Boiler Draught: Natural vs. Artificial Methods
B
O
I
L
E
R
D
R
A
U
G
H
T
D
E
FINITI
O
N
Boiler
dr
a
ug
h
t
may
b
e
defined
a
s
the
s
m
all
d
i
f
ference
between
the
pressure
o
f
outsi
d
e
a
i
r
and
that
o
f
gas
e
s
within
a
furnace
o
r
chimney
a
t
the
grate
l
e
v
e
l,
w
h
ich
causes
flow
o
f
a
i
r
/
hot
fl
u
e
gases
to
take
p
l
ace
throu
g
h
bo
i
l
e
r
.
Th
e
draught
i
s
nec
e
ssary
to
f
o
rce
a
i
r
thr
o
ugh
the
fuel
bed
/
grate
to
a
i
d
i
n
proper
combustion
o
f
fu
e
l
an
d
to
remove the
products
o
f
co
m
bustion
i
.
e
.
flue
gases
to
the
a
t
mosphere
a
f
ter
they
have
g
i
ven
the
i
r
hea
t
to
w
a
ter
b
e
i
n
g
evaporated
i
n b
o
i
l
e
r
.
Draught
a
lso
provides
vel
o
c
ity
t
o
flue
g
ases
an
d
so
i
n
creases
the
hea
t
t
r
ansfer
c
o
-e
f
ficie
n
t
i
n
the
bo
i
l
e
r
.
Thu
s
dra
u
ght
i
s
ess
e
ntial
l
y
req
u
ired
i
n
a
boiler
and
can
b
e produced
b
y a
number
of
metho
d
s.
N
A
T
U
R
A
L
D
R
A
U
GHT
Natural
draught
i
s
obta
i
ned
nat
u
ra
l
ly
b
y
the
use
of
a chimne
y
.
Ch
i
m
n
ey
i
s
a
con
i
c
a
l
shape
vertic
a
l
tub
u
l
a
r
steel
or
masonry
o
r
concre
t
e
s
t
ructu
r
e
having
a
l
a
rge
he
i
ght.
The
flue
g
ases
after
t
r
ansferri
n
g
their
hea
t
i
n
the
boiler
are
guided
b
y
chimney
to
a
considera
b
le
he
i
ght
i
n the atmosph
e
r
e
.
A
s
chimney
ha
s
a
l
a
r
g
e
hei
g
ht
an
d
i
s
the
on
l
y
outl
e
t
f
r
om
bo
i
l
e
r
,
i
t
rema
i
ns
fil
l
ed
b
y
ho
t
flue gas
e
s.
These
exha
u
st
gases,
howe
v
er
have
g
iven
th
e
ir
hea
t
to
water
i
n
the
boile
r
,
are
still
h
o
tter
t
h
a
n
the
bo
i
l
e
r
room
a
i
r
.
Due
to
this,
the
e
x
ha
ust
gas
e
s
are
lighter
i
n
weight
than
outside
a
i
r
an
d
so
l
i
f
t
s
u
p
naturally
i
n
the
ch
i
m
n
ey
and
fina
l
ly
esca
p
e
out
i
n
the
atmosphere
f
r
om
top
o
f
the chimne
y
.
A
s
the
flue
gas
e
s
l
i
ft
u
p
i
n the ch
i
mne
y
, more
gases
f
r
om
fuel
be
d
fl
o
w
to
w
ards
c
h
imney
to
t
a
ke
their
p
l
ac
e
.
D
u
r
ing
th
i
s
process,
boiler
an
d
d
o
the
they
first
flow
throu
g
h
the
j
o
b
o
f
hea
t
i
n
g
o
f
w
a
ter
and
produce
steam.
A
s
flue
gas
e
s
flow
throu
g
h
boiler
an
d
then
to
chimne
y
,
fresh
a
ir
f
r
o
m
outsi
d
e
naturally
e
nters
the
boiler
and
help
i
n
bu
r
ning
o
f
fuel
an
d
pr
o
duction
of
ho
t
flue gas
e
s.
So,
i
n
this
w
a
y
,
a
sma
l
l
pressure
d
i
f
ference
is
naturally
created
between
the
b
a
se
o
f
chimney
and
the
a
i
r
i
n
let
po
i
nt
o
f
boiler
becau
s
e
o
f
de
n
sity
d
i
f
ference
betw
e
e
n
ho
t
flue
gas
e
s
inside
the
chimney
an
d
f
r
esh
col
d
er
a
i
r
outsi
d
e
.
T
h
is
press
u
re
d
i
f
ference
i
s
ca
l
led
n
a
tural
draught,
bec
a
use
i
t
i
s produced
natural
l
y
.
A
R
TIFICIAL
DR
A
UGHT
When
the
draught
i
s
produced
b
y
some
e
x
ter
na
l
agency
i
.
e
.
mechan
i
c
al
fan/blower
o
r
b
y
s
t
eam
jet
itself,
i
t
i
s cal
l
ed
artificial
draugh
t
.
In
mod
e
rn
commercial
bo
i
l
e
r
s,
more
v
alue
of
draught
i
s
req
u
ir
e
d
to
i
n
crease
t
h
e
hea
t
t
r
ansfer
c
o
-
e
f
ficie
n
t
an
d
he
n
ce
the
ther
m
al e
f
ficienc
y
.
S
o
,
a
r
tificial
draught
i
s
mu
s
t
to
u
s
e
to
overcome
the
flow
resistance
o
f
fe
r
ed
b
y
l
a
rge
flue
passa
g
e
s
.
FORC
E
D
D
R
A
U
GHT
It
i
s a
pos
i
tive
pressure
draugh
t
.
Th
e
fan
i
s
i
n
st
a
lled
a
t
the
base
o
f
the
boiler
before
grate
whi
c
h
forces
the
outside
air
throu
g
h
fu
e
l
bed,
furnace
and
a
i
r
pre-he
a
ter
and
then
fl
u
e
gases
throu
g
h
flue
passa
g
e,
econ
o
mizer
etc.
Th
e
enclo
s
ure
for
the
furnace
has
to
b
e
ve
r
y
tightly
sealed
so
t
h
at
gases
f
r
om
the
furnace
d
o
n
ot
l
e
ak
ou
t
i
n
the
bo
i
l
e
r
hous
e
.
IND
U
C
E
D
D
R
A
U
GHT
In
this
sys
t
em
a
fan
o
r
b
l
ower
i
s
l
o
c
ated
a
t
o
r
near
the
base
o
f
the
chi
m
ney
w
h
ich
creates
a
partial
vacuum
i
n
the
furnace
an
d
flue passa
g
e
.
Thus
the
a
i
r
an
d
fl
u
e
gas
e
s
a
r
e
drawn
throu
g
h
t
h
e
boiler
du
e
to
co
m
para
t
iv
e
ly
h
i
g
her
pressure
of
outsi
d
e
a
i
r
.
It
i
s
convenient
to
produce
induc
e
d
draught
and
like
i
n
forced
dra
u
ght,
an
y
type
o
f
fan/blower
may
b
e use
d
.
BAL
A
N
C
ED D
R
AUGHT
It
i
s a
combin
a
tion
o
f
forced
an
d
i
n
duc
e
d
draught.
Forced
d
r
aught
fan
overcomes
t
h
e
resista
n
ce
i
n
air
pre-he
a
ter
an
d
gra
t
e
.
Induc
e
d
d
raught
fan
overcom
e
s
draught
l
o
sses
through
b
o
ile
r
,
ec
o
n
o
miz
e
r
and
co
n
n
e
ct
i
ng
flue
passa
g
es
etc.
Dep
e
n
d
i
n
g
o
n
the
type
o
f
fuel
burnt
an
d
type
of
boile
r
,
the
f
a
n
o
r
b
l
ower
used
m
a
y
b
e
o
f
an
y
type
as
radi
a
l
o
r
a
x
i
a
l
et
c
.
AD
V
AN
T
AGES
OF
MECHANICAL
DRAUGHT
It
i
s more
econ
o
mical
an
d
its
control
i
s eas
y
.
Desir
e
d
value
o
f
draught
can
b
e
pro
du
ced
by
mechanic
a
l
means
w
h
ich
cann
o
t
b
e
produced
by
means
o
f
natural
draugh
t
.
I
t
i
n
creas
e
s
the
rate
o
f
comb
u
stion
b
y
w
h
ich
low
grade
fuel can a
l
so
b
e use
d
.
It
reduces
the
s
m
oke
l
e
v
el
and
increases
the
h
eat
t
r
ansfer
c
o
-e
f
ficient
o
n
flue
gas
e
s
side
thus
i
n
creases
the ther
m
al e
f
ficie
n
cy
o
f
bo
i
l
e
r
.
It
saves
t
h
e
en
e
rgy
a
nd
the
heat
o
f
flue
gases
can
b
e best
util
i
zed
by
i
t.
DISA
D
V
A
N
T
AGES
OF ME
C
H
A
NIC
A
L
D
R
A
U
GHT
Initial
costs
o
f
mechan
i
cal
draught system are
h
i
g
h
.
R
u
nning
cost
i
s
a
l
so
h
i
g
h
due
to
requ
i
r
e
ment
of
electricity
bu
t
that
i
s
eas
i
ly
c
o
mpensat
e
d
b
y
the
savi
n
gs
i
n
fuel consumptio
n
.
Maintenance
cost
i
s a
l
so
o
n h
i
g
h
er
si
d
e
.
Noise
l
e
v
el
o
f
boiler
i
s
a
l
so
high
d
ue
to
n
o
isy
fan/blo
w
er
etc.
ST
E
AM JET
DR
A
UGHT
It
i
s
a
ve
r
y
simp
l
e
a
nd
easy
method
o
f
p
r
od
u
c
ing
artificial
d
r
aught
w
i
t
h
out
the
need
o
f
a
n
e
l
ectric
moto
r
.
Steam
unde
r
pressure
i
s ava
i
l
a
b
l
e
i
n the
bo
i
l
e
r
.
When
a
small
p
o
rtion
o
f
steam
i
s
p
a
s
s
ed
through
a
jet
o
r
nozz
l
e,
pressure
en
e
rgy
converts
to
kinetic
en
e
rgy
an
d steam comes
ou
t
w
i
th
a h
i
gh
vel
o
cit
y
.
Th
i
s
h
igh
velocity
s
t
eam
carries,
a
l
ong
with
it,
a
l
a
r
g
e
mass
o
f
a
i
r
o
r
f
l
ue
gas
e
s
a
nd
makes
i
t
to
flow
throu
g
h
bo
i
l
e
r
.
Steam
jet
i
s
d
i
rect
e
d
towards
a
fi
x
e
d
d
i
rection
and
carri
e
s
a
l
l
its en
e
rgy
i
n
ki
n
etic
for
m
.
I
t
creates
s
ome
vacu
u
m
i
n
its
surroun
d
ings
an
d
so
attra
c
ts the
a
i
r
o
f
flue gas
e
s
e
ither
b
y carryi
n
g
alo
n
g
w
i
th
i
t.
Thu
s
i
t
h
a
s
the
capac
i
ty
to
make
flow
o
f
the
flue
gas
e
s
e
ither
by
c
arryi
n
g
o
r
i
n
ducing
towards
chimne
y
.
FORC
E
D
ST
E
AM JET
DR
A
UGHT
Steam
f
r
om
the
bo
i
ler
a
fter
having
been
thr
o
t
t
l
e
d
to
a
ga
u
g
e
pressure
o
f
1
.5
to
2
ba
r
i
s
suppli
e
d
to
the
jet
o
r
nozz
l
es
i
n
stall
e
d
i
n ash
p
i
t
.
Th
e
steam
emerging
ou
t
o
f
nozz
l
es
with
a
great
velocity
drags
a
i
r
al
o
ng
the
fuel
bed
,
furnac
e
,
flue
passa
g
e
an
d
then
to
the
chimne
y
.
Here
ste
a
m
jet
i
s
forcing
the
air
an
d
flue
gas
e
s
to
flow
through
boil
e
r
he
n
c
e
i
t
i
s
forced
steam
j
e
t
draugh
t
.
IND
U
C
E
D
ST
E
AM J
E
T
DR
A
UGHT
The
jet
o
f
s
team
i
s
d
i
verted
into
smoke
bo
x
or
chimne
y
.
Th
e
kinetic
hea
d
o
f
t
h
e
steam
i
s
high
bu
t
s
ta
t
ic
head
i
s
l
o
w
i
.
e
.
i
t
c
r
e
ates
a
par
t
i
a
l
vacu
u
m
wh
i
c
h
draws
the
a
i
r
throu
g
h
the
grate,
ash
p
i
t,
flues
and
then to
motor
bo
x
an
d
chimne
y
.
Th
i
s
type
o
f
arran
g
e
m
ent
i
s
employed
i
n
l
o
c
omotive
bo
i
l
e
r
s
.
H
e
re
ste
a
m
j
e
t
i
s
s
u
cking
the
f
l
u
e
gas
e
s
throu
g
h
bo
i
l
e
r
,
so
i
t
i
s Induced
Steam
Jet
Drau
g
ht.
A
D
V
A
N
T
AGES
It
i
s qu
i
te
simp
l
e
an
d chea
p
.
It
ha
s the
capa
b
i
l
ity
o
f
usi
n
g
l
o
w
grade
fuel
s
.
It
occup
i
es
very
l
e
ss spac
e
.
Initial
cost
i
s l
o
w
.
Mai
n
tena
n
ce
cost
i
s l
o
w
.
Ex
h
a
ust
s
t
e
am
from
s
team
en
g
ine
o
r
turb
i
ne
can
be
used
eas
i
ly
i
n
Steam
Jet
Drau
g
ht.
DISA
D
V
A
N
T
AGES
I
t
can
op
e
rate
only
when
some
s
t
e
am
is
ge
n
erated.
Drau
g
ht
produced
i
s very
l
o
w
.
D
R
A
U
GHT
LOSSES
Loss
du
e
t
o
the
f
r
ictio
n
a
l
resist
a
nce
o
f
fered
b
y
flue
ga
s
passa
g
e
to
the
flow
o
f
flue gase
s
.
Loss
du
e
to
bend
s
i
n
ga
s
fl
o
w
circu
i
t,
which
a
l
s
o
o
f
fer flow
resistanc
e
.
Loss
du
e
to
frict
i
on
hea
d
i
n
grate,
ec
o
n
o
mize
r
,
super he
a
ter
et
c
.
Loss
du
e
to
flow resistance
o
f
fered
b
y chimne
y
.
Loss
du
e
t
o
imparting
some
velocity
to
flue
gas
e
s,
which
i
s
required
to
i
n
crease
h
e
at
tra
n
sfer
i
n
boil
e
r
an
d
a
l
s
o
to
throw
away
the
flue
gases
f
r
o
m
chimne
y
.
Boiler draught is the pressure difference that facilitates airflow through a boiler to aid in combustion and heat transfer. Natural draught relies on chimney height and temperature differentials, while artificial draught involves external mechanisms. Both methods play crucial roles in boiler operation and efficiency.
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DEFINITION Boiler draught may be defined as the small difference between the pressure of outside air and that of gases within a furnace or chimney at the grate level, which causes flow of air/hot flue gases to take place through boiler. The draught is necessary to force air through the fuel bed/ grate to aid in proper combustion of fuel and to remove the products of combustion i.e. flue gases to the atmosphere after they have given their heat to water being evaporated in boiler.
Draught also provides velocity to flue gases and so increases the heat transfer co-efficient in the boiler. Thus draught is essentially required in a boiler and can be produced by a number of methods.
NATURAL DRAUGHT Natural draught is obtained naturally by the use of a chimney. Chimney is a conical shape vertical tubular steel or masonry or concrete structure having a large height. The flue gases after transferring their heat in the boiler are guided by chimney to a considerable height in the atmosphere. As chimney has a large height and is the only outlet from boiler, it remains filled by hot flue gases.
These exhaust gases, however have given their heat to water in the boiler, are still hotter than the boiler room air. Due to this, the exhaust gases are lighter in weight than outside air and so lifts up naturally in the chimney and finally escape out in the atmosphere from top of the chimney. As the flue gases lift up in the chimney, more gases from fuel bed flow towards chimney to take their place. During this process, boiler and do the job of heating of water and produce steam. they first flow through the
As flue gases flow through boiler and then to chimney, fresh air from outside naturally enters the boiler and help in burning of fuel and production of hot flue gases. So, in this way, a small pressure difference is naturally created between the base of chimney and the air inlet point of boiler because of density difference between hot flue gases inside the chimney and fresh colder air outside. This pressure difference is called natural draught, because it is produced naturally.
ARTIFICIAL DRAUGHT When the draught is produced by some external agency i.e. mechanical fan/blower or by steam jet itself, it is called artificial draught. In modern commercial boilers, more value of draught is required to increase the heat transfer co- efficient and hence the thermal efficiency. So, artificial draught is must to use to overcome the flow resistance offered by large flue passages.
FORCED DRAUGHT It is a positive pressure draught. The fan is installed at the base of the boiler before grate which forces the outside air through fuel bed, furnace and air pre-heater and then flue gases through flue passage, economizer etc. The enclosure for the furnace has to be very tightly sealed so that gases from the furnace do not leak out in the boiler house.
INDUCED DRAUGHT In this system a fan or blower is located at or near the base of the chimney which creates a partial vacuum in the furnace and flue passage. Thus the air and flue gases are drawn through the boiler due to comparatively higher pressure of outside air. It is convenient to produce induced draught and like in forced draught, any type of fan/blower may be used.
BALANCED DRAUGHT It is a combination of forced and induced draught. Forced draught fan overcomes the resistance in air pre-heater and grate. Induced draught fan overcomes draught losses through boiler, economizer and connecting flue passages etc. Depending on the type of fuel burnt and type of boiler, the fan or blower used may be of any type as radial or axial etc.
ADVANTAGES OF MECHANICAL DRAUGHT It is more economical and its control is easy. Desired value of draught can be produced by mechanical means which cannot be produced by means of natural draught. It increases the rate of combustion by which low grade fuel can also be used. It reduces the smoke level and increases the heat transfer co-efficient on increases the thermal efficiency of boiler. It saves the energy and the heat of flue gases can be best utilized by it. flue gases side thus
DISADVANTAGES OF MECHANICAL DRAUGHT Initial costs of mechanical draught system are high. Running cost is also high due to requirement of electricity but that is easily compensated by the savings in fuel consumption. Maintenance cost is also on higher side. Noise level of boiler is also high due to noisy fan/blower etc.
STEAM JET DRAUGHT It is a very simple and easy method of producing artificial draught without the need of an electric motor. Steam under pressure is available in the boiler. When a small portion of steam is passed through a jet or nozzle, pressure energy converts to kinetic energy and steam comes out with a high velocity. This high velocity steam carries, along with it, a large mass of air or flue gases and makes it to flow through boiler.
Steam jet is directed towards a fixed direction and carries all its energy in kinetic form. It creates some vacuum in its surroundings and so attracts the air of flue gases either by carrying along with it. Thus it has the capacity to make flow of the flue gases either by carrying or inducing towards chimney.
FORCED STEAM JET DRAUGHT Steam from the boiler after having been throttled to a gauge pressure of 1.5 to 2 bar is supplied to the jet or nozzles installed in ash pit. The steam emerging out of nozzles with a great velocity drags air along the fuel bed, furnace, flue passage and then to the chimney. Here steam jet is forcing the air and flue gases to flow through boiler hence it is forced steam jet draught.
INDUCED STEAM JET DRAUGHT The jet of steam is diverted into smoke box or chimney. The kinetic head of the steam is high but static head is low i.e. it creates a partial vacuum which draws the air through the grate, ash pit, flues and then to motor box and chimney. This type of arrangement is employed in locomotive boilers. Here steam jet is sucking the flue gases through boiler, so it is Induced Steam Jet Draught.
ADVANTAGES It is quite simple and cheap. It has the capability of using low grade fuels. It occupies very less space. Initial cost is low. Maintenance cost is low. Exhaust steam from steam engine or turbine can be used easily in Steam Jet Draught.
DISADVANTAGES It can operate only when some steam is generated. Draught produced is very low.
DRAUGHT LOSSES Loss due to the frictional resistance offered by flue gas passage to the flow of flue gases. Loss due to bends in gas flow circuit, which also offer flow resistance. Loss due to friction head in grate, economizer, super heater etc. Loss due to flow resistance offered by chimney. Loss due to imparting some velocity to flue gases, which is required to increase heat transfer in boiler and also to throw away the flue gases from chimney.
