cement mill drives

7/30/2019 Cement Mill Drives

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Process Description

Crusher Drives

Compressor Drives

Waste Gas Fan Drives

Rotary Kiln Drives

Cement Mill Drives


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The raw materials for producing cement contain lime and silica as main

components and ferric oxide as fluxing components.

The limestone mixed from the quarries is crushed and transported to theplant by dumper wagons.

The crushed limestone together with the required amounts of corrective

additives like clay bauxite, iron ore etc. is ground in grinding mills.

The fine dry powder coming out is homogenized by passage of air from

bottom and through the medium.


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It is then fed into the kiln for producing cement clinker at high

temperatures.

The clinker coming out of the kiln is air cooled in special types of coolers

and then transported to the storage.

After aging in storage for atleast three days the clinker, mixed with theright amounts of gypsum is fed to the cement grinding mills and ground

to required fineness.

The cement is stored in silos, drawn for packing in gunny bags and

dispatched by wagons to the dealers.


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Block Diagram Dry Process


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The driving motors used in the cement industry can be broadly classified as

follows:

Cement Mill drives Kiln drives

Crusher drives

Waste Gas Fan drives

Compressor drives


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Function:Mining blasting limestone and clay. Rock fragments transported. Cement is

typically produced from limestone, clay, sand and iron ore. Before the raw

material is ground, high-powered stationary or mobile crushers reduce the

material to a processable size. The drive systems used not only have to handle

the significant mass moment of inertia of the crusher rotor during startup,

but also be able to withstand continuously high torque peaks.

Crusher chunks obtained this way are approximately 1.5 inches in size.

Requirements of crusher motors: The starting torque should be limited to 160% of the full load torque.


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Breakdown torque should be 200 to 250% of full load torque.

Overload capacity of 15% for 15s and 20% for 10s taking intoconsideration the adverse loading conditions encountered in practice.

The rotor should be able to withstand locked rotor current, without any

external resistance, for one minute. (Important as entrapped boulders may

jam the crushers.)

The motors that meet the above requirements are:

1. Slip ring induction motor with rotor resistance starters and speed control

are suited as crusher drives. A dc chopper is used to control the resistance.

2. They are available in multiple jaw, conical and roll crusher variety.


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Process


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The air compressor motors have ratings in the range 300 kW to 450 kW.

The compressors when started on load require high starting torque and

equipment for limiting the starting current. TEFC enclosed motors with

speeds varying between 1000 and 750 rpm are employed.

Generally, the compressors are started on no load. Squirrel cage motors

with reduced voltage starting are preferred.

ac drives with speed control achieved by static frequency converter may

also be used as they provide better starting conditions. The operation of compressor is a variable torque operation. At low speeds

torque required is very low and simple control of V/f is sufficient.


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Function:The preheater tower has a series of cyclone chambers through which the raw

material passes on its way to the kiln. To save energy, modern cement plants

preheat the material before it enters the kiln. The induced draft (ID) fans

extract the hot gases from the kiln, heating the raw material as it swirls

through the cyclones.

Requirements of crusher motors: The starting torque should be limited to 120% of the full load torque.

Breakdown torque should be 200 to 250% of full load torque.

Speed variation should be between 1000 and 1500 rpm.


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1. The motors used are of the slip-ring type.

2. The cast iron grid resistance controllers are normally used for starting and

controlling the speed of these drives, although subsynchronous converter

cascade is also used.3. As the motors are located outdoor or in semi-outdoor locations totally

enclosed motors of TEFC are employed. The sliprings and brush gears are

totally enclosed and kept external to the motor enclosures for ease in

maintenance.

4. Due to higher frequency and finer control, thyristorized dc drives are

replacing slipring motors in fan drives too.


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Function:Heating the rawmix to sintering temperature (up to 1450 C) to produce

cement clinkers.There are different types of rotary kilns depending on whether the cement is

manufactured by means of wet or dry process. But, in general, they are

tubular, slightly tilted from the horizontal and have a ring gear fitted around

them, which engages with one or two pinions. Each pinion is driven by a

variable speed motor.


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Clinker Production


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Process:The rotary kiln consists of a tube made from steel plate, and lined with

firebrick. The tube slopes slightly (14) and slowly rotates on its axis at

between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end,

and the rotation of the kiln causes it gradually to move downhill to the otherend of the kiln. At the other end fuel, in the form of gas, oil, or pulverized

solid fuel, is blown in through the "burner pipe", producing a large concentric

flame in the lower part of the kiln tube.


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As material moves under the flame, it reaches its peak temperature, before

dropping out of the kiln tube into the cooler. Air is drawn first through thecooler and then through the kiln for combustion of the fuel. In the cooler the

air is heated by the cooling clinker, so that it may be 400 to 800 C before it

enters the kiln, thus causing intense and rapid combustion of the fuel.Requirements of kiln motors: Power requirement is very high.

Speed ratio control is 1:10.

Starting torque should be in the range of 200 to 250% of full load torque.

The acceleration of the drive should be completed in about 5 seconds.

For small periods an overload capacity of 200-250% may be required.


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ac commutator motors and Ward Leonard controlled dc motors.

Due to requirements of higher output ratings and of speed range in excess

of 1:2, higher capital cost, lower efficiencies and greater maintenance

problems; the commutator motors and Ward-Leonard drives have beensuperseded by dc motors with static supply.

For higher power ratings, cycloconverter-fed synchronous motor is

deployed as the drive motor.

To cope up with increasing kiln capacities, twin motor dc drives are used.

Two dc motors with separate pinions drive the same gear wheel at the

periphery of the kiln drum.


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Function:A cement mill is a drive used to grind the hard, nodular clinker from

the cement kiln into the fine grey powder that is cement. Most cement iscurrently ground in ball mills and also vertical roller mills which are more

effective than ball mills.

Process:Raw materials are fed to the mill in right proportion with the help of

weighfeeders. The output of the ball mill is taken to a separator, which

separates out unacceptable coarse cement. The acceptable product is fed to the

cyclone which separates the air dust from the cement.


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1. Weighfeeders:A separate drive motor (dc series motor) for each raw material is used to

drive the feed table under which strain gauges are placed for measurement

of weight of material. The strain gauges are deformed by the load

consequently modifying the resistances. This unbalances the Wheatstone

Bridge giving an output voltage which changes the speed of the drive.

capacity(%) = loading(%) * speed(%)

If the loading increases, then speed will decrease. In modern cement mills,a microprocessor-based control scheme is used.


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2. Mill Drives:Requirements of mill drives are: High starting torque. The starting current must be limited to a maximum

of two times the full load value to minimize voltage dips.

An overload capacity of 50% for 1 min occurring for 4 times an hour.


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Three starts from cold conditions and two consecutive starts from hot

conditions per hour against full load.

The drive system used keeping in mind the above requirements are:

1. 3 phase slip-ring induction motors of 6.6 kV; though salient pole

synchronous motor can also be employed.

2. Liquid resistance starters are employed to start the motor and bring it

upto full speed.

3. Gear boxes are also attached in order to get the desired mill speed of about

15 rpm.

4. In order to improve the power factor of the line current drawn, high

voltage capacitors of adequate reliability and automatic capacitor control

switchgear and circuit breakers are used.


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TWIN Drives: Due to large ratings (above 300 kW) required for thedrive and due to the limitations of the availability of large size gear boxes

and motors, twin drives are employed. The two motors have identical

capacity as also their liquid resistance starters.

GEARLESS Drives: The rotor is shrunk onto the mill and the air gapbetween the rotor and the overlapping stator is maintained by levitation

using sophisticated electronic closed loop control. Variable frequency drives

are obtained by the use of cycloconverters and/or dc link converters. This

arrangement completely dispenses with the gearbox which is generally the

source of maintenance problems.


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3. Separator:A converter-fed dc motor/variable speed slip ring induction motor can be

employed. The speed is controlled between 150-300 rpm to adjust the

fineness of the cement product. The speed range is 1:10 by armature voltage

control at constant torque.4. Blower Drive:Squirrel cage induction motors with start/stop push buttons, overload relay,

fuses and contactors are used for all blower drives.5. Cyclone:The drive used is usually a slip-ring induction motor.


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Thank You!

cement mill drives

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