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Rotary Kilns

Rotary Kilns
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Rotary Kilns are used to heat solids to the point where a required chemical reaction(s) takes place. The Rotary Kiln is basically a rotating inclined cylinder, solids retention time in the kiln is an important factor and is set by proper selection of diameter, length, speed, slope and the internal design.

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.

PRINCIPLE

Such a flame is projected from a burner-pipe. The fuel for this may be gas, oil or pulverized coal. Solids retention time in the kiln is an important factor and is set by proper selection of diameter, length, speed, slope and the internal design.

CONSTRUCTION CONSTRUCTION :
Rotary Kilns are normally direct fired and lined with refractory material to protect the steel shell, reduce radiation losses and transfer heat to the material. A typical Rotary kiln is cylindrical shell, slightly inclined from the horizontal and supported on two or more tyres, which in turn rotate on rollers. One of the tyre is fixed axially by a set of thrust rollers to care of the thrust load during kiln movement. The drum assembly along with the tyres are rotated by a girth gear/ pinion arrangement. Shell diameter and length are generally designed to suit specific products.Heat Transfer in Rotary kiln : All kilns are heat exchangers and the heat is transferred in a variety of ways by radiation, convection and conduction.The most effective way of transferring heat is by gas radiation from the hot flame and a smaller amount of transfer from refractory. The hot bricks transfer by radiation to material and also by conduction to the charge



SEALING ARRANGEMENT

SEALING ARRANGEMENT :
A seal is essential at each end to prevent ambient air from entering the process. Air leakage can have varying effects on different process. Sometimes it reduces efficiency and sometimes it can create unsafe or unstable conditions. To maintain proper control of the volume and temperature of gases flowing through the kiln it is necessary to limit the leakage of the cold air in the kiln particularly where the rotating shell enters the feed and the discharge end hoods. Seal rings are provided for this purpose. Various types of seal rings are used which include spring loaded types, where two finished metal surfaces are in contact under spring pressure. The sealing segments require regular adjustment and replacement.


Refractory :

Because of high temperature and abrasive material liner is required and call it as refractory. Refractory is installed as solid block to fit in to different areas tightly in round formation from sliding or falling out. There are many types of bricks in different composition and have different properties such as insulation value, maximum temperature and resistance to wear. Areas where brick can not be used liquid refractor called castables are used with anchor support. Kiln can also have insulation material under the brick and this extra layer helps to reduce shell losses.

BEARINGS & DRIVE COMPONENTS Bearings & Drive Components :
The most common drive components are gear reducers. Gear reducers are used when reduced speed or increased torque is required. Gear reducers can be designed differently for applications. Rotary Kilns are normally gear driven through girth gear mounted over the shell on gear support base, assembled on a section of the shell away from the shell end where high temperatures are encountered. To compensate the thermal expansion the girth gear is mounted on tangential spring plates attached to the shell. The driving pinion is mounted on the output shaft of a worm / helical gear unit.

Bearings are in every moving piece of equipment. Bearings are also uniquely designed for there application. The most common bearings are the anti-friction or roller bearings. These bearings have become standardized and perform excellent in almost any condition. Anti-friction bearings do not work well with heavy loads. Different kind of bearing are used on the kiln where high loads are seen.



REFRACTORY LINING 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. The refractory selected depends upon the temperature inside the kiln and the chemical nature of the material being processed. Mostly refractory is installed as solid block to fit in to different areas tightly in round formation from sliding or falling out. There are many types of bricks in different composition and have different properties such as insulation value, maximum temperature and resistance to wear. Areas where brick cannot be used liquid refractor called castables are used with anchor support. Also provided insulation material under the brick and this extra layer helps to reduce the heat transfer to the shell. A typical refractory will be capable of maintaining a temperature drop of 1000°C or more between its hot and cold faces. The shell temperature needs to be maintained below around 350°C 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.

FIRING SYSTEM FIRING SYSTEM :
The hot gases may be generated in an external furnace, Hot Air Generator (HAG), or may be generated by a flame inside the kiln. Such a flame is projected from a burner-pipe. The fuel for this may be gas, oil or pulverized coal. The fuel could be mixed with air before or in the burner. The flame temperature is a factor of fuel type, fuel/air ratio and other burning conditions.




SCRUBBER & DUST COLLECTION SCRUBBER & DUST COLLECTION :
Exhaust gases from an oil fired kiln contain certain proportion of very fine dust, entrained in the air stream while in contact with materials being calcined and there may be undesirable constituents such as sulfur dioxide or hydrogen chloride. Equipment is installed to scrub these out before the exhaust gases pass to atmosphere.

A Dust Collector of high efficiency cyclone type is used to trap the fine dust. Very fine particles which escape the dry collector may be recovered by means of a wet collector in the form of a sludge or alternatively by means of Bag Filter.

ROTARY COOLER ROTARY COOLER :
ROTARY COOLER is a part of Rotary kiln system used to lower the temperature of the hot products discharged from the kiln to a temperature approaching ambient. The Rotary Cooler is installed beneath the Rotary Kiln and interconnected with the kiln discharge hood. The construction is similar to Rotary kiln with a cylindrical shell inclined to the horizontal with tyres, support rollers, feed hood and driven by girth gear & pinion. A part of the cooler shell is castable lined to take care of the shell from the hot discharge.

ROTARY COOLER ROTARY COOLER :
ROTARY COOLER is a part of Rotary kiln system used to lower the temperature of the hot products discharged from the kiln to a temperature approaching ambient. The Rotary Cooler is installed beneath the Rotary Kiln and interconnected with the kiln discharge hood. The construction is similar to Rotary kiln with a cylindrical shell inclined to the horizontal with tyres, support rollers, feed hood and driven by girth gear & pinion. A part of the cooler shell is castable lined to take care of the shell from the hot discharge.

ROTARY COOLER The Rotary Cooler operates in counter-flow, the hot gases being substituted by ambient or chilled air, which is in direct contact with the hot material. For high temperature applications the material is cooled to a temperature approaching ambient by continuous water spray over the shell and operates on the principle of heat transfer through the rotating cylinder wall which is continuously irrigated by a series of external water sprays mounted over the unit. The water is partially evaporated as steam while the excess is collected from the bottom of the unit and re-circulated. The material thus cooled is discharged from the other end of the shell.

ROTARY CARBONISER (KILN) FOR ACTIVATED CARBON UNITS :

ROTARY CARBONISER


ROTARY CARBONISER Rotary carbonisers (kilns) are used in Activated carbon units for the steam activation of carbon granules (charcoal) at a controlled temperature. The kiln is unique in design with a spiral wound cooling coil arrangement fixed over the discharge end of the shell to lower the product temperature during the discharge through cooling coil. The system is complete with moveable type feed hood for easy maintenance, rubbing type, spring loaded sealing arrangement at both ends of the carboniser to seal and to control the atmospheric air entry in to the burning zone through adjustable damper valve provided on the feed hood, Steel casted girth gear with back-up plate to take-up the expansion during operation, cone at discharge end for free flow of flue gas through flue gas pipe etc.

APPLICATION :
Rotary kilns are used for the calcinations of Titanium dioxide, Illeminate, kaolin, Barium Carbonate, Magnesium Carbonate, Silica Gel, Lime, Activated Carbon, Vermiculite, Barium Sulphide, Magnesium Oxide, Petroleum Coke, Clay, Gypsum, Alumina etc.





Rotary Dryers

Rotary Dryers
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Rotary Dryers are used to remove moisture from material using hot gases to heat the feed material and to evaporate the water. Rotary Dryers are suitable for drying wide range of materials continuously because of its ability to process material having considerable variation in size and composition

Construction :
Rotary Dryers are slightly inclined cylinder shell supported by two nos. riding rings(tyres) running on a set of support Rollers. Fabricated in carbon, stainless steel and carbon+Ebonite lined with flights or lifters are welded or bolted internally to produce cascade of particles falling through hot gas stream. The mechanical lifting of the material allows to dry materials from filter cake to coarse minerals also helps in breaking-up lumps for uniform drying. The length of the drum may range from 4 to 10 times its diameter. Normal dryers the length will be 10 times its diameter and usually operate with 12 to 15% of their volume filled with the material.

ROTARY DRYER CONSTRUCTION End closer seals :
Both the ends of the drum are closed by steel fabricated hoods with air inlet/outlet chute with suitable seals in between the hood and the dryer drum. If slight ingress of air is acceptable simple felt seals can be used, bolted to a stationary end plate and positioned in between to seal rings. If air is unacceptable spring loaded friction seals are provided, where two finished metal surfaces are in contact under spring pressure. The sealing segments require regular adjustment and replacement.

LIFTERS Operation :
Dying is achieved by passing the material being processed through the horizontal rotating drum which also passes a current of hot air. Lifters or flights fitted internal surface of the drum lift and cascade the material through the hot air stream. The hot air steam can be directed through the cylinder either in the same direction as material (co-current) or in the opposite direction (counter current). Counter current applications are more efficient in terms of energy input but give rise to high product temperature, with the associated handling problem.

Diagram: The diagram above illustrates how flights create the curtain in a rotary dryer to maximize heat transfer

Air Handling System :
In Rotary Dryer the flow of air is induced by a centrifugal fan located up stream of the air heater with filter at fan inlet. The exhaust air is removed either by a second centrifugal fan at the outlet or direct at outlet end of the dryer and through dust collector.

The exhaust gas stream from any drying operation inevitable include a significant amount of fine particles and these fine particles may be collected by passing the gases through cyclones or bag filter or wet scrubber or any combination on any or all of these, depending upon the nature and quantity of dust involved.

Air Heater :
The air may be heated indirectly by means of thermal oil or electricity-heated or steam heat exchanger.

Process Control :
For any system there is relationship between the product moisture content and exhaust air temperature. Therefore product moisture can be controlled through the control of exhaust air temperature by regulating the flow of heat by means of temperature controller with a thermocouple located in the exhaust air duct. The optimum performance can be achieved if the dryer operates as per design conditions and with constant evaporative load. Any variation in evaporation load due to variation in feed rate or moisture content results in a corresponding variation in exhaust temperature which in turn increases or decreases air heater output and it raises or lowers the inlet air temperature. Thus evaporative load may be controlled by measurement of inlet air temperature using controller with temperature probe located in the inlet air duct and output signal to a variable rate feeder, which generally provides control of the drying system.

MATERIAL OF CONSTRUCTION:
Carbon Steel, Stainless Steel & MS + Rubber lined.

PRINCIPLE OF OPERATION PRINCIPLE OF OPERATION :
The principle of operation is based on showering or cascading the wet material through a hot stream, flowing either co-current or counter-current to the solids. The hot gas induces the evaporation of the moisture. The heat lost to the material and evaporation of water vapour reduces the gas temperature rapidly, such that it leaves the dryer at a comparatively low temperature.

The efficiency of the dryer is largely dependent on the differential on the differential between the inlet and exhaust temperatures, although the heat transfer rate is also influenced by the relationship between the design of flights and the speed of rotation. However, irrespective of the gas and material temperature, the drying or residence time may be critical, as this is governed by the rate of diffusion of water from the core to the surface of the material.

Rotary dryer adopts the direct flow type of heating with heat source from the heat exchanger. The material to be dried enters the rotating dryer shell, in a regulated way through a screw feeder and through a feed chute attached with the feed hopper, is mounted over the stationary feed hood. As the dryer is installed in slope the material flows to the back end under gravity and rotation force and is lifted by lifters repeatedly and thrown down, which make the material form even curtain and exchange heat sufficiently with the inner heat flow and repeated scattering. The water separated by the hot air flow is changed in to vapour and discharged in to the atmosphere either direct or through a dust collection system. The dried material is discharged from the other end of the shell through a stationary discharge hood.

CO-CURRENT & COUNTER CURRENT FLOW
Depend on character of material, the gas stream moving toward the discharge end from the feed end known as co-current flow or toward the feed end from the discharge end known as counter-current flow.

CO-CURRENT DRYER CO-CURRENT DRYER
Co-current dryers are particularly suitable for drying materials containing high moisture content, which are heat sensitive or have a tendency to stick or cake.

The wet material is in contact with the gas at its highest temperature, which rapidly evaporates surface moisture. The initial heat transfer rate is high, causing an immediate and considerable drop in gas temperature, which prevents overheating of the material and the dryer shell.

CO-CURRENT DRYER The final product is in contact with the gas at its lowest temperature, enabling the moisture content to be readily controlled, usually by maintaining the dryer exhaust gas temperature at a pre set value.

CO-CURRENT APPLICATION
Wet feed in contact with the hottest drying gases supplied from an external source, where the heat source is by convection. Suitable for filter cakes, minerals, fertilizers, floatation concentrates, coal/coke, clay, phosphates, animal feeds & sludge.

COUNTER CURRENT DRYER COUNTER CURRENT DRYER
Counter Current dryers are more suitable for materials that must be dried to very low levels of moisture, where the last traces of moisture are difficult to remove, or where an elevated product temperature is desirable. They are also used effectively as combined dryer/pre-heaters. However, since the final product is in contact with the gas at its highest temperature the counter-current dryer is often unsuitable for heat sensitive materials. Although this system can be more efficient, moisture which is to remain in the product is not so readily controlled.

COUNTER-CURRENT APPLICATIONS COUNTER-CURRENT APPLICATIONS
Wet feed in contact with the hottest drying gases supplied from an external source, where the heat source is by convection. Suitable for silica gel, sugar, chemical salts & crystalline products (low moisture range), ammonium nitrate, potassium nitrate, barium nitrate, potassium chloride, ores & minerals, pigments, removal of floatation reagents etc.





DRIVE ARRANGEMENT

DRIVE ARRANGEMENT
The drives of Rotary Dryers are designed for the reliable operation of the dryer under any operating conditions. All drive components are designed for the installed motor torque plus all safety factors and not only for effective torque generated at nominal capacity. The installed reserves and safety factors ensure the extremely flexible operation of the Rotary Dryer, including higher product throughput, without overloading the drive.

END CLOSER SEALS
Both ends of the drum are closed by steel fabricated hoods with air inlet/outlet chute with suitable seals in between the hood and the dryer drum. Normally spring loaded friction seals are provided where two rubbing surfaces are in contact under spring pressure.

AIR FILTER AIR FILTER
It is an essential component of the air intake system of Rotary Dryer and is tasked with trapping any dirt or debris in the air entering the intake system. It is important that foreign material is not allowed to enter the Rotary Dryer.







AIR HEATING SYSTEM AIR HEATING SYSTEM
In co-current and counter-current systems, the material when drying or pre-heating which are unaffected by heat or exposure to a flame, an Oil or Gas Burner is used to fire directly into the drum. For low temperature and heat sensitive applications, indirectly heated air can be supplied via Finned Tube Heat Exchangers and induced by a centrifugal fan located at up steam of the air heater with filter at fan inlet.



OIL OR GAS BURNER
An oil burner is a heating device which burns oils, diesel fuel and other similar fuels. The fuel is atomized into a fine spray usually by forcing it under pressure through a nozzle which gives the resulting flame a specific flow rate, angle of spray and pattern (variations of a cone shape). This spray is usually ignited by an electric spark with the air being forced through around it at the end of a blast tube, by a fan driven by the oil burner motor. The fuel pump is typically driven via a coupling connecting its shaft and motor. Oil burners also include combustion-proving devices to prevent out-of-control combustion - Primary Control; Safety Control; Cad Cell Control; Master Control; Fire-Eye Control are all common names for the 'combustion safety control'.

FINNED TUBE HEAT EXCHANGERS FINNED TUBE HEAT EXCHANGERS
Heat exchanger with finned heating surfaces, so-called finned tube heat exchangers are widely used in Rotary Dryer which offers the possibility of heat transfer between Air and liquids significantly space-saving and is more. Finned tube heat exchangers are designed to transfer heat from clean air and gases with high efficiency on liquids or vapors. In this way the Air can be heated in a closely space.



AIR HANDLING SYSTEM
In Rotary Dryer the flow of air is induced by a centrifugal fan located at upstream of the air heater and the exhaust air is removed either direct at outlet or by a second centrifugal fan at the outlet through Dust Collector.

INDUCED DRAFT FAN INDUCED DRAFT FAN (ID Fan)
Induced draft fan (ID fan) is located between dust collector and chimney of Rotary Dryer. ID fan will take the hot flue gases from Rotary Dryer via dust collector (dust separation system or Fume Extraction system) and will deliver to chimney. ID fan will handle the flue gases i.e. hot air. ID fan will produce the negative pressure in the Rotary Dryer to remove the flue gases from Dryer via Cyclone and bag Filter and to push the flue gases to chimney.

FORCE DRADT FAN (FD Fan)
Force draft (FD fan is used basically for providing the required quantity of hot air to the Rotary Dryer for smooth and uniform combustion of fuel. FD fan will produce the positive pressure inside the Rotary Dryer.

DUST COLLECTION SYSTEM
The choice of dust collection equipment will vary according to the application, but in addition to the range of high efficiency and high capacity Cyclone Separators is used.

CYCLONE SEPARATORS CYCLONE SEPARATORS
In Rotary Dryer application this is the most widely used type of dust collection equipment, in which dust-laden gas from the rotary dryers enters a cylindrical or conical chamber tangentially at one or more points and leaves through a central opening . The dust particles, by virtue of their inertia, will tend to move toward the outside separator wall, from which they are led into a receiver. A cyclone is essentially a settling chamber in which gravitational acceleration is replaced by centrifugal acceleration. At operating conditions in rotary dryers commonly employed, the centrifugal separating force or acceleration may range from 5 times gravity in very large diameter.

APPLICATION
Rotary dryers are suitable for a wide variety of products, ranging from granular, powdered and crystalline materials, through to filter cakes and sludges for the food, chemical and mineral industries.





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