General Layout of the Plant & Circuits

8/13/2019 General Layout of the Plant & Circuits

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General Layout of the Plant: Though each plant is uniquein itself in terms of specific features and functionalities, still

there is a broad outline to which all thermal power plants

confirm to and the general layout of a typical power plant.

There are four main circuits in any thermal power plantand these are

1. Coal & Ash Circuit this circuit deals mainly with feeding

the boiler with coal for combustion purposes and taking care

of the ash that is generated during the combustion process

and includes equipment and paraphernalia that is used to

handle the transfer and storage of coal and ash.

. Air & !as Circuit we know that air is one of the maincomponents of the fire triangle and hence necessary for

combustion. "ince lots of coal is burnt inside the boiler it

needs a sufficient quantity of air which is supplied using

either forced draught or induced draught fans. The e#haust

gases from the combustion are in turn used to heat the

ingoing air through a heat e#changer before being let off in

the atmosphere. The equipment which handles all these

processes fall under this circuit.

$. %eed ater & "team Circuit this section deals with

supplying of steam generated from the boiler to the turbines

and to handle the outgoing steam from the turbine by cooling

it to form water in the condenser so that it can be reused in

the boiler plus making good any losses due to e'aporation

etc.

(. Cooling ater Circuit this part of the thermal power plant

deals with handling of the cooling water required in the

system. "ince the amount of water required to cool theoutgoing steam from the boiler is substantial, it is either taken

from a nearby water source such as a ri'er, or it is done

through e'aporation if the quantity of cooling water a'ailable

is limited.


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The abo'e breakdown of the plant would gi'e you a clear idea

about the components of the plant but a complete picture shown

below would be more useful in getting an idea how these circuits

are integrated together to form the complete power plant.

THE AIR/GAS CIRCUIT
http://www.brighthub.com/engineering/mechanical/articles/23141.aspx?image=7191


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The circuit or path of airflow to the boiler and gasses of

combustion from the boiler furnace back

to the atmosphere is known as the Air/Gas Circuit.

Air for combustion is drawn from the atmosphere by the %orced

)raught *%)+ fan and deli'ered to the wind bo# after passing

through the Air eater. The Air eater is a heat e#changer where

heat is reco'ered from the flue gasses to pre-heat the combustion

air. The combustion air flows from the wind bo# into the burner

assemblies where the fuel and air are mi#ed in the correct ratio for

stable, efficient combustion. The fuelair mi#ture burns in the

boiler furnace in a continuous process releasing heat energy, which

is absorbed by the boiler components *water walls, super heater,

reheater, economiser and air heater+ The hot gasses from

combustion are drawn from the furnace and through the boiler and

air heater by the /nduced )raught */)+ fan and discharged up the

stack. 0oilers burning coal ha'e a precipitator or some other

method of particulate collection located in the flue gas path

between the air heater and /) fans to pre'ent fly ash discharge into


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the atmosphere. The circuit or path of the steam flow from the

boiler to the turbines and water flow from the condenser to the

boiler is known as the Thermal Cycle.

THE THERMA C!CE

The 0oiler %eed ump raises the pressure of the feed water

sufficiently for the water to flow through the igh ressure *+

%eed eaters, the feed water regulating 'al'e and into the boiler.

The feed heaters use steam bled from the igh and /mmediate

ressure * & /+ turbines topre-heat the feed water.

The feedwater-regulating 'al'e controls the amount of water

entering the boiler to maintain thecorrect water le'el in the boiler

drum. The economiser is situated in the flue gas path2 it is a tubular

heat e#changer, which further preheatsthe feed water using the flue

gasses lea'ing the boiler as the heat source. The boiler drum

distributes the feed water into down comers *large bore e#ternal

pipes+, which deli'ers the water to the bottom distribution headers,

where the water enters the tubes forming the water walls of the

boiler furnace. The boiler water in the water walls is heated by the


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heat released by the combustion of fuel in the furnace. The water

rises in the water walls gaining more heat2 hence some of it is

changed into steam. The steamater mi#ture is collected from the

water walls in the top or collection headers and returned to the

drum where it is separated, the water returning to the boiler water

circulation pattern, the steam passing to a heat e#changer known as

the super heater where its temperature *thus energy le'el+ is

increased. The superheated steam flows from the boiler to the

turbine 'ia the go'ernor 'al'es, which controls the steam flow to

the turbine ie" the energy input to the turbine. "team entering the

turbine is e#panded through the no33les and the rotating turbine

blades producing torque at the turbine shaft. ence, the steam

pressure and temperature is reduced that is, heat energy

*enthalpy+ is transformed to mechanical energy *torque+. The

steam lea'es the turbine to return to a heat e#changer located in

the boiler flue gas path known as the 4reheater5 where its

temperature *energy le'el+ is increased. The 4hot reheat steam5

flows to the / and finally to the 6ow ressure *6+ turbine,


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e#panding on the way through as it does its work spinning the

turbine blades.

The steam e#hausts from the 6 turbine to the condenser, where

the steam space is held at a high 'acuum ensuring ma#imum

e#pansion *work+ from the steam through the 6 turbine. The

e#haust steam is condensed in the condenser turning back into

water known as condensate. The Cooling ater *C+ system

supplies 'ast 'olumes of seawater to the condenser to condense the

e#haust steam. The e#traction pump remo'es the condensate from

the condenser and deli'ers it to the deaerator 'ia the 6 feed

heaters. The deaerator and 6 feed heaters use steam bled from the

/ and 6 turbines to pre-heat the condensate *6 feed water+.The

0oiler %eed ump draws water from the deaerator storage tank and

so the cycle is complete.


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Steam #Thermal$ power %lant

A steam power plant, also known as thermal power plant, is

using steam as working fluid. "team is produced in a boilerusing coal as fuel and is used to dri'e the prime mo'er,

namely, the steam turbine. /n the steam turbine, heat energy

is con'erted into mechanical energy which is used for

generating electric power. !enerator is an electro-magnetic

de'ice which makes the power a'ailable in the form of

electrical energy.

ayout of steam power plant&

The layout of the steam power plant is shown in figure

below. /t consists of four main circuits. These are7

Coal and ash circuit.

Air and flue gas circuit

ater and steam circuit and

Cooling water circuit
http://mechanical-engineering-info.blogspot.in/2011/11/steam-thermal-power-plant.htmlhttp://3.bp.blogspot.com/-F5WRm_ji9l4/TsJINYANAiI/AAAAAAAAAW0/eKkBY9AbxOQ/s1600/power.jpghttp://mechanical-engineering-info.blogspot.in/2011/11/steam-thermal-power-plant.html


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Coal and ash circuit&

Coal from the storage yard is transferred to the boilerfurnace by means of coal handling equipment like belt

con'eyor, bucket ele'ator, etc., ash resulting from the

combustion of coal in the boiler furnace collects at the back

of the boiler and is remo'ed to the ash storage yard through

the ash handling equipment.

Ash disposal7

The indian coal contains $89 to (89 ash. A power plant of

188: 8 to ; tonnes of hot ash per hour. ence

sufficient space near the power plant is essential to dispose

such large quantities of ash.

Air and flue 'as circuit&

Air is taken from the atmosphere to the air preheater. Air is

heated in the air preheater by the heat of flue gas which is

passing to the chimney. The hot air is supplied to the

furnace of the bolier.

The flue gases after combustion in the furnace, pass around

the boiler tubes. The flue gases then passes through a dust

collector, economi3er and pre-heater before being

e#hausted to the atmosphere through the chimney. 0y this

method the heat of the flue gases which would ha'e been

wasted otherwise is used effecti'ely. Thus the o'erall

efficiency of the plant is impro'ed.


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Air pollution&

The pollution of the surrounding atmosphere is caused bythe emission of ob


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or lake, pro'ided adequate water supply is a'ailable from

the ri'er or lake throughout the year.

/f adequate quantity of water is not a'ailable at the plant

site, the hot water from the condenser is cooled in thecooling tower or cooling ponds and circulated again.

Ad)anta'es of thermal power plants

1. /nitial cost is low compared with hydro-plant.

. The power plant can be located near load center, so

the transmission losses are considerably reduced.

$. The generation of power is not dependent on thenature=s mercy like hydro plant.

(. The construction and commissioning of thermal plant

requires less period of time than a hydro plant.

Modern Hi'h %ressure *oilers

A boiler which generates steam at a pressure of >;

kgfsq.cm or abo'e is termed as a+hi'h pressure ,oiler-.

The present tendency is towards the use of high pressure

boilers in power plants. The modern high pressure boilers

used for power generation ha'e capacities of (8 to 1?88

tonneshr of superheated steam with a pressure upto 18

kgfsq.cm and a temperature of about ?;8=C. @ne of the

largest modern steam power plants in the world is in


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ater tube boilers are generally preferred for high pressure

and high output whereas fire tube boilers for low pressure

and low output.Ad)anta'es of hi'h pressure ,oilers&

Method of water circulation

ater circulation through the boiler may be either natural

circulation due to density difference or by forced

circulation. /n high pressure boilers, water circulation is

made with the help of a centrifugal pump which forceswater through the boiler tubes. This is called 4forced

circulation of water5. The use of natural circulation is

limited upto 18 kgfsq.cm. "team pressure and forced

circulation upto 18 kgfsq.cm. %orced circulation increases

the rate of heat transfer and hence increases the steam

generating capacity of boilers.

Si.e of drumsThe high pressure boilers are characteri3ed by the use of

'ery small steam separating drums or by the complete

absence of any drum.

Type and arran'ement of tu,es

The heat of combustion is utili3ed more efficiently by the

use of small diameter and light weight tubes in large

numbers. To a'oid large resistance to the flow of water ,the high pressure boilers ha'e a parallel set of arrangement

of tubes.

Compactness


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The boiler components can be arranged hori3ontally, gi'ing

greater accessibility and operational con'enience as high

head required for natural circulation is eliminated by using

forced circulation. The space required is hence less andarrangement is compact.

(oundation cost

)ue to the light weight tubes and small si3e drums required

and the arrangement being compact, the cost of foundation

is reduced.

Efficiency

The efficiency of the power plant is increased upto (89, by

using high pressure superheated steam. Also steam can be

raised quickly after the boiler is fired.

Cost of electricity

"ince efficiency of the plant is increased by using high

pressure boilers, the cost of electricity production is

reduced.)erheatin'

All the parts are uniformly heated2 therefore the danger of

o'erheating is reduced. Also thermal stress problem is

a'oided.

Scale formation

The tendency of scale formation is eliminated due to the

high 'elocity of water through the boiler tubes.

Types of hi'h pressure ,oilers

1. 6a-:ont 0oiler
http://mechanical-engineering-info.blogspot.in/2012/03/la-mont-boiler-working-and-construction.htmlhttp://mechanical-engineering-info.blogspot.in/2012/03/la-mont-boiler-working-and-construction.html


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. 6oeffler 0oiler

$. 0enson 0oiler
http://mechanical-engineering-info.blogspot.in/2012/04/loeffler-boiler.htmlhttp://mechanical-engineering-info.blogspot.in/2012/04/benson-boiler.htmlhttp://mechanical-engineering-info.blogspot.in/2012/04/loeffler-boiler.htmlhttp://mechanical-engineering-info.blogspot.in/2012/04/benson-boiler.html

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General Layout of the Plant & Circuits