Boiler

12/31/2016 Jahangirabad institute of technology 1
 By:
Naphis ahmad
Assistant professor
JIT, Barabanki

STEAM GENERATOR OR BOILER
A steam generator or boiler is usually a closed vessel made of
steel. Its function is to transfer the heat produced by the
combustion of fuel to water and ultimately to generate steam.

BOILER PROPERTIES:
 (i) Safety. The boiler should be safe under operating conditions.
 (ii) Accessibility. The various parts of the boiler should be
accessible for repair and maintenance.
 (iii) Capacity. Should be capable of supplying steam according to
the requirements.

(iv) Efficiency. Should be able to absorb a maximum amount of heat
produced due to burning of fuel in the furnace.
(v) It should be simple in construction .
(vi) Its initial cost and maintenance cost should be low.
(vii) The boiler should have no joints exposed to flames.
(viii) Should be capable of quick starting and loading.

Generate maximum steam.
Light in weight and not occupy large space.
Proper safety regulations.
Cheaper in cost.
Easy cope up with fluctuating demands of
requirements.
Easily accessible for inspection and repair.

The source of heat for a boiler are :
Wood
Coal
Diesel
Natural Gas
Nuclear Energy

 Though there are many classifications of steam boilers, yet
the following are important.
 ACCORDING TO THE CONTENTS IN THE TUBE:
a) fire tube or smoke tube boiler
b) water tube boiler.

FIRE – TUBE BOILERS

 ACCORGING TO POSITION OF FURNACE:
a) Internally fired boilers
b) Externally fired boilers
 ACCORDING TO AXIS OF FURNACE:
a) Vertical boilers
b) Horizontal boilers
 ACCORDING TO NUMBER OF TUBES:
a) Single tube boilers
b) Multi tube boilers

 ACCORDING TO METHOD OF
CIRCULATION OF WATER:
a) Natural circulation boilers
b) Forced circulation boilers
 ACCORDING TO THE USE
a) Stationary boilers
b) Mobile boilers

 Following factors should be considered
 Quality of steam required
 Steam Generation rate
 Accessibility of repair and inspection
 Area
 Cost
 Erection facility
 Fuel and Water available

Boiler shell
Combustion chamber
Furnace
Grate
Heating surface
Mounting
Accessories

Pressure Gauge
Water level indicator
Safety valve
Fusible plug
Blow of cock
Steam stop valve
Feed check valve

Air preheater : It utilize the heat of exhaust gases.
Economizer: Heat is used to raise the temperature
of feed water supplied to the boiler.
Steam super heater: Used to super heat the
steam.
Feed pump: Raise the pressure of water and force it into the boiler.
Injector: Used to feed water in boilers.

5

Considered to be one of the most efficient type
multi tubular boilers.
Consists of external cylindrical shell and fire
box. The shell and fire box both are
hemispherical.
The fire box and combustion chamber is
connected through a short pipe.
The flue gases from the combustion chamber
flow to the smoke box through a number of
smoke tubes.

The gases from the smoke box pass to the
atmosphere through a chimney
The external diameter of pipe is 62.5mm and are
165in number.
Manhole is provided for cleaning.

6

 The boiler consists of a long cylindrical external
shell (1) built of steel plates in sections riveted
together.
 It has two large internal flue tubes (2).
 A fire grate (3) is also called furnace is provided at
one end of the flue tubes on which solid fuel is burnt.
 At the end of the fire grate there is a brick arch (5)
to deflect the flue gases upwards.
 The hot flue gases leaving internal flue tubes passes
through the bottom tube(6).

 These flue gases move to the front of the boiler where
they divide and flow into side tube (7).
 The flue gases then enter the main flue (9) which
leads them to chimney.
 The damper (8) is fitted at one side flues to control
the draught and thus regulate the rate of generation of
steam.
 A spring loaded safety valve (10) and stop valve (11)
are mounted.

 Locomotive boiler is a horizontal fire tube type mobile
boiler. The main requirement of this boiler is that it
should produce steam at a very high rate. Therefore,
this boiler requires a large amount of heating surface
and large grate area to burn coal at a rapid rate. In order
to provide the large heating surface area, a large
number of fire tubes are setup and heat transfer rate is
increased by creating strong draught by means of steam
jet

 Large rate of steam generation per square metre of
heating surface. To some extent this is due to the
vibration caused by the motion.
 It is free from brickwork, special foundation and
chimney. This reduces the cost of installation.
 It is very compact.
The pressure of the steam is limited to about 20 bar.

1. This is an example of water tube boiler and is used
in stationary and marine engine.
2. The efficiency of this boiler is much greater than that
of the fire tube boiler.
3. This boiler is used when pressure is above 10bar
and steam generating capacity is required higher then
7000kg/hr.

 SAFETY MOUTINGS
 Safety valves (2 Nos)
 Fusible plug
 Control mountings
 Water level indicator
 Pressure Gauge
 Blow of cock
 Steam stop valve
 Feed check valve
 Inspection Mountings
 Man hole
 Mud hole

 Air preheater : It utilize the heat of exhaust gases.
 Economizer: Heat is used to raise the temperature
of feed water supplied to the boiler.
 Steam super heater: Used to super heat the
steam.
 Feed pump: Raise the pressure of water and force
it into the boiler.
 Injector: Used to feed water in boilers.

 Air preheater
9

 Economizer
10

 Draught is defined as “the pressure difference (between absolute gas
pressure at any point in a gas flow passage and the atmospheric air outside
the boiler) required to maintain flow of air and exhaust gases through the
boiler setting.”
 Function of draught:
1. To supply required quantity of air to the boiler furnace for the combustion
of fuel.
2. To remove exhaust gases from the boiler passage to maintain flow of air
and gases.
3. To create pressure difference against pressure losses in the flow passage.
4. To discharge the exhaust gases such that they will not be objectionable or
injurious to the surroundings.

Draught
Natural
Draught
Produced by chimney
Artificial
Draught
Steam Jet
Draught
Produced by jet of stream
Forced
Draught
Induced
Draught
Mechanical
Draught
Produced by fans
Forced
Draught Fan
Induced
Draught Fan

 It produces draught which required to force air and gases
through the furnace, boiler accessories & setting.
 It carries the product of combustion to such a height that
they will not be objectionable or injurious to surrounding.
 The chimney is vertical tabular structure build either of
masonry concrete or steel.
 The draught produces by the chimney is due to density
difference between the column of hot gases inside the
chimney & cold air outside the chimney & also on the
height of the chimney above the level of the furnace grate.
 The density difference depends on the temperature of flue
gases passing in chimney & temperature of atmospheric air.

Natural Draught
Natural Draught = Difference of pressure due to cold air column and
hot gas column

 Suppose,
T0 = Absolute Zero Temperature = 273 K
𝜌0 = Density of air at absolute temperature = 1.293
𝑘𝑔
𝑚3
Tg = Temperature of hot gas
𝜌 𝑔 = Density of hot gas
Now from PV = m R T P = 𝜌 R T
∴ 𝜌 𝑇 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
o Density of air at temp. Tg
𝜌0 𝑇0 = 𝜌 𝑎𝑖𝑟 𝑇𝑔 ∴ 𝜌𝑎𝑖𝑟 ℎ𝑜𝑡 =
𝜌0 𝑇0
𝑇 𝑔

o 𝑇 𝑎 = Atm. temp. & 𝜌 𝑎 = 𝑎𝑖𝑟 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 𝑎𝑡 𝑇𝑎 (𝑐𝑜𝑙𝑑)
𝜌0 𝑇0 = 𝜌 𝑎 𝑇𝑎 ∴ 𝜌 𝑎 =
𝜌0
𝑇0
𝑇 𝑎
o We know that P = 𝜌𝑔ℎ
∴ ∆𝑃 = 𝜌 𝑔 𝑔 ℎ 𝑔 = 𝜌 𝑤 𝑔 ℎ 𝑤
(Where, ℎ 𝑔 = 𝐶𝑜𝑙𝑢𝑚𝑛 𝑜𝑓 𝑔𝑎𝑠
ℎ 𝑤 = 𝐶𝑜𝑙𝑢𝑚𝑛 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟)
o 𝑚 =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑎𝑖𝑟
𝑘𝑔 𝑜𝑓 𝑓𝑢𝑒𝑙
=
𝑚 𝑎
𝑚 𝑐
= 𝐴𝑖𝑟 𝐹𝑢𝑒𝑙 𝑅𝑎𝑡𝑖𝑜
∴
𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑓 𝐻𝑜𝑡 𝑎𝑖𝑟
𝑘𝑔 𝑜𝑓 𝑓𝑢𝑒𝑙
=
𝑚 𝑎
+ 𝑚 𝑐
𝑚 𝑐
= 𝑚 𝑔 = 𝑚 + 1

o So, 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐻𝑜𝑡 𝑔𝑎𝑠 𝑎𝑡 𝑇𝑔 = 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑎𝑖𝑟 𝑎𝑡 𝑇𝑔
=
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑎𝑖𝑟
𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑎𝑖𝑟 𝑎𝑡 𝑇𝑔
=
𝑚
𝜌0𝑇0
𝑇𝑔
=
𝑚𝑇 𝑔
𝜌0 𝑇0
o Density 𝑜𝑓 𝐻𝑜𝑡 𝑔𝑎𝑠 𝑎𝑡 𝜌g =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝐻𝑜𝑡 𝐺𝑎𝑠
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐻𝑜𝑡 𝐺𝑎𝑠
=
𝑚+1
𝑚𝑇 𝑔
𝜌0
𝑇0
∴ 𝜌g =
𝑚+1 (𝜌0
𝑇0
)
𝑚 𝑇𝑔
o Pressure of Hot Gases in Chimney at Grate level = 𝜌g g H
∴ 𝑃 𝐻𝑜𝑡 =
𝑚+1 (𝜌0
𝑇0
)
𝑚 𝑇𝑔
𝑔 𝐻
o Pressure due to Cold air (outside) column at Grade level
∴ 𝑃 𝐶𝑜𝑙𝑑 =
𝜌0
𝑇0
𝑇𝑎
𝑔 𝐻

o So,
𝑁𝑎𝑡𝑢𝑟𝑎𝑙 𝐷𝑟𝑎𝑢𝑔ℎ𝑡 = 𝑃𝐶𝑜𝑙𝑑 − 𝑃𝐻𝑜𝑡
∴ ∆𝑃 = 𝜌0 𝑇0 𝑔 𝐻
1
𝑇𝑎
−
𝑚+1
𝑚
1
𝑇𝑔
∴ ∆𝑃 = 353 𝑔 𝐻
1
𝑇𝑎
−
𝑚+1
𝑚
1
𝑇𝑔
(Where,
𝜌0 𝑇0 =1.293 * 273 = 353)
o Now, We Know that
∴ ∆𝑃 = 𝜌 𝑔 𝑔 ℎ𝑔 = 353 𝑔 𝐻
1
𝑇𝑎
−
𝑚+1
𝑚
1
𝑇𝑔
∴
𝑚+1 (𝜌0
𝑇0
)
𝑚 𝑇𝑔
𝑔 𝐻 = 353 𝑔 𝐻
1
𝑇𝑎
−
𝑚+1
𝑚
1
𝑇𝑔
∴ 𝐻 𝑔 = 𝐻
𝑚+1
𝑚
𝑇𝑔
𝑇𝑎
− 1 m of gas

∴ ∆𝑃 = 𝜌 𝑤 𝑔 ℎ𝑤 = 353 𝑔 𝐻
1
𝑇𝑎
−
𝑚+1
𝑚
1
𝑇𝑔
∴ hw =
353
1000
𝐻
1
𝑇𝑎
−
𝑚+1
𝑚
1
𝑇𝑔
∴ ℎ𝑤 = 353 𝐻
1
𝑇𝑎
−
𝑚+1
𝑚
1
𝑇𝑔
mm of water
o Now, Hot Gas Velocity in Chimney 𝑉 =
2𝑔(ℎ𝑔 − ℎ𝑓)
(where hg = hot gas column
hf = loss due to friction)
𝑚𝑔 = 𝜌 𝑔 𝐴 𝑐ℎ𝑖𝑚𝑛𝑒𝑦 𝑉 = 𝜌𝑔 𝐴 2𝑔 ℎ𝑔
𝑚𝑔 =
𝑚+1
𝑚
𝜌0
𝑇0
𝑇𝑔
𝐴 2𝑔 ℎ𝑔

∴ 𝑚𝑔 =
𝑚 + 1
𝑚
𝜌0𝑇0
𝑇𝑔
𝐴 2𝑔 𝐻
𝑚
𝑚 + 1
𝑇𝑔
𝑇𝑎
− 1
∴ 𝑚𝑔 =
𝑚 + 1
𝑚
𝜌0𝑇0 𝐴 2𝑔𝐻
𝑚
𝑚 + 1
1
𝑇𝑔𝑇𝑎
−
1
𝑇𝑔2
∴ 𝑚𝑔 = 𝐾
𝑚
𝑚 + 1
1
𝑇𝑔𝑇𝑎
−
1
𝑇𝑔2
o For max. discharge diff. w.r.t Tg & equate zero
∴ The condition for max. discharge will be
𝑇𝑔
𝑇𝑎
=
2 (𝑚+1)
𝑚

𝑛 =
9.81 𝐻
𝑚
𝑚+1
𝑇𝑔
𝑇𝑎
−1
𝐶 𝑝𝑔
( 𝑇𝑔−𝑇2
)
Where, T2 = Temp. in artificial draught
In Chimney draught, the efficiency if always less than
1%

1. Chimney draught does not require any external power
to produce draught.
2. Simple in construction, less cost& has long life.
3. No mechanical parts & hence maintenance cost is
negligible.
4. Chimney keeps flue gases at a high place in the
atmosphere which prevents contamination of
atmosphere & maintains the cleanliness.

1. The draught produced by chimney depends on height of chimney, atmospheric
air tem & tem of flue gases leaving through the chimney. Hence in order to get
appropriate draught at given height off chimney, the flue gases have to be
discharged at comparatively high temp resulting in the reduction in overall
efficiency of plant.
2. The maximum pressure available for producing natural draught by chimney is
hardly 10 to 20 mm of water.
3. Draught created by chimney is dependent on atmospheric air &flue gases
conditions, hence there is no flexibility to create more draught under peak load
conditions because draught available is constant for given height of chimney.
4. Due to low velocity of air, the mixing process of air & fuel is not proper &
hence combustion is very poor. This increases the specific fuel combustion of
boiler.

 Steam super heater

1. Evaporative capacity
2. Equivalent evaporation
3. Factor of evaporation
4. Boiler efficiency

 Ability to evaporate water.
 Generally it is expressed in terms of
A. Kg of steam/hour
B. Kg of steam /hour/area of heating surface
C. Kg of steam/kg of fuel is fired

 It can be defined as the amount of water evaporated
from water at 100 deg C to dry and saturated steam at
100 deg C.
 Simply it means that the capacity of boiler to generate
same evaporation.

 Ratio of heat received by 1kg water under working
condition to that received by 1kg of water evaporated
from and at 100deg C.
 Fc=h-hf/2257

 Ratio of heat utilized to the heat supplied.
 Efficiency= ma ( h – hf )/c
 Where ma=mass of water actual evaporated to the
system
 C= calorific value to the fuel
 hf=enthalpy of water at given feed temp.
 h=enthalpy of fuel.

 Condenser is a device in which steam is condensed to
water at a pressure less than atmosphere.
 Condensation can be done by removing heat from
exhaust steam using circulating cooling water.
 During condensation, the working substance changes
its phase from vapour to liquid and rejects latent
heat.
 The exhaust pressure in the condenser is maintaned
nearly 7 to 8 kpa which corresponds to condensate
temperature of nearly 313 kelvin.

 To reduce the turbine exhaust pressure so as to increase
the specific output and hence increase the plant
efficiency and decrease the specific steam consumption.
 To condense the exhaust steam from the turbine and
reuse it as pure feed water in the boiler. Thus only make
up water is required to compensate loss of water
 Enables removal of air and other non condensable gases
from steam. Hence improved heat transfer.

 Condenser
 Air Extraction Pump
 Condensate Extraction Pump
 Cooling Water Circulating Pump
 Hot Well
 Cooling Tower
 Make up Water Pump
 Boiler Feed Pump

It increases the work output per kg of steam supplied to the
power plant. It also reduces the specific steam consumption,
therefore reduces the size of power plant of given capacity.
 It improves the thermal efficiency of the power plant.
 It affects the saving in cost of water to be supplied to
the boiler since the condensate is returned to the
boiler.
 Cost of water softening plant is also reduced since
pure feed water is available for the boiler.

According to the type of flow:
 Parallel flow , Counter flow & Cross flow
According to the Cooling Action:
 Jet Condensers or mixing type
 Low Level Parallel Flow Jet Condenser
 Low Level Counter Flow Jet Condenser
 High Level Jet Condenser
 Ejector Jet Condenser
 Surface Condensers or Non-mixing type
1. Down Flow 2. Central Flow 3. Inverted Flow 4.
Evaporative type 5. Regenerative type

 In jet condensers exhaust steam and cooling water come
in direct contact and mix up together. Thus, the final
temperature of condensate and cooling water leaving the
condenser is same.
 Such condensers are normally used for small power units.
 It can be used when cooling water is cheaply and easily
available.
 These condensers are not usually employed since the
Condensate collected can not be reused in boiler, because
it contains impurities like dust, oil, metal particles etc in
the condensate.

 Exhaust steam and
cooling water both flow
in the same direction.
 Wet air pump is used to
extract the mixture of
condensate, air &
coolant. This limits the
vacuum created in the
condenser up to 600 mm
of Hg.

The cooling water is supplied
from the top of the condenser and
steam from side of the condenser.
The water flows in downward
direction through a series of
perforated trays. Steam gets
condensed while it comes in
contact with the falling water.
The air pump always maintains
the required vacuum in the
condenser and induces the cooling
water to be lifted into the
condenser up to a height of 5.5m.
The excess amounts of
condensate from hot well flows
into the cooling pond by an
overflow pipe.

Condenser shell is installed at
height greater than that of
atmospheric pressure in water
column i.e. 10.33 m.
A tall pipe more than 10.33m
length is attached to the bottom of
the condenser. This allows the
condensate and coolant to be
discharged from condenser under
the gravity action; hence a
condensate extraction pump is not
required.
The water from the hot well will
not be able to rise into the
condenser and flood the turbine
due to vacuum pr. Maintained in
condenser.

 In this cooling water enters from
the top of the condenser at least
under a head of 6m of water
pressure with the help of a
centrifugal pump and then it passes
over a series of convergent nozzles
and finally it leaves through a
convergent divergent nozzle.
 The non return valve helps in
preventing the rush of water from
hot well to the engine in case the
cooling water supply fails.
 Momentum of flowing water is
used to remove the mixture of
condensate & coolant from
condenser without the use of any
extraction pump.

Advantages:
 Simple in design & cheaper.
 Less floor area is required.
Disadvantages:
 Condensate is not pure hence can not be reused.
 Low vacuum efficiency.

 In surface condenser, the exhaust steam and cooling water
do not come in physical contact, rather they are separated
by heat transfer wall. Hence condensate remains pure &
can be reused.

 Exhaust steam enters the
top of condenser shell &
flows downward over
water tubes.
 Water tubes are double
passed. The cold water
flows in lower side first
& then in upper side in
the reverse direction,
which enables the
maximum heat transfer.

 The steam flows radially inward
 The condensate is collected
at the bottom of the shell from
where it is taken out by the
condensate extraction pump.
 The steam gets access to the
entire periphery of tubes, and
thus a large surface area for the
hear transfer is available as
compared to the down flow.

 The steam enters the bottom of the shell and
air extraction pump connected at the top.
 Steam flows upward first and subsequently,
returns to the bottom of the condenser.
 The condensate extraction pump is connected
at the bottom of the shell to extract the
condensate.

 The evaporation of some
cooling water provides the
cooling effect, thereby
steam condenses.
 Steam to be condensed is
passed through grilled tubes
& cooling water is sprayed
over outer surface of tubes.
 The evaporative condensers
are most suitable for small
plants, where supply of cold
water is limited.

Advantages:
 High vacuum efficiency.
 Pure condensate.
 Low quality cooling water can be used.
 It allows the expansion of steam through a higher pressure
ratio.
Disadvantages:
 Large amount of water is required.
 Construction is complicated.
 Costly maintenance and skilled workers.
 Large floor area.

Jet Condensers Surface Condensers
1) Cooling water and steam are mixed up
2) Requires small floor space
3) The condensate cannot be used as feed water
to boiler unless it is free from impurities
4) More power is required for air pump
5) Less power is required for water pump
6) Requires less quantity of cooling water
7) The condensing plant is simple
8) Less suitable for high capacity plants due to
low vacuum efficiency
1) Cooling water & steam aren’t mixed up
2) Requires large floor space
3) The condensate can be used as feed
water to boiler as it is not mixed with
cooling water
4) Less power is required for air pump
5) More power is required for water pump
6) Requires large quantity of cooling water
7) The condensing plant is complicated
8) More suitable for high capacity plants
as vacuum efficiency is high

 The ambient air leaks to the condenser chamber at the joints &
glands which are internally under pressure lower than that of
ambient.
 Another source of air is the dissolved air with feed water. The
dissolved air in feed water enters into boiler and it travels with
steam into condenser.
Effects of Air Leakage:
 The presence of air lowers vacuum in the condenser. Thus back
pressure of the plant increases, and consequently, the work
output decreases.
 Air has very poor thermal conductivity. Hence, the rate of heat
transfer from vapour to cooling medium is reduced.
 The presence of air in the condenser corrodes to the metal
surfaces. Therefore, the life of condenser is reduced.

• When the steam condenses in a closed vessel, the vapour phase of
working substance changes to liquid phase, and thus its specific
volume reduces to more than one thousand times.
• Due to change in specific volume, the absolute pressure in the
condenser falls below atmospheric pressure and a high vacuum is
created.
• This minimum pressure that can be attained depends on the
temperature of condensate and air present in the condenser.
The absolute pressure = Atmospheric pressure – Vacuum Gauge
in the condenser Pressure

 Lowering the condenser pressure
will increase the area enclosed by
the cycle on a T-s diagram which
indicates that the net work will
increase. Thus, the thermal
efficiency of the cycle will be
increased
 Lowering the back pressure causes
an increase in moisture content of
steam leaving the turbine.
 Increase in moisture content of
steam in low pressure stages, there
is decrease in efficiency & erosion
of blade may be a very serious
problem and also the pump work
required will be high.

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