ENGINEERING COLLEGEmahalakshmiengineeringcollege.com/pdf/eee/IVsem/EE2252/UNIT 1.… · Types of...

EEE DEPT /MAHALAKSHMI ENGINEERING COLLEGE,TRICHY-621213

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI – 621213

QUESTION BANK

--------------------------------------------------------------------------------------------------------------

Sub. Code : EE2252 Semester : IV

Subject : POWER PLANT ENGINEERING Unit : I

--------------------------------------------------------------------------------------------------------------

PART-A

1.Mention the various ash handling system in the thermal power plant?

(AU NOV 11)

(i)Mechanical handling system

Use of screw/belt conveyors and

Bucket elevators

(ii)hydraulic system

(iii)pneumatic system

(iv)steam jet system

2.Define Fluidized bed Combustion(FBC)? (AU NOV 10)

Fluidized Bed Combustion evolved from efforts to find a combustion process able

to control pollutant emissions without external emission controls (such as

scrubbers).The technology burns fuel at temperatures of 1,400 to 1,700 degrees F,well

below the threshold where nitrogen oxides form.


FBC systems fit into essentially two major groups:

(i)Atmospheric systems(FBC)

(ii)Pressurized systems(PFBC) and

Two minor subgroups

(iii)Bubbling or circulating fluidized bed(BFB or CFB)

FBC -Atmospheric fluidized beds use a shy limestone or dolomite to capture sulfur

released by the combustion of coal.

3.What are the factors affecting the cooling of water in cooling tower?

(AU NOV 11)

(i)Temperature of air

(ii)humidity of air

(iii)Size and height of tower

(iv)velocity of air entering the tower

(v)temperature of hot air

(vi)Accessibility of air to all parts of tower.

4.What are the process in Rankin cycle? (AU NOV 12)

Reversible adiabatic expansion in the turbine.

Constant pressure heat transfer in the condenser.

Reversible adiabatic pumping process in the feed pump.

Constant pressure heat transfer in the boiler.

5.State and explain the Carnot cycle process? (AU APR 11)

The Carnot cycle consists of four reversible process namely two

isothermal process and two adiabatic process. But there is no heat losses in this cycle.

6.Define boiler mountings and boiler accessories? (AU APR 10)

Boiler mountings are required for the operation and safety of the boiler

Boiler accessories are used as integral parts of boiler and help in running

successfully.


7.What is the necessity of feed pump in thermal power plant? (AU NOV 13)

Feed pump is used to circulate the demineralized water (or) condensed steam

once again to boilers.

The closed circuit operation can be achieved by feed pump Which reduces cost

and increases efficiency of thermal plants.

8.Mention the advantage of self pulverized fuel firing system? (AU NOV 12)

It is possible to use any grade of coal as coal is powdered before use.

Boiler unit can be started from coal rapidly and efficiently

High efficiency due to complete combustion

This method is more flexible to respond for fluctuating loads on plants.

9.State the zeroth law of thermodynamics? (AU NOV 19)

Zeroth law states that "Two systems in thermal equilibrium with the third system

are in thermal equilibrium to each other.

10.What is thermodynamic cycle? (AU APR 09)

A thermodynamic cycle is a serious of thermodynamic process transfering

heat and work, while varying pressure,temperature,and other state variables ,eventually

returning a system to its initial state.

Two primary cycles of thermodynamic cycles are

(i)Power cycles (ii)Heat pump cycles.

11.What is boiler ?

Boiler is the enclosed vessel in which water is heated and circulated until the

water is turned into steam at the required pressure. Sometimes this stream is further

heated in the super heater as higher the steam pressure and temperature the greater

efficiency the engine will have in converting the heat in steam in to mechanical work.

Types of boilers:

(i)Fire tube boilers

(ii)Water tube boilers

12.What is meant by Pulveriser ?

A pulveriser is a device for grinding coal for combustion in a furnace in power

plant. The coal is put in the boiler after pulverisation.


Types of pulverisers:

(i)Ball mill

(ii)Tube mill

(iii)Ring and ball

13.List out the various types of air heaters?

(i)Tubular Air heater

(ii)Plate Type air heater (iii)Regenerative air heater

PART-B

1.Give the layout of modern steam power plant and explain it briefly?

(AU APR10, 11,12)

The schematic line diagram of modern steam power station is

shown. The whole arrangement can be divided into the following stages for the sake of

simplicity :

(i)Coal and Dust Handling Arrangement

(ii)Steam Generating Plant

(iii)Steam turbine

(iv)Alternator Generator

(v)Feed Water

(vi)Cooling arrangement

Cooling and dust handling arrangement:

The coal is transported to the power station by road or rail and is stored

in the coal storage plant .

From the coal storage plant ,coal is delivered to the coal handling plant where it

is crushed into small pieces in order to increase its surface exposure ,thus

promoting rapid combustion without using large quantity of excess air.

The crushed coal is fed to the boiler by belt conveyers. The coal is burnt in the

boiler and the ash produced after the complete combustion of coal is removed to


the ash handling plant and then delivered to the ash storage plant for disposal.

The removal of the ash from the boiler furnace is necessary for proper burning of

the coal.

Steam Generating Plant:

The steam generating plant consists of a boiler for the production of steam and

other auxiliary equipment for the utilisation of flue gases.

Boiler:

The heat of combustion of coal in the boiler is utilised to convert water into steam at

high temperature and pressure .

The flue gases from the boiler is utilised to convert water into steam at high

temperature and pressure. The flue gases from the boiler make their path

through super heater,economiser,air preheater and are finally exhausted to

atmosphere through the chimney.

Super heater:

The steam produced in the boiler is generally wet and is passed through a super

heater where it is dried and super heated.

The superheating provides two principal benefits .Firstly ,the overall efficiency is

increased. Secondly ,too much condensation in the last stages of turbine ,which

would cause blade corrosion ,is avoided.

The superheated steam from the super heater is fed to steam turbine through

the main valve.

Economizer:

An economizer is essentially a feed water heater and derives heat from

the flue gases for this purpose.

The feed water is fed to the economizer before supplying to the boiler.

The economizer extracts a part of heat of flue gases to increases the

feed water temperature.

Air Preheater:

An air preheater increases the temperature of the air supplied for coal

burning by deriving heat from the flue gases

.Air is drawn from the atmosphere by a forced draught fan and is passed

through air preheater before supplying to the boiler furnace. The air


preheater extracts heat from the flue gases and increases the

temperature of air used for coal combustion .

The principle benefits of pre heating the air are: the increased thermal

efficiency and increased steam capacity per square meter of bolier

surface.

Steam Turbine:

The dry and superheated steam from the super heater is fed to the steam turbine

through main valve .

The heat energy of the steam when passing over the blades of the turbine is

converted into mechanical energy.

After giving heat energy to the turbine ,the steam is exhausted to the condenser

which condenses the exhausted steam by means of cold water circulation.

Alternator:

The steam turbine is coupled to an alternator .The alternator converts mechanical

energy of turbine into electrical energy. The electrical output from the alternator is

delivered to the bus bars through the transformer, circuit breakers and isolators.

Feed Water:

The condensate from the condenser is used as feed water to the boiler .Some water

may be lost in the cycle which is suitably made up from the external source. The feed

water on its way to the boiler is heated by water heaters and economiser.This helps in

raising the overall efficiency of the plant.

Cooling arrangement:

To improve the efficiency of the plant, the steam exhausted from the turbine is

condensed by means of condenser. Water is drawn from a natural source of

supply such as a river ,canal or lake and is circulated through the condenser.

The circulating water takes up the heat of the exhausted steam and itself

becomes hot. This hot water coming out from the condenser is discharged at a

suitable location down the river .

In case the availability of water from the source is not assured throughout the

year ,cooling towers are used.

During the scarcity of water in the river, hot water from the condenser is passed

on to the cooling towers where it is cooled. The cold water from the cooling tower

is reused in the condenser.


2.How are the cooling towers classified? Explain any one of them with the neat

sketch? (AU APR 10)

As the size of the power plant is increased ,the space required for the spray

pond increases considerbly.Therefore ,cooling towers are very much required for large

size power stations. Lack of large water flows makes these plants rely on cooling towers

,to remove heat energy from the circulating water in a closed circuit.

A cooling tower is a semi -enclosed device for evaporative cooling of water by

contact with air .


It is a concrete or wooden structure inside which an arrangement of corrugated

surfaces ,troughs or bufflers are provided. At bottom ,there is a reservoir for

storage of cooled water.

Warm water is allowed to fall from the top and is allowed to tickle in drops

through trays, while air flows from bottom to the tower of the top.

Drift eliminators are provided at the top of the tower to prevent the escape of

water particles with air.

The eliminators are made of few rows of blades, placed inclined to each other

and providing a zigzag path for the air steam. The moisture is deposited on the

blades and falls back into tank.

The cooling of water by means of cooling tower can again be divided into two

types :

(i)Natural Draft Cooling towers

(ii)Forced Draft Cooling towers.

Natural Draft Cooling towers :

In the cooling tower ,the water cools by (i) evaporation (ii) heat transfer to the air.

In many towers about 75% of the cooling takes place by evaporation and the

remainder by heat conduction. Performance of the tower depends upon the

difference between the hot water temperature entering the tower and the cold

water leaving it.

The natural draft towers are closely built structures which are quite tall and have

the top portion working as a stack. The cooling depends upon the tower height

and temperature of air and so these towers have their limitations when large

cooling capacity is required.

The circulating water to be cooled is pumped at about 10m above the ground into

troughs and the nozzles at the bottom of the troughs, spray the water in thin

sheets.

These sheets of water then break under force of gravity and on striking with

hurdles. The circulation of the air for cooling the water is induced by enclosing

the heated air in the chimney.

The heated air is lighter than the surrounding atmosphere which creates a

difference of pressure.

There are two types of construction of natural draft cooling tower(i) rectangular

timber tower (ii) reinforced concrete hyperbolic type.

The timber cooling towers have much lower initial costs but is inferior to concrete

cooling towers in the following respects:


1.The timber is liable to the mechanical injury ,etc.,if some of the boards break it will

stop circulation of air .

2.There is a possibility of fire hazard when there is no circulation of water at the time of

repairs.

3.The life is 10 to 15 years.

Nowadays concrete towers are used and they have the following advantages:

1.Better circulation of air.

2.Wide variation of load is possible and much larger amount of water per hour can be

cooled as compared to timber type.

3.Maintenance cost is very much less.

4.No fire hazard.

5.Chimney shape creates its own draft assuring efficiency operation even when there is

no wind.


Forced Draft cooling tower:

When the generating capacity of the power station is high, it requires very high

rate of cooling water per hour .

In such cases the natural draft type of cooling towers are not useful and thus it

necessitates large amount of cooling air provided by fans. The principle of

forced draft cooling tower ,except that a motor driven fan is also provided at the

base.

If however impeller is used at the top of the cooling tower it is called induced

draft cooling tower.

The forced draft cooling towers have a horizontal shaft fan on the side of the

cooling tower. The fan discharges air towards the back of tower and then air is

turned upwards towards by means of baffles and in the process it cools the

falling water.

The velocity of the air passing out is generally not high so the fan speed of the

order of 1000 r.p.m. can be used.

The induced draft type cooling towers are most popular for very large capacity

installations. They consists of large fans with vertical shafts located at the top of

the tower and the fans pull air upwards from the sides of the cooling tower.

The warm air is exhausted at a considerable velocity upward after cooling the

water on its way.It necessitates low speed fans and mostly require speed

reduction gears to be used along with the motors.

In order to increase the efficiency of the tower, the water flow over the tower

should be uniformly distributed and divided into fine droplets and the large

wetted area should be used.

In order to increase the air contact time ,a series of baffles ,shelves etc.are used

to obstruct the flow of air.

The following are the disadvantages of the forced draft cooling towers:

(i) High velocity from the impeller makes difficult the air distribution over the whole of

area.

(ii)The open nature of the top of the cooling tower with low humidity conditions

produces a great load on the spray eliminator.

(iii)Due to the low height ,low discharge velocity along with light winds may cause

recirculation of the heated air.


3.Draw the layout of thermal power plant and the explain the working of different

circuits? (AU NOV 10)

In the thermal power plant ,one of coal, oil or natural gas is used to convert the

water into steam. The steam is used to turn a turbine, which is connected to a

generator, which is then supplied to the customers through high voltage power lines.

Detailed process of power generation in a thermal power plant:

(i)Water Intake:

Firstly, water is taken into the boilers through a water source. If water is

available in a plenty of in the region, then the source is open pond or river. If water is

scarce ,then it is recycled and the same water is used over and over again.


(ii)Boiler heating:

The boiler is heated with the help of oil, coal or natural gas. A furnace is used to

heat the fuel and supply the heat produced to the boiler. The increase in temperature

helps in the transformation of water into steam.

(iii)Steam turbine:

The steam is generated in the boiler is sent through the steam turbine. The turbine

has blades that rotate when high velocity steam flows across them. This rotation of

turbine blades is used to generate electricity.

(iv)Generator:

A Generator is connected to the steam turbine. When the turbine rotates,the

generator produces electricity which is then passed on to the power distribution

systems.

(v)Steam mountings:

There is some other equipment like the economiser and air pre heater. An

economiser uses the heat from the exhaust gases to heat the feed water. An air pre-

heater heats the air sent into the combustion chamber to improve the efficiency of the

combustion process.

(vi)Ash collection system :

There is separate residue and ash collection system in place to collect all the

waste materials from the combustion process and to prevent them from escaping into

the atmosphere.

Apart from this, there are various other monitoring systems and instruments in

place

to keep track of the functioning of all the devices .This prevents any hazards from taking

place in plant.


4.Name the various methods of ash handling? Describe the pneumatic system of

ash handling? (AU NOV 10)

The widely used ash handling systems are

(i)mechanical system

(ii)hydraulic system

(iii)pneumatic system

(iv)steam jet system

Pneumatic type:

Pneumatic type ash handling is the most popular method used in medium level

power plants. It uses dense phase conveying system for conveying ash is totally

enclosed without any leakage. Conveying material moves through the pipe line slowly

as a coherent slug at a low velocity, which ensures minimum wear and tear. The

system can convey materials upto a distance of around 200-250 mts.

Principal operation for dense phase pneumatic conveying:

(i).Conveying material is collected in hopper and is sensed by the permissive level

probe provided in hopper and initiates conveying cycle. The steam has dual operability

either through level probe or through an auto timer. In timer mode of operation the

system goes on continuous cycle after a present time interval. Moreover the system has

manual override facility.

(ii)The inlet valve on the top of the vessel opens and allows material to gravitate into the

vessel. The valve closes after the preset adjustable time delay.


(iii)Once the valve closing is ensured through the limit switch valve its circumferential

inflatable seal inflates and ensures complete leak proof.

(iv)Now ,convey air is injected in to the vessel after ensuring seal pressure. Thus vessel

is pressurized and the material resistance leads to pressure build up which conveys

material to the destination point.

(v)When conveying is complete ,the pressure drops down to near atmospheric

pressure and is sensed by control system .The conveying air supply to the system is

then stopped.

(vi)Now the system is ready to accept the next change of material on command from

the level probe/autotimer.

(vii)Normally the destination side is also provided with high level switch ,which gives an

alarm when filled up and prevents any further transfer of material into it.

(viii)The system is provided with the various accessories like bag filters, ash conditioner

to make the system environmental friendly. In a typical ash handling plant ,the ash

storage silo is designed for 1-day storage. The ash is silo is discharged on a lorry


through an ash conditioner, which sprays water in mist form and prevents the ash from

escaping to the atmosphere.

5.Explain the various steps involved in "in plant coal" handling in a thermal

power plant? (AU NOV 10)

In power plant coal handling is a very important aspect of power plant safety.In

variably the coal is not exposed as it can pollute the air and release the

poisonous gases like carbon monoxide.

The coal from the heaps is moved in the plant by means of long conveyers that

are electrically operated. Basically coal /lignite handling system consists of a

hopper for receiving coal from -end loaders or dumpers.

A grating is provided above the hopper, which limits the maximum size of the

coal /lignite going to the conveyer.

A vibro -feeder that controls the flow from the hopper to belt conveyer and carries

in to a primary crusher or a vibrating screens depending on the size of ROM

coal/lignite.(i.e. run of mine (ROM) coal the coal that comes directly from the

mine has impurities associated with it).

From the double deck -vibrating screen -6 mm present in the raw material directly

goes to the bunker.

Any material above +6 mm gets crushed for the desired sizing and is

recirculated to the screen.

Crushers used are hammer mills, impactors or roll crusher. It should be ensured

that no metallic particles get mixed with coal and gas to the crusher who would

reduce the crusher life to great extent.

Now -a-days metal detectors ,electromagnetic separators can be used for

removing foreign metallic items from the ROM coal/lignite. The crusher should

also ensure that only correct size particles go to the boiler bunker.

Higher size particle would not get fluidized and fines won escape without getting

burnt. The crusher and the screen is thus the heart of the plant and should be

chosen with utmost care with due consultation with specialists.


Layout of the coal handling system:

Coal is supplied to the power plant in railway wagons.

The coal is then unloaded into underground hoppers or bunkers. The wagons

can be unloaded either manually or through rotary wagon trippers.

From the bunkers, the coal is lifted by conveyor to transfer tower from where it

can be delivered either to the fuel store or by a conveyor to a crusher.

The coal is then passed through the magnetic separators and screens and

crushed in crushers into pieces 25 to 30 mm in size for stoker firing and 10 to 20

mm when pulverized fuel is fired in boiler furnaces.

The crushed coal in the latter case is milled to fine powder and then it is carried

through automatic weigher to a transfer tower fuel is lifted and distributed

between boiler hoppers by a conveyor.


6.Briefly explain the air cooled cooling system? (AU NOV 12)

There are two types of air cooled cooling system:They are

(i)The direct dry cooling system

(ii)The indirect dry cooling system

Direct Dry cooling system:

In the direct dry system ,the turbine exhaust steam is piped directly to the air

cooled finned tube,condenser.The finned tubes are usually arranged in the form

of an ' A' frame or delta over a forced draught fan to reduce the land area.

The steam trunk main has a large diameter is as short as possible to the turbine.


The whole power plant then uses less than 10% of the water required for a wet -

cooled plant, but a lot of power (around one to 1.5% of power station's output ) is

consumed by the large fans required .This is direct dry cooling ,using air cooled

condenser.(ACC)

The direct system is the most commonly used as it has the lowest capital cost,

but significantly higher operating costs. The power required to operate the fans of

this system is several times that required for wet towers, being typically 4 to 5

MW for a 420 MW unit.

Indirect dry cooling system :

Indirect dry cooling systems have a condenser and a turbine exhaust system as for

wet systems, with the circulating water being passed through finned tubes in a natural

draught cooling tower. The water pipe work allows the towers to be sited away from the

station.

A variation on this type of indirect system is the system that uses a direct contact

condenser in place of the traditional tube condenser. In the spray condenser ,the water

from the cooling cycle mixes with the boiler water. The maintenance of the water quality

to suit all the circuits to the successful operation of the system.


7.Draw the Rankine cycle for a coal and steam power plant ?((AU APR 11)


8.Describe the different types of overfeed stokers ?(AU APR 13)

ENGINEERING COLLEGEmahalakshmiengineeringcollege.com/pdf/eee/IVsem/EE2252/UNIT 1.… · Types of...

Documents

Transcript of ENGINEERING COLLEGEmahalakshmiengineeringcollege.com/pdf/eee/IVsem/EE2252/UNIT 1.… · Types of...