improve thermal efficiency of candu cycle
TRANSCRIPT
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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
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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
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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
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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 .
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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.
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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
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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 -
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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 .
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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.
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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
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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
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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 .
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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
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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
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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
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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.
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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
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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 .
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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 .
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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 .