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.