CHAPTER-1

ALUMINIUM SMELTING

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Chapter-1 Aluminium Smelting1.1. INTRODUCTION

Aluminium compounds make up 7.3% of the earth's crust, making it the third most common crustal element and the most common crustal metal on earth.

Aluminium was first produced in 1808. There are three main steps in the process of aluminium production. First is the mining of aluminium ore, most commonly bauxite, referred to as bauxite mining. Second is the refining of

bauxite into aluminium oxide trihydrate (Al2 O3), known as alumina, and third is the electrolytic reduction of alumina into metallic aluminium. The process requires approximately two to three tonnes of bauxite for the production of one tonne of alumina, and in turn, approximately two tonnes of alumina is

required for making one tonne of aluminium.

Aluminium occupies a special place in extractive metallurgy because it can be produced as a high-purity product, enabling its special properties to be

utilized. It has many economically attractive applications in the construction sectors, in the transportation sector, in numerous industrial products, packaging, and containers. The substitution of aluminium for common

materials such as steel, copper, and certain composites can generate large energy savings over the net life of various products. It also reduces the

production of the greenhouse gas, carbon dioxide, particularly in transportation applications because lightweight aluminium-intensive vehicles

will use less fuel than conventional vehicles.In an aluminium smelter, direct current (DC) is fed into a line of electrolytic cells connected in series. These electrolytic cells are the nerve centre of the

process. While the cells (pots) vary in size from one plant to another, the fundamental process is identical and is the only method by which aluminium

is produced industrially. It is named the Hall-Heroult process after its inventors.

Each smelting cell is a large carbon-lined metal container, which is maintained at a temperature of around 960°C and forms the negative

electrode (or cathode). The cell contains an electrolytic bath of molten salt called 'cryolite' (Na3AlF6), into which a powder of aluminium oxide (Al2O3) is fed and becomes dissolved to form a solution. Aluminium fluoride (AlF3) is added to maintain the target bath chemistry. Large carbon blocks, made

from calcined petroleum coke and liquid coal tar pitch, are suspended in the solution; and serve as the positive electrode or anode.

The electrical current passes from the carbon anodes via the bath, containing alumina in solution, to the carbon cathode cell lining. The current then passes to the anode of the next pot in series. As the electrical current passes through the solution, the aluminium oxide is dissociated into molten

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aluminium (Al) and oxygen (O2). The oxygen consumes the carbon (C) in the anode blocks to form carbon dioxide (CO2), which is released.

The electrolytic reaction can be expressed as follows: 2 Al2O3 + 3 C → 4 Al + 3 CO2

1.2. HISTORY

A. Chemical displacementHans Christian Oersted first heated potassium amalgam with aluminum chloride and produced tiny pieces of aluminum in 1825. This was twenty

years after Sir Humphrey Davy first named the metal “aluminum.” Davy was the first to use electrolysis to produce samples of potassium, sodium,

strontium, calcium, barium, magnesium and boron. He tried unsuccessfully for many years to produce aluminum by electrolysis. Even though he could not isolate it he was convinced that it existed and named it anyway. (Later,

in much of the world, the name was changed to "aluminium" to be consistent with other metals. In North America the original spelling is still used.)

Twenty years later Wohler passed aluminum chloride vapor over molten potassium and managed to produce larger globules of aluminum. Each

globule weighed only between 10 and 15 milligrams.It was not until 1854 that a French schoolteacher, Henri Sainte-Claire Deville,

substituted cheaper sodium as the reductant. He managed to prepare a small bar of aluminum. It was so precious that it was displayed next to the

Crown Jewels at the Paris Exposition the next year.The French Emperor, Napoleon III, financed Deville's work on aluminum. He hoped that aluminum could be used to make lightweight armor. Napoleon used aluminum cutlery on special occasions and had an aluminum rattle

made for his young son.The displacement of aluminum from its chloride by a more active metal worked adequately. It was of course expensive because the sodium or

potassium had to be produced by electrolysis. Aluminum chloride also had to be made from naturally occurring aluminum compounds and it is a difficult

compound to handle. It is volatile and tends to absorb water, which interferes with the production of aluminum.

B. ElectrolysisThen, in the 1880s, a young American student, Charles Martin Hall, became

interested in aluminum. He decided to develop a commercial process for extracting aluminum using an electric current. In his first experiments he

electrolyzed solutions of aluminum salts in water. All he managed to produce were the gases hydrogen and oxygen.

He tried electrolyzing molten aluminum oxide. It did not work. The oxide's high melting point prevented its electrolysis. Hall tried many other

substances without success.

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He needed something that would dissolve aluminum oxide producing a solution that could conduct an electric current, but which would not itself be

decomposed.He began a systematic search of different salts for this purpose. In February 1886 Hall passed a direct current through a solution of alumina dissolved in cryolite (Na3 AIF6) in a carbon crucible. After several hours he allowed the contents to solidify. When he broke up the solid he found several small

buttons of aluminum.Within a few weeks Paul Heroult in France had independently produced

aluminum by an almost identical process. Both Hall and Heroult were only 22 years old. Two years later Karl Bayer developed his process for the

extraction of pure aluminum oxide from bauxite.In 1888 Hall set up a company to manufacture aluminum. That company

later became known as the Aluminum Company of America or, Alcoa.The new process made aluminum production so much easier and cheaper

that by 1891 the last Deville chemical reduction plant had closed. From only a few tons annually, five years earlier, production reached well over 300 tons

in 1891.Although Hall and Heroult are credited with the development of the

electrolytic extraction of aluminum, they were not the first to have the idea. Davy, Deville and Bunsen (of burner fame) all attempted to extract

aluminum using electrolysis. Deville even experimented with electrolysis of cryolite and alumina, but his only source of electric current was batteries. At that time electrolysis was too expensive for commercial production. It was Thomas Edison's invention of the dynamo and its development that made

electrical power available to Hall and Heroult.

C. Other MethodsGiven the high energy and other costs associated with the Hall-Heroult

process, over the years the industry has looked at alternative methods of extracting aluminum. Research has explored carbothermal processes, which require temperatures greater than 2000oC for direct reduction to take place,

and alternatively a process involving electrolysis of anhydrous aluminum chloride.

While such processes have been shown to produce aluminum, for a variety of technical reasons they have not been translated into viable commercial scale plants. At the same time considerable effort has been made to improve the

efficiency of the Hall-Heroult process. Before the Second World War the consumption of electricity in aluminum smelting averaged around 23.3

kilowatt hours per kilo of metal produced, whereas today, the most efficient pots run at close to 13 kilowatt hours per kilo.

These improvements owe in part to the computerization of smelting cells, an improvement that has largely been unrecognized outside the industry.

The computer takes into account the various current operating variables so that the voltage in the pot is always the best for prevailing conditions.

Other energy saving advances include -

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Improvements in bath chemistry to lower both the smelting temperature and heat losses and to increase the efficiency of the use

of electrical current Improved insulation to reduce heat losses

Improved baking technology for carbon anodes Reduced carbon anode consumption per kilogram of aluminum

produced

1.3. THE PRIMARY ALUMINIUM PRODUCTION PROCESS

All modern primary aluminium smelting plants are based on the Hall-Heroult process, invented in 1886. Alumina is reduced into aluminium in electrolytic

cells, or pots. The pot consists of a carbon block (anode), formed by a mixture of coke and pitch, and a steel box lined with carbon (cathode). An electrolyte consisting of cryolite (Na3AlF6) lies between the anode and the

cathode. Other compounds are also added, among those are aluminium fluoride and calcium fluoride. The latter to lower the electrolyte's freezing

point. This mixture is heated to approximately 9800C. At this point the electrolyte melts and refined alumina is added. Reduction of aluminium ions produce molten aluminium metal at the cathode and oxygen at the anode,

which react with the carbon anode itself to produce CO2.

Aluminium smelting process is an electrolysis process, so an aluminium smelter uses prodigious amounts of electricity. The smelters tend to be

located near large power stations, often hydroelectric ones and near ports since almost all of them use imported alumina. A large amount of carbon is

also used in this process. Once aluminium is formed, the hot, molten metal is alloyed with other metals to make a range of primary aluminium products

with different properties and suitable for processing in various ways to make end-user products.

It takes about 2 tonnes of bauxite to produce 1 tonne of alumina; and approximately 2 tonnes of alumina to produce 1 tonne of aluminium.

1.4. ALUMINIUM SMELTING

In an aluminium smelter, direct current (DC) is fed into a line of electrolytic cells connected in series. These electrolytic cells are the nerve centre of the

process. While the cells (pots) vary in size from one plant to another, the pg. 5

fundamental process is identical and is the only method by which aluminium is produced industrially. It is named the Hall-Heroult process after its

inventors.

Each cell is a large carbon-lined metal container, which is maintained at a temperature of around 960°C and forms the negative electrode (or cathode). The cell contains an electrolytic bath of molten salt called 'cryolite' (Na3AlF6), into which a powder of aluminium oxide (Al2O3) is fed and becomes dissolved to form a solution. Aluminium fluoride (AlF3) is added to maintain the target bath chemistry. Large carbon blocks, made from calcined petroleum coke and liquid coal tar pitch, are suspended in the solution; and serve as the

positive electrode or anode.

The electrical current passes from the carbon anodes via the bath, containing alumina in solution, to the carbon cathode cell lining. The current then passes to the anode of the next pot in series. As the electrical current passes through the solution, the aluminium oxide is dissociated into molten aluminium (Al) and oxygen (O2). The oxygen consumes the carbon (C) in the

anode blocks to form carbon dioxide (CO2), which is released.

The electrolytic reaction can be expressed as follows: 2 Al2O3 + 3 C → 4 Al + 3 CO2

Figure:1.1. Aluminium smelting pots

Aluminium is formed at about 900°C, but once formed has a melting point of only 660°C. In some smelters this spare heat is used to melt recycled metal,

which is then blended with the new metal. Recycled metal requires only 5 percent of the energy required to make new metal. Blending recycled metal with new metal allows considerable energy savings, as well as the efficient

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use of the extra heat available. When it comes to quality, there is no difference between primary metal and recycled metal.

The smelting process required to produce aluminium from the alumina is continuous, the potline is usually kept in production for 24 hours a day year around. A smelter cannot be easily stopped and restarted. If production is interrupted by a power supply failure of more than four hours, the metal in

the pots will solidify, often requiring an expensive rebuilding process.

The hot, molten, metallic aluminium obtained in the process sinks to the bottom of the reduction cell, while the gaseous by-products form at the top

of the cell. The aluminium is siphoned from the bottom of the cell in a process called tapping (done by rotation every 24 hours), and transported to dedicated casting operations where it is alloyed; then cast into ingots, billets

and other products.

In addition to carbon dioxide, the aluminium smelting process also emits hydrogen fluoride (HF) an extremely toxic gaseous emission. Fume

treatment plants ("FTPs") are used to capture the hydrogen fluoride and recycle it as aluminium fluoride for use in the smelting process. During abnormal smelting conditions, known as anode effects, perfluorocarbon

("PFC") gases are emitted. Two PFC compounds are released during anode effects, namely tetrafluoromethane (CF4) and hexafluoroethane (C2F6), which

have greenhouse gas warming potential of 6,500 and 9,200 times greater than CO2 respectively.

The aluminium smelting process is extremely energy intensive, which is why most primary aluminium smelters are located where there is ready access to abundant energy/power resources. It is also a continuous process: a smelter

cannot be stopped and restarted easily. To the contrary, if production is interrupted by a power outage for more than four hours, the molten

aluminium in the cells will solidify. This is because metallic aluminium is formed at 900°C but, once formed, has a melting point of only 660°C. When

cells 'freeze' in this way, the only recourse for recovery is to rebuild the smelter.

There are two main types of aluminium smelting technologies, known as Prebake and Soderberg.

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Figure 1.2. Illustrations of Prebake cell (left) and Soderberg cell (right)

The principal difference between them is the type of anode used. Soderberg technology uses a continuous anode which is delivered to the cell in the form

of a paste, and which bakes in the pot itself. Prebake technology, on the other hand, uses multiple anodes in each cell. These anodes are pre-baked in

a separate facility and then suspended in the smelting cell.

1.5. SMELTING POT

Smelting cell or pot is the furnace where aluminium is produced in hot molten form from powder alumina.

Industrial production of primary aluminium is carried out in alumina reduction cells (Hall- Héroult process) adopted in the late nineteenth century,

and continues as the process in commercial use today. The Hall-Héroult process involves the electrolytic reduction of alumina (Al2 O3) dissolved in a

molten cryolite(Na3 AlF6) bath operating at temperatures about 960 ºC. Carbon anode is consumed in the reaction that makes CO and CO2. Molten

aluminium is reduced at the cathode.

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Figure 1.3. prebaked smelting pot

1.6. LAYOUT OF AN ALUMINIUM SMELTING POTS

The Hall-Heroult electrolysis process is the major production route for primary aluminium. An electrolysis cell is made of a steel shell with a series of insulating linings of refractory materials. The cell consists of a brick-lined

outer steel shell as a container and support.

Inside the shell, cathode blocks are cemented together by ramming paste. The top lining is in contact with the molten metal and acts as the cathode.

The molten electrolyte is maintained at high temperature inside the cell. The prebaked anode is also made of carbon in the form of large sintered blocks suspended in the electrolyte. A single Soderberg electrode or a number of prebaked carbon blocks are used as anode, while the principal formulation

and the fundamental reactions occurring on their surface are the same.

An aluminium smelter consists of a large number of cell (pots) in which the electrolysis takes place. A typical smelter contains anywhere from 300 to 720 pots, each of which produces about a ton of aluminium a day, though

the largest proposed smelters are up to five times that capacity. Smelting is run as a batch process, with the aluminium metal deposited at the bottom of the pots and periodically siphoned off. Power must be constantly available,

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since the pots have to be repaired at significant cost if the liquid metal solidifies.

1.7. POT COMPONENTS

Electrolyte: The electrolyte is a molten bath of cryolite (Na3AlF6) and dissolved alumina. Cryolite is a good solvent for alumina with low melting point, satisfactory viscosity, low vapour pressure. Its density is also lower

than that of liquid aluminium (2 vs 2.3 g/cm3), which allows natural separation of the product from the salt at the bottom of the cell. The cryolite ratio (NaF/AlF3) in pure cryolite is 3, with a melting temperature of 1010 °C, and it forms a eutectic with 11% alumina at 960 °C. In industrial cells the

cryolite ratio is kept between 2 and 3 to decrease its melting temperature to 940-980 °C.

Figure 1.4. components of pot

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Cathode: The carbon lining which forms the pot cavity consists of carbon blocks, preformed by external manufacturers. These blocks are placed in the

steel pot shell and cemented together with a paste similar to that used in making the blocks. Thermal insulation consisting of firebrick, vermiculite, or

similar materials is placed between the cavity lining and the steel shell. Large steel bars, serving as electrical current collectors, are embedded in the bottom portion of the cavity lining and extend through openings in the shell

to connect with the electrical bus which links one pot to the next.

Figure 1.5. pot cathodes

Carbon pot linings normally last from 4 to 6 years. When failure of a lining occurs, usually via the penetration of aluminum metal through to the

cathode collectors, the collectors dissolve. Then, the metal and sometimes the fused cryolite bath leak around the collectors. A sudden increase in iron levels in the aluminum usually indicates that a pot is nearing the end of its service life. The lining must then be repaired at the point(s) of failure by a

procedure called "patching", or else the entire lining, insulation, and collector assembly is replaced. The latter procedure is called "relining". Pot patching

and pot relining are a significant part of the production expense.

Anode: Carbon anodes are a major requirement for the Hall-Heroult process. About 0.5 tons of carbon is used to produce every ton of aluminum. There are two main types of carbon anode used. Both types are made from the

same basic materials and react in the same way.

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A mixture of petroleum coke and pitch is strongly heated causing the pitch to bind the coke particles together. "Pre-baked" anodes are made before they

are added to the pot, but "Soderberg" anodes are actually formed and baked in the pot.

The Soderberg anode uses the waste heat of reaction in each pot to pyrolyze the coke and pitch. As the lower part of the anode is consumed in the reaction, more raw materials are added at the top. During the baking

process many volatile products are driven off as the pitch hydrocarbons are dehydrogenated. Solid carbon is left as the anode.

Although the Soderberg anode may be more energy efficient it is easier to treat the volatile wastes if they are not mixed with the other emissions from

the pot. Dehydrogenation is often less complete in the Soderberg anode causing more hydrogen fluoride to be formed during the anode reaction. So,

for environmental reasons, modern smelters use prebaked anodes.

Prebaked Anodes: In prebaked technology the anodes used are termed as prebaked anodes which are made from a mixture of petroleum coke, aggregate and coal tar pitch binder moulded into blocks and baked in

separate anode baking furnace at about 1120 °C. Prebaked anodes consist of solid carbon blocks with an electrically conductive rod (e.g. copper) inserted and bonded in position usually with molten iron. An aluminium rod with iron studs is then cast or rammed into grooves in the top of the anode block in order to support the anode and conduct the electric current to the anode

when it has been positioned in the cell. Prebaked anodes have to be removed at regular intervals, when they have reacted down to one third or

one fourth of their original size. These remaining anodes are termed as butts and are usually cleaned outside the cell in a separate cleaning station to be able to recirculate the adhering bath materials removed from the cell. The

cleaned butts are then crushed and used as a raw material in the manufacturing of new anodes.

The carbon block consists of high purity calcined petroleum coke and the crushed remnants of used anode blocks bound together with pitch. The

petroleum coke usually used is a by-product of petroleum refining. Its purity is important as the carbon is actually consumed in the electrolytic reaction. Any impurities present in the finished anode can pass into the metal in the smelting cell. Separating the crushed spent anode material by size allows

different sized particles to be mixed so that the greatest density of packing is achieved.

The component materials are mixed together in heated containers to enable the melted pitch to blend completely with the coke particles. The resulting "green" mixture is weighed accurately and formed into the required anode

shape.

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The green anodes are delivered to in-ground baking furnaces, which consist of a series of refractory brick lined pits with hollow, surrounding

interconnected flue walls.

Anodes are packed into the pits with a blanket of coke covering the anodes and filling the space between the anode blocks and the walls of the pits. The

coke often used is termed fluid coke and consists of small spherical coke particles the size of fine sand. Appropriately sized petroleum coke can also

be used.

The pits are heated with natural gas for a period of several days. The flue system of the furnace is arranged so that hot gas from the pits being fired is

drawn through the next few sections of pits to preheat the next batch of anodes before they are fired. Air for combustion of the gas travels through the flues of previously fired sections, cooling these anodes while reheating

the air. The anodes are fired to approximately 1150°C, and the cycle of placing green anodes, preheating, firing, cooling, and removal is

approximately two weeks.

The so called "ring" type furnace uses flues under draft, and since the flue walls are of dry type construction, volatile materials released from the

anodes during the baking cycle are drawn into the flues. Once in the flues they burn, providing additional heat.

The baked anodes are removed from the furnace pits by means of an overhead crane on which pneumatic systems for loading and removing the

pit packing coke may also be mounted.Because the crushed, recycled anode component of a new anode has taken

up fluorides during its life in the pot environment, this gives rise to a potential emission of fluorides to air during the baking process. Scrubbing

equipment traps these additional fluorides for return to the smelting process.Cleaned baked anode blocks are transferred from the bake plant storage

area by conveyors to the rodding area to be made into rodded anode assemblies.

1.8. THE HALL-HEROULT SMELTING PROCESS

A. The Process at Work: Simply, the Hall-Heroult process is the method by which alumina is separated into its component parts of aluminum metal and oxygen gas by electrolytic reduction. It is a continuous process with alumina

being dissolved in cryolite bath material (sodium aluminum fluoride) in electrolytic cells called pots and with oxidation of the carbon anodes. The bath is kept in its molten state by the resistance to the passage of a large electric current. Pot temperatures are typically around 920°- 980°C. The

aluminum is separated by electrolysis and regularly removed for subsequent casting. The pots are connected electrically in series to form a ‘potline.’

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In each pot, direct current passes from carbon anodes, through the cryolite bath containing alumina in solution, to the carbon cathode cell lining and then to the anodes of the next pot and so on (see Figure 1.1). Steel bars embedded in the cathode carry the current out of the pot while the pots

themselves are connected through an aluminum bus-bar system. The pot consists of a steel shell in which the carbon cathode lining is housed. This

lining holds the molten cryolite and alumina in solution and the molten aluminum created in the process. An electrically insulated superstructure

mounted above the shell stores alumina automatically delivered via a sealed system and holds the carbon anodes, suspending them in the pot.

The electrolyte, which fills the space between the anodes in the pot, consists of molten cryolite containing dissolved alumina. A solid crust forms at the surface of the electrolyte. The crust is broken periodically and alumina is

stirred into the electrolyte to maintain the alumina concentration.

As the electrolytic reaction proceeds, aluminum which is slightly denser than the pot bath material is continuously deposited in a metal pool on the bottom of the pot while oxygen reacts with the carbon material of the

anodes to form oxides of carbon. As the anodes are consumed during the process, they must be continuously lowered to maintain a constant distance between the anode and the surface of the metal, which electrically is part of

the cathode. The anodes are replaced on a regular schedule.

The vigorous evolution of carbon dioxide at the anode helps mix the added alumina into the electrolyte but carries off with it any other volatile materials and even some fine solids. If any carbon monoxide does form it usually burns to carbon dioxide when it contacts air at the surface of the crust. Compounds

of fluoride formed in side reactions are the other main volatile product. Approximately 13 -16 kilowatt-hours of direct current electrical energy, one

half kilo of carbon, and two kilos of aluminum oxide are consumed per kilo of aluminum produced.

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Figure 1.6. Cross section of an aluminum producing pot containing pre-baked carbon anodes

As electrolysis progresses, the aluminum oxide content of the bath is decreased and is intermittently replenished by feed additions from the pot's

alumina storage to maintain the dissolved oxide content at about 2 to 5 percent. If the alumina concentration falls to about 1.5 to 2 percent, the

phenomenon of "anode effect" may occur. During anode effect, the bath fails to wet the carbon anode, and a gas film forms under and about the anode.

This film causes a high electrical resistance and the normal pot voltage, about 4 to 5 volts, increases 10 to 15 times the normal level. Correction is

obtained by computer controlled or manual procedures resulting in increased alumina content of the bath.

Reducing the prevalence of anode effects produces process benefits and also reduces the potential emissions of perfluorocarbons (CF4 and C2 F6) that are

greenhouse gases.

B. Electrolyte: The molten electrolyte bath consists principally of cryolite (sodium aluminum fluoride) plus some excess aluminum fluoride, 6 to 10

percent by weight of fluorspar and 2 to 5 percent aluminum oxide.

The control of bath composition is an important operation in the aluminum production process. To reduce the melting point of bath (pure cryolite melts at 1009° C), the bath contains fluorspar and some excess aluminum fluoride,

which along with the dissolved alumina, reduces the melting temperature sufficiently to permit the pots to be operated in the 920° to 980°.C range.

The reduced operating temperature improves pot efficiency.

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The weight ratio of sodium fluoride/ aluminum fluoride in cryolite is 1.50; the excess aluminum fluoride in the electrolyte is adjusted to yield a sodium

fluoride/aluminum fluoride ratio in the 1.00 to 1.40 range by weight.In the first few weeks after a newly lined pot is placed in operation, the

electrolyte is rapidly absorbed into the lining and insulation with a marked preferential absorption of a high-sodium- containing portion, tending to reduce the sodium fluoride/aluminum fluoride ratio below that desired.

Compensation for this is made by adding soda ash.After the first few weeks of operation the electrolyte tends to become

depleted of aluminum fluoride through volatilization of aluminum fluoride rich compounds, through reaction with residual caustic soda in the alumina, and, through hydrolysis from moisture in the air or added materials to give

hydrogen fluoride.

C. Fluoride Recovery: Gases and solids evolved from the pot and its electrolyte are controlled by various treatment processes. The most efficient of the commercially used methods is the Alcoa A398 Process that utilizes a highly effective pot hooding system and removes more than 99 percent of

the fluoride emission from the captured pot gases. The A398 Process prevents air pollution, conserves valuable resources for recycling and,

because it is a dry process, there are no liquid wastes to be disposed of.

Fluoride gases are passed through a bed of alumina where fluoride is adsorbed. The particulate matter is then collected in a fabric filter baghouse.

The reacted or fluoride-containing alumina is recycled into the aluminum production process.

Relatively small amounts of fluoride are able to escape from the smelting process, typically during operations such as anode changing when sections

of pot hooding are removed. These emissions are subject to E. P .A. discharge licenses. Continual efforts are made to improve work practices and

processes to keep these emissions to a minimum.At Alcoa operated plants, a regular check on the efficiency of emission

control equipment is made by plant technical staff using a range of laboratory based and portable equipment.

1.9. CALCULATION OF PRODUCTION AND ENERGY EFFICIENCY OF POT

Production EfficiencyFor a smelting cell of 100 percent efficiency just maintained at the reaction

temperature a theoretical production number can be calculated.Avogadro's number = 6.0221 X 1023

Elementary charge = 1.6022 x 10-19 CoulombsThese numbers give the accepted value for a "Faraday" of:

One Faraday = 96485 Coulombs

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Atomic weight of aluminum = 26.9815Valence of aluminum = 3

One-gram equivalent weight contains Avogardro's number of atoms.It takes 3 electrons to liberate one atom of aluminum.

Therefore every Faraday liberates 26.9815/3 grams of aluminum.Therefore, if "I" is the current through a smelting cell, and all of this current

produces aluminum, then in 24 hours every such cell makes:(Ix24x60x60/96485) x (26.9815/3)/1000 kilograms of aluminum

Say the current is 180,000 amperes (typical of many smelters); then the theoretical production per cell as predicted by Faraday's Law is 1450

kilograms/day.

However, owing to electrical shorting and other electrolytic reactions and a certain amount of reoxidation of aluminum, this theoretical production is not

realized. In practice the term "current efficiency" is used, this being the percentage of the current that actually results in aluminum produced.

In practice, current efficiency is generally in the order of 90 -95% so that:

Actual production per cell = Theoretical Production x CE/100.

Therefore, for example, the actual production in a 180,000 ampere cell at 90% current efficiency over 24 hours would be 1450x90/100 kilograms; i.e.

1305 kilograms.

A simplified equation for daily cell production is, therefore

Production per cell per day = (Current x CE/100 x 0.008054) kilograms

Energy Efficiency

If energy consumption is to be taken into account, then knowledge of the operating voltage of the cell is required. An estimate of this for some cells

would be 4.5 volts.

The above production would therefore occur with a (DC) energy consumption of 180000 x 4.5 x 24/1000 kilowatt-hours.

The electrical energy consumption per unit mass under the preceding scenario would be

(Current x Volts x 24/1000)/(Daily Production)

= (Current x Volts x 24/1000)/(Current x CE/100 x 0.008054)

So, kWH/kg = Volts/CE x 298 kilowatt-hours/kilogram

pg. 17

With the numbers given, the energy efficiency is 14.9 kWH/kg, although the most efficient plants would achieve numbers between 13 and 14 kWH/kg.

Most modern plants consist of one to six potlines, each potline consisting of 100 to 300 individual cells connected in series.

1.10. AVERAGE PRODUCTION PER DAY

Alumina powder feeding is done in every 4 minutes.

In every feeding 1.8 kg powder is fed through a single feeder.

Every pot has 2 feeders. i.e. in every 4 minutes 3.6 kg powder is fed to a single pot.

In one hour powder feeding

3.6 * 60/4 = 3.6 * 15 = 54 kg

Per day powder feeding to a single pot

54 * 24 = 1296 kg

Daily aluminium production from a single pot 640 kg.

Hindalco has 720 working pots

Daily powder feeding

1296 * 720 = 933120 kg

Daily aluminium production

640 * 720 = 460800 kg

1.11. POT ABNORMALITY

There are different pot abnormalities those create problems in aluminum production. They are,

1. Side covering missing: Due to this anode erosion takes place, carbon dusting increases and pot cooling starts.

2. Pot voltage fluctuation: If the pot voltage exceeds 100milli volt then pot shaking starts and if it is less than 100milli volt then noise occurs.

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3. Pot temperature fluctuation: Generally the pot temperature should be maintained 9600c. The allowable tolerance is +- 10. If the

temperature exceeds this range then the production decreases.4. Bath and metal height: Bath and metal height is another important

issue for aluminum production. The bath height should be maintained 16-18 cm and the metal height should be maintained 14-16 cm. If the bath height increases then the pot temperature increases and if the

metal height increases then the bath height decreases and pot cooling starts.

5. Bath ratio: The bath ratio should be maintained as 1.16.6. Current distribution: The current distribution should be proper. The

pot current should be maintained 85kamp.7. Clamping: Loose clamping may lead to sparking.

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CHAPTER-2

POINT FEEDER

pg. 20

Chapter-2 POINT FEEDERPoint feeder is the device through which the powder alumina is fed to the

smelting cell (pot). The feeder is a fully pneumatic component which works on compressed air.

2.1. FUNCTIONAL UNITS

A feeder has the following functional units

1) Cylinder assembly2) Clevis

3) Guide cylinder4) Cone assembly

Figure 2.1. point feeder components

There is a cylinder assembly at the top of the point feeder which is operated by compressed air at a pressure of 7 bar .There is one inlet and return

channel at the top and bottom head of the cylinder respectively. The cylinder assembly is mainly responsible of the movement of the cone assembly for

feeding purpose. (Each feeding nearly 1.8 kg of powder is charged)

pg. 21

The clevis consists of a male clevis and a female clevis connected to each other through a clevis pin.

The hopper is the chamber in which the alumina powder is stored, and is refilled through air slide channels at regular intervals.

2.2. WORKING

The feeder is a fully pneumatic component operated on compressed air. A breaking hole is present in the smelting cell through which the powder

alumina is fed in to the pot. The feeder is present just above the hole. The powder is first carried to the feeder through the channels and the feeder feds

the powder alumina directly into the hole.

Figure 2.2. point feeder

The feeder has two ports one for air intake and the other for air return at the top head and the bottom head of the cylinder respectively. The piston has a

piston rod which is connected to another rod inside the guide tube. The guide tube provides support to the piston rod assembly and has sealing

elements to avoid air leakage to the opposite side of the piston. The piston rod assembly is connected to a cone at the bottom of the guide tube. The

cone is provided to open and close the feeder passage for feeding the alumina powder.

pg. 22

The whole process is digitalized and operated by computerized systems (EPC- Electronic process control) . There is a definite time interval for the

cyclic feeding process which is preset in the computer. Here the time interval for feeding is set as 4 minutes (i.e. feeding will be done in the feeder in every

4 minutes).

When the compressed air is supplied through the inlet channel, the piston moves towards the bottom head which provides a forward movement to the

piston rod and as a result the cone connected at the bottom is pushed downward and closes the powder passage. At that time the alumina powder

is fed to the feeder through the hopper.

At the return stroke when the piston moves upward the cone is dragged up and the alumina powder present inside the feeder is fed to the feeding hole

of the smelting cell (pot).

Both for the feeder and the breaker the air pressure is given as 75psi.

Here every single pot has two point feeders placed at one line. The feeding is done in every 4 minutes and in every feeding 1.8 kg alumina powder enters into the pot. The feeding timing and feeding amount has been preset and

controlled through the computerized system.

2.3. FEEDER PROBLEMS

Feeding is done continuously, so some problems arise in the point feeder.

1. Seal failure (cone failure)2. Piston sticking

pg. 23

CHAPTER-3

pg. 24

PNEUMATIC CRUST BREAKER

Chapter-3 PNEUMATIC CRUST BREAKERCrust breaker is a pneumatic device used for breaking. Here breaking means

clearing the path for the feeding of alumina powder. So the crust breaker clears the path for the alumina powder to enter into the pot and take part in

the electrolysis process.

The crust breaker and the point feeder both work in a cyclic manner. First the crust breaker clears the hole for the feeding of alumina powder and then

the feeder feeds the powder.

3.1. COMPONENTS

The crust breaker mainly comprises of heat resistive pneumatic cylinder, hammer and guide pipe. Other components are,

1) Cylinder and piston assembly2) Clevis

3) Hammer4) Guide pipe

3.1.1. Cylinder and Piston Assembly

pg. 25

The cylinder and piston assembly consists of the cylinder, piston, piston rod, top head, magnets, bottom head, return tube, long and short studs and

middle support.

The cylinder has a bore of diameter 125mm and a stroke of length 550mm.

The upper head of the cylinder is called top head and the lower head of the cylinder is called bottom head.

The air enters into the cylinder through the inlet channel present at the top head of the cylinder. This air pressure pushes the piston downward. The

piston rod is connected with the hammer through the clevis. As the piston moves towards the bottom head it pushes the hammer downward and

breaking occurs.

The air flow for cylinder operation is controlled by solenoid valve (5/2 or 4/2 poppet valves)

pg. 26

Figure 3.1. cylinder and piston assembly of pneumatic crust breaker

There is a hollow return tube through which the air pushes the piston back to the top head.

10 small magnets are fitted at the top head in order to hold the piston after the breaking. Once breaking has been completed there is a definite time gap

after which breaking continues again and this cyclic process goes on. The magnets are used for holding the piston at the top head in that gap period,

i.e. it holds the piston at the top head after breaking finishes in the first cycle upto the beginning of breaking at the second cycle. As breaking is done in

every four minutes the magnets hold the piston at the top head for 4 minutes.

There are four long and four short studs at the sides of the cylinder for providing support to the cylinder. They are connected to each other through

the middle support.

A gland follower is fitted below the bottom head of the cylinder for preventing air and oil leakage.

For the lubrication of the parts lubricating oil is supplied along with the compressed air. The cylinder is made of heat resistive materials to sustain

the high heat produced (9600c) inside the smelting cell (pot).

pg. 27

3.1.2. Clevis

Clevis is the connector which connects the piston rod and the hammer. It plays a very crucial role in transmitting the motion smoothly from the

cylinder to the hammer.

There are two types of clevis for connecting the shafts, i.e. the male clevis and the female clevis and together they transmit motion. The female clevis is attached to the piston rod and the hammer contains the male clevis. Both

the male and female clevis are connected to each other with the help of clevis pin to transmit the constrained motion from the cylinder to the

hammer.

3.1.3. Hammer

Hammer is that part of the crust breaker which comes in contact with the hole and breaks the powder and clears the path for feeding alumina powder.

Figure 3.2. Hammer attached in crust breaker

The hammer is connected with the piston rood of the crust breaker through the clevis. The hammer is a hollow cylinder. A hammer bit is welded at the edge of the hammer. The tip of the hammer bit is made cone shaped for ease in operation. After use when the hammer bit becomes unusable the

whole hammer is changed.

pg. 28

Figure 3.3. crust breaker assembly

3.1.4 Guide Tube

The guide tube is the cylinder covering the piston rod, clevis and the hammer. The guide tube smoothens the motion of the hammer by protecting

it from vibration. The guide tube is also made of heat resisting material to sustain the high heat produced inside the pot.

Figure 3.4. guide tube

3.2. WORKING

pg. 29

The air enters into the crust breaker cylinder through a small inlet channel present at the top head. The pressure of this compressed air pushes the

piston downward towards the bottom head. As the piston moves towards the bottom head the hammer attached to the piston rod moves downward and breaking takes place. After breaking the air returns through the return pipe

and the piston returns to the top head thus the cyclic process goes on.

There are 10 magnets fitted at the top head for holding the piston. Breaking is a cyclic process and there is a definite time interval between the first cycle

to the next cycle. The magnets hold the piston in this time interval. Here breaking is done in every 4 minutes so the magnet holds the piston at the

top head for this 4 minutes.

3.3. BREAKER PROBLEMS

1. Gland leakage2. Tophead leakage3. Bucket leakage

4. Hammer bit damage

pg. 30

CHAPTER-4

ANODE JACK

pg. 31

Chapter-4 Anode Jack

In a smelting pot anode jack is used for lifting the anode up and downward i.e. for adjusting the height of the anode. It is also called as pot jack.

The anode is made of carbon. As the anode is used the lower part of the anode is consumed in the reaction. Due to which there are chances that the anode may not come in contact with the molten metal and the electrolysis

process may stop. To eliminate this problem aluminium smelters are provided with anode jacks.

There are 12 anode beams on both sides of a prebake pot and 2 anode buses on two sides to hold 6 anode beams on each side. There are 4 jacks in a pot and are connected to the anode bus. In case of requirement the jacks lift the

bus bar as a result of which all the anodes are lifted at a time.

The anode jacks are operated electrically. After tapping whenever the molten metal level inside the pot decreases the anode jack is automatically adjusted

and the beams moves downward to continue the process.

4.1. COMPONENTS

The anode jack assembly has the following components;

1. Motor2. Intermediate shaft

3. Jack

pg. 32

Figure 4.1. anode jack

4.1.1. Motor

The motor is the prime source from which the entire operation of anode jack starts. There is a motor fitted at one end of the pot. The motor provides

motion to the intermediate shaft. The shaft connected to the motor is the driving shaft and the intermediate shafts are the driven shafts. The motion of the driving shaft is transmitted to the intermediate shafts through worm and

worm wheel arrangement.

4.1.2. Intermediate shaft

The intermediate shaft is the medium of transmission motion from the motor to the jack. The intermediate shaft is divided into two parts, the long shaft

and the short shaft. There are 4 gear boxes, which connect the shafts. Among which two of them have bevel gear arrangement and the other two have worm gear arrangement. The two bevel gears are used to transmit the

motion form the driving shafts to the short shafts at 900 angle. The short shafts are connected to the long shafts by worm gear arrangement. The

rotational motion of the shaft is converted to translational motion of the jack and it lifts the anode bus.

4.1.3. Jack

There are 4 jacks in a single pot, two on each side. The bus is clamped to the jack. As the shaft rotates the rotational motion is converted to translational motion at the jack due to which the anode bus is lifted up and downward.

pg. 33

Figure 4.2. anode jack assembly in a pot

4.2. OPERATION

When the motor switch is on the driving or motor shaft rotates. The driving shaft is connected to the short intermediate shaft through bevel gear

arrangement. So the motion of the driving shaft provides motion to the short intermediate shaft. The short intermediate shaft is connected to the long

intermediate shaft through worm gear arrangement. So when the short shaft rotates the long shaft also rotates simultaneously. The jacks are connected to the long intermediate shaft at its two ends. The jacks hold the anode bus.

The rotational motion of the intermediate shafts is converted to reciprocating motion of the anode jack as a result of which it moves up and down resulting

in raising and lowering of anodes.

pg. 34

CHAPTER-5

pg. 35

COMPRESSOR

Chapter-5 CompressorCompressors are widely used in industries to transport fluids. It is a

mechanical device that compresses a gas.Generally, the compression of gases may be accomplished in device with rotating blades or in cylinders with reciprocating pistons. There are many

types of compressors, thus a proper selection is needed to fulfil the typical necessity of each industry.

5.1. TYPES OF COMPRESSORS

Compressor is a device used to increase the pressure of compressible fluid, either gas or vapor, by reducing the fluid specific volume during passage of

the fluid through compressor.

One of basic aim of compressor usage is to compress the fluid, then deliver it to a higher pressure than its original pressure. The inlet and outlet pressure

level is varying, from a deep vacuum to a high positive pressure, depends on process’ necessity. This inlet and outlet pressure is related, corresponding

with the type of compressor and its configuration.

compressors are generally classified into two separate and distinct categories: dynamic and positive displacement.

pg. 36

5.1.1. DYNAMIC COMPRESSORDynamic compressor is a continuous-flow compressor which includes centrifugal compressor and axial flow compressor. It is widely used in

chemical and petroleum refinery industry for specifies services. They are also used in other industries such as the iron and steel industry, pipeline booster,

and on offshore platforms for reinjection compressors.The dynamic compressor is characterized by rotating impeller to add velocity

and pressure to fluid. Compare to positive displacement type compressor, dynamic compressor are much smaller in size and produce much less

vibration.

(i) Centrifugal CompressorThe centrifugal compressor is a dynamic machine that achieves compression

by applying inertial forces to the gas (acceleration, deceleration, and turning) by means of rotating impellers.

It is made up of one or more stages; each stage consists of an impeller as the rotating element and the stationary element, i.e. diffuser.

There are two types of diffuser: vaneless diffusers and vaned diffusers.Vaneless diffuser is widely used in wide operating range applications, while

the vaned diffuser is used in applications where a high pressure ratio or high efficiency is required.

The parts of centrifugal compressor are simply pictured below.

pg. 37

Figure 5.1. Centrifugal compressor

In centrifugal compressor, the fluid flow enters the impeller in an axial direction and discharged from an impeller radially at a right angle to the axis

of rotation. The gas fluid is forced through the impeller by rapidly rotating impeller blades. The gas next flows through a circular chamber (diffuser),

following a spiral path where it loses velocity and increases pressure.

The deceleration of flow or “diffuser action” causes pressure build-up in the centrifugal compressor. Briefly, the impeller adds energy to the gas fluid,

and then the diffuser converts it into pressure energy.The maximum pressure rise for centrifugal compressor mostly depends on

the rotational speed (RPM) of the impeller and the impeller diameter. But the maximum permissible speed is limited by the strength of the structural

materials of the blade and the sonic velocity of fluid; furthermore, it leads into limitation for the maximum achievable pressure rise. Hence, multistage

centrifugal compressors are used for higher pressure lift applications.

A multistage centrifugal compressor compresses air to the required pressure in multiple stages

Typical centrifugal for the single-stage design can intake gas volumes between 100 to 150,000 inlet acfm. A multi-stage centrifugal compressor is

normal considered for inlet volume between 500 to 200,000 inlet acfm.

pg. 38

It designs to discharge pressures up to 2352 psi, which the operation speeds of impeller from 3,000 rpm to higher. There is limitation for velocity of

impeller due to impeller stress considerations; it is ranged from 0.8 to 0.85 Mach number at the impeller tip and eye.

Centrifugal compressors can be driven by electrical motor, steam turbine, or gas turbines.

Based on application requirement, centrifugal compressors may have different configurations. They may be classified as follows:

i. Compressors with Horizontally-split CasingsHorizontally-split casings consisting of half casings joined along the

horizontal centerline are employed for operating pressures below 60 barsii. Compressors with Vertically-split Casings

Vertically-split casings are formed by a cylinder closed by two end covers. It is generally multistage, and used for high pressure services (up to 700

kg/cm2).iii. Compressors with Bell Casings

Barrel compressors for high pressures have bell-shaped casings and are closed with shear rings instead of bolts.

iv. Pipeline CompressorsThey have bell-shaped casings with a single vertical end cover and are

generally used for natural gas transportation.v. SR Compressors

These compressors are suitable for relatively low pressure services. They have the feature of having several shafts with overhung impellers.

(ii) Axial Flow CompressorAxial flow compressors are used mainly as compressors for gas turbines. They are used in the steel industry as blast furnace blowers and in the

chemical industry for large nitric acid plants.Compare to other type of compressor, axial flow compressors are mainly

used for applications where the head required is low and with the high intake volume of flow. The efficiency in an axial flow compressor is higher than the

centrifugal compressor.

The component of axial flow compressor consist of the rotating element that construct from a single drum to which are attached several rows of

decreasing-height blades having airfoil cross sections. Between each rotating blade row is a stationary blade row. All blade angles and areas are designed

precisely for a given performance and high efficiency.

One additional row of fixed blades (inlet guide vanes) is frequently used at the compressor inlet to ensure that air enters the first stage rotors at the

desired angle. Also, another diffuser at the exit of the compressor might be added, known as exit guide vanes, to further diffuse the fluid and control its

velocity.pg. 39

Axial flow compressors do not significantly change the direction of the flow stream; the fluid flow enters the compressor and exits from the gas turbine

in an axial direction (parallel with the axis of rotation). It compresses the gas fluid by first accelerating the fluid and then diffusing it to increase its

pressure. The fluid flow is accelerated by a row of rotating airfoils (blades) called the rotor, and then diffused in a row of stationary blades (the stator). Similar to the centrifugal compressor, the stator then converts the velocity

energy gained in the rotor to pressure energy. One rotor and one stator make up a stage in a compressor. The axial flow compressor produces low

pressure increase, thus the multiple stages are generally use to permit overall pressure increase up to 30:1 for some industrial applications.

Figure 5.2. Axial flow compressor

Driver of axial flow compressor can be steam turbines or electric motors. In the case of direct electric motor drive, low speeds are unavoidable unless

sophisticated variable frequency motors are employed. Here are the advantages and disadvantages of axial flow compressor.

5.1.2. POSITIVE-DISPLACEMENT COMPRESSORPositive displacement compressors deliver a fixed volume of air at high

pressures; it commonly can be divided into two types: rotary compressors and reciprocating compressors. In all positive displacement machines, a certain inlet volume of gas is confined in a given space and subsequently compressed by reducing this confined space or volume. At this elevated

pressure, the gas is next expelled into the discharge piping or vessel system.

pg. 40

(i) Rotary CompressorRotary compressor is a group of positive displacement machines that has a

central, spinning rotor and a number of vanes. This device derives its pressurizing ability from a spinning component. The units are compact,

relatively inexpensive, and require a minimum of operating attention and maintenance. In a rotary compressor, the pressure of a gas is increased by trapping it between vanes which reduce it in volume as the impeller rotates

around an axis eccentric to the casing.

The volume can be varied only by changing the speed or by bypassing or wasting some of the capacity of the machine. The discharge pressure will

vary with the resistance on the discharge side of the system. Rotary compressors are generally classified as screw compressor, vane type

compressor, lobe and scroll compressor. The main difference between each type is their rotating device.

(ii) Reciprocating CompressorThe reciprocating, or piston compressor, is a positive displacement

compressor that uses the movement of a piston within a cylinder to move gas from one pressure level to another higher pressure level. Reciprocating compressors might be considered as single acting when the compressing is accomplished using only one side of the piston, or double acting when it is

using both sides of the piston.They are used mainly when high-pressure head is required at a low flow.

Generally, the maximum allowable discharge-gas temperature determines the maximum compression ratio.

Reciprocating compressors are furnished in either single-stage or multistage types.

For single stage design, the entire compression is accomplished with a single cylinder or a group of cylinders in parallel.

Intercoolers are provided between stages on multistage machines. These heat exchangers remove the heat of compression from the gas and reduce

its temperature to approximately the temperature existing at the compressor intake. Such cooling reduces the volume of gas going to the

high-pressure cylinders, reduces the power required for compression, and keeps the temperature within safe operating limits.

Typical compression ratios are about 3 per stage to limit discharge temperatures to perhaps 300oF to 350°F. Some reciprocating compressors have as many as six stages, to provide a total compression ratio over 300.

pg. 41

Figure 5.3. Reciprocating compressor

The intake gas enters the suction manifold into the cylinder because the vacuum condition is created inside the cylinder as the piston moves

downward. After the piston reaches its bottom position it begins to move upward. The intake valve closes, trapping the gas fluid inside the cylinder. As the piston continues to move upward it compresses the gas fluid, increasing its pressure. The high pressure in the cylinder pushes the piston downward.

When the piston is near the bottom of its travel, the exhaust valve opens and releases high pressure gas fluid.

5.2. MAJOR COMPONENTS OF COMPRESSOR

Reciprocating compressor:Major components of reciprocating compressor are

i. CrankcaseThe crankcase or frame of reciprocating compressor is generally made from

cast iron or steel plate. Crankcase is the housing of crankshaft and also serves as the oil reservoir.

ii. PistonPiston is a commonly component of reciprocating devices. It is the moving

component that is contained by a cylinder which purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod or

connecting rod.iii. Cylinder

pg. 42

A compressor cylinder is the housing of piston, suction and discharge valves, cooling water passages (or cooling fins), lubricating oil supply fittings and

various unloading devices.There are two types of compressor cylinder designs: valves in bore and valves out of bore. The valves in bore design has the compressor valves located radially around the cylinder bore within the length of the cylinder

bore.These cylinders have the highest percentage of clearance due to the need for scallop cuts at the head-end and crank end of the cylinder bore to allow

entry and discharge for the process gas.The valves out of bore design consists of compressor valves at each end of the cylinder. While this design provides a lower percent clearance it is more

maintenance intense.iv. Crankshaft

The crankshaft is the part of a reciprocating compressor which translates reciprocating linear piston motion into rotation. It is typically made of forged

steel, consists of crankpins and bearing journals.v. Connecting rod

In a reciprocating compressor, the connecting rod connects the piston to the crankshaft; thus form a simple mechanism that converts linear motion into

rotating motion.vi. Crosshead

The crosshead rides in the crosshead guide moving linearly in alternate directions with each rotation of the crankshaft. The piston rod connects the crosshead to the piston. Therefore, with each rotation of the crankshaft the

piston moves linearly in alternating directions.vii. Piston rod

A piston rod joins a piston to a connecting rod. It may have a collar on the end that connects to the piston.

viii. Intercooler and AftercoolerThe intercooler is a heat exchanger situated in between the LP cylinder and the HP cylinder in case of a multi stage double acting compressor. When the air is compressed in the LP cylinder the temperature of the air increases. The air again moves to the HP cylinder for further compression which increases

its temperature further. In such case there are chances of bursting of cylinder. To avoid such problem an intercooler is placed in between the LP and HP cylinder. After the air being compressed in the LP cylinder it passes

through the intercooler and loses some temperature and goes to the HP cylinder for further compression

Similarly the aftercooler is placed after the HP cylinder. The compressed air is further compressed in the HP cylinder and its temperature again increases. To reduce this air temperature the air after the HP cylinder is passed through

the aftercooler and is stored in the receiver.ix. lubricator

The lubricator or lubricating cylinder supplies oil for lubricating the moving parts inside the compressor.

pg. 43

Figure 5.4. reciprocating compressor

x. oil filterThe oil filter filters the lubricating oil before it is passed to the moving parts

of the compressor.

5.3. DESIGN CONSIDERATION

5.3.1. Fluid propertiesa. Gas Composition: In design of compressor, gas compositions data are very important. It should be analysis and listed in compressor specification

sheet with each component name, molecular weight, boiling point. This data are important determined the volume flow rate of the compressed gas, average molecular weight, compressible factor, and specific heat ratio.

b. Corrosiveness: Corrosive gas stream constituents must be identified for all operating conditions including transients. This important because

corrosion gas as wet H2S in compression service can cause stress corrosion cracking of high strength materials.

c. Fouling tendency: The compressor design specification sheet should include the fouling tendency of the gas and specify compressor flushing

facilities if required.This is an important parameter for the selection of the type of compressor

design.Axial and high speed, single stage centrifugal is not suitable for fouling

service.

pg. 44

Flushing allows helical screw and conventional centrifugal compressors to be used in a fouling service.

d. Liquid in gas stream: Liquid in the gas stream should be avoided because is harmful to compressors. For feed stream into compressor that

content liquid in gas stream, liquid separators and heat tracing and insulation of compressor inlet lines should be provided when ambient

temperature is below the dew point of the gas at the compressor inlet or when handling hydrocarbon gas components heavier than ethane.

e. Inlet pressure: Gas stream inlet pressure should be specified in compressor specification sheet for the lowest value; this is to meet the

guarantee performance of compressor. The allowance pressure drop of 0.3 psi in through compressor inlet hood, screen, filter and piping should be

expected.f. Discharge pressure: Centrifugal and axial compressors, the discharge pressure specified is at the discharge flange. Meanwhile reciprocating and

rotary compressors, the specification should note that the discharge pressure specified is downstream of the pulsation suppression device for

reciprocating compressors and downstream of the discharge silencing device on rotary compressors.

g. Inlet temperature: For gas stream temperature is affects the volume flow rate, compression service head requirements, and required power. Because of this inlet temperatures for compression process should be

specified full range.h. Discharge temperature: Discharge temperature of Compressor is

affected by inlet temperature, pressure ratio, gas specific heat values, and the efficiency of compression. This temperature is important in determining compressor mechanical design, gas fouling tendency, process compression

stage selection, and cooler and piping design.

5.3.2. process compression stagesCompression ratio is the relation of discharge pressure (P2) over the suction pressure (P1) for a compressor, P2/P1. For the high-pressure compression services the compressor is design for multiple process compression stages and sometime the coolers are included between the stages to remove the

heat of compression.Reasons for providing process compression staging are:

a. To limit the discharge temperature of each stage to acceptable levels from the standpoints of both compressor design restraints and the fouling

tendency of the compressed gas.b. To make side streams available in the compression sequence at

intermediate pressure levels such as in process refrigeration systems.

c. To reduce “compressor stage" inlet temperatures thereby reducing the amount of work (head) required to achieve a given pressure ratio.

pg. 45

d. To satisfy differential pressure and pressure ratio limits of various compressor types, e.g., axial thrust load limitations for centrifugal and axial,

piston rod stress limits in reciprocating compressors, and rotor lateral deflection and axial thrust in screw compressors.

e. Provide the condition for include intercooler between stages, that will help reduce horsepower require for compression, and keeps the temperature

within safe operating limits.

5.4. DEFINITIONSAdiabatic / Isentropic: This model assumes that no energy (heat) is

transferred to or from the gas during the compression, and all supplied work is added to the internal energy of the gas, resulting in increases of

temperature and pressure.Aftercooler: Aftercooler is a heat exchanger which is used when discharge gas temperature leaving compressor shall be decreased before entering to

other equipment or system.Bearing: Is a device to permit constrained relative motion between two

parts, typically rotation or linear movement. Compressors employ at least half a dozen types of journal bearings.

Essentially all of these designs consist of partial arc pads having a circular geometry.

Blades- Rotating airfoils for both compressors and turbines unless modified by an adjective.

Capacity: The amount of air flow delivered under specific conditions, usually expressed in cubic feet per minute (CFM).

Clearance - Some volume which is remains vacant between the top position of the piston and the cylinder

Compression Ratio: The ratio of the discharge pressure to the inlet pressure.

Compressor Efficiency: This is the ratio of theoretical horse power to the brake horse power.

Discharge Pressure: Air pressure produced at a particular point in the system under specific conditions measured in psi (pounds per square inch).Discharge Temperature: The temperature at the discharge flange of the

compressor.Gauge Pressure: The pressure determined by most instruments and

gauges, usually expressed in psi. Barometric pressure must be considered to obtain true or absolute pressure (psig).

Brake Horsepower: Brake Horsepower delivered to the output shaft of a motor or engine, or the horsepower required at the compressor shaft to

perform work.Impeller: Is a rotor inside a shaped housing forced the gas to rim of the

impeller to increase velocity of a gas and the pressure in compressor.Inlet Pressure: The actual pressure at the inlet flange of the compressor

typically measure in psi.

pg. 46

Inlet volume flow: The flow rate expressed in volume flow units at the conditions of pressure, temperature, compressibility, and gas composition,

including moisture content at the compressor inlet flange.Intercooler: After compression, gas temperature will rise up but it is limited

before entering to the next compression. Temperature limitation is depending to what sealing material to be used and gas properties.

Intercooler is needed to decrease temperature before entering to the next compression.

Isentropic process: An adiabatic process that is reversible. This isentropic process occurs at constant entropy. Entropy is related to the disorder in the

system; it is a measure of the energy not available for work in a thermodynamic process.

Isobaric process: Means that the volume increases, while the pressure is constant.

Isochoric process: Is a constant-volume process, meaning that the work done by the system will be zero. In an isochoric process, all the energy added as heat remains in the system as an increase in internal energy.Isothermal: This model assumes that the compressed gas remains at a

constant temperature throughout the compression or expansion process. In this cycle, internal energy is removed from the system as heat at the same

rate that it is added by the mechanical work of compression. Isothermal compression or expansion more closely models real life when the

compressor has a large heat exchanging surface, a small gas volume, or a long time scale (i.e., a small power level). Compressors that utilize inter-

stage cooling between compression stages come closest to achieving perfect isothermal compression. However, with practical devices perfect isothermal

compression is not attainable. For example, unless you have an infinite number of compression stages with corresponding intercoolers, you will

never achieve perfect isothermal compression.Maximum allowable temperature: The maximum continuous temperature for the manufacturer has designed the equipment.

Maximum allowable working pressure (MAWP): This is the maximum continuous pressure for which the manufacturer has designed the

compressor when it is operating at its maximum allowable temperature.Maximum inlet suction pressure: The highest inlet pressure the

equipment will be subject to in service.Multi-Stage Compressors: Compressors having two or more stages

operating in series.Normal operating condition: The condition at which usual operation is expected and optimum efficiency is desired. This condition is usually the

point at which the vendor certifies that performance is within the tolerances stated in this standard.

Piston Displacement: The volume swept by the piston; for multistage compressors, the piston displacement of the first stage is the overall piston

displacement of the entire unit.

pg. 47

Polytropic: This model takes into account both a rise in temperature in the gas as well as some loss of energy (heat) to the compressor's components. This assumes that heat may enter or leave the system, and that input shaft

work can appear as both increased pressure (usually useful work) and increased temperature above adiabatic (usually losses due to cycle

efficiency). Compression efficiency is then the ratio of temperature rise at theoretical 100 percent (adiabatic) vs. actual (polytropic).

Process compression stage: Is defined as the compression step between two adjacent pressure levels in a process system. It may consist of one or

more compressor stages.Radially split: A joint which is perpendicular to the shaft centerline.

Rated discharge pressure: Is the highest pressure required to meet the conditions specified by the purchaser for the intended service.

Rated discharge temperature: Is the highest predicted operating temperature resulting from any specified operating condition.

Rotor: The rotors are usually of forged solid design. Welded hollow rotors may be applied to limit the moment of inertia in larger capacity compressors.

Balancing pistons to achieve equalization of rotor axial thrust loads are generally integral with the rotor. Rotating blades are located in peripheral

grooves in the rotor.Volumetric Efficiency: This is the ratio of the capacity of a compressor to

the piston displacement of compressor.

5.5. THEORY

5.5.1. Thermodynamic processThermodynamic is a branch of science which deals with energy. It is core to

engineering and allows understanding of the mechanism of energy conversion. Compression theory is primarily defined by the Gas Laws and the

First and Second Laws of Thermodynamics.(I) Gas Laws

The general law of state for gases is based on the laws of Charles, Boyle, Gay-Lussac and Avogadro. This states how pressure, volume, and

temperature affect each other. It can be written:p x v = R × T Eq (1a)

where R is the gas constantThe constant R only concerns the properties of the gas. If the mass m of the

gas takes up thevolume V, the relation can be written:

p x V = n x R x T Eq (1b)An additional term may be considered at this time to correct for deviations

from the ideal gas laws. This term is the compressibility factor “Z.” Therefore, the ideal gas equation becomes:

pg. 48

p x v = Z x R x T Eq (1c)

(II) First Law of ThermodynamicsThermodynamics’ first main principle says that energy can neither be

created nor destroyed, but it can be changed from one form to another.Q W x E h w= D Eq (2a)

In thermodynamic, system might be classified as isolated, closed, or open based on the possible transfer of mass and energy across the system

boundaries. The system in which neither the transfer of mass nor that of energy takes place across its boundary with the surroundings is called as

isolated system. A closed system has no transfer of mass with its surroundings, but may have a transfer of energy (either heat or work) with

its surroundings.And an open system is the system in which the transfer of mass as well as

energy can take place across its boundary.When the variables of the system, such as temperature, pressure, or volume

change, the system is said to have undergone thermodynamic process. There are various types of thermodynamic process:

1. Isobaric process2. Isochoric process

3. Isothermal process4. Adiabatic process

The heat flow can be prevented either by surrounding the system with thermally insulating material or by carrying out the process so quickly that

there is not enough time for appreciable heat flow.Here is a graphic for various types of thermodynamic process above.

Figure:10. Thermodynamic processes on a pressure-volume diagram

5. Isentropic process6. Reversible and irreversible process

5.5.2. Compression ProcessCompressor is a work absorbing device used for increasing the pressure of a fluid. When gas is compressed, its molecules are made to come closer, by

pg. 49

which they occupy less space. As the number of molecules of gas increases in a given volume, its mass and density also increases.

Increasing in density would affect to pressure increment.Pressure of a fluid is increased by doing work upon it, which is accompanied

by increase in temperature depending on the gas properties.Figure:11 below presents a compression schematic layout.

Figure:11. Compression schematic layoutIt is mentioned before that there are two types of compressor: positive

displacement and dynamic. They compress the gas fluid in different principle of operation. Positive displacement compressor compresses the fluid by

trapping successive volumes of fluid into a closed space then decreasing its volume.

Compression occurs as the machine encloses a finite volume of gas and reduces the internal volume of compression chamber.

The other type of compressor, dynamic compressor, compresses the fluid by the mechanical action of rotating vanes or impeller imparting velocity and

pressure to the fluid.The larger the diameter of impeller, the heavier the molecular weight of gas

fluid, or the greater the speed rotation would produce greater pressure. Generally, positive displacement compressor is selected for smaller volume

of gas and higher pressure ratios.Dynamic compressor is selected for higher volume of gas fluid and smaller

pressure ratios.

5.6. CALCULATIONSHere at HINDALCO 2 types of compressors are used

1. Ingersollrand compressor (IR) (Reciprocating compressor)2. Screw compressor (Rotary compressor)

1. IR(Ingersollrand) compressorIt is a reciprocating compressor.

The main components of this compressor are the air filter, LP cylinder, HP cylinder, intercooler, aftercooler, lubricating cylinder.

pg. 50

In the IR compressor there are 4 suction and delivery valves in LP cylinder. Similarly in the HP cylinder there are 2 suction and delivery valves.

There is a non-return valve (NRV) present between the compressor and the receiver. When the compressor is off the NRV doesn’t allow the air to return

to the compressor so as to avoid bursting of compressor.As the piston reciprocates continuously a lot of heat is developed inside the

cylinder to reduce that temperature water jackets are provided.

IR COMPRESSORMake:- siemensKw/hp:- 110/150

Volt:- 415 +- 10%Amp:- 193A

Rpm:- 1485 rpmDelivery pressure:

LP side:- 3kgHP side:- 4 kg

Suction valve temperature:- 500cDelivery valve temperature:- 1200c

Coil temperature:- 60-650cIntercooler temperature:- below 400cAftercooler temperature:- below 400c

Intercooler pressure:- 28 psiOil pressure:- 1.5-2 psi

Potroom delivery pressure:- 85 psi

2. Screw compressorModel:-ZR 250Kw:- 250 kw

Compressor outlet pressure:- 6 barAir filter:- -0.038 bar(when it becomes -0.044 bar, a warning rings and the

filter is cleaned and fitted again)Oil pressure:- 2.32 barIntercooler:- 2.3 bar

Compressor outlet temperature:- 310cElement-1(intercooler outlet) temperature:- 1700c

Element-2(intercooler inlet) temperature:- 520cAftercooler outlet temperature:- 1550c

Cooling water in:- 260cLP Cooling water out:- 400c

Cooling water out:- 380cOil temperature:- 650c

pg. 51

CHAPTER-6

pg. 52

VACUUM CRUCIBLE

CHAPTER-6 VACUUM CRUCIBLE

Vacuum crucible is the carriage used for extracting the molten aluminium from the smelting pot and for carrying the liquid aluminium to the casting

plant.

Alumina is fed inside the pot and molten aluminium is produced. The aluminium produced in the pot is extracted with the help of vacuum crucible and this metal extraction operation is called metal tapping. Metal tapping is done in all the pots in every 32 hours. This tapped metal is then carried out

to the casting plant.

pg. 53

6.1. COMPONENTS

The vacuum crucible has four major components:

1. The VC chamber2. Lead

3. Syphon and Elbow

6.1.1. VC Chamber

The vacuum crucible chamber is a steel casing and inside the chamber heat resisting bricks are fitted in order to protect the outer casing from high heat

of molten metal. The metal tapped is stored in the chamber.

Figure 6.1. VC chamber

6.1.2. Lead

The upper cover of the vacuum crucible is called lead. All the components that help in extracting the metal are attached to the lead.

Figure 6.2. lead

The lead is attached to the chamber at the time of tapping. There are 4 adjustable clamps to connect the VC and the lead. There is a rope at the

bottom of the lead for air tighten purpose. 2 hooks are there at the top of the

pg. 54

lead for holding the VC.

Figure 6.3. vacuum crucible

6.1.3. Syphon and Elbow

The syphon is the channel that enters into the pot and through the syphon the molten aluminium is tapped into the VC.

The elbow is connected to the syphon and is attached to the lead.

6.2. SPECIFICATION

Weight of empty VC - 5 tonnes

Maximum capacity of VC - 5 tonnes

But generally 4 to 4.5 tonnes of metal is syphoned to avoid overflow.

The air pressure in the VC is maintained as 90 psi at the time of tapping.

6.3. METAL TAPPING

There are 4 holes at the four edges of a pot for metal tapping. There is also an air point in between every two pots.

pg. 55

Figure 6.4. tapping with vacuum crucible

There are two lines in a vacuum crucible. In one line air is injected into the VC and another rejecter channel throw which the air is rejected. The air pipe of the VC is connected to the air point and pressurized air enters into the VC and the air is rejected at high pressure through the rejecter. The rejected air creates vacuum inside the VC and the liquid aluminium is ejected into the VC

at this pressure. Not only metal, bath is also extracted with the help of VC.

pg. 56

CHAPTER-7

pg. 57

ELECTRIC OVERHEAD TRAVELLING CRANE

(EOTC)

CHAPTER-7 EOT Crane7.1. INTRODUCTION

E.O.T. crane stands for Electric Overhead Travelling crane. The most adaptable and the most widely used type of power driven crane for indoor

service is undoubtedly the three motion EOT crane. It serves a larger area of floor space within its own travelling restrictions than any other permanent

type hoisting arrangement.

This used in industries for handling and moving a maximum specified weight of the components called capacity of the crane within a specified area. Use

of E.O.T. cranes in industries is both an efficient and cost effective method of handling the materials. As obvious from the name, these cranes are

electrically operated by a control pendant, radio/IR remote pendant or from an operator cabin attached with the crane itself.

As the name implies, this type of cranes are electrically operated by a control pendant, radio/IR remote pendant or from an operator cabin attached

with the crane itself and provided with movement above the floor level.

pg. 58

Hence it occupies no floor space and this can never interface with any movement of the work being carried out at the floor of the building.

An overhead crane consists of parallel runways with a travelling bridge spanning the gap. A hoist, the lifting component of a crane, travels along the

bridge. If the bridge is rigidly supported on two or more legs running on a fixed rail at ground level, the crane is called gantry crane. Unlike

construction cranes overhead cranes are used for either manufacturing or maintenance applications, where efficiency or downtime are critical factors.

The three motions of such crane are the hoisting motion and the cross travel motion. Each of the motions is provided by electric motors.

The above characteristics have made this type of crane suitable for medium and heavy workshop and warehouses. No engineering erection shop,

machine shop, foundry, heavy stores is complete without an EOT crane.

In a steel plant, rolling mill, thermal power plant, hydraulic power plant, nuclear power plant, this type of crane is considered indispensable. In short in all industries, wherein heavy loads are to be handled, EOT crane find its

application.

7.2. APPLICATIONOverhead cranes are commonly used in the refinement of steel and other

metals such as copper and aluminium. At every step of manufacturing process, until it leaves a factory as a finished product, metal is handled by

overhead cranes. From raw material pouring to the lifting of finished products to trucks or trains every work is done by overhead cranes.

In steel industries E.O.T. cranes are used in almost every sectors starting from the pouring of raw material to repair and maintenance of every

machinery.In aluminium industries E.O.T. cranes are used in most of the operations

such as metal tapping, anode beam raising, anode changing, breaker and feeder changing and in other maintenance operations.

Almost all paper mills use EOT cranes for regular maintenance needing removal of heavy press rolls and other equipment. These are used in initial construction of paper machines for installing heavy cast iron paper drying

drums.In all other industries also EOT cranes are used in most of the fields for

holding and travelling large weights.

7.3. EOT CRANE PARTS

pg. 59

A EOT crane consists of two distinct parts

1. Bridge2. Hoisting trolley or Crab

7.3.1. Bridge

The Bridge consists of two main girders fixed at their ends and connected to another structural component called the end carriage. The two end carriages

are mounted the main runners or wheels (four or more) which provide the longitudinal motion to the main bridge along the length of the workshop. The

motion of the bridge is derived from an electric motor which is geared to a shaft running across the full span of the bridge and further geared to a wheel at each end. In some design separate motors may be fitted at each corner of

the main bridge. The wheels run on two heavy rails fixed above the floor level along the length of the shop on two girders, called gantry girder.

7.3.2. CrabThe Crab consists of the hoisting machinery mounted on a frame, which is in turn mounted on at least four wheels and fitted with suitable machinery for traversing the crab to and fro across the main girders of the crane bridge. Needless to mention that the crab wheels run on two rail sections fixed on the top flange of the main bridge. Thus the load hook has three separate

motions, these being the hoisting, cross traverse of the crab, and longitudinal travel of the whole crane. Each motion is controlled

independently of the other motions by separate controllers situated in a

pg. 60

control cage or in a suitable position for controlling from the floor by pendent chains.

The essential parts are:

1. Bridge– 2 No’s2. End carriage– 2 No’s

3. Wheel of the bridge– At least 4 No’s4. Crab (without auxiliary hoist)– 1 No’s

5. Hoisting machinery set– 1 No’s6. Wheels of crab– At least 4 No’s

7. Bottom Block (without auxiliary hoist)– 1 No’s8. Lifting hook– 1 No’s

9. Rail on the gantry girder for crane movement– 2 No’s10. Rail on the bridge for crab movement– 2 No’s

11. Operators cabin– 1 No’s

7.4. OPERATION

pg. 61

7.4.1. Mechanical

Before operation, check all parts are lubricated properly as per lubricating chart. Electrical wiring is to be completed as per wiring diagram. During initial test it should be checked that bridge, crab & other components

mounted on crab are clear of roof beam & walls. All motors are connected properly & that the limit switches cut off the supply to motors in proper direction. In case the limit switches don’t cutoff the supply in the proper direction make the necessary changes in wiring. The crane should be run

light for a little while before loading the same & it should be checked that all limit switches should work satisfactorily.

Commence lifting the load in stages, starting with not more than 5% of the safe working load & then increasing this gradually in succeeding trails, till

you have reached the full load. During this we must ensure that any part of the crane does not show any sign of giving way while going through all motions of hoisting, traverse & travel. Finally, test the crane with 25%

overload before the same is put into operation.

7.4.2. Electrical

Before pressing ‘ON’ Push button of main contact or see that all drum controllers or master controllers are in off position. There are 4, 6, 8 steps in drum controller depending on HP. of motor. On the 1st step full resistance of

resistance box is inserted & smoothly all resistance is cut off by the controller. Whenever motor gets supply, brake is released, thus allowing motor to accelerate smoothly. Whenever motor supply is cut off, thrust or

brake applies brake & brings the motor to stand-still. whenever load reaches extreme position, limit switch cuts off the supply to that motor in that

particular direction & load can’t be moved further in that direction. The operator can move the load in backward direction by moving the drum

controller in reverse direction, or pressing the related push button.

7.5. SAFETY FEATURES

7.5.1. Built in safety features1. Emergency switches at corners to stop the crane in case of emergency.

Provisions can be made to warn operator through indicating lamp.2. Reversing contactors are inter-locked electrically to avoid short circuit.3. Bell/Warning horn is provided for signaling crane operation & to warn

people at floor level.4. Display of sign board like danger board, instruction regarding the opening

of panel doors for safety of maintenance personnel/operator.5. Use of master controllers to enable operation of crane at lower control

voltage there by avoiding danger of line voltage to operators.6. Adequate earthing of all electrical components.

pg. 62

7. Interlocking of master controllers, starter contractors, overload relays, over hoist limit switches with main circuit breakers/ contactors to avoid

accidental starting of various motions of crane.8. Provision of anti-drop circuit in case of hoist motion for preventing, drifting

of load.9. Provision of plugging circuit for cross & long travels to avoid jerking &

smooth stopping of travel motion.10. Selection of motor brakes, switch gears.

11. Equipping the cabin with adequate lighting & provision of fan & exhaust fan as needed.

12. Design of the cabin worked out taking into consideration of ergonomical aspects like sufficient head room, suitable chair & placement of control equipments like master controllers P.B. Stations within easy reach of

operator.13. Provision of door switches in case of cross contactors being angle

iron/copper contactors & wherever considered necessary as safety measures.

7.5.2. Operation safety features

1. In individual motion panels, provision is made for protecting motors against short circuit. This is achieved either by providing H R C fuses or MCB

or MCCB.2. Every motor is protected against O/L relays by providing thermal or

magnetic O/L relays.3. Single phase preventions are provided in selective cases where supply

conditions & operational safety demands for.4. Undervoltage protection. Main incoming circuit breaker/contactor is

provided with under voltage protection.5. Limit switches are provided for excess movement in respective direction.

This avoids toppling, hitting, damage to other machineries.6. Selection of motors, brake, clutches & other switch gear & control gear equipments done carefully taking into account repeated reversals. Higher

inertia loads & frequent starting & stopping suitable safety factors are considered for selection.

7.6. CALCULATION

7.6.1. ElectricalTo determine the relationship between rotor weight, MVA rating & speed.

The rotor weight was proportional to the output & inversely proportional to the square root of the

speed. However, a wide variation in rotor weight was found, which could only be explained by variations in unit design & method of rating.

The formula is given by:Rw = 50(MVA/n0.5) 0.74

pg. 63

Where, Rw= Rotor weight in tones for rotors with standard inertia.MVA= Rotor rating at 60ºC temperature rise.

n = Rotor speed, 90 rev/min minimum.

This equation is used to determine the weight of a generator rotor for units with standard inertia & speeds in excess of 90 rev/min. Data obtained for units with slower speeds indicated a wide variation in rotor weight .when plotted in the same manner, & therefore it was not possible to derive a

formula for large slow speed units. The study had to be confined to relatively small rotors with ratings below about 100MVA.since large rotors are

connected to major power networks where added inertia is not a requirement.it is only on small & isolated systems where extra inertia is

required for stability.

Standard inertia for generator rotors can be determined from the following equation.

GD2= 310 000(MVA/n1.5)1.25Where,

GD2 = Standard inertia (tonne/m2)G = Rotor weight (tonne)

D = Diameter of gyration (m)So as to allow the effect of extra inertia on rotor weight.

Equation was expanded to include a coefficient as follows:Rw = 50(MVA/n0.5)0.74{1+ C(K-1)}

Where,C= Coefficient of added inertia.

K = Inertia ratio defined as rotor inertia divided by standard inertia.

In the case of an overhauling load when using an adjustable frequency control & a squirrel cage motor, the speed of the motor & load is directly a

function of the applied frequency to the motor. By changing the applied frequency to the motor, the synchronous speed of the motor changes in

accordance with the following equation:Synchronous speed = 120 * f/P

Where,f is the applied frequency & P is the number of poles in the machine.

7.6.2. Mechanical

TorqueThe horsepower equation may be used to determine the maximum

continuous full-load torque a motor can produce.The equation is:

pg. 64

T= (HP×5250)/NWhere,

HP= Power in horse power.N= Speed in r.p.m.

7.7. EOT CRANES AT HINDALCO ACCORDING TO TONNAGE CAPACITY

Basing on the tonnage capacity HINDALCO has 4 types of EOT Cranes.

1. 10 tonne capacity (6 fall sealing)

2. 15 tonne capacity (6 fall sealing)

3. 15 tonne capacity (8 fall sealing)

4. 30 tonne capacity (8 fall sealing)

The main difference between 6 fall sealing and 8 fall sealing is that in 6 fall sealing idler pulley is not required while in 8 fall sealing idler pulley is

required.

pg. 65

CHAPTER-8

BEAM RAISING MACHINE (BRM)

pg. 66

CHAPTER-8 BEAM RAISING MACHINE (BRM)Beam raising machine is the device used in the aluminium smelters for

raising the anode beams used in the smelting pots.

Here all the pots are prebake pots and the life span of a prebake anode is 28 to 30 days. We also know that the carbon anode is consumed in the process. As the anode gets consumed it should be held downward so that it will take

part in the electrolysis process. So the height of the anode is maintained with the help of beam raising machine.

8.1. COMPONENTS

The prime components of beam raising machine are

1. Legs2. Stands

3. Diaphragm valve

pg. 67

The beam raising machine has 12 legs, 6 on each side to hold the 12 beams of a pot. Holding clamps are attached on the legs for holding the beams.

It has 4 stands, 2 on each side for maintaining the height and keeping the BRM stable.

There are 36 diaphragm valves present, 3 on each leg for operating the legs of the beam raising machine.

8.2. OPERATION

The beam raising machine is fully operated by compressed air. First the BRM is fitted in the pot with the help of EOT Crane. After that the air pipe of the

BRM is connected to the air point present beside the pot.

There are 3 diaphragm valves on each leg, two are at the lower end and one at the upper end. The valves at the lower end helps to tilt the holding clamp and the valve at the upper end help to tilt the leg so that the legs hold the beams tightly. All these valves are operated by compressed air. The beam

raising machine has air channels through which the air operates the valves. The legs tilt inside opposite to the holding clamp to hold the beam tightly.

The stand helps to maintain the height of the beam.

The operating air pressure for BRM is maintained as 80 psi.

pg. 68

CHAPTER-9

COOLING TOWER

pg. 69

CHAPTER-9 COOLING TOWER9.1. INTRODUCTION

Cooling towers are a very important part of many chemical plants. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-

grade heat from cooling water. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to

the exchangers or to other units for further cooling.

Cooling tower extracts waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use

the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry

cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature.

Typical closed loop cooling tower system is shown in Figure 9.1.

pg. 70

Figure 9.1 closed loop cooling tower

Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power

stations and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers

are natural draft and induced draft cooling towers.Cooling towers vary in size from small roof-top units to very

large hyperboloid structures (as in the adjacent image) that can be up to 200 meters (660 ft) tall and 100 meters (330 ft) in diameter, or rectangular

structures that can be over 40 meters (130 ft) tall and 80 meters (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power

plants, although they are also used to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units

installed on or near buildings to discharge heat from air conditioning.9.2. COOLING TOWER TYPES

Cooling towers fall into two main categories:1. Natural draft and2. Mechanical draft

Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size of these towers, they are generally

used for water flow rates above 45,000 m3/hr. These types of towers are used only by utility power stations.

pg. 71

Mechanical draft towers utilize large fans to force or suck air through circulated water. The water falls downward over fill surfaces, which help

increase the contact time between the water and the air - this helps maximize heat transfer between the two. Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation. Since, the

mechanical draft cooling towers are much more widely used, the focus is on them in this chapter.

Mechanical draft towersMechanical draft towers are available in the following airflow arrangements:

1. Counter flows induced draft.

2. Counter flow forced draft.

3. Cross flow induced draft.

In the counter flow induced draft design, hot water enters at the top, while the air is introduced at the bottom and exits at the top. Both forced and

induced draft fans are used.

In cross flow induced draft towers, the water enters at the top and passes over the fill. The air, however, is introduced at the side either on one side (single-flow tower) or opposite sides (double-flow tower). An induced draft fan draws the air across the wetted fill and expels it through the top of the

structure.

pg. 72

Figure 9.2. cooling tower types

The Figure 9.2 illustrates various cooling tower types. Mechanical draft towers are available in a large range of capacities. Normal capacities range

from approximately 10 tons, 2.5 m3/hr flow to several thousand tons and m3/hr. Towers can be either factory built or field erected – for example

concrete towers are only field erected.

Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., an eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the

pg. 73

individual cells and whether the air inlets are located on the sides or bottoms of the cells.

In HINDALCO most of the cooling towers are cross flow induced draft type.

Figure 9.3. cross flow induced draft cooling tower

9.3. TOWER MATERIALS

In the early days of cooling tower manufacture, towers were constructed primarily of wood. Wooden components included the frame, casing, louvers, fill, and often the cold water basin. If the basin was not of wood, it likely was

of concrete.Today, tower manufacturers fabricate towers and tower components from a variety of materials. Often several materials are used to enhance corrosion

resistance, reduce maintenance, and promote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fiber, and concrete are widely used in tower construction as well as aluminum and various types

of plastics for some components.

pg. 74

Wood towers are still available, but they have glass fiber rather than wood panels (casing) over the wood framework. The inlet air louvers may be glass

fiber, the fill may be plastic, and the cold water basin may be steel.Larger towers sometimes are made of concrete. Many towers casings and

basins are constructed of galvanized steel or, where a corrosive atmosphere is a problem, stainless steel. Sometimes a galvanized tower has a stainless

steel basin. Glass fiber is also widely used for cooling tower casings and basins, giving long life and protection from the harmful effects of many

chemicals.Plastics are widely used for fill, including PVC, polypropylene, and other polymers. Treated wood splash fill is still specified for wood towers, but

plastic splash fill is also widely used when water conditions mandate the use of splash fill. Film fill, because it offers greater heat transfer efficiency, is the fill of choice for applications where the circulating water is generally free of

debris that could plug the fill passageways.

9.4. COMPONENTS OF COOLING TOWER

The basic components of an evaporative tower are: Frame and casing, fill, cold water basin, drift eliminators, air inlet, louvers, nozzles and fans.

1. Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With

some smaller designs, such as some glass fiber units, the casing may essentially be the frame.

2. Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. Fill can either be splash or film

type.

With splash fill, water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill

surface. Plastic splash fill promotes better heat transfer than the wood splash fill.

Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the

splash fill.

3. Cold water basin: The cold water basin, located at or near the bottom of the tower, receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold water discharge

pg. 75

connection. In many tower designs, the cold water basin is beneath the entire fill.

Some forced draft counter flow design, however, the water at the bottom of the fill is In channeled to a perimeter trough that functions as the cold water basin. Propeller fans are mounted beneath the fill to blow the air up through

the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors.

4. Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere.

5. Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower—cross flow design— or be located low

on the side or the bottom of counter flow designs.

6. Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the

tower. Many counter flow tower designs do not require louvers.

7. Nozzles: These provide the water sprays to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed in place and have either round or square spray patterns or can be part of a rotating assembly as found in

some circular cross-section towers.

8. Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their

size, propeller fans can either be fixed or variable pitch.

A fan having non-automatic adjustable pitch blades permits the same fan to be used over a wide range of kW with the fan adjusted to deliver the desired

air flow at the lowest power consumption.

Automatic variable pitch blades can vary air flow in response to changing load conditions.

9.5. COOLING TOWER PERFORMANCE

The important parameters, from the point of determining the performance of cooling towers, are:

pg. 76

Figure 9.4. Range and Approach

i) “Range” is the difference between the cooling tower water inlet and outlet temperature. (Figure 9.4).

ii) “Approach” is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although, both range and

approach should be monitored, the `Approach’ is a better indicator of cooling tower performance. (Figure 9.4).

iii) Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal range, i.e., difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range +

Approach).

iv) Cooling capacity is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.

v) Evaporation loss is the water quantity evaporated for cooling duty and, theoretically, for every 10,00,000 kCal heat rejected, evaporation quantity

works out to 1.8 m3. An empirical relation used often is:

Evaporation Loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2)

T1-T2 = Temp. difference between inlet and outlet water

vi) Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to the dissolved solids in make-up water.

pg. 77

vii) Blow down losses depend upon cycles of concentration and the evaporation losses and is given by relation:Blow Down = Evaporation Loss / (C.O.C. – 1)

viii) Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the water and the air mass flow rates. Against design values, seasonal variations

require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness through measures like water box loading

changes, blade angle adjustments.Thermodynamics also dictate that the heat removed from the water must be

equal to the heat absorbed by the surrounding air: \L(T1 – T2) = G(h2 – h1)

L/G = (h2 – h1) / (T1 – T2)where:

L/G = liquid to gas mass flow ratio (kg/kg)T1 = hot water temperature (0C)

T2 = cold water temperature (0C)h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature

(same units as above)h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature

(same units as above)Thermodynamics also dictate that the heat removed from the water must be

equal to the heat absorbed by the surrounding air.

9.6. FACTORS AFFECTING COOLING TOWER PERFORMANCE

1. Capacity

Heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are not sufficient to understand cooling tower performance. Other factors, which we will see, must be stated along with flow rate m3/hr. For example, a cooling

tower sized to cool 4540 m3/hr through a 13.9oC range might be larger than a cooling tower to cool 4540 m3/hr through 19.5 0C range.

2. Range

Range is determined not by the cooling tower, but by the process it is serving. The range at the exchanger is determined entirely by the heat load and the water circulation rate through the exchanger and on to the cooling

water.

Range 0C = Heat Load in kcals/hour / Water Circulation Rate in LPH

pg. 78

Thus, Range is a function of the heat load and the flow circulated through the system.

Bureau of Energy Efficiency 140

3. Approach

Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature.

For example, the cooling tower might be specified to cool 4540 m3/hr from 48.9 0C to 32.2 0C at 26.7 0C wet bulb temperature.

Cold Water Temperature 32.2 0C – Wet Bulb Temperature (26.7 0C) = Approach (5.5 0C)

As a generalization, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.8 0C approach to the design wet bulb is the coldest water temperature that cooling tower

manufacturers will guarantee. If flow rate, range, approach and wet bulb had to be ranked in the order of their importance in sizing a tower, approach

would be first with flow rate closely following the range and wet bulb would be of lesser importance.

4. Heat Load

The heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired

operating temperature level of the process. In most cases, a low operating temperature is desirable to increase process efficiency or to improve the

quality or quantity of the product. In some applications (e.g. internal combustion engines), however, high operating temperatures are desirable.

The size and cost of the cooling tower is proportional to the heat load. If heat load calculations are low undersized equipment will be purchased. If the calculated load is high, oversize and more costly, equipment will result

inefficient performance.

Process heat loads may vary considerably depending upon the process involved. Determination of accurate process heat loads can become very complex but proper consideration can produce satisfactory results. On the

other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy.

pg. 79

5. Wet Bulb TemperatureWet bulb temperature is an important factor in performance of evaporative

water cooling equipment. It is a controlling factor from the aspect of minimum cold water temperature to which water can be cooled by the

evaporative method. Thus, the wet bulb temperature of the air entering the cooling tower determines operating temperature levels throughout the plant,

process, or system. Theoretically, a cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load.

However, a thermal potential is required to reject heat, so it is not possible to cool water to the entering air wet bulb temperature, when a heat load is

applied. The approach obtained is a function of thermal conditions and tower capability.

Initial selection of towers with respect to design wet bulb temperature must be made on the basis of conditions existing at the tower site. The

temperature selected is generally close to the average maximum wet bulb for the summer months. An important aspect of wet bulb selection is, whether it is specified as ambient or inlet. The ambient wet bulb is the

temperature, which exists generally in the cooling tower area, whereas inlet wet bulb is the wet bulb temperature of the air entering the tower. The later can be, and often is, affected by discharge vapors being recirculated into the

tower. Recirculation raises the effective wet bulb temperature of the air entering the tower with corresponding increase in the cold water

temperature. Since there is no initial knowledge or control over the recirculation factor, the ambient wet bulb should be specified. The cooling

tower supplier is required to furnish a tower of sufficient capability to absorb the effects of the increased wet bulb temperature peculiar to his own

equipment.

It is very important to have the cold water temperature low enough to exchange heat or to condense vapors at the optimum temperature level. By evaluating the cost and size of heat exchangers versus the cost and size of the cooling tower, the quantity and temperature of the cooling tower water can be selected to get the maximum economy for the particular process.

6. Approach and Flow

Suppose a cooling tower is installed that is 21.65 m wide × 36.9 m long × 15.24m high, has three 7.32 m diameter fans and each powered by 25 kW

motors. The cooling tower cools from 3632 m3/hr water from 46.1 0C to 29.4 0C at 26.7 0C WBT dissipating 60.69 million kCal/hr.

pg. 80

For meeting the increased heat load, few modifications would be needed to increase the water flow through the tower. However, at higher capacities, the

approach would increase.

7. Range, Flow and Heat Load

Range is a direct function of the quantity of water circulated and the heat load. Increasing the range as a result of added heat load does require an

increase in the tower size. If the cold water temperature is not changed and the range is increased with higher hot water temperature, the driving force between the wet bulb temperature of the air entering the tower and the hot

water temperature is increased, the higher level heat is economical to dissipate.

If the hot water temperature is left constant and the range is increased by specifying a lower cold water temperature, the tower size would have to be

increased considerably. Not only would the range be increased, but the lower cold water temperature would lower the approach. The resulting change in

both range and approach would require a much larger cooling tower.

8. Approach & Wet Bulb Temperature

The design wet bulb temperature is determined by the geographical location. Usually the design wet bulb temperature selected is not exceeded over 5

percent of the time in that area. Wet bulb temperature is a factor in cooling tower selection; the higher the wet bulb temperature, the smaller the tower

required to give a specified approach to the wet bulb at a constant range and flow rate.

9. Fill Media Effects

In a cooling tower, hot water is distributed above fill media which flows down and is cooled due to evaporation with the intermixing air. Air draft is

achieved with use of fans. Thus some power is consumed in pumping the water to a height above the fill and also by fans creating the draft.

An energy efficient or low power consuming cooling tower is to have efficient designs of fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor. Power savings in a cooling tower, with use of efficient fill

design, is directly reflected as savings in fan power consumption and pumping head requirement.

Function of Fill media in a Cooling Tower:

pg. 81

Heat exchange between air and water is influenced by surface area of heat exchange, time of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. Fill media in a cooling tower is

responsible to achieve all of above.

Splash and Film Fill Media:

As the name indicates, splash fill media generates the required heat exchange area by splashing action of water over fill media and hence

breaking into smaller water droplets. Thus, surface of heat exchange is the surface area of the water droplets, which is in contact with air.

Film Fill and its Advantages:

In a film fill, water forms a thin film on either side of fill sheets. Thus area of heat exchange is the surface area of the fill sheets, which is in contact with

air.

Due to fewer requirements of air and pumping head, there is a tremendous saving in power with the invention of film fill.

Recently, low-clog film fills with higher flute sizes have been developed to handle high turbid waters. For sea water, low clog film fills are considered as

the best choice in terms of power saving and performance compared to conventional splash type fills.

9.7. CHOOSING A COOLING TOWER

The counter-flow and cross flows are two basic designs of cooling towers based on the fundamentals of heat exchange. It is well known that counter flow heat exchange is more effective as compared to cross flow or parallel

flow heat exchange.Cross-flow cooling towers are provided with splash fill of concrete, wood or perforated PVC. Counter-flow cooling towers are provided with both film fill

and splash fill.

Performance Assessment of Cooling TowersIn operational performance assessment, the typical measurements and

observations involved are: Cooling tower design data and curves to be referred to as the basis. Intake air WBT and DBT at each cell at ground level using a whirling

pyschrometer. Exhaust air WBT and DBT at each cell using a whirling psychrometer.

pg. 82

CW inlet temperature at risers or top of tower, using accurate mercury in glass or a digital thermometer.

CW outlet temperature at full bottom, using accurate mercury in glass or a digital thermometer.

Process data on heat exchangers, loads on line or power plant control room readings, as relevant.

CW flow measurements, either direct or inferred from pump motor kW and pump head and flow characteristics.

CT fan motor amps, volts, kW and blade angle settings TDS of cooling water.

Rated cycles of concentration at the site conditions. Observations on nozzle flows, drift eliminators, condition of fills, splash

bars, etc.

9.8. TYPICAL PROBLEMS AND TROUBLE SHOOTING FOR COOLING TOWERS

Problem / Difficulty

Possible Causes Remedies/Rectifying Action

Excessive absorbed current /

electrical load

1. Voltage Reduction Check the voltage

2a. Incorrect angle of axial fan blades

Adjust the blade angle

2b. Loose belts on centrifugal fans (or speed reducers)

Check belt tightness

3. Overloading owing to excessive air flow-fill has

minimum water loading per m2 of tower section

Regulate the water flow by means of the valve

4. Low ambient air temperature

The motor is cooled proportionately and hence delivers more

than name plate powerDrift/carry-over of water outside the

unit

1. Uneven operation of spray nozzles

Adjust the nozzle orientation and

eliminate any dirt2. Blockage of the fill pack Eliminate any dirt in the

top of the fill3. Defective or displaced

dropleteliminators

Replace or realign the eliminators

4. Excessive circulating water Adjust the water flow-pg. 83

flow (possibly owing to too high

pumping head)

rate by means of the regulating valves.

Check for absence of damage to the fill

Loss of water from basins/pans

1. Float-valve not at correct level

Adjust the make-up valve

2. Lack of equalizing connections

Equalize the basins of towers operating in

parallelLack of cooling and hence increase in

temperatures owing to increased temperature range

1. Water flow below the design valve

Regulated the flow by means of the valves

2. Irregular airflow or lack of air

Check the direction of rotation of the fans and/or belt tension

(broken belt possible)3a. Recycling of humid

discharge airCheck the air descent

velocity3b. Intake of hot air from

other sourcesInstall deflectors

4a. Blocked spray nozzles (or even blocked spray tubes)

Clean the nozzles and/or the tubes

4b. Scaling of joints Wash or replace the item

5. Scaling of the fill pack Clean or replace the material (washing with

inhibited aqueous sulphuric acid is

possible but long, complex and expensive)

9.9. CALCULATION

Cooling tower pump

Kw – 55 kw

Volt – 415 +- 10%

RPM – 1480

Amps – 93.9

pg. 84

Inlet water temperature – 40 0c

Outlet water temperature – 30 0c

pg. 85