues and the temperature is reduced until
it exits the kiln at near room temperature.
Kilns in this type include:
• Hoffman kiln
• Bull’s Trench kiln
• Habla (Zig-Zag) kiln
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Ceramic Kilns
• Kilns are an essential part of the manufacture
of all ceramics, which require heat treatment,
often at high temperatures. During this
process, chemical and physical reactions
occur that permanently alter the unfired body.
In the case of pottery, clay materials are
shaped, dried and then fired in a kiln.
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• Wood Drying Kiln
A variety of wood drying kiln technologies exist
today:
1. Conventional wood dry kilns - are either
package-type (side loader) or track-type
(tram) construction.
2. Dehumidification kilns - Heat comes
primarily from an integral dehumidification
unit that also removes humidity.
3. Solar kilns - are conventional kilns, typically
built by hobbyists to keep initial investment
costs low.
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• Rotary Kilns
- is a pyroprocessing device used to raise
materials to a high temperature (calcination) in
a continuous process. Materials produced using
rotary kilns include:
• Cement
• Lime
• Refractories
• Metakaolin
• Titanium dioxide
• Alumina
• Vermiculite
• Iron ore pellet
22. OPERATING PRINCIPLE
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• The kiln is a cylindrical vessel, inclined slightly to
the horizontal, which is rotated slowly about its
axis. The material to be processed is fed into the
upper end of the cylinder. As the kiln rotates,
material gradually moves down towards the lower
end, and may undergo a certain amount of stirring
and mixing. Hot gases pass along the kiln,
sometimes in the same direction as the process
material (co-current), but usually in the opposite
direction (counter-current). The hot gases may be
generated in an external furnace, or may be
generated by a flame inside the kiln. Such a flame
is projected from a burner-pipe (or "firing pipe")
which acts like a large bunsen burner.
24. APPLICATION & USES
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• Tunnel kilns are used to vitrify clay bricks,
particulate solids, and large solid objects.
They are also used to sinter capacitors, soft
ferrite, composite materials, and capacitors
used in computers and cellular telephones
also. Rotary kilns are used to make cement
and to calcine small waste stone and free-
flowing, granular solids. Downdraft kilns are
used to produce brick, pipe, tile, and
stoneware, while updraft kilns are used for
pottery burning.
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Other uses of kiln include:
• To dry green lumber so it can be used immediately
• Drying wood for use as firewood
• Heating wood to the point of pyrolysis to produce charcoal
• For annealing, fusing and deforming glass, or fusing metallic
oxide paints to the surface of glass
• For cremation (at high temperature)
• Drying of tobacco leaves
• Drying malted barley for brewing and other fermentations
• Drying hops for brewing (known as a hop kiln or oast house)
• Drying corn (grain) before grinding or storage, sometimes
called a corn kiln, corn drying kiln.
• Smelting ore to extract metal
• Heating limestone with clay in the manufacture of Portland
cement
• Heating limestone to make quicklime or calcium oxide
