IPE 301E: Fundamental of Mechanical Engineering
Dr. M. Muhshin Aziz KhanProfessor, Dept. of IPE, SUST
Steam generation unit
Steam Generation Unit Boiler
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.
Also known as steam generation unit.
Purpose of Boiler The steam generated in the boiler is
then supplied: to an external combustion engine
i.e. steam engine and steam turbines for power generation,
for industrial process work in cotton mills, sugar mills, textile industries etc., at low pressures, and
for heating the installations and producing hot water for hot water supply at much lower pressures.
Primary Requirements for a Boiler The water must be contained safely. The steam must be safely delivered
in desired condition (as regard its pressure, temperature, quality and required rate)
Steam Generation Unit Important Boiler Terms
Shell Consists of one or more steel plates
bent into a cylindrical form and riveted or welded together.
The shell ends are closed with end plates.
Setting: The primary function is to
confine heat to the boiler and form a passage for gases.
It is made of brick work and may form the wall of the furnace and combustion chamber
Grate it is a platform in the furnace upon
which fuel is burnt.
Furnace It is the chamber formed by the space
above the grate and below the boiler shell, in which combustion takes place.
Mountings The items which are used for safety of
boiler. Accessories The items which are used for increasing
the boiler efficiency.
Steam Generation Unit Classification of Steam Boilers
Though there are many classifications of steam boilers, yet the following are important: ACCORDING TO THE CONTENTS IN THE
TUBE Fire tube or smoke tube boiler Water tube boiler.
ACCORDING TO POSITION OF FURNACE Internally fired boilers Externally fired boilers
ACCORDING TO AXIS OF FURNACE Vertical boilers Horizontal boilers
ACCORDING TO NUMBER OF TUBES Single tube boilers Multi tube boilers
ACCORDING TO METHOD OF CIRCULATION OF WATER Natural circulation boilers Forced circulation boilers
ACCORDING TO PRESSURE OF STEAM High pressure boiler (> 80 bar) Low pressure boiler (≤ 80 bar)
ACCORDING TO THE USE Stationary boilers Mobile boilers
Steam Generation Unit Fire tube boiler
Flames and hot gases (flue gases: products of combustion) pass through the tubes, and
Tubes are surrounded by water. Examples: Cochran Boiler, Lancashire
Boiler etc.
Water tube boiler
Water passes through the tube, and Flames and hot gases pass outside the
tubes. Examples: Babcock and Wilcox Boiler,
Stirling Boiler etc.
Comparisonbetween water tube and fire tube boiler
Criterion Fire Tube Boiler Water Tube Boiler
Flow of hot gases
Gases in tubes surrounded by water outside.
Water in tubes and hot gases flow outside.
Location of furnace Internal External
Floor area for a given output
Large Small
Capacity 10000 kg/hr 50000 kg/hr
Evaporation Slow Fast
Criterion Fire Tube Boiler Water Tube Boiler
Pressure range 15 ~ 20 bar 170 ~ 200 bar
Efficiency 80% 92%
SafetyLarge water content and low
steam generation. Better safety.
Small water content and large steam generation. Needs
expert attention.
Explosion Lesser risk due to lower pressure.
Higher risk due to higher pressure.
Application to large power
plantNot suitable Suitable
Comparison ….Cont’d
Criterion Fire Tube Boiler Water Tube Boiler
Skill requirement
Require less skill for efficient and economic working
Require more skill and careful attention
Water treatment Not so necessary Necessary
Construction Difficult Simple
Shell diameter Large for same power. Small for same power.
Comparison ….Cont’d
Steam Generation Unit Classification of Steam Boilers
Though there are many classifications of steam boilers, yet the following are important: ACCORDING TO THE CONTENTS IN THE
TUBE Fire tube or smoke tube boiler Water tube boiler.
ACCORDING TO POSITION OF FURNACE Internally fired boilers Externally fired boilers
ACCORDING TO AXIS OF FURNACE Vertical boilers Horizontal boilers
ACCORDING TO NUMBER OF TUBES Single tube boilers Multi tube boilers
ACCORDING TO METHOD OF CIRCULATION OF WATER Natural circulation boilers Forced circulation boilers
ACCORDING TO PRESSURE OF STEAM High pressure boiler (> 80 bar) Low pressure boiler (≤ 80 bar)
ACCORDING TO THE USE Stationary boilers Mobile boilers
Steam Generation Unit Cochran Boiler Salient Features
Fire tube Internally fired Vertical Natural circulation Multi-tubular Widely used in marine practice
(mobile boiler) Efficiency: 70% ~ 75% Capacity: 20 kg/hr ~ 3000 kg/hr
(suitable for fluctuating loads) Low initial cost
Steam Generation Unit Components of Cochran Boiler
It consists of Shell
It is hemispherical on the top, where space is provided for steam.
GrateIt is placed at the bottom of the furnace where coal is burnt.
Fire box (furnace )It is also dome-shaped like the shell so that the gases can be deflected back till they are passed out through the flue pipe to the combustion chamber.
Flue pipeIt is a short passage connecting the fire box with the combustion chamber.
Steam Generation Unit Components of Cochran Boiler
Fire tubesA number of horizontal fire tubes are provided, thereby the heating surface is increased.
Combustion chamberIt is lined with fire bricks on the side of the shell to prevent overheating of the boiler. Hot gases enter the fire tubes from the flue pipe through the combustion chamber.
ChimneyIt is provided for the exit of the flue gases to the atmosphere from the smoke box.
ManholeIt is provided for inspection and repair of the interior of the boiler shell.
Steam Generation Unit Working Principle of Cochran Boiler
Coal is fed into the grate through the fire hole and burnt. Ash formed during burning is collected in the ash pit provided just below the grate and then it is removed manually.
The hot gases from the grate pass through the flue pipe to the combustion chamber.
The hot gases from the combustion chamber flow through the horizontal fire tubes and transfer the heat to the water by convection.
The flue gases coming out of fire tubes pass through the smoke box and are exhausted to the atmosphere through the chimney.
Smoke box is provided with a door for cleaning the fire tubes and smoke box.
Steam Generation Unit Cochran Boiler Salient Features
Fire tube Internally fired Vertical Natural circulation Multi-tubular Widely used in marine practice
(mobile boiler) Efficiency: 70% ~ 75% Capacity: 20 kg/hr ~ 3000 kg/hr
(suitable for fluctuating loads) Low initial cost
Steam Generation Unit Babcock and Wilcox Boiler Salient Features
Water tube Externally fired Horizontal Natural circulation Multi-tubular Efficiency: ~ 92% Capacity: 50000kg/hr Suitable for power plant)
Steam Generation Unit Components of Babcock and Wilcox Boiler
It consists of Steam and water drum (Boiler shell)
One half of the drum is filled up with water and steam remains on the other half. It is about 8 meters in length and 2 meters in diameter.
Water tubesWater tubes are placed between the drum and furnace in an inclined position (at an angle of 10 to 15 degree) to promote water circulation. These tubes are connected to the uptake-header and the down-comer.
Steam Generation Unit Components of Babcock and Wilcox Boiler
Uptake- and downtake-headersThe drum is connected at one end to the uptake-header by short tubes and at the other end to the down-take header by long tubes.
GrateCoal is fed through fire door to the grate and is burnt.
FurnaceIt is kept just below the uptake header.
BafflesThe fire-brick baffles, two in number, are provided to deflect the hot flue gases.
Steam Generation Unit Components of Babcock and Wilcox Boiler
Super-heaterThe boiler is fitted with a super-heater tube which is placed just under the drum and above the water tubes.
Mud boxMud box is provided at the bottom end of the down-take header. The mud or sediments in the water are collected in the mud box and blown-off time to time by means of a
blow–off cock.
Inspection doorInspection doors are provided for cleaning and inspection of the boiler.
Steam Generation Unit Working Principle of Babcock and Wilcox
Boiler Flow of flue gases
The hot flue gases rise upward and pass across the left-side portion of the water tubes.
The baffles guided the flue gases to travel in the zig-zag manner (i.e., the hot gases are deflected by the baffles to move in the upward direction, then downward and again in the upward direction) over the water tubes and along the super-heater.
The flue gases finally escape to atmosphere through chimney.
Steam Generation Unit Working Principle of Babcock and Wilcox
Boiler Water circulation
Water tubes which is just above the furnace is heated comparatively at a higher temperature than the rest of it.
Water rises into the drum through the uptake-header.
In the drum, steam being lighter is collected in the upper part of the drum. The water from the drum comes down through the down–take header into the water tubes.
A continuous circulation of water from the drum to the water tubes and water tubes to the drum is thus maintained.
Steam Generation Unit: Mountings
Steam Generation Unit Functions of Boiler Mountings
Blow-off cockUsed to discharge mud and other sediments deposited in the bottom most part of the water space in the boiler, while boiler is in operation. Can also be used to drain-off boiler water. It is mounted at the lowest part of the
boiler. Water level indicator
Required to indicate the safety level of water inside the boiler shell. Consists of three cocks: Steam, Water
and Drain cocks. It is fitted in front of the boiler.
Pressure gaugeRequired to indicate the steam pressure inside the boiler shell. It is fitted in front of the boiler. Pressure gauges generally used are of
Bourden type
Safety valves Required to release the internal boiler pressure if the pressure exceeds a predetermined safety level. It is attached to steam chest.
Steam Stop ValveUsed to regulate flow of steam from boiler to steam pipe and to shut off the steam completely when required. It is fitted to the highest part of the shell.
Fusible plug Required to stop the boiler if the boiler gets dry to prevent damage to the boiler. It is a metallic plug installed in the water
pathway. If the boiler temperature exceeds the
safety limit, it melts and allows shell water to pass through and extinguish the furnace fire.
Steam Generation Unit Functions of Boiler Accessories
Feed Water PumpUsed to deliver the water into the boiler. The quantity of water supply
should be at least equal to that evaporated and supplied.
The pressure of feed water should be 20% more than that in the boiler.
Feed pumps commonly used are of two types: Centrifugal type Reciprocating type
Superheater
Used to increase the temperature of saturated steam without raising its pressure. It is generally used as integral
part of the boiler. It extracts heat from hot flue
gases. It is placed in the path of hot
flue gases from the furnace. Advantages of superheated
steam Eliminates the erosion of
turbine blade. Increases efficiency of steam
plant. Reduces the losses due to
condensation in the steam pipe.
Steam Generation Unit Functions of Boiler Accessories
EconomizerUsed to heat the feed water by using the heat in exhaust flue gases before leaving the chimney. It improves the economy of the
boiler. Feed water is passed through the
tubes of the economizer while the hot flue gases are passed over the tubes.
Advantage of using an economizer: Causes 15% – 20% of the coal saving. Increases the steam generating
capacity. Prevents formation of scale in the
boiler water tubes. Minimizes the strains due to unequal
expansion.
Air PreheaterUsed to heat the air required for the combustion. Installed between the economizer
and chimney. Air is drawn through the tubes of
the heater while the hot flue gases are passed over the tubes.
Air is then passed through the duct to the furnace.
Advantages of using a air preheater: Increases the boiler efficiency by
2% for each 30° - 40° rise in air temp.
Results in better combustion with less soot, smoke and ash.
Enables lower grade fuel to be burnt with less excess air.