Boilers

UNIT 2 Boilers
Dr S MuraliDr. S. Murali
Professor & Head
Department of Mechanical Engineering

Boiler
is an apparatus to produce steam.
– Thermal energy released by combustion of fuel is
d k h d i dused to make steam at the desired temperature
and pressure.

Applications
• The steam produced is used for:
– Producing mechanical work by expanding it in
steam engine or steam turbine.
H i h id i l d i d i l b ildi– Heating the residential and industrial buildings
– Performing certain processes in the sugar mills,
h i l d t til i d t ichemical and textile industries.

Boiler Requirements
• Safety.
Th b il h ld b f d ti– The boiler should be safe under operating
conditions.
A ibilit• Accessibility.
– The various parts of the boiler should be accessible
f i d i tfor repair and maintenance.
• Capacity.
– Should be capable of supplying steam according to
the requirements.

Boiler Requirements
• Efficiency.
Sh ld b bl t b b i t f– Should be able to absorb a maximum amount of
heat produced due to burning of fuel in the
furnacefurnace.
• Construction
i l i t ti– simple in construction .
• Cost
– Its initial cost and maintenance cost should be
low.

Boiler Requirements
• Joints.
Th b il h ld h j i t d t– The boiler should have no joints exposed to
flames.
E f t ti• Ease of starting.
– Should be capable of quick starting and loading.

Boiler Classification
1. According to the flow of water and hot gases
Fi t b ( k t b )a. Fire tube (or smoke tube)
Eg: Cochran, Lancashire and Locomotive boilers
b Water tube boilersb. Water tube boilers
Eg: babcock & Wilcox boiler, Sterling boiler .
2 A di t th i f th h ll2. According to the axis of the shell
a. Vertical boilers
b. horizontal boilers
c. Inclined

3. According to location or position of the
furnacefurnace.
a. Externally fired boilers.
ti l t b l l ti d th t h i b ilvertical tubular, locomotive and the scotch marine boilers
b. internally fired boilers
Cochran lancashireCochran, lancashire
4. According to the application
ia. stationery
b. mobile boilers.

5. According to steam pressure
L (15 20b )a. Low (15 – 20bar)
b. medium and (20 – 80bar)
h h b l ( h b )c. high pressure boilers(more than 80 bar)
a. Subcritical(less than 221 bar)
b S iti l( th 221b )b. Supercritical(more than 221bar)
6. According to water circulation arrangement
a. Natural Circulation
Lancashire, babcock and wilcox boiler
b d i l ib. Forced circulation
Velox, lamont, benson boiler

advantages of fire tube boilers
• Low cost
Fl i f d d b• Fluctuations of steam demand can be met
easily
• It is compact in size.

Simple Vertical boiler
Capacity‐2500 kg/hr
Pressure 7‐10 barPressure 7 10 bar

Cochran Boiler
Salient features
• The dome shape of the furnace
causes the hot gases to deflectcauses the hot gases to deflect
back and pass through the flue.
The un‐burnt fuel if any will also
be deflected backbe deflected back.
• Spherical shape of the top of the
shell and the fire box gives higher
area by volume ratioarea by volume ratio.
• It occupies comparatively less
floor area and is very compact.
• It is well suited for small capacity• It is well suited for small capacity
requirements.

Cochran Boiler- features
• very compact and requires minimum floor
areaarea
• Any type of fuel can be used with this boiler
• Well suited for small capacity requirements
• gives about 70% thermal efficiency with coal g y
firing and about 75% with oil firing

Cochran Boiler- features
• Pressure Gauge
W L l I di• Water Level Indicator
• Safety Valve
• Fusible Plug
• Blow‐off CockBlow off Cock
• Steam Stop Valve
F d Ch k V l• Feed Check Valve

Cornish Boiler
No. of Flue tubes one
Diameter of Shell 1.25 to 1.75 m
Length of shell 4 to 7m
Max. Working Pressure 10.5 bar
Steam Capacity 6500 kg/hr

Lancashire Boiler
Di f Sh ll 2 3Diameter of Shell 2 to 3 m
Length of shell 7to 9m
Max. Working Pressure 16 bar
Efficiency 50 to 60 %

Lancashire boiler
• It is stationary fire tube, internally fired, horizontal, natural
circulation boiler.
hi i id l d b il b f i d i• This is a widely used boiler because of its good steaming
quality and its ability to burn coal of inferior quality.
• These boilers have a cylindrical shell 2 m in diameters and
l h fits length varies from 8 m to 10 m.
• It has two large internal flue tubes having diameter
between 80 cm to 100 cm in which the grate is situated.
• This boiler is set in brickwork forming external flue so that
the external part of the shell forms part of the heating
surface.

AdvantagesAdvantages
• The feed pipe projecting into the boiler is perforated to
ensure uniform water distribution.
• Its heating surface area per unit volume at the boiler is
considerably large.
• Its maintenance is easy.Its maintenance is easy.
• It is suitable where a large reserve of hot water is
needed. This boiler due to the large reserve capacity
can easily meet load fluctuationscan easily meet load fluctuations.
• Super‐heater and economizer can be easily
incorporated into the system, therefore; overall
ffi i f h b il b id bl i defficiency of the boiler can be considerably increased
(80‐85%).

LOCOMOTIVE BOILERLOCOMOTIVE BOILER

Muhammad Fadhli Mustaffa & Ahmad Lutfi Mohayiddinff f y

• horizontal fire tube type mobile boiler.
• produce steam at a very high rate Therefore• 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 asurface 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 g , g
tubes are setup
• heat transfer rate is increased by creating y g
strong draught by means of steam jet.

AdvantagesAdvantages
• Large rate of steam generation per square
metre of heating surface To some extent thismetre of heating surface. To some extent this
is due to the vibration caused by the motion.
I i f f b i k k i l f d i• It is free from brickwork, special foundation
and chimney. This reduces the cost of
i ll iinstallation.
• It is very compact.
• The pressure of the steam is limited to about
20 bar.

Scotch Boiler Max. Working Pressure 17 barg

Merits Of Water Tube BoilersMerits Of Water Tube Boilers
1. Generation of steam is much quicker due to
small ratio of water content to steam content.
This also helps in reaching the steaming
temperature in short time.
2 Its evaporative capacity is considerably larger2. Its evaporative capacity is considerably larger
and the steam pressure range is also high‐200
bar.
3. Heating surfaces are more effective as the hot
gases travel at right angles to the direction of
water flowwater flow.

4. The combustion efficiency is higher because
complete combustion of fuel is possible as the p p
combustion space is much larger.
5 Th h l i h b il5. The thermal stresses in the boiler parts are
less as different parts of the boiler remain at
uniform temperature due to quick circulationuniform temperature due to quick circulation
of water.
6. The boiler can be easily transported and
erected as its different parts can be separated.

7. Damage due to the bursting of water tube is
less serious. Therefore, water tube boilers are
sometimes called safety boilers.
8 All f h b b il il8. All parts of the water tube boilers are easily
accessible for cleaning, inspecting and
repairing.repairing.
9. The water tube boiler's furnace area can be
easily altered to meet the fuel requirements.

Demerits
1. It is less suitable for impure and sedimentary
water as a small deposit of scale may causewater, as a small deposit of scale may cause
the overheating and bursting of tube.
Therefore use of pure feed water is essentialTherefore, use of pure feed water is essential.
2. They require careful attention. The
i hi hmaintenance costs are higher.
3. Failure in feed water supply even for short
period is liable to make the boiler over‐
heated.

Water tube boilers classification
1. Horizontal straight tube boilers
L it di l da. Longitudinal drum
b. Cross‐drum.
2. Bent tube boilers
a. Two drum
b. Three drum
c. Low head three drum
d. Four drum.
3. Cyclone fired boilersy

BABCOCK WILCOX BOILERBABCOCK WILCOX BOILER

Features
• The evaporative capacity of this boilers is high compared with
other boilers (20,000 to 40,000kg/hr). The operating pressure
lies between 11.5 to 17.5 bar.
• The draught loss is minimum compared with other boilers.
h d f b b l d l• The defective tubes can be replaced easily.
• The entire boiler rests over an iron structure, independent of
brick work, so that the boiler may expand or contract freely.brick work, so that the boiler may expand or contract freely.
The brick walls which form the surroundings of the boiler are
only to enclose the furnace and the hot gases.

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