FURNACES I. GENERAL DESCRIPTION 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. Several industries like iron and steel making, non-ferrous metals production, glass making, manufacturing, ceramic processing, calcination in cement production etc. employ furnace. 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. Generally, furnaces that operate at temperatures under 1000°F are called ovens. Furnaces and ovens have very similar features. Both are primarily used to heat treat metals, using gas, oil, or electricity. II. GENERAL FUNCTION 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.
Transcript
1. FURNACES I. GENERAL DESCRIPTION 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. Several industries like iron and steel making, non-ferrous
metals production, glass making, manufacturing, ceramic processing,
calcination in cement production etc. employ furnace. 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. Generally, furnaces that operate at temperatures under
1000F are called ovens.Furnaces and ovens have very similar
features. Both are primarily used to heat treat metals, using gas,
oil, or electricity. II. GENERAL FUNCTION 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. In a continuous furnace, the hearth may be stationary or
rotary. Rotation speed can be adjusted based on the size, weight,
and load of the charge. Open spaces beneath the hearth circulate
air. Sidewalls support the arch-shaped roof. Heat in furnaces is
generated by combustion of fuel or conversion of electric energy.
Fuel and air enter via burners fired through burner tiles. Heat is
transferred to the material by or a combination ofinduction,
conduction, convection, and radiation. The products of fuel
combustion exit through vents, flues, and a high temperature stack,
carrying with it some heat. To recover this heat, flue gases are
used to preheat the stock or material being heated, the combustion
air, or the fuel. The flue is located at the top or bottom,
depending on whether the furnace is updraft or downdraft
respectively.
2. 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. III. TYPES / CLASSIFICATION / CLASSES
Furnaces are broadly classified into two types based on the heat
generation method: combustion furnaces that use fuels, and electric
furnaces that use electricity. Combustion furnaces can be
classified in several based as shown in Table 1: type of fuel used,
mode of charging the materials, mode of heat transfer and mode of
waste heat recovery. Table 1 Classification of Combustion Furnaces
Classification Method Types and Examples Combustion Type Oil-fired
Gas-firedType of fuel used 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
3. Rotary hearth furnaces Mode of heat transfer Radiation (open
fire place) Convection (heated through medium) Mode of waste heat
recovery Recuperative Regenerative Oil Fired Furnace Furnace oil is
the major fuel used in oil fired furnaces, especially for reheating
and heat treatment of materials. LDO is used in furnaces where
presence of sulphur is undesirable. The key to efficient furnace
operation lies in complete combustion of fuel with minimum excess
air. Furnaces operate with efficiencies as low as 7% as against up
to 90% achievable in other combustion equipment such as boiler.
This is because of the high temperature at which the furnaces have
to operate to meet the required demand. For example, a furnace
heating the stock to 1200C will have its exhaust gases leaving at
least at 1200C resulting in a huge heat loss through the stack.
However, improvements in efficiencies have been brought about by
methods such as preheating of stock, preheating of combustion air
and other waste heat recovery systems. Gas Furnace Gas furnaces
have a thermostat that signals the furnace to ignite the gas once
the temperature drops below the specified level. Natural gas
supplies heat in a convenient and cost-efficient manner. It
consumes less energy compared to other types of furnaces. Another
way to utilize furnaces more efficiently is having a heat exchange
for warming water. A separate water heater running in your house
gives a remarkable increase on the electric bill. Typical Furnace
System 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 1250C. Forging
furnaces use an open fireplace system and most of the heat is
transmitted by radiation. The typical loading in a forging furnace
is 5 to 6 tonnes with the furnace operating for 16 to 18 hours
daily. The total operating cycle can be divided into (i) heat-up
time (ii) soaking time and (iii) forging time. Specific fuel
consumption depends upon the type of material and number of reheats
required.
4. 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. The chargingand discharging of the material
is done manually and the final product is in the form of rods,
strips etc. The operating temperature is about 1200C. The total
cycle time can be further categorized into heat-up time and
rerolling time. During heat-up time the material gets heated upto
the required temperature and is removed manually for rerolling. The
average output from these furnaces varies from 10 to 15 tonnes /
day and the specific fuel consumption varies from 180 to 280 kg. of
coal / tonne of heated material. b) Continuous Pusher Type. The
process flow and operating cycles of a continuous pusher type is
the same as that of the batch furnace. The operating temperature is
about 1250C. Generally, these furnaces operate 8 to 10 hours with
an output of 20 to 25 tonnes per day. The material or stock
recovers a part of the heat in flue gases as it moves down the
length of the furnace. Heat absorption by the material in the
furnace is slow, steady and uniform throughout the cross-section
compared with batch type. 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 900C and
1250C, until it is plastic enough to be pressed or rolled to the
desired section, size or shape, The furnace must also meet specific
requirements and objectives in terms of stock heating rates for
metallurgical and productivity reasons. Incontinuous reheating, the
steel stock forms a continuous flow of material and is heated to
the desired temperature as it travels through the furnace. Types of
Continuous Reheating Furnace Continuous reheating furnaces are
primarily categorized by the method by which stock is transported
through the furnace. There are two basic methods: Stock is butted
together to form a stream of material that is pushed through the
furnace. Such furnaces are called pusher type furnaces. Stock is
placed on a moving hearth or supporting structure which transports
the steel through the furnace. Such types include walking beam,
walking hearth, rotary hearth and continuous recirculating bogie
furnaces. The major consideration with respect to furnace energy
use is that the inlet and outlet apertures should be minimal in
size and designed to avoid air infiltration. i) Pusher Type
Furnaces The pusher type furnace is popular in steel industry. It
has relatively low installation and maintenance costs compared to
moving hearth furnaces. The furnace may have a solid hearth, but it
is also possible to push the stock along skids with water-cooled
supports that allow both
5. the top and bottom faces of the stock to be heated. The
design of a typical pusher furnace design is shown schematically in
Figure 4.5. Pusher type furnaces; however, do have some
disadvantages, including: Frequent damage of refractory hearth and
skid marks on material Water cooling energy losses from the skids
and stock supporting structure in top and bottom fired furnaces
have a detrimental effect on energy use; Discharge must be
accompanied by charge Stock sizes and weights and furnace length
are limited by friction and the possibility of stock pile-ups. All
round heating of the stock is not possible. ii) Walking Hearth
Furnaces The walking hearth furnace (Figure.4.6) allows the stock
to be transported through the furnace in discrete steps. Such
furnaces have several attractive features, including: simplicity of
design, ease of construction, ability to cater for different stock
sizes (within limits), negligible water cooling energy losses and
minimal physical marking of the stock. The main disadvantage of
walking hearth furnaces is that the bottom face of the stock cannot
be heated. This can be alleviated to some extent by maintaining
large spaces between pieces of stock. Small spaces between the
individual stock pieces limits the heating of the side faces and
increases the potential for unacceptable temperature differences
within the stock at discharge. Consequently, the stock residence
time may be long, possibly several hours; this may have an adverse
effect on furnace flexibility and the yield may be affected by
scaling. iii) Rotary hearth furnace The rotary hearth furnace
(Figure 4.7) has tended to supersede the recirculating bogie type.
The heating and cooling effects introduced by the bogies are
eliminated, so heat storage losses are less. The rotary hearth has,
however a more complex design with an annular shape and revolving
hearth. iv) Continuous Recirculating Bogie type Furnaces These
types of moving hearth type furnaces tend to be used for compact
stock of variable size and geometry. In bogie furnaces (Figure
4.8), the stock is placed on a bogie with a refractory hearth,
which travels through the furnace with others in the form of a
train. The entire furnace length is always occupied by bogies.
Bogie furnaces tend to be long and narrow and to suffer from
problems arising from inadequate sealing of the gap between the
bogies and furnace shell, difficulties in removing scale, and
difficulties in firing across a narrow hearth width. v) Walking
Beam Furnaces The walking beam furnace (Figure 4.9) overcomes many
of the problems of pusher furnaces and permits heating of the
bottom face of the stock. This allows shorter stock heating times
and furnace lengths and thus better control of heating rates,
uniform stock discharge temperatures
6. and operational flexibility. In common with top and bottom
fired pusher furnaces, however, much of the furnace is below the
level of the mill; this may be a constraint in some applications.
Electric Furnace Nowadays an electric furnace is taking major role
both for domestic and industrial application. A chamber heated by
electric current is known as Electric Furnace. Electric furnaces
are cheaper than oil fired furnaces and gas fired furnaces.
Operating Principle The source of heat is a continuous electric arc
that is formed between the electrodes and the charged metal (Fig.
1). Temperatures as high as 1925C (3500F) are generated in this
type of furnace. There are usually three graphite electrodes, and
they can be as large as 750 mm (30 in.) in diameter and 1.5 to 2.5
m (5 to 8 ft.) in length. Their height in the furnace can be
adjusted in response to the amount of metal present and the amount
of wear of the electrodes. 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. Electric furnace capacities range from 60 to 90 tons
of steel per day. For smaller quantities, electric furnaces can be
of induction type. The metal is placed in a crucible, a large pot
made of refractory material and surrounded with a copper coil
through which alternating current is passed. The induced current in
the charge melts the metals. These furnaces are also used for
re-melting metal for casting. Fig. 1 Schematic illustration of
types of Electric Furnaces: (a) direct arc, (b) indirect arc , and
(c) induction.
7. Material of Construction To make Electric Furnace refractory
bricks, heating elements, compensating cables, thermocouples,
temperature indicator-cum-controller etc. are required. Refractory
Bricks: Best quality refractory bricks must be used in the furnaces
either for coil holding or for insulation purpose, now a days, also
ceramic bricks or ceramic blankets are used for insulation and this
is also found to be the best insulation material for furnace.
Resistance temperature of insulation material should be more than
working temperature of furnace. Heating Elements: The main function
of heating elements is to convert electricity to heat. Nichrome
80/20 and also Kanthal A1 found to be the best heating elements
within continuous working temperature of 1050 to 1100C; available
both strip, ribbon and also wire form. Another heating element is
Silicon Carbide. Silicon Carbide heating elements is also a best
heating element. Silicon Carbide works continuously at a high
temperature up to 1450C & no support require this heating
element. Common Maximum Temperature of Heating Elements: Kanthal A1
max 1400C Kanthal AF max 1300C Silicon Carbide max range between
1300 to 1500C Thermocouples: Thermocouples are pair of dissimilar
material wired and joined at least one end. The function of any
thermocouple wire is to convert heat or cool to millivolt. In 1821,
the German-Estonian Physicist John Allen discovered that when any
conductor such as metal is subjected to a thermal gradient, it will
generate a Voltage. Compensating Cables: It is also very important
that the right compensating cable is used between the controller
and the thermocouple probe. Every different type of thermocouple
has its own compensating cables. Compensating cable use the actual
thermocouple materials but in cheaper forms. When connecting a
thermocouple to temperature controller through compensating cable
most important to know what is positive and what is negative wire
of compensating cable. Continuous Working Temperature Range for
Most Common Use Thermocouples: Type-K (Cromel - Alumel) Temperature
Range 0 to +1200C Type-J Temperature range 0 to +750C. Type-R
Pt/Pt-Rh13% Temperature range 0 to +1600C Type-B Used Platinum and
Rhodium contain each conductor one 30% rhodium and another 6%
rhodium +200 to 1200C Type-S Suitable for higher temperature range
between 0 to 1600C Type-T Copper and Constantant, both conductors
are non-magnetic (-)185 to +300C Type-E Cromel and Constantan, both
conductors are non-magnetic 0 to +800C
8. RTDs (Resistance Temperature Detectors) used in widely
industrial sector for its more accuracy within its range (-)200 to
+500C Temperature Controllers: To control the temperature of any
furnace, a temperature controller is inevitable. It may be blind
temperature controller or Digital Temperature
indicator-cum-controller. Price Quotation IV. COMPONENTS &
PARTS All furnaces have the following components as shown in Figure
2: A refractory chamber constructed of insulating materials for
retaining heat at the high operating temperatures. A hearth to
support or carry the steel. This can consist of refractory
materials or an arrangement of metallic supports that may be
water-cooled. Burners that use liquid or gaseous fuels to raise and
maintain the temperature in the chamber. Coal or electricity can be
used in reheating furnaces. Chimney to remove the combustion
exhausts gases from the chamber. Charging and discharging doors
through which the chamber is loaded and unloaded. Loading and
unloading equipment include roller tables, conveyors, charging
machines and furnace pushers.
9. Figure 2: Typical Furnace Components V. APPLICATION AND USES
Furnaces are primarily used to heat treat metals. High temperatures
soften, melt, and annealthe 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 1200F 1200-1600F 1500-1600F up to 2300F up
to 2300F 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. Semiconductor ingots
are grown within furnaces.
10. KILNS I. GENERAL DESCRIPTION A kiln is a thermally
insulated chamber, a type of oventhat produces temperatures
sufficient to complete some process, such as hardening, drying, or
chemical changes. Various industries and trades use kilns to harden
objects made from clay into pottery, bricks etc. Various industries
use rotary kilns for pyroprocessingto calcinate ores, produce
cement, lime, and many other materials. A rotary kiln is a
cylindrical steel tube lined with insulating brick. The large ones
can be as long as 760 feet with a diameter of 25 feet. The kiln
turns on a horizontal axis at an angle of 2 to 3 percent to the
horizontal. II. GENERAL FUNCTION Charge, or material to be heated,
enters tunnel kilns on trays or trucks at one end, contacts the
gas, and exits at the other end. The trays or trucks move on tracks
or monorails. Heating is provided by reheat coils, and large
propeller-type fans circulate the combustion gases. III. TYPES /
CLASSIFICATION / CLASSES 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. The final characteristics are determined by the
composition and preparation of the clay body, by the temperature at
which it is fired, and by the glazes that may be used. Although
modern kilns often have sophisticated electrical systems to control
the firing temperatures, pyrometric devices are also frequently
used. Types of ceramic kiln In the broadest terms, there are two
types of kiln, both sharing the same basic characteristics of being
an insulated box with controlled inner temperature and atmosphere.
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.
11. Kilns in this type include: Clamp kiln Skove kiln Scotch
kiln Down-Draft kiln 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. A continuous kiln is
energy-efficient, because heat given off during cooling is recycled
to pre-heat the incoming ware. In some designs, the ware is left in
one place, while the heating zone moves across it. Kilns in this
type include: Hoffman kiln Bulls Trench kiln Habla (Zig-Zag) kiln A
special type of kiln, common in tableware and tile manufacture, is
the roller-hearth kiln, in which ware placed on batts is carried
through the kiln on rollers. Anagama kiln - the Asian anagama kiln
has been used since medieval times and is considered the oldest
style of production kiln, brought to Japan from China via Korea in
the 5th century. This kiln usually consists of one long firing
chamber, pierced with smaller ware stacking ports on one side, with
a firebox at one end and a flue at the other. Firing time can vary
from one day to several weeks. Traditional anagama kilns are also
built on a slope to allow for a better draft. Bottle kiln - a type
of intermittent kiln, usually coal-fired, formerly used in the
firing of pottery; such a kiln was surrounded by a tall brick hovel
or cone, of typical bottle shape. Catenary arch kiln, typically
used for the firing of pottery using salt, these by their form (a
catenary arch) tend to retain their shape over repeated heating and
cooling cycles, whereas other types require extensive metalwork
supports. Electric kilns - kilns operated by electricity were
developed in the 20th century, primarily for smaller scale use such
as in schools, universities, and hobby centers. The atmosphere in
most designs of electric kiln is rich in oxygen, as there is no
open flame to consume oxygen molecules. However, reducing
conditions can be created with appropriate gas input, or by using
saggars in a particular way. Feller kiln brought contemporary
design to wood firing by re-using un-burnt gas from the chimney to
heat intake air before it enters the firebox. This leads to an even
shorter firing cycle and less wood consumption. This design
requires external ventilation to prevent the in-
12. chimney radiator from melting, being typically in metal.
The result is a very efficient wood kiln firing one cubic meter of
ceramics with one cubic meter of wood. Microwave assisted firing -
this technique combine microwave energy with more conventional
energy sources, such as radiant gas or electric heating, to process
ceramic materials to the required high temperatures.
Microwave-assisted firing offers significant economic benefits.
Noborigama kiln - the Noborigama is an evolution from Anagama
design as a multi- chamber kiln, usually built on a slope, where
wood is stacked from the front firebox at first, then only through
the side-stoking holes with the benefit of having air heated up to
600 C from the front firebox, enabling more efficient firings. The
Svres kiln was invented in Svres, France and efficiently generated
high- temperatures (1280 C) to produce water-proof ceramic bodies
and easy to obtain glazes. It features a down-draft design that
produces high temperature in shorter time, even with wood-firing.
Top-hat kiln - an intermittent kiln of a type sometimes used to
fire pottery. The ware is set on a refractory hearth, or plinth,
over which a box-shaped cover is lowered. Wood Drying Kiln A
variety of wood drying kiln technologies exist today: conventional,
dehumidification, solar, vacuum and radio frequency. Conventional
wood dry kilns (Rasmussen, 1988) are either package-type
(sideloader) or track-type (tram) construction. Most hardwoodlumber
kilns are sideloader kilns in which fork trucks are used to load
lumber packages into the kiln. Most softwood lumber kilns are track
types in which lumber packages are loaded on kiln/track cars for
loading the kiln. Modern high-temperature, high-air-velocity
conventional kilns can typically dry 1-inch-thick (25 mm) green
lumber in 10 hours down to a moisture content of 18%. However,
1-inch- thick green Red Oak requires about 28 days drying down to a
moisture content of 8%. Heat is typically introduced via steam
running through fin/tube heat exchangers controlled by on/off
pneumatic valves. Less common are proportional pneumatic valves or
even various electrical actuators. Humidity is removed via a system
of vents, the specific layout of which is usually particular to a
given manufacturer. In general, cool dry air is introduced at one
end of the kiln while warm moist air is expelled at the other.
Hardwood conventional kilns also require the introduction of
humidity via either steam spray or cold water misting systems to
keep the relative humidity inside the kiln from dropping too low
during the drying cycle. Fan directions are typically reversed
periodically to ensure even drying of larger kiln charges.
Dehumidification kilns are similar to other kilns in basic
construction. Drying times are usually comparable. Heat comes
primarily from an integral dehumidification unit that also removes
humidity. Auxiliary heat is often provided early in the schedule,
where the heat required may exceed the heat generated by the
dehumidification unit.
13. Solar kilns are conventional kilns, typically built by
hobbyists to keep initial investment costs low. Heat is provided
via solar radiation, while internal air circulation is typically
passive. Rotary Kilns A Rotary kiln 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 pellets Operating Principle 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. The fuel for this may be gas, oil or
pulverized coal. Material &Construction The basic components of
a rotary kiln are the shell, the refractory lining, support tyres
and rollers, drive gear and internal heat exchangers. Kiln Shell
This is made from rolled mild steel plate, usually between 15 and
30 mm thick, welded to form a cylinder which may be up to 230 m in
length and up to 6 m in diameter. This will be usually situated on
an east/west axis to prevent eddy currents. Upper limits on
diameter are set by the tendency of the shell to deform under its
own weight to an oval cross section, with consequent flexure during
rotation. Length is not necessarily limited, but it becomes
difficult to cope with changes in length on heating and cooling
(typically around 0.1 to 0.5% of the length) if the kiln is very
long. This is cylindrical.
14. Refractory Lining The purpose of the refractory lining is
to insulate the steel shell from the high temperatures inside the
kiln, and to protect it from the corrosive properties of the
process material. It may consist of refractory bricks or cast
refractory concrete, or may be absent in zones of the kiln that are
below around 250C. The refractory selected depends upon the
temperature inside the kiln and the chemical nature of the material
being processed. In some processes, such as cement, the refractory
life is prolonged by maintaining a coating of the processed
material on the refractory surface. The thickness of the lining is
generally in the range 80 to 300 mm. A typical refractory will be
capable of maintaining a temperature drop of 1000C or more between
its hot and cold faces. The shell temperature needs to be
maintained below around 350C in order to protect the steel from
damage, and continuous infrared scanners are used to give early
warning of "hot-spots" indicative of refractory failure. Tyres&
Rollers Tyres, sometimes called riding rings, usually consist of a
single annular steel casting, machined to a smooth cylindrical
surface, which attach loosely to the kiln shell through a variety
of "chair" arrangements. These require some ingenuity of design,
since the tyre must fit the shell snugly, but also allow thermal
movement. The tyre rides on pairs of steel rollers, also machined
to a smooth cylindrical surface, and set about half a kiln-diameter
apart. The rollers must support the kiln, and allow rotation that
is as nearly frictionless as possible. A well-engineered kiln, when
the power is cut off, will swing pendulum-like many times before
coming to rest. The mass of a typical 6 x 60 m kiln, including
refractories and feed, is around 1100 tonnes, and would be carried
on three tyres and sets of rollers, spaced along the length of the
kiln. The longest kilns may have 8 sets of rollers, while very
short kilns may have only two. Kilns usually rotate at 0.5 to 2
rpm, but sometimes as fast as 5 rpm. The Kilns of most modern
cement plants are running at 4 to 5 rpm. The bearings of the
rollers must be capable of withstanding the large static and live
loads involved, and must be carefully protected from the heat of
the kiln and the ingress of dust. In addition to support rollers,
there are usually upper and lower "retaining (or thrust) rollers"
bearing against the side of tyres, that prevent the kiln from
slipping off the support rollers. Friction between tyre and rollers
causes concave, convex or conical wear on both surfaces of tyre and
rollers. This wear deforms the cylindrical shape of these units and
causes vibration, shell deformation, more power consumption and if
not resurfaced these problems takes the level up to changing the
shell and tyre which takes more budget and shut down time. Drive
Gear The kiln is usually turned by means of a single Girth Gear
surrounding a cooler part of the kiln tube, but sometimes it is
turned by driven rollers. The gear is connected through a gear
train to a variable-speed electric motor. This must have high
starting torque in order to start the kiln with a large eccentric
load. A 6 x 60 m kiln requires around 800 kW to turn at 3 rpm. The
speed of material flow through the kiln is proportional to rotation
speed, and so a variable speed drive is needed in order to control
this. When driving through rollers,
15. hydraulic drives may be used. These have the advantage of
developing extremely high torque. In many processes, it is
dangerous to allow a hot kiln to stand still if the drive power
fails. Temperature differences between the top and bottom of the
kiln may cause the kiln to warp, and refractory is damaged. It is
therefore normal to provide an auxiliary drive for use during power
cuts. This may be a small electric motor with an independent power
supply, or a diesel engine. This turns the kiln very slowly, but
enough to prevent damage. Internal Heat Exchanger Heat exchange in
a rotary kiln may be by conduction, convection and radiation, in
descending order of efficiency. In low-temperature processes, and
in the cooler parts of long kilns lacking preheaters, the kiln is
often furnished with internal heat exchangers to encourage heat
exchange between the gas and the feed. These may consist of scoops
or "lifters" that cascade the feed through the gas stream, or may
be metallic inserts that heat up in the upper part of the kiln, and
impart the heat to the feed as they dip below the feed surface as
the kiln rotates. The latter are favoured where lifters would cause
excessive dust pick-up. The most common heat exchanger consists of
chains hanging in curtains across the gas stream. Price
Quotation
16. IV. COMPONENTS & PARTS V. APPLICATION & USES 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. 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