Chapter 5_Steam Turbines and Condensers

8/13/2019 Chapter 5_Steam Turbines and Condensers

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Thermal Engg.

CHAPTER 5 Steam Turbines and Condensers

STEAM NOZZLESIntroductionThe main use of a steam nozzle in steam turbines is to produce a jet of steam with a high

velocity.

Continuity EquationConsider a nozzleMass flow rate at enter of nozzle =!"Mass flow rate at e#it of nozzle =$!$"$where% = density of fluid% ! = area " = "elocity of flow!ccording to continuity e&uation% mass flow rate remains constant!" = constant 'for steady flow(

ors

AV

= constant 'where

s

1

= % where vs= specific volume(

Types of steam nozzles! steam nozzle is a passage of varying cross)section% which converts heat energy of steaminto *inetic energy. The increase in velocity of steam jet at e#it of nozzle is obtained due todecrease in total enthalpy of steam. This conversion of energy is done with minimum loss.+ollowing three types of nozzles are important,1. Convergent Nozzle

-hen the cross section of a nozzle decreases continuously from entrance to e#it% it is

called a convergent nozzle. This acts as subsonic nozzle.

2. Divergent Nozzle

-hen cross section of nozzle increases continuously from entrance to e#it% it is calleddivergent nozzle. This acts as supersonic nozzle.

3. Convergent-Divergent Nozzle


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Thermal Engg.

-hen the cross section of nozzle first decreases from its entrance to throat and thenincreases from throat to e#it% it is called convergent divergent nozzle.

ThroatThe smallest cross section of the nozzle is called the throat."elocity '"$( of steam at any section considered in ms is calculated using steady flowe&uation%

"$= ( ))(200021

2

1

hhV + converting enthalpy from */*g to /*g

"$= ( ))(2000 dh

since "is negligible and hd= Enthalpy or heat drop during e#pansion of steam in a nozzle

"$= dh72.44

"$= dKh72.44

-here 0 = the nozzle coefficient or nozzle efficiency due to friction.

Concept of Mach Number

Mach 1umber% M=a

V

Mach number is the ratio of velocity '"( at a state in a flowing fluid to the value of sonicvelocity 'a( ie velocity of sound at the same state.-hen M2% flow is called supersonic%-hen M= % flow is called sonic%-hen M3% flow is called subsonic%-hen M22% flow is called 4ypersonic.

Critical Pressure Ratio! nozzle is normally designed for ma#imumdischarge by designing a certain throat pressure%

which produces this condition.

The one value of the ratio1

3

P

Pwhich will produce

ma#imum discharge is called critical pressureratio.

1

1

3

1

2

+=

n

n

nP

P

-here 56= 5ressure of steam at throat in 1m$

5= 7nitial 5ressure of steam in 1m$

n = 5olytropic inde#

$


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Thermal Engg.

Ma#imum discharge is given by

+

+=

12

1

1

max

1

2

1

2 n

nv

P

n

nAm

-here% v= volume of *g of steam at pressure '5( in m6

! = cross sectional area at throat in m$

5= 7nitial 5ressure of steam in 1m$

n = 5olytropic inde#

Applications or Uses of Nozzle. 7n Turbo Machines 'ie 8team Turbines(

The high velocity stream of fluid is directed on the curved blade and *inetic energy istransferred into mechanical wor*.

$. 7n /et 5ropulsionThe thrust produced by the jet issuing from a nozzle provides the propulsive effort.

6. 7n +low MeasurementThe differential pressure drop is correlated to velocity to fluid discharge.

9. 7t is also used in injectors for pumping feed water into boiler.:. 7t is also used in ejector condenser.

STEAM TURBINESIntroduction8team turbine is a prime mover which converts heat energy of high pressure% hightemperature steam into mechanical wor*.

Classification of team Turbines. !ccording to the mode of steam action

i. 7mpulse Turbineii. ;eaction Turbine

$. !ccording to the direction of steam flowi. !#ial +low Turbineii. ;adial +low Turbine

6. !ccording to e#haust condition of steami. Condensing Turbineii. 1on Condensing Turbine

9. !ccording to the pressure of steam

i. 4igh 5ressure Turbineii. Medium 5ressure Turbineiii.


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Thermal Engg.

!n impulse turbine is a turbine which runs by the impulse of steam jet. 7n this turbine% the steam is first made to flow through a nozzle.

Then the steam jet impinges on the turbine blades 'which are curved li*e buc*ets( and

are mounted on the circumferences of the wheel.

The steam jet after impinging glides over the concave surface of the blades and finally

leaves the turbine.

The buc*ets move in the direction of jet. This movement of the blades ma*es the runner

to rotate.

De-Level Impulse Turbine

7t is most commonly used steam turbine. 7t has the following main components,i. No!e

9


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Thermal Engg.

7t is a circular guide mechanism% which guides the steam to flow at the designed directionand velocity. 7t also regulates the flow of steam.

ii. Runner and B!ades7t consists of a circular disc fi#ed to a horizontal shaft. n the periphery of the runner% anumber of blades are fi#ed uniformly.

iii. Casin"7t is an air tight metallic case% which contains the turbine runner and blades. 7t controlsthe movement of steam from the blade to the condenser and does not permit it to moveinto the space. 7t is essential to safeguard the runner against any accident.

Pressure and !elocity of team in an Impulse Turbine

The pressure of steam jet is reduced in the nozzle and remains constant while passing

through the moving blade.

The velocity of steam is increases in the nozzle and is reduced while passing through the

moving blades.

This type of turbine is used for small power generation.

Reaction Turbine

:


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Thermal Engg.

7n a reaction turbine% the steam enters the wheel under pressure and flows over the

blades and ma*es them to move. The turbine runner is rotated by the reactive forces of steam jets.

The pressure drop is affected partly in fi#ed guide blades which are designed to wor* as

nozzles and partly in moving blades.

The jet steam leaves the fi#ed blades at high velocity and produces impulse as well as

reaction on moving blades.

!bsolute reaction turbine is rarely in actual practice.

Parsons ea!tion Turbine

7t is :>? reaction turbine 'ie impulse)reaction turbine(.

7t has the following main components,

i. Casin"7t is an air tight metallic case% in which the steam from the boiler% under a highpressure and temperature is distributed around the fi#ed blades 'guide mechanism(in the casing.

ii. #uide Me$%anismThese are made up with the help of guide blades% in the form of a wheel. This wheelis generally fi#ed to the casing that is why these guide blades are also called fi#edblades. These blades guides and regulates &uantity of steam entering the runner.

iii. Turbine Runner

7t consists of runner blades fi#ed to a shaft. The blades fi#ed to the runner allow thesteam to enter and leave the runner without shoc*.

i&. 'ra(t TubeThe steam after passing through the runner flows into the condenser through a tubecalled the draft tube.

Pressure and !elocity of team in a Reaction Turbine

The pressure in a reaction turbine is reduced in the fi#ed blades as well as in the moving

blades.

The velocity of steam is increased in the fi#ed blades of a reaction turbine.

@


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Thermal Engg.

Im)u!se Turbine Rea$tion Turbine. The steam flows through the nozzle and . The steam flows first through guideimpinges on the moving blades. mechanism and then through the moving

blades.$. The steam impinges on the buc*ets with $. The steam glides over the moving vanes

with*inetic energy. pressure and *inetic energy.

6. The steam may or may not be admitted 6. The steam must be admitted over thewholeover the whole circumference. circumference.

9. The steam pressure remains constant 9. The steam pressure is reduced during its

flowduring its flow through the moving blades. through the moving blades.

:. The relative velocity of steam while gliding :. The relative velocity of steamwhile glidingver the blades remains constant 'assuming over the moving blades increases'assumingno friction(. no friction(.

@. The blades are symmetrical. @. The blades are not symmetrical.A. The number of stages re&uired are less for A. The number of stages re&uired are

more forThe same power developed. the same power developed.

A


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Thermal Engg.

Compoundin" of team Turbines

4igh pressure and temperature steam is used in the power plants to increase their

thermal efficiency.

7f the entire pressure drop from boiler pressure to condenser pressure is carried out in

one stage only% then the velocity of steam entering into the turbine will be e#tremely high.

This will lead to high turbine rotor speed which is not useful for practical purposes and a

reduction gearing is necessary between turbine and e&uipment driven by turbine.

!lso there is danger of structural failure of blades.

The velocity of steam at e#it is also high leading to loss of *inetic energy.

These difficulties associated with use of single stage turbines for large pressure drop can

be solved by compounding ie use of more than one stage.

These methods are

. "elocity Compounding$. 5ressure Compounding6. 5ressure)"elocity Compounding

!elocity Compoundin"

There is only one set of nozzle and two or more rows of moving blades.

There is a row of fi#ed guide blade in between the moving blades.

The function of fi#ed blade is to direct the steam coming from first moving row to ne#t

moving row.

The enthalpy drop ta*es place only in the nozzle at first stage and is converted to *inetic

energy or velocity.

The velocity of steam gained in nozzle is successively absorbed in stages by rows of

moving blades and finally steam is e#hausted.

This is used in BCurtis turbine.

D


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Thermal Engg.

Pressure Compoundin"

The turbine is provided with one row of fi#ed blades 'wor*s as nozzles( at the e#it of

each row of moving blade.

The total pressure drop of steam does not ta*e place in a single nozzle but is divided

among all the rows of fi#ed blades which wor* as nozzles.


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Thermal Engg.

This method is used in B;ateau and BFoelly turbine.

Pressure#!elocity Compoundin"

This is a combination of pressure and velocity compounding.

The total pressure drop of steam from boiler to condenser pressure is divided into

number of stages as done in pressure compounding and velocity obtained in each stageis also absorbed in several stages.

This arrangement re&uires fewer stages and a compact turbine can be designed for a

given pressure drop.

This method has the advantage of pressure compounding to provide higher pressure

drop in each stage and hence fewer stages.

The advantage of velocity compounding to reduce velocity of each blade row.

This is used in BCurtis turbine.

Re"enerati$e %eed &eatin"

;egenerative +eed 4eating is one of the methods to improve the efficiency of cycle.

The dry saturated steam from boiler enters the turbine at a higher temperature and then

e#pands isentropically to a lower temperature and then enters into the condenser.

The condensate from the condenser is pumped bac* and circulated around turbine in the

direction opposite to the direction of steam flow in the turbine.

The condensate is thus heated before entering into the boiler.

8uch heating is *nown as ;egenerative +eed 4eating.

7n actual practice it is impossible to achieve this cycle.

;eason for not being able to achieve this cycle,

1ot possible to effect the necessary heat transfer from steam in turbine to li&uidfeed water.

>


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Thermal Engg.

Moisture content of steam leaving turbine is considerably increased due to heattransfer.

Re"enerati$e %eed &eatin" '(y (leedin" of team)

7n this system% the steam is removed from turbine at one or more points and then fed to

the feed water heater.

! small amount of steam 'm*g( is drained from turbine and enters the feed water heater. The remaining steam ')m( *g is ta*en out from the turbine and fed to condenser.


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Thermal Engg.

+rom condenser% condensate e&ual to ')m( *g is fed to feed heater.

4ere it e#changes heat from m*g of steam coming from turbine and this m *g of steam

mi#es with the ')m( *g of steam to become *g.

This *g of steam is fed to the boiler with the help of feed pump.

The heating arrangement may comprise of number of heaters depending on capacity of

turbine.

Ad$anta"es of Re"enerati$e %eed &eatin" or (leedin". 7t reduces size of Condenser.$. Thermal efficiency is improved because the average temperature of heat addition is

increased.6. The thermal stresses set up in boiler are minimized as the temperature ranges in the

boiler are reduced.

*isad$anta"es of Re"enerati$e %eed &eatin" or (leedin"

. 5lant becomes more complicated.$. Greater maintenance re&uired due to addition of heaters.6. 4eaters are costly thus increasing the overall capital cost.

Nozzle Control +o$ernin"

The main function of the governing is to maintain the turbine speed constant irrespective

of load on the turbine.

7n this method% nozzles are grouped together in 6 to : numbers or more and each group

of nozzle supplies steam through valve which controls the flow to the wheel.

This governing is used only for first stage of turbine.

This is used for medium and large steam turbines.

Hnder full load condition all regulating valves are open and when load on turbine is

reduced the suitable valve is closed to reduce the supply of steam.

STEAM CON'ENSERSIntroduction! steam condenser is a closed vessel into which the steam is e#hausted and condensedafter doing wor* in an engine cylinder or turbine. ! steam condenser has the following twoobjectives,

$


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Thermal Engg.

The primary objective is to maintain a low pressure 'below atmospheric pressure( so as

to obtain the ma#imum possible energy from steam and thus to secure a high efficiency.

The secondary objective is to supply pure feed water to hot well from where it is pumped

bac* to the boiler.

Ad$anta"es of a team Condenser. 7t increases e#pansion ratio of steam% and thus increases efficiency of the plant.$. 7t reduces the bac* pressure of the steam% and thus more wor* can be obtained.6. 7t reduces temperature of the e#haust steam% and thus more wor* can be obtained.9. The reuse of condensate as feed water for boilers reduces the cost of power generation.:. The temperature of condensate is higher than that of fresh water. Therefore the amount

of heat supplied per *g of steam is reduced.

Classification of CondensersThe steam condensers are classified into two types depending upon the way in which thesteam is condensed. /et Condensers or Mi#ing Type Condensers$. 8urface Condensers or 1on Mi#ing Type Condensers

*. +et Condensers or Mi,in" T-)e Condensersn the basis of the direction of flow of the condensate and the arrangement of the tubingsystem jet condensers are classified into fourtypes,

a. Parallel "lo# $et Con%ensers

7n parallel flow jet condenser% both the steam and

water enter at the top and the mi#ture is removedfrom the bottom.

The principle of this condenser is that the e#haust

steam is condensed when it mi#es up with water.

The condensate% cooling water and air flow

downwards and are removed by two separatepumps *nown as air pump and condensate pump.8ometimes% a single pump *nown as wet air pumpis also used to remove both air and condensate.

The condensate pump delivers the condensate to

the hot well% from where surplus water flow to thecooling water tan* through an overflow pipe.

b. Counter&lo# or Lo# Level $et Con%ensers

7n counterflow or low level jet condensers% the e#haust steam enters at the bottom% flows

upwards and meets the down coming cooling water.

6


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Thermal Engg.

The vacuum is created by the air pump% placed at the

top of the condenser shell. This draws the supply ofcooling water% which falls in a large number of jets%through perforated conical plate.

The falling water is caught in the trays% from which it

escapes in a second series of jets and meets thee#haust steam entering at the bottom.

The rapid condensation occurs% and the condensate

and cooling water descends through a vertical pipe tothe condensate pump% which delivers it to the hot well.

!. 'arometri! or (igh Level $et Con%ensers These condensers are provided at a high level

with a long vertical discharge pipe.

7n high level jet condensers% e#haust steam enters

at the bottom% flows upwards and meets the downcoming cooling water in the same way as that oflow level jet condenser.

The vacuum is created by the air pump% placed at

the top of the condenser shell.

The condensate and cooling water descends

through a vertical pipe to the hot well without theaid of any pump. The surplus water from the hotwell flows to the cooling water tan* through anoverflow pipe.

%. )*e!tor Con%ensers

7n ejector condensers% the steam and water mi# up

while passing through a series of metal cones.

-ater enters at the top through a number of guide

cones. The e#haust steam enters the condenser through

non)return valve arrangement.

The steam and air then passes through the hollow

truncated cones.

!fter that it is dragged into the diverging cones

where its *inetic energy is partly transformed topressure energy.

The condensate and cooling water is then

discharged to the hot well.

. Sur(a$e Condensers or Non Mi,in" T-)e Condensers

9


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Thermal Engg.

8urface Condensers consists of a horizontal cast iron cylindrical vessel pac*ed with

tubes% through which the cooling water flows.

The ends of the condenser are cut off by vertical perforated type plates into which water

tubes are fi#ed. This is done in such a manner that the lea*age of water into the centrecondensing space is prevented.

The water tubes pass horizontally through the main condensing space for the steam.

The steam enters at the top and is forced to flow downwards over the tubes due to the

suction of the e#traction pump at the bottom.

The cooling water flows in one direction through the lower half of the tubes and returns in

opposite direction through the upper half.

n the basis of the direction of flow of the

condensate% the arrangement of tubing systemand the position of the e#traction pump thesurface condensers are classified into fourtypes,

a. Do#n "lo# +ur&a!e Con%ensers

7n down flow surface condensers% the e#haust

steam enters at the top and flow downwardsover the tubes due to force of gravity as wellas suction of the e#traction pump fitted at thebottom.

The condensate is collected at the bottom and

then pumped by the e#traction pump.

The dry air pump suction pipe% which is

provided near the bottom% is covered by abaffle so as to prevent the entry of condensedsteam into it.

:


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Thermal Engg.

!s the steam flows perpendicular to the direction of flow of cooling water 'inside the

tubes(% this is also called a cross surfacecondenser.

b. Central "lo# +ur&a!e Con%ensers

7n central flow surface condensers% thee#haust steam enters at the top and flowdownwards.

The suction pipe of the air e#traction pump is

placed in the centre of the tube nest. Thiscauses the steam to flow radially inwards overthe tubes towards the suction pipe.

The condensate is collected at the bottom and

then pumped by the e#traction pump.

This condenser is an improvement over the

down flow type as the steam is directedradially inwards by a volute casing around thetube nest thus giving an access to the wholeperiphery of the tubes.

!. egenerative +ur&a!e Con%ensers

7n this condenser% the condensate is heated by a regenerative method.

The condensate after leaving the tubes is passed through the e#haust steam from the

engine or turbine.

7t thus% raises its temperature for use as feed water for the boiler.

%. )vaporative Con%enser

The steam to be condensed

enters at the top of a series ofpipes outside of which a film ofcold water is falling.

!t the same time% a current of

air circulates over the waterfilm% causing rapid evaporationof some of the cooling water. !sa result of this% the steam

circulating inside the pipe iscondensed.

The remaining cooling water is

collected at an increasedtemperature and is reused.

7ts original temperature is

restored by the addition of there&uisite &uantity of cold water.

The evaporative condensers are provided when the circulating water is to be used again

and again.

These condensers consist of sheets of gilled piping% which is bent bac*wards andforwards and placed in a vertical plane.

@


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Thermal Engg.

+et Condensers Sur(a$e Condensers. Cooling water and steam are mi#ed up. . Cooling water and steam are not

mi#ed up.$.


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Thermal Engg.

a. The dissolved air in the feed water enters into the boiler which in turn enters into thecondenser with the e#haust steam.

b. The air lea*s into the condenser% through various joints% due to high vacuum pressure inthe condenser.

c. 7n case of jet condensers% dissolved air with the injection water enters into the

condenser.

Effects of Air -ea0a"eThe effects of air lea*age on the performance of condensing plants area. 7t reduces the vacuum pressure in the condenser.b. 8ince air is a poor heat conductor% particularly at low densities% it reduces the rate of heat

transmission.c. 7t re&uires a larger air pump. Moreover% an increased power is re&uired to drive the

pump.

Coolin" To.ers

The function of the cooling tower is to cool the hot water from the condenser by e#posing

it to the atmospheric air% so that the cold water may be used again for circulation.

The cooling towers are used in steam power plants where there is a limited supply of

cooling water.

7t is placed at a certain height 'at about meters from the ground level(.

The hot water falls down in radial sprays from a height and the atmospheric air enters

from the base of the tower.

The partial evaporation of water ta*es place which reduces the temperature of circulating

water.

This cooled water is collected in the pond at the base of the tower and pumped into the

condenser.

Cooling Towers are classified as follows

a. A$$ordin" to t%e t-)e o( drau"%ti. Natural Draught Cooling To#ers

7n this% the circulation of air is produced by the pressure difference of air inside andoutside the cooling tower.

ii. "or!e% Draught Cooling To#ers

D


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Thermal Engg.

7n this% the circulation of air is produced by means of fans placed at the base of thetower.

/or$ed 'rau"%t Coo!in" To0ers Indu$ed 'rau"%t Coo!in"To0ersiii. In%u!e% Draught Cooling To#ers

7n this% the circulation of air is produced by means of fan placed at the top of the tower.

b. A$$ordin" to t%e materia! Usedi. Timber Cooling To#ers

These towers have short life% high maintenance charges% limited cooling capacity% rotseasily due to e#posure to sun% wind% water% etc% and design does not facilitate propercirculation of air.

ii. Con!rete Cooling To#ers

These towers have large capacity% improved draught and air circulation% increasedstability under pressure and low maintenance.

iii. +teel Du!t T,pe Cooling To#ersThe duct type cooling towers are rarely used in case of modern power plants due to theirsmall capacity.

CHAPTER 5 Steam Turbines and Condensers1uestions /rom Pre&ious Board Pa)ers

. 8tate the Kaltons law of partial pressure. May > '@( Kec $ May Kec > May >Kec >D '$($. -rite basic function of condenser. 4ow condensers are classifiedL

May $ '9(6. E#plain construction and wor*ing of down flow type surface condenser with neat s*etch.

May >D'@(

E#plain construction and wor*ing of surface condenser with neat s*etch.Kec > '9(Give classification of steam condensers. E#plain the wor*ing of surface condenser.Kec $ 'D(

9. -rite sources of air lea*age into the condenser and its effects on the performance ofcondenser.


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Thermal Engg.

May Kec % Kec >% May > '@(% Kec>D '9(-hat are sources of air lea*ages in condenserL 4ow these can be detectedLMay $ '9(

:. Compare jet and surface condensers on the basis of construction% performance and

application.

Kec >D '9(@. 8tate function of steam condenser. May >

'$(A. -hat is condenserL 8tate its function. 1ame two types and two demerits of it.

May > '@(D. -hat is vacuum efficiency of condenserL May $

'$(. -hat is vacuum efficiency and condenser efficiencyL

Kec '9(

>.-hat are the primary and secondary functions of condenserL E#plain.Kec '9(

.Kefine nozzle and its function. Give its types and write two application of nozzle.May Kec

>D% May >D '9($.Kefine and state significance of Mach number.

May >D '9(6.8tate applications of steam nozzles and e#plain critical pressure. Kec $

'9(9.-hat is nozzleL Kraw the s*etches of different types of nozzles and also write four

applications of it. May $ '9( May > '9( Kec>D Kec > '@(

:.-hy compounding of steam turbine is necessaryL 8tate the methods of compounding.Kescribe any one method with neat s*etch. Kec $ May Kec > '9( May> Kec >D 'D(-hat do you mean by compounding of steam turbineL May> '9(8tate necessity of compounding of steam turbines. E#plain pressure compounding withs*etch.

May $ 'D(

E#plain the pressure) velocity compounding method of steam turbine.Kec '9(@.E#plain the term Bleeding of steam used in steam turbine plant with a neat s*etch.

May '9(A.-hat is regenerative feed heatingL -hat are its advantages and disadvantagesL E#plain

this process with neat s*etch.May >D 'D(E#plain with neat s*etch regenerative feed heating and state its advantages.

May $ '9( Kec% May > 'D(D. Kifferentiate between impulse turbine and reaction turbine.

Kec $ '9(

$>


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Thermal Engg.

.Kescribe the construction of impulse turbine with neat s*etch.May $'9(

$>.E#plain wor*ing of impulse steam turbine by using pressure velocity variation diagram.May

'9(

$. E#plain clearly with the help of neat s*etch the wor*ing of a reaction turbine. !lso showpressure and velocity variation for the same. Kec >'D(

$$. -ith pressure)velocity diagram% e#plain how stored energy of steam is converted intomechanical energy in case of reaction turbine.Kec 'D(

$6.Classify steam turbine with respect to a( !ction of steam over moving blade% b(E#pansion stages% c( 5ressure of steam entering% d( E#haust steam pressure.

Kec > '9($9.E#plain with neat s*etch cooling tower any oneL 8tate any two merits and demerits of

cooling tower.

May > '9(-hat is function of cooling tower in steam power plantL Give various types of coolingtowers only by s*etch.

May '9($:.E#plain natural draught cooling tower with s*etch. May $

'9([email protected] s*etch e#plain wor*ing of forced draught cooling tower.

Kec $ '9($A.Kifferentiate between natural draught and forced draught cooling tower. E#plain with neat

s*etch induced draught cooling tower.Kec 'D(

$

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Chapter 5_Steam Turbines and Condensers

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