Improve Thermal Efficiency of CANDU cycle

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PI

25-7

Heat and

Thermodynamics

Course

PI 25

EFFICIENCY

ND

THE NDU

Y LE

Objec t ives

1 .

Def ine :

a eff ic iency

b

thermal ef f ic iency

2.

Given a l l inpu ts and outpu ts of a

system,

determine

the.

thermal ef f ic iency .

3. a Expla in how th e t he rmal e f f i c i ency of th e NDU

y le

can be improved

by r ising

bo i l e r

pre ssure .

b

Sta t e th e

main

l imi ta t ion

on

th e improvement in

a

4. a Expla in how

th e thermal e f f i c i ency

o f

the

NDU

cycle can

be improved

by l owe ri ng c onde ns er pre s -

sure.

b Sta te

two l imi ta t ions

on the

improvement in

a .

5.

a

Expla in

how

th e

t he rmal

e f f ic ien c y o f

th e

NDU

cycle can be

improved by superheat ing in

the

bo i l e r .

b

Sta t e th e

main l imi t a t ion on

the

improvement in

a

6.

a

Expla in how th e thermal ef f ic iency

o f the NDU

cycle can be improved by:

b

Sta te

a

c

Sta t e

a

rehea t ing between the high and low pressure

tu rb ines .

using ex t rac t ion

steam

f or f ee dh ea tin g.

the main

l imi ta t ion

on each improvement

in

two p rac t i c a l bene f i t s o f

each

improvement in

7. a Expla in how the

thermal

e f f i c i ency of the NDU

cycle

can be improved

by

moisture

separa t ion .

b Sta te the p rac t i c a l bene f i t o f moisture

separa t ion .

M arch 1984



PI 25 7

Module Seven dea l s with e f f i c i en cy Afte r de f i n i t i on s

and

s imple ca l cu l a t i ons

a re

de a l t

wi th th e r e s t o f th e

module conce rns th e t h e rma l e f f i c i en cy o f th e NDU cyc l e

The i n t e n t of t h i s module i s

to

make you aware o f various

cons ide ra t ions t h a t

have been

t aken

i n to

accoun t

in orde r to

make

NDU genera t ing

s t a t i on s

as

e f f i c i e n t

as

pos s ib l e

Eff i c i ency can be def ined as o utp ut d iv id ed by i npu t

o f t en expressed as p erc en ta ge v alu e This de f i n i t i on can

apply to

many th ings :

the e f f i c i ency of hea t t r an s f e r in th e

bo i l e r s

th e

e f f i c i en cy

o f

pump

th e

e f f i c i en cy

of t u r -

b ine

e t c So

t h a t you can co nsid er th e thermodynamic cyc l e

t h a t

r e p r e s e n t s

NDU

un i t we wi l l de f i ne pa r t i c u l a r

type

of

e f f i c i ency thermal e f f i c i ency

The thermal e f f i c i ency

of

system

i s def ined to

be

th e

n e t

work

ou tpu t o f the

system

divided

by

the

t o t a l

hea t

i npu t

to the sys tem o f t en

expressed

as percen tage

The

ne t work

ou tpu t i s th e work produced by the system

minus

the work put

in to th e system in

orde r

to make

opera t e



PI

25-7

- - - - .

Answer ques t ions

7.1 and 7.2

before you proceed , then

check your

NSWERS · •

7.1 Define the

fo l lowing:

a

Eff ic iency:

b Thermal

e f f i c i ency :

in the

answers

space prov ided

wi

th e TEXT

7 .2

2390 o f

hea t

a re

added in

th e

bo i l e r s o f a C NDU

un i t .

I f

the

un i t

produces 788 MW

of

e lec t r i c i ty and

i f 5 .5

a re pu t in

to pump

the feedwater from th e

condenser

to the

bo i l e r s

what i s the thermal

e f f i c i en

cy o f the

cycle?

At t h i s

po in t

you

should

be

able to

do

the

f i r s t

two

ob

j ec t ives

fo r t h i s module. I f you f ee l you need

more

prac

t i c e consu l t

with

the course manager.

3



PI

25-7

NDU Cycle Thermal Eff ic iency

The secondary

hea t t r anspor t system

i s

th e

thermodynamic

cycle

o f

a

NDU

un i t .

t

i s

t h i s

system

t h a t

th e

hea t p ro -

duced nuc lea r f i s s ion

i s

used

to

produce sha f t mechanical

energy. Needless to say, it i s important t h a t the thermal

e f f i c i e ncy o f

t h i s cyc l e be

as high as

can be

reasonab ly

ach ieved . This s ec t i on dea l s

with the cyc le

thermal e f f i c -

iency o f a NDU un i t how it can

be

maximized and

what

p r a c t i c a l

l imi t s a re imposed

on it

Boi le r

Pressure

The pressure and

with

it the tempera ture o f steam pro-

duced in the

bo i l e r

should

be

as high as poss ib l e .

Overa l l

as

th e

sa tu ra t ion

tempera ture

and

pressure

o f

the

steam

l eav-

ing th e bo i l e r a re r a i s ed

the

r a t i o o f ne t work

ou tpu t

to

hea t inpu t i s i n c reased .

In

o the r words,

the h igher the

tempera ture and p res su re

o f the

s team,

the

h igher

th e

propor -

t i on o f hea t

energy

made ava i l ab l e to

produce

work.

There

i s

an upper

l im i t on

s team

t empera ture and

thus

p re ss ure in th e

NDU system . This

upper l im it i s imposed

because

o f cons ide ra t ions invo lv ing the

fue l

and fue l

shea th . The ho t t e r the s team to be genera ted in the bo i l e r s

th e ho t t e r th e

P T

D20

must

be.

This

r e su l t s in an i n c rease

in the su r face

t empera ture

o f the fu e l in

the

r eac to r . The

fue l i s uranium diox ide

U02

manufactured in cy l i nd r i c a l

pe l l e t s .

As

V02

i s

a ceramic

mate r i a l

i t s

thermal

conduc

t i v i t y i s very low and th e pe l l e t c ore tempe ra tu re i s

t h e r e -

fore much

h ighe r

than

th e su r face tempera ture . The t empera -

t u r e

p ro f i l e o f

a

fue l pe l l e t inc lud ing the sh ea th ing . i s

shown in F ig ure 7 .1 .



PI 25 7

u l

ellet

u l heath

Figure

7.1

Note t h a t tempera tures

in

the order o f

2000 2200°C

occur

in

the cent re

o f

the fue l

in

order to

produce

250 260°C

steam. higher

steam

tempera tures

were

used the pe l l e t

tempera ture would

have

to

increase and the danger o f

re l ea se

o f

f i s s ion products

due to fue l

sheath f a i lu re would

s i gn i f i -

cant ly

inc rease Thus

the steam temperature is

l imited

to

a

maximum of about 250 260°C.



PI 25 7

Check

Answer th e

y ou r answ ers

fo l lowing

w ith tho se

q ue st io ns b efo re you

i n

th e TEXT NSWERS

proceed.

7 3

Expla in how th e

the rmal

e f f i c i ency

o f

the NDU

cyc l e

can

be

improved ra i s ing bo i l e r

pre ssure

7.4

Sta t e

th e

ef f ic iency

l imi t a t ion on the improvement

due to ra is ing

bo i l e r

p res su re

in thermal



PI 25-7

Condenser Pressure

The pressure

and temperature)

o f

steam

ente r ing th e

condenser from th e LP turb ine should be

as

low

as

poss ib l e

As

th e

pressure and temperature a t the LP

t u rb ine

o u t l e t

decrease

th e

work

t h a t

has

been

ext rac ted

from

the

stearn

i nc rea se s The r a t i o

of

n et work output

to

hea t inpu t

increases

s ig n i f i c an t l y

because o f the ex t ra work made

av a i l -

ab l e Thus th e thermal e f f i c i ency

i n c reas es

There a re

two main

l imi t a t ions

on lower ing

the

condenser

pressure :

a)

The temperature of

the condenser

cooling

water

(CCW) i s th e

main

l im i t a t i o n The wate r used to

condense th e steam

i s

lake water In summer th e

temperature

of

the

W a t

the

condenser

i n l e t

is

about 20°C.

Remember from

Module PI 25-2 t h a t t h i s

l eads to steam tempe ra tu re a bo ut

30·C

in th e

con

denser I t

i s

i n t e res t ing to note t ha t as

the

W

temperature

drops with

th e

approach of winter

the

NDU cyc le

e f f i c i ency

increases

b) The lower

the

condenser

pressure and

t empera tu re ,

th e higher

the

moisture

con ten t

of

the

steam leav-

ing

th e

LP t u rb ine Remember t ha t mois ture con ten t

above about 10

i s

undes i rab le Without

the

use

of

mois tu re

separa t ion rehea t ing

feedheat ing ,

and

some other

fea tures

w hich you w i l l consider in PI

34), th e mois ture con ten t

o f

steam en te r in g th e

condenser

a t

30·C

would

be

21 .

Mois ture

con ten t

would have to be

reduced

in

order

to have a low

condenser pressure

_ < Answer the, following

quest ions

in

the

space provided,

then check your answers with

those

in

the

TEXT ANSWERS .

- 7 -



PI

5 7

7.5 Explain

how

th e thermal

e f f i c i ency

o f th e NDU cyc le

can

be improved

lowering

condenser pres su re .

7 .6 Sta t e two l im i t a t i on s on th e improvement in

ques t i on

• •

Superheat ing

Superheat ing steam in

the

bo i l e r s i e ,

heat ing the

steam

to a t empe ra tu r e

h ighe r

than

th e s a t u r a t i on t empe ra tu r e fo r

th e steam

p res su r e

wi l l i n c rea se

cyc l e

the rma l

e f f i c i e n c y .

Here i s an

ex t r a

hea t in pu t re qu ired

to

supe rhea t

th e

steam

bu t th e r a t i o of ava i l ab l e work from t h i s steam

to

th e t o t a l

hea t i npu t r equ i red i s i n c rea sed . Thus th e ove r a l l cyc l e

thermal e f f i c i ency

i s inc reased .



PI 25 7

The

main

l imi ta t ion on superheat ing i s

the l im i t

of

250 260°C imposed

by the fuel and fuel

sheath

considerat ions

mentioned in

ra i s ing

bo i l e r

pres su re . Superheated steam

a t

250 260°C

could be produced

bu t

would be a t a lower

p re s

sure than sa tu ra t ed steam a t the same temperature . The r a t io

o f

ava i l ab l e

work

to hea t

i npu t

would

be l e s s

because

of

the

lower pre ssu re .

Another

opt o would be to use

a foss i l - fue l l ed

super

hea te r .

The

pr i ce of such fue l

i s

however the main reason

why fO s i l - fue l led superhea te rs

are no t used.



l ~ ~ Answer quee

t iona

then

check

your

answers

PI 25-7

7 .7 and 7 .8

in

th e space provided,

wi th t os

in

the T XT NSW RS

7.7 Expla in

how

th e

the rmal

e f f i c i ency

o f th e NDU cycle

can be

improved by superheat ing in the

bo i l e r .

7.8 S ta t e

the

m n

l imi t t ion

o f th e improvem ent due to

superhea t ing .

Reheat ing

Remember

t h a t

r ehea t i ng between th e HP t u rb i n e and th e

LP

t u rb ine produces superheated

steam

en te r ing

th e

LP

t u r

b ine .

s a

r e su l t , the mois ture conten t

o f

the s team

in

the

LP t u rb ine

i s reduced

and

the

tu rb ine e f f i c i ency i s t h e r e

fo re . inc reased .

Thus

the

r a t i o

o f

ne t

work

outpu t

to

hea t

input ,

ie ,

the cycle thermal e f f i c i ency ,

i s increased .

The main l imi t a t ion on rehea t ing i s due to

the

tempera

t u re o f

th e l i v steam used to hea t th e main steam . This

l i ve steam i s taken from the steam leav ing th e bo i l e r , so

i s l im ited to

about

250-260°C.

Although

rehea t ing

to high r tempera tures could be

achieved by means o f f o s s i l - f ue l l ed

rehea te r s ,

economical

cons ide ra t ions

fu el p rice make

t h i s opt ion

una t t r a c t i ve .

10



PI

25 7

_ < ~ Answer the

Check your answers

following

with those

q ue stio ns b efore

you

in the TEXT NSWERS

proceed

7 .9

Expla in

how

th e

the rmal

e f f i c i ency

o f

the

NDU

cyc le

can be improved

by

reheat ing between the

high

pressure

and low pressure tu rb ines .

7.10

Sta te

th e

main l imi t a t ion on

th e

im provement due to

reheating

Reheat ing

prov ides p r a c t i c a l

bene f i t s bes ides an

increase in

cycle

thermal efficiencYi

these

bene f i t s

are

the

main

reason reheat ing i s

done The f i r s t

bene f i t

i s

a reduc-

t ion in moisture con ten t in

the

steam as goes through the

LP t u rb ine . The r e su l t of having dr i e r

steam

is th t the

steam can be u t i l i zed

to a

lower temperature

and

p ressure

This

al lows

a

lower

condenser

pressure

to

be

used

and

t h i s

reduct ion

i t s e l f

a l so improves

the cycle

thermal

e f f i c i ency .

The second bene f i t o f rehea t ing i s

t h a t

the

steam

flow

i s smal le r

to

produce

a

cer ta in power Remember

t h a t

r ehea t -

ing

increases the

enthalpy o f

the

steam

before

en te rs the

LP t u rb ine .

The steam can produce more work than steam t h a t

has

no t

been

rehea ted

and

thus l es s

flow i s necessary

to

produce

a

ce r t a in power

11



PI 25-7

Answer th e fo l lowing

ques t ions

in th e space

prov ided

then check

your

answer

wi th

th e one in th e

llTEXT ANSWERS .

7.11 Sta te two

prac t i ca l

benef i t s

of

reheat ing .

Feedhea t ing

The water t h a t i s re tu rned from the

bo i l e r s

i s

heated

from

about 30°C to about

two r easons

fo r

t h i s

feedhea t ing :

condenser

to

175°C. There

th e

are

I f

th e

wate r

i s

re tu rned

to

th e

bo i l e r s

opera ted a t abou t 250

C a t

30

e

l a r g e

s t r e s s e s

wi l l occur in the bo i l e r s To

t h e se

s t r e s s e s

the wate r must

be

p rehea t ed

which

t h e rma l

reduce

2 Less

f u e l

i s burned

to

produce

a

g iven

amount

o f

power ,

s i nce

most o f th e se ns ib le h ea t needed

to

change th e tempera ture o f th e wate r from condenser

cond it ion s to

bo i l e r

cond i t i ons

comes from f eed-

hea t ing

Genera l ly

steam ex t rac ted from

th e

tu rb in e to hea t

th e feedwater

in

the f eedhea t e r s Using th e

ex t rac t ion

steam

increases

NDU

cycle

the rmal

e f f i c i ency The

i n c rease

i s

due to th e

use

o f

the

hea t energy o f

the

s team.

I f th e

ex t rac t ion

s team i s not ex t r ac t ed bu t al lowed to

cont inue

through

the t u rb ine

would

produce

some

work

and

t hen

en t e r

the

condenser In the

condenser

about 90 o f

th e

l a t e n t hea t o f

the

steam i s re je cte d to the

lake as the s team

i s

condensed .



PI 25 7

hen

th e ex t r ac t ion steam goes

to

the feedhea te r the

work t h a t

could produce

in the t u rb ine i s lo s t However

th e stearn condenses in the feedhea te r and i t s hea t i s given

up to th e feedwater . Thus a t the expense o f th e

lo ss o f

some work

produc t ion

a

s i gn i f i c an t

amount o f hea t

i s con-

served

w ith in the cyc le

This

conservat ion

reduces

the

hea t

inpu t in the

bo i l e r s

and the r a t i o

o f

net work output

to

hea t

inpu t ie th e

cycle

thermal ef f i c i ency i s inc reased

The in crea se in thermal ef f i c i ency depends on the

number

o f

feedhea te r s

used

and th e p re ss ure s a t

which the steam i s

ext rac ted

from

th e

tu rb ine The l a rge r the number o f

feed-

hea t e r s th e l a r ge r the

inc rease

in e f f i c i ency

However the

ga in

in e ff ic ie nc y

due to i n s t a l l i ng

each success ive

feed-

h ea te r ge ts smal le r and smal le r

For

a given number

o f

feed-

hea t e r s i s

poss ib le to ca lcu la te

op t imal pre ssu re s

fo r

th e

ex t r ac t ion steam

so

t h a t the

inc rease

in

the thermal

e f f i c i ency wi l l

be

maximum.

The

main

l im i t a t i on

on the improvement due

to

using

ex t r ac t ion steam

fo r

feedheat ing i s economic. Using

a l a rge r

number o f feedhea te rs makes poss ib le to achieve

a

l a rge r

inc rease

in

th e cycle therm al

e f f i c i ency however in -

creases

the cap i t a l cos t s At

some

po in t th ere i s

a

balance

between th e benef i t from e f f i c i ency inc rease and cos ts

incur r ed In a

NDU s t a t i on

t h i s occurs

when

5 6 feedhea t -

e r s are used and th e feedwater

i s heated to about

l75°c

13



PI 25-7

__ Answer the fo l lowing ques t ions then check your answers

wi

th

t hose

in th e TEST ANSWERS

7.12

Expla in how using

ex t rac t ion steam

fo r

feedheat ing

can

improve

th e thermal e f f i c i ency

o f

th e

NDU

cyc le

7.13} Sta te th e

main

l imi ta t ion on

the

improvement in e f f i c -

iency due to using ex t rac t ion

steam

fo r f ee dh ea tin g



PI 25 7

Using the

extrac t ion

steam for

feedheat ing also

provides

prac t i ca l benef i t s The f i r s t

i s

a reduc t ion in moisture

con ten t o f

the

steam in

the

tu rb ine

When

wet steam flows

through th e tu rb ine has a v ery B wirlin g motion.

Cent r i -

fu gal fo rces are exerted

both

on

vapor

and

water

droplets

but

due to

the dif ference between the i r dens i t i es the

l iquid

is

cent r i fuged

outwards i a

towards the

tu rb ine cas ing As

ex t rac t ion

steam

i s

removed

from

th e

cas ing i t s mois ture

content i s

much

l a rge r

than

the

a ve ra ge m o is tu re

con ten t

of

the stearn flowing through

the

tu rb ine The r e su l t of th i s is

a

reduction

in

moisture content in the

turbine

The

second benef i t

has

to do

with

the amount o f

steam

ext rac ted

Up

to

3 of

the steam flow i s

removed from

the

turb ine se t

fo r

feedheat ing. This

reduction in

steam

flow

through the

lo w pressure

turb ine enables a smal ler and

th ere fo re le ss cos tly

low

pressure turb ine

to

be

used.

____ •• Answer quest ions 7.14 then check your answer with the

one in the TEXT ANSWERS .

1.5



PI

25-7

7 .14 S ta t e two

p r a c t i c a l

bene f i t s o f using ex t rac t ion

steam

fo r

f e edhea t i ng .

Mois ture Separa t ion

Moisture separa t ion

causes a s ign i f i can t increase in

cycle

thermal

e f f i c i ency . The

main

e f f ec t of moisture in th e

steam

i s

a

t endency

to r e ta rd

th e

motion

o f

the

t u rb ine

blades

t h i s

causes a lo ss in work produc t ion in the t u r -

b in e . Reducing th e

mois ture

con t en t al lows the t u rb ine

to

produce

more work fo r th e same hea t inp u t . Thus the cyc le

thermal e f f i c i ency i s

improved.

The p r a c t i c a l

bene f i t of moisture

sepa ra t ion i s the same

as

th e rst b en e f i t l i s t e d in reh ea t in g: mois tu re s epa r a -

t i on

r educes th e mois ture

conten t o f the steam going

th rough

th e

t u rb ine which al lows a lower condenser p ressu re to be

used without

exceeding the maximum accep tab le moisture

con

t e n t

of the steam

a t

the tu rb ine

exhaust

about 10-12 ).

Check

Answer

the

your

answers

fol lowing

with

those

ques tions be fo re you

in the TEXT ANSWERS

proceed.



PI

25 7

7.15}

Explain

how

moisture

separation can increase the

ther-

~ ~ l eff ioiency of ~

CANDU

cycle.

1 16 S t a t e th e p r a c t i c a l b en ef i t o f mois ture

sepa ra t i on

You

have now

completed t.he l a s t module

o f

PI 25. I f

yOL

fee l you can answer th e objec t ives

fo r t h i s module

obta in c r i t e r ion

t e s t

a nd

answer

it I f you

are

r ot

con-

f iden t

attais

poin t

pleaae

consul t

with the

c o u ~ s e m a n a g e r

17



PI 25-7

TEXT

NSWERS

7 .1 a

Eff ic iency

expressed

as

a

outpu t d iv ided

percen tage .

by

i npu t

o f ten

b)

Thermal Eff ic iency

net work

outpu t o f

a

system

div ided t o t a l hea t

inpu t

to th e system,

of ten

ex pressed as a

percen tage .

7.2) The hea t input in

t h i s

ques t ion i s

2390 MW

The

net work output

i s 7B8

5 .5 = 782.5

MW

The thermal

e f f i c i ency

i s 782 .5 2390)

x 100

32.7

7.3)

As

bo i l e r

pressure

i s

ra i sed

the

steam

temperature

and

pressure both increase . As

they

inc rease

the

ra t io

o f

work

ava i l ab le

to hea t i npu t i n c reases . This w il l in -

c rease

th e r a t i o o f net work output to hea t inpu t i e

the thermal ef f ic iency of

the

cycle .

7 .4 The main

l imi t a t ion i s a

maximum achievable s te ar n te mp

e ra tu re

o f

abou t 250 -260°C,

impose::l

by

fu e l am fue l

sheath

cons idera t ions .

7.5}

As

th e steam

pressure

am tempera ture

in

the condenser

a re

lowered,

the

work

produced

in

th e

tu rb ine

in -

c reases . The r a t i o o f

ne t

work outpu t to hea t inpu t

i e th e the rmal e f f i i e n ~ y i n c reases .

7.6} The i n c rease in e ff ic ie n cy due to low ering c onde nser

pressure i s

l imi ted

by

th e tem pera ture o f the lake

wate r used

as

th e condenser coolant am

by the maximum

a l lowable mois ture conten t

in

the LP t u rb i ne .

7.7} I f superhea t ing i s done

in

the bo i l e r the

stearn

wi l l

have

a

h ighe r amount

o f work ava i lab le to

hea t i npu t

r a t i o .

The

the rmal

e f f i c i ency

o f

the

cycle

can

thus

be

i n c reased .

7.S}

The

main l im i t a t i on on superhea t ing

in

the bo i l e r

i s

250-260°C tempera ture

l im i t

due to the

mater ia l

cons id-

e ra t ions o f th e fue l am fue l shea th .



PI 25-7

T XT

NSWERS

7 .9 Reheat ing

between HP

and LP tu rb in es in cre ase s cycle

thermal

e f f i c i ency

by

providing

superheated

steam

to

th e LP t u rb i ne .

As

th e mois tu re con ten t o f

th e

s team

in

the

turb ine i s reduced the tu rb ine eff ic iency

i n c r e a s e s ,

and

t he r e fo re

th e

cyc l e the rma l

e f f i c i e ncy

i s inc reased .

7.10}

The main l im i t a t i on on improving cyc le

the rma l e f f i -

ciency by rehea t ing the l i ve stearn t empera tu re . The

l i v e

s team

i s t aken from th e main s team f low from th e

bo i l e r s ;

i t s

tempera ture i s l imi ted to 250-260°C.

7 .11

Rehea t ing

red uc es m o is tu re

con t en t in th e Ip t u rb ine

which

al lows

a low condenser p res su re to be

main t a i n -

ed . A lso ,

th e

s team

f low l e s s wi th r ehea t i ng to

p rov ide

a

given

power

th an the

flow

wi thou t

r ehea t i ng

-

t hu s

th e whole secondary sys tem s i z e i s sma l l e r

and

le s s c o st ly .

7 .12 Using ex t r a c t i on s team

fo r

f eedhea t ing i n c rea se s NDU

cyc l e

the rma l

e f f i c i e ncy because , a t th e

expense

o f

some l o ss o f

work

product ion , a

l a rge

amount o f h ea t i s

conserved with

th e

cyc l e . This

hea t i s

conserved

be -

cause th e s team i s condensed by f eedwate r r a t h e r than

by lake w ate r . The

r a t i o o f

n e t work ou tpu t to hea t

i npu t

i s i n c r eased

because

much l e s s hea t i npu t i s

r equ i r ed .

7 .13

The

l im i t a t i on

on th e e f f i c i e ncy

improvement

due to th e

use

o f ex t r a c t i on s team fo r feedhea t ing i s economic.

t

i s th e ba l ance between th e i n c rea s ing co s t o f

th e

equ ipment

and

th e in cre as in g ga in in e f f i c i e ncy as th e

number o f fee dh ea te rs { nc re ase s.

7 .14 The

irst bene f i t o f us ing

ex t r ac t ion s team

fo r

feed-

hea t i ng

i s

t h a t mois tu re con t en t in th e LP t u r b i ne i s

r educed .

The

second

bene f i t i s

t h a t

th e

s i ze

o f

th e

t u r b i ne

s e t i s

reduced

due to sm alle r f low

t h rough

th e

o u t l e t

p or t io n o f

th e

t u r b i n e .

7 .15

The e f f e c t o f

mois tu re

s epa ra t i on i s t h a t l e s s work i s

l o s t in th e t u rb ine th e

work

l o s t be ing due to r e t a r -

da t i o n o f th e moving

b l ade s because

o f l i qu id

imping

ment

• The LP

t u rb ine

thu s has a h ighe r

work ou tpu t

fo r th e same hea t in pu t ,

and

th e

cyc l e

t he rmal e f f i -

c iency i n c r e a s e s .



5 7

TEXT ANSWERS

7.16 The pr ct ic l

enef i t o f moisture

separation i s that

low condenser pressure can be maintained without

exceeding the

m x mum acceptable

moisture

content

o f

the steam at the turbine eXhaust about l O ~ 1 2 .

Improve Thermal Efficiency of CANDU cycle

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