turbine, generator & auxiliaries - course 334
TRANSCRIPT
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334.00-7
Tur bin e, G e ne ra to r
Aux i l i a r i e s
Course 334
THE CONDENSER
In th e
sec t ion
on th e stearn
tu rb ine
we discussed how
th e tu rb ine
conver ted
th e hea t
energy
of the stearn pass ing
through it to th e mecha nic al e ne rg y o f th e ro ta t in g sh af t .
t i s obviously to
o ur ad van tag e
to ex t ra c t as
much
work as
poss ib le from th e sa tu ra ted stearn
t h a t
i s genera ted in th e
stearn
gene ra to r .
In
the
s imples t terms, the lower the temp
e ra tu re and pre ssure a t
th e
ou t l e t
o f
the
low
pre ssure t u r -
bine the
g rea te r w i l l be
the amount
of
energy
which
can
be
ex t rac ted from th e stearn. In f a c t if
th e
exhaus t
of th e
tu rb ine i s near a pe r fec t vacuum ra the r t h a t a t atmospher ic
pres su re roughly
5
more
energy can be
ex t rac ted from th e
stearn pass ing through
the
tu rb ine . t i s
the condenser
which
prov ides
th e
means
o f ma in ta in in g
t h i s
low
a bs olu te p re ss ur e
a t the
exhaus t
of th e low pressure
t u rb ine .
The way
in
which
th e condenser provides t h i s low pre ssure i s th ro ugh c onde ns a
t i on of
th e
stearn i n to
water .
At th e ou t l e t
of
the
low
pressure
tu rb ine a ki logram of
wet steam
occupies cub ic meters . When t h i s steam
condens
es
to water th e one
ki logram
of
water
occupies
.001
cubic
m eters about one qua r t . Thus as one
ki logram
of steam con
denses ,
a lmost
38 cub ic meters
of empty space
which was
prev ious ly
f i l l e d
with steam
i s
l e f t
behind. t i s t h i s
crea t ion of
empty sp ace th ro ug h
th e
condensing
of steam which
provides
the
vacuum
in
th e
condenser .
Figure
7 .1 shows
a
sec t iona l view of
a
condenser . The
cool ing water
which
removes the l a t e n t hea t of vapour iza t ion
and causes th e steam to condense f lows through th e tubes .
This
condenser
cool ing water i s taken from the
lake
or r i ve r
pumped
through th e condenser tubes and then
f
lows back
to
th e lake or r i ve r . The condenser
cool ing
water i n l e t to the
condenser i s known as th e
i n l e t
water box and th e condenser
cool ing water ou t l e t as th e ou t l e t water box.
The type of
condenser
shown in Figure 7.1 i s known as
a
s ing le
pass condenser s ince th e coo ling
water
passes through
th e
condenser only
once.
Double
pass
condensers
are
also
found
in th e Nuclear
Generat ing
Divis ion but only on the
smal le r
un i t s .
The
tubes
a re
supported
a t
e i t he r
end by
tube
shee ts
and along t h e i r l eng th by sagging p la t e s .
In
add i t ion to g iv in g support
to
th e condenser tubes
the
sagging p l a t e s
are spaced
to ensure
t ha t the
tubes
are
damped a g ain st v ib ra ti on
induced
e i t h e r by
resonance with
the
tu rb ine running f requenc ies or by
steam pass ing
over the
March 1984
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tubes . The sagging
pla tes
are bolted or welded to the con
denser
she l l
and they cont r ibute to
the
condenser she l l
s t rength When
the
condenser
i s under
fu l l vacuum, the she l l
must
withstand an
externa l
pressure
of
almost
100 kPa.
The
sagging pla tes
h elp co un te rac t th i s pressure force which
is
tending
to
col lapse the condenser she l l Axial
support
i s
g i
ven
to
the
tube
sheets
by
staybars
which
run
pa ra l l e l
to
the
tubes .
These s taybars
tend
to prevent any s ign i f i can t
a xia l s tre ss es from being carr ied by the
tubes.
On
the steam
side of the
condenser,
the steam leaves
the
low pressure
turbine
through
an
exhaust trunk and
enters
the
she l l of
the
condenser where t
passes
around the tubes. The
condensed
water
o r condensate f a l l s in to
the
bottom of the
condenser and i s
col lec ted
in
a
hotwel l .
J
A la rge tu rb ine
uni t
general ly has one condenser
for
each low
pressure turbine
However,
to prevent var ia t ions
in
the
back
pressures of
each condenser, the she l l s
of
the
con
densers
are
joined
by
a large
say
2
metre
diameter
balance
This balance
pipe a lso
al lows a
condenser
ha l f
to be
shutdown
for
cleaning while the
turbine un i t
remains a t
power.
A
Exhaust
Trunk
B
Tubes
C Sagging Pla tes
D
n l e t
Water
Box
E. Tube
Sheet
F.
Out le t Water Box
G
She l l
H
Hotwell
Inspec t ion
Doors
J Staybars
K Expansion
Jo in t s
Single Pass Condenser
Figure
7.1
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334 00 7
xl ust Trunk
ir
xtraction
0 0
o
0 0
Steam
o
eating
o
Lane
00( )
0 0 0 ( 1 0
OOGOOOOOO
o
o
0 0
0 ( ) 0000
0 0 0 ,
OOO( )OOOOO
O
rr
c
\ \ . . .
j
Condenser
Cross Sect ion
Figure
7 2
Figure
7 2
shows
a cross
sect ion of
a
t yp i ca l
s ing le
pass
condenser The
exhaust trunk may be
bol ted
or
welded to
the bottom
of
the
turbine
cas ing
l t ernate ly may
be
supported
independently
and
connected
to
the
turbine
by
a
f l ex ib l e bel lows The la t ter method i s more typ i ca l of large
turbine un its s ince al lows the condenser to expand inde-
pendent
of the turbine
There
i s
a large
steam heating
lane or steam
excess
lane
down the cen tre of the condenser
sh e l l
which contains
no
cool ing
water tubes The purpose of th i s lane
i s
to ensure
tha t
a
port ion of the exhaust steam goes
to
the bottom of the
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condenser.
to reach
achieved:
334.00-7
By
allowing
a s ign i f ican t percentage of
the
steam
the
bottom
of the
condenser, the following
i s
1. The
water in
the hotw ell is kept a t
sa tura t ion
temper
a ture
I f the
water
in the h otw ell was al lowed to be
come
subcooled,
would be an
unnecessary
loss
of
heat
from the condensate. This unnecessary
loss
of
heat
energy
would make the s team/feedwater
cycle
less e f f i -
c i en t as th is lo s t hea t would have to be put back in to
the
condensate by e i ther the fee dh eatin g system
or
the
steam genera tor .
2.
The lower condenser tubes
are forced
to t ra nsfe r as much
steam
out of
the condenser
as
the upper tubes .
This
en
sures the en t i r e
tube
surface
i s
used
to
t r ans fe r
heat
3.
The
thermal
expansion
of the tubes i s
equal ized.
I f the
lower
tubes were
cooler
than
the upper tubes, i t
would
produce
a
bending
force
on
the
condenser
she l l
and
tube
shee ts
Condenser Air Extract ion
The steam
wi l l
continue to condense and
maintain
a good
vacuum as long as
four
c onditio ns a re fu l f i l l ed :
1.
a i r
enters the she l l steam s ide of the condenser.
2. The
tubes
car ry a
normal flow of
r e l a t ive ly cool lake
water .
3.
The
tubes
r emain expos ed to the steam.
4. The
tube surfaces are
not fouled
by
corrosion
products
or o ther
mater ia ls
ny
change
in the system which inva l ida te s one of these
sta tements
wi l l
re su l t
in
a decrease
in
the vacuum in the
condenser.
Apart
from
a
decrease
in
turbine
eff ic iency a
decreasing
vacuum w il l re su lt
in overheat ing of
the low
pres-
sure turbine blading as the
blades must pass
through higher
densi ty steam than they were designed to encounter .
I f a i r enters the condenser,
most
of
wi
remain
there
because
cannot condense with the
steam.
The a i r
wi l l gradua l ly
f i l l
up the empty space l e f t by the
condensing
steam un t i l
the vacuum
in
the
condenser is destroyed. In
addi t ion some of
the
a i r
which leaks
in to the condenser wi l l
disso lve in
the condensing
water.
The oxygen in
the
a i r can
cause
con sid er ab le c or ro si on problems
in
the condensate
and
feedwater systems. Since the condenser operates below
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334.00-7
atmospher ic pres su re ,
there i s a tendency fo r a i r to
leak
in to
the condenser
and
t h i s a i r must be removed.
The
normal
method
of
s t a r t i ng up a tu rb ine un i t i s
to
evacuate th e condenser
she l l and
turb ine
to a pa r t i a l vacuum
and then
to
evacuate the
remaining a i r
a f t e r ro l l ing
th e
t u r
bine w ith
steam.
Once
the
condenser
i s a t
normal
operat ing
vacuum [about kPa a ] ,
th e a i r
ext rac t ion
system
must re
move only th e small amount of a i r which leaks
in to the
con
denser .
The
a i r
ex t rac t ion system has to be
capable of
deal ing
both with vacuum r a i s ing and normal maintenance of vacuum.
When ra i s ing vacuum, the
a i r ex t rac t ion system has to remove
not
only the a i r
which
f i l l s
the
condenser she l l during shu t
down, but a lso the a i r
in
th e
P and LP
tu rb ines
and pipework
back to the
emergency
sto p valves. The
volume of
a i r which
must
be removed i s
t yp i c a l ly
about
6500
cubic m eters. To
p rev en t ex cessiv e s ta r tup
t ime being
expended in drawing a
vacuum, t h i s
a i r
must
be
removed
in
something
on
the order
of
an
hour .
n the o ther hand, the maximum
amount of a i r
leakage
in to
an operat ing
un i t i s
something l ike one cubic
meter
per
minute. This
requires nowhere near the
a ir
removal
capaci ty t h a t i n i t i a l evacuat ion requ i res .
Whatever
the
method of
a i r
ex t rac t ion , there
wi l l
typ ica l ly be
a
la rge a i r
removal ca pa city fo r i n i t i a l evacuat ion and a smal le r capaci
ty
fo r ma in ta in in g
vacuum.
The
a i r
ext rac t ion
system removes
a i r and other non-con
d en sa ble g ase s from
the
condenser
by e i t he r a steam a i r e jec
to r
or
vacuum
pumps. Whatever
the
method of a i r
removal, the
system
c rea tes
area
of
lower
pressure
than
condenser
vacuum.
Air flows from the
condenser
to th i s low pressure area where
t can be removed. In Figure 6.2 , you can see the
a i r
ex
t rac t ion
po in t s
in the condenser.
They
are located within
the
tube
bundles
to ensure t ha t
gases moving toward these
poin ts must pass over many tubes pr io r
to
removal from the
condenser. The a i r ex t rac t ion system wi l l remove any gas and
would j u s t
as
soon suck
out steam as
air .However,
in
passing
over
the tubes ,
the steam
i s condensed and the a i r
ex t rac t ion
sy stem only removes non-condensable gases .
Steam Air Ejectors
Figure
7.3
shows
a
sec t iona l
view
of
a
t yp ica l
steam
a i r
e jec tor .
Steam a t about 2000 kPa g enters the nozzle where
steam hea t
energy i s conver ted to v elo ci ty . The high veloci
ty
steam
j e t i s d i rec ted i n to a di f fuse r . The d i f fuse r i n l e t
has
a much l a rge r
cross
sec t iona l area
than
t ha t of the high
speed
j e t
enter ing t This
crea tes
an extremely low p re s
sure
a t
the
i n l e t
to the di f fuse r .
This
low pressure
area is
connected
to the condenser. The low
pressure
ai r -vapour mix
tu re
from
the
condenser comes in to p hy sic al c on ta ct with the
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Steam
Air
Ejec to r
F igure 7.3
high speed
j e t
and i s
bat ted
along by
the steam enter ing
the
di f fuser
The a i r
t rave l s through the di f fuse r with the
steam
and i s discharged
to a higher
pressure
region. This
cons tant removal of
low
p re ssu re airv ap ou r mixture a t the
di f fuser
i n l e t
encourages
more
a i r
to
flow
in to
the
d i f fus -
e r This es tab l i shes
a continuous
flow
from
condenser in to
e jec tor
For s ta r tup purposes when
la rge
volumes of
a i r
must be
removed a
high capaci ty
steam
e jec tor ca l led
a hogging
e jec tor
i s used. This high capaci ty e jec tor can evacuate the
condenser and tu rb ine un it to
normal
vacuum in
something
l ike
45 minutes.
The
hogging e jec tor uses l a rge
quant i
t i e s of
steam.
After
passing through the
e jec tor
the
steam
is
re -
leased
to
atmosphere. While th i s improves
the capac ity
of
th e
hogging
e jec tor
makes
the
e jec tor wastefu l of steam.
Thus
while
the hogging e jec tor
i s
des irable
for
rapid vacuum
ra i s ing is
fa r
too
inef f ic ien t
for
normal
operat ion .
During
normal
operat ion a much smal ler steam e jec tor
is
used.
This holding
e jec tor
as
may
be ca l led i s normally
a two
s tage steam
e jec tor
as shown in Figure
7.4 .
When the
a i r e j ec to r i s working
proper ly the
a i r from
the
main con
denser
i s drawn in to
the
f i r s t s tage
The
steam and
a i r
mixture from
the f i r s t s tage
pass in to
an in tercondenser
where most
of
the
steam
i s condensed. The a i r is then drawn
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econd
Stage
jector
Water
Tubes
~ r i n
Vent To
Atmosphere
team
In l e t
Second
Stage I so la t ion
Valve
Drain to
Condenser
F i r s t Stage
s o l t i o n ~ v e
Suction from
c o n d e n s e r ~
F i r s t Stage
jector
Typical Two-Stage Condensing Air
Ejec to r
Figure 7.4
in to a
second
s tage
The
stearn and a i r
mixture
from the
second
s tage
pass
in to
an
aftercondenser
where
the
steam
is
condensed and the a i r
re leased
to atmosphere. The
cooling
water for both
th e
intercondenser and af tercondenser is main
condens at e t aken from the condensate extrac t ion pump pr ior to
the low
pressure
feedheaters . The advantage
of having two
s tages
in
the a i r e jec tor i s
t ha t
the
a i r is
ra i sed from
condenser vacuum to atmospheric pressure in two s teps ra the r
than one. The f i r s t s tage
ra ises
pressures
from
5 kPa a to
about 35 kPa a , while
the
second
s tage
ra i ses
pressure
from
35 kPa a to
normal
atmospheric pressure 101.3 kPa a .
Vacuum Pumps
a l t e rna t ive
method
of
a i r
extrac t ion i s
use
of
vacuum
pumps.
These pumps
are
of a
varie ty
of designs although they
are
normally r ot at in g p o si ti ve displacement pumps. The usua l
conf igurat ion
is
to have more
than
one
pump.
The number of
pumps
running depends on the vacuum
condi t ions .
Picker ing
NGS A fo r example, has th ree screw type pos i t ive disp lace-
ment vacuum
pumps fo r
each condenser. All th ree
pumps
are
used for i n i t i a l vacuum ra i s ing During
normal
operat ion ,
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only one pump i s required . However, i f
backpressure r i ses
to
8 kPa a , a second pump wi l l
s t a r t ;
i f the
backpressure
r i ses
to 12 kPa a , the th i rd pump
wil l s t a r t .
Another possib le combination i s
to
use vacuum pumps fo r
normal
vacuum maintenance
and
a hogging
e j ec t o r
fo r i n i t i a l
vacuum
ra i s ing .
There i s rea l ly no c lea rcut advantage of one a i r ext rac -
t ion method
over the
o ther . Vacuum pumps allow eas ie r
on o f f
cont ro l and tend
to
increase p lan t e ff ic ie ncy s ince they
don
t
use steam. However, in p rac tice they requi re
more
maintenance and
are
g en er ally le ss
r e l i ab l e .
Condenser Cooling
Water
The condenser cool ing water system, or system as t
i s most often
ca l led
i s
used
to remove the l a ten t heat of
vapour izat ion
from
the exhaust
steam
enter ing the
condens
e r . The r i s e in
temperature across the condenser i s
l imi ted to about 10C for th ree
reasons:
1. The
higher the temperature r i s e
of
across the con
denser , the
higher
condenser pressure and therefore the
lower cyc le e f f ic iency .
2.
The
h igher the
temperature r i s e
the
grea ter
the
t enden
cy for
a i r
to come out of
solut ion and
co l l ec t in the
high poin ts
of
the
system.
3.
I f
the
water
ente r s
there
warm
outf low.
has re su l ted
r i s e .
leaves the
condenser
much
warmer
than
t
i s a rapid
growth in marine l i fe in
the
The prevention of
t h i s
b io lo g ic p ol uti on
in
l ega l
l imi t s
on
condenser temperature
The
hea t energy which must be removed from
each
kilogram
of exhaust
steam
i s about 2450 KJ. In increas ing i t s temper
a ture
by 10C, a ki logram of water can remove about 4
KJ. Thus t requi res
about
58 kilogram s of water
2450
KJ/42 KJ to condense one kilogram
of
exhaust
steam.
I t re -
qui res about
60 cubic
meters of flow
per
second fo r a
s ingle la rge tu rb in e
uni t .
This extremely l a rge flow ra te
presents
two
problems:
1. an extremely l a rge amount of
pumping
power must be
con
sumed, and
2.
the re
can
be
l i t t l e placed in
the
system fo r
water
t r ea tmen t
and pur i f i ca t i on
which
impedes flow.
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For
a la rge NDU
genera t ing
s t a t i on ,
th e req uired
W
pumping power i s on the order o f 4 pe r un i t . To keep t h i s
p um pin g p ow er
within economic l imi t s , the
pumps
are genera l ly
of high capac i ty
and
low discharge
head.
Since th e
condenser
i s
normally
loca ted above th e l eve l of the lake
or
r i ve r ,
apprec iable
pumping
power
may
be expended to ra i se the water
up
to
the
condenser .
Much of
t h i s
power
requirement
can
be
e l imina ted
by opera t ing the
W
system as a s iphon. This
means
the
pump
i s only requi red to overcome the f r i c t i on flow
lo s ses .
When a W
system
i s opera ted as a
siphon the W
s ide
of
the
condenser
operates
below atmospher ic pres su re . As
water
passes
through
the condenser
a i r
which
has
dissolved
in the
W
water comes out of
so lu t ion .
These
gases
tend to
co l l e c t
in th e
condenser tubes and water
boxes
blocking flow
and
des t roying the
siphon. The W system
i s
pro tec ted
ag ain st the a i r
binding of the
tube s ide of
the
condensers by
an a i r
removal system.
This vacuum pr iming system
as it i s
ca l l ed , removes a i r from
the
condenser
water
boxes.
t i s
worth mentioning t ha t
th i s
system i s
complete ly
separa te from
the a i r removal system fo r
the
she l l
s ide
of th e
condenser .
Because of
the
extremely
l a rge
flow ra te of W water ,
the re
i s
littl water
t rea tment
except i n t e rm i t t en t ch lo r ina -
t ion and
t r a sh removal. Water
f i r s t
passes
through a coarse
sc reen. The spacing between bars must be small
enough to
prev en t lo gs ,
boa t s , people
or
ice from being swept i n to
the
W
system.
On
the othe r hand the spacing must
be
wide
enough
to prevent
a
l a rge
quant i ty of sm all f i sh or seaweed
from r ap id ly plugging
the
screen . The c oa rse scre en spacing
i s
genera l ly
2
to 15
cm with
exact
spacing
l a rge ly
determin-
ed
by
the water
condi t ions
near
the
s t a t i on :
the
d i r t i e r
the
water
th e
f i n e r
th e mesh
of
the coarse sc reen.
The
W
flow
next
passes
through a
band screen
or
t r ave l l i ng
screen
as it
i s
a l t e rna t ive ly ca l led . This screen
has
a
f ine mesh typ ica l ly
cm
or
l e s s ) and i s
driven
by a
motor.
When
the
screen accumulates
su f f i c i en t t ra sh
to
ra i se
the d i f f e r en t i a l pres su re ,
th e
motor i s
turned on
and
the
screen
s lowly ro t a t e s to a new
pos i t ion .
The screen i s wash-
ed
with high pressure water
j e t s
near th e
top
of i t s
t r ave l .
The
purpose
of the screen i s
to block any
t ra sh l a rge
enough
to block the tubes .
Some
marine l i f e
may
pass
both
th e
coarse and
t r ave l l i ng
screens and accumulate on
the
condenser
tube
shee ts
and
tube
su r faces . The
warm condi t ions
and
cons tant
renewing
of
wate r
r e su l t in rap id
growth of
th i s
marine
l i f e .
This growth can
r ap id ly
r e su l t
in tu be p lu gg in g.
To r e s t r i c t the
growth
ra te
chlor ine i s in te rm i tte nt ly in je cte d in to the
W
in take . Th e
chlor ine
in je ct io n r ate
must
be low enough to p re ve nt p ois on -
ing
of
the
marine
l i f e in the
W
outf low.
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8/10/2019 Turbine, Generator & Auxiliaries - Course 334
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334.00 7
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Trave l l ing Screen
Figure
7 .5
Regardless of what
methods
are used to r e s t r i c t tube
and
tube shee t fou ling , even tua lly the
tube
shee ts pa r t i cu l a r ly
the i n l e t tube sheet must be c leaned. To f ac i l i t a t e c lean-
ing
of
condenser CCW s ides while
ope ra t ing , the c i rcu la t ing
water
system
of
la rge s t a t i ons
supply
ind iv idua l condenser
ha lves which can be i s o l a t ed
from
each o ther . The ar range
ment
shown
in
Figure 7 .6
al lows
ha l f
of
one
condenser to be
i s o l a t ed and
opened
fo r
cleaning.
The balance piping between
condensers
ensures
the
remaining
f ive
condenser
ha lves are
su f f i c i en t to
keep
the
un i t
a t
power.
10
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8/10/2019 Turbine, Generator & Auxiliaries - Course 334
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334.00-7
Discharge
Line To ommon
Discharge
Channel
I
l l
I
l l
l l
Q
I
Q
Q
I
I
~
Q l l
I
r
Q l l
N
Q
l l
M
0 Q
=l l=
O Q
=l l=
OQ
=l l=
~
~
~
o
~
Al
o
Al
o
~
Al
UO
H
UO
H
UO
H
I
I
uct ion
From ommon
Screen
House
and
hlor ina to r
W System For
ingle
Turbine
Uni t
with Three P
Turbines
Figure 7 .6
11
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8/10/2019 Turbine, Generator & Auxiliaries - Course 334
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334.00-7
SSIGNM NT
1 .
Why does the
LP turbine exhaust
to
a
vacuum
ra ther
than
to atmosphere?
How
i s
th i s vacuum
maintained?
2.
raw a
cross sec t ion of
a typ ica l
condenser showing:
a
exhaust
t runk
b tubes
c
tube
sheets
d i n l e t
and
out le t water boxes
e sagging
pla tes
f s taybars
g
she l l
h
hotwel l
i
expansion
jo in t s
j waterbox inspect ion doors
3. Why
are tu rb ine
uni ts
with more than one
condenser
f i t -
ted with balance pipes between condensers?
4.
Why
a re
the
a i r
e xtra ctio n p oin ts for
the
condenser
she l l surrounded by
tubes?
5.
What
i s
the
purpose
of
a
steam
heat ing
lane .
6. Why
must
a ir be extrac ted from a
condenser
shel l?
From
condenser waterboxes?
7. What
is
a
hogging
ejector?
8.
How
does
a
steam
a i r e jec tor work?
9.
What
provisions
are
made
to
prevent
blockage of condens
er
tubes?
R.O. Schuelke
12