STEAM BOILERS

INTRODUCTIONINTRODUCTION

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.

The steam generated may be supplied :

1.To an external combustion engine, i.e., steam engine

and steam turbines

2.At low pressures for industrial process work in

cotton mills, sugar factories , breweries etc., and

3.For producing hot water, which can be used for

heating installations at much lower pressures

Purpose of boilers

• For generating power in steam engines or steam turbines

• In textile industries for sizing and bleaching

• For heating the buildings in cold weather and for producing hot water for hot water supply

Primary requirements of 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)

IMPORTANT TERMS:IMPORTANT TERMS:

• Boiler shell

• Combustion chamber

• Grate

• Furnace

• Heating surface

• Mountings

• Accessories

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

Furnace: it is the chamber formed by the space above the grate and below the boiler shell, in which combustion takes place.

Grate: it is a platform in the furnace upon which fuel is burnt

BURNER - A device for the introduction of fuel and air into a furnace at the desired velocity.The burner is the principal device for the firing of oil and/or gas.

Ash PitThe ash pit is the space where ashes accumulate under the fire. The ashes fall through the fire grate into a (often) removable box.

Mud Ring or Mud DrumA typically cylindrical shaped space at the bottom of the water space. Sediment, mud, and other impurities gather there.

SAFETY VALVE - A spring loaded valve that automatically opens when pressure attains the valve setting. Used to prevent excessive pressure from building up in a boiler.

• Pressure Gauge. This indicates the pressure of the steam in the boiler.

• Fusible Plug . A fusible plug operates as a safety valve when dangerous temperatures, rather than dangerous pressures, are reached in a closed vessel.

• Feed Check Valve. The high pressure feed water is supplied to the boiler through this valve. This valve opens towards the boiler only and feeds the water to the boiler.

• Steam Stop Valve. • It regulates the flow of

steam supply outside.

Classification of boilers

• Horizontal, vertical or inclined• Fire tube and water tube• Externally fired and internally fired• Forced circulation and natural circulation• High pressure and low pressure• Stationary and portable• Single tube and multi tube

Horizontal, vertical or inclined

• If the axis of the boiler is horizontal, vertical or inclined then it is called horizontal, vertical or inclined boiler respectively

Fire tube and water tube

• If hot gases are inside the tube and water is outside the tube, it is called fire-tube boiler.

• Examples: Cochran, Lancashire and locomotive boilers

• If water is inside the tube and hot gases are outside the tube, it is called fire-tube boiler.

• Examples: Babcock and Wilcox, Stirling, Yarrow boiler etc

Externally fired and internally fired

• The boiler is known as externally fired if the fire is outside the shell.

• Examples: Babcock and Wilcox, Stirling

• The boiler is known as internally fired if the furnace is located inside the boiler shell.

• Examples: Cochran, Lancashire

Forced circulation and natural circulation

• In forced circulation type of boilers, the circulation of water is done by a forced pump

• Examples: Velox, Lamont, Benson boiler• In natural circulation type of boilers, circulation

of water in the boiler takes place due to natural convection currents produced by the application of heat

• Examples: Lancashire, Babcock and Wilcox

High pressure and low pressure• The boilers which produce steam at pressures

of 80 bar and above are called high pressure boilers

• Examples: Babcock and Wilcox, Velox, Lamont, Benson boilers

• The boilers which produce steam at pressure below 80 bar are called low pressure boilers

• Examples: Cochran, Cornish, Lancashire and locomotive boilers

Stationary and portable

• Stationary boilers are used for power plant-steam, for central station utility power plants, for plant process steam etc

• Mobile or portable boilers include locomotive type, and other small unit for temporary use at sites

Single tube and multi tube

• The fire tube boilers are classified as single tube or multi-tube boilers, depending upon whether the fire tube is one or more than one

• Examples of single tube boilers are Cornish and simple vertical boiler

WATER TUBE STEAM BOILER:WATER TUBE STEAM BOILER:

The water is contained inside the tubes(called water tubes)which are surrounded by flames and hot gases from outside.

Ex: Babcock and Wilcox boiler, stirling boiler, la - Mont boiler, yarrow boiler and loeffler boiler

FIRE TUBE BOILER

In fire tube steam boilers, the flames and hot gases produced by the combustion of fuel pass through the tubes(called multi tubes) which are surrounded by water. The heat is conducted through the walls of the tube from the hot gases through the surrounding water.

Ex: Cochran boiler, Lancashire boiler, Cornish boiler,

Comparison of fire tube and water tube boilers

Particulars Particulars Fire-tube boilersFire-tube boilers Water-tube boilersWater-tube boilers

Position of water and hot Position of water and hot gasesgases

Hot gases inside the tubes Hot gases inside the tubes and water outside the and water outside the tubestubes

Water inside the tubes Water inside the tubes and hot gases outside the and hot gases outside the tubestubes

Mode of firingMode of firing Generally internally firedGenerally internally fired Externally firedExternally fired

Operation pressureOperation pressure Limited to 16 barLimited to 16 bar Can go up to 100 barCan go up to 100 bar

Rate of steam productionRate of steam production LowerLower HigherHigher

SuitabilitySuitability Not suitable for large Not suitable for large power plantspower plants

Suitable for large power Suitable for large power plantsplants

Risk on bursting Risk on bursting Involves lesser risk of Involves lesser risk of explosion due to lower explosion due to lower pressurepressure

More risk on bursting More risk on bursting due to high pressuredue to high pressure

Floor areaFloor area For a given power it For a given power it occupies more floor areaoccupies more floor area

For a given power it For a given power it occupies less floor areaoccupies less floor area

ConstructionConstruction DifficultDifficult Simple Simple

Cont…

ParticularsParticulars Fire-tube boilersFire-tube boilers Water-tube boilersWater-tube boilers

TransportationTransportation Difficult Difficult SimpleSimple

Shell diameterShell diameter Large for same powerLarge for same power Small for same powerSmall for same power

Chances of explosionChances of explosion Less Less MoreMore

Treatment of waterTreatment of water Not so necessaryNot so necessary More necessaryMore necessary

Accessibility of various Accessibility of various partsparts

Various parts not so easily Various parts not so easily accessible for cleaning, accessible for cleaning, repair and inspectionrepair and inspection

More accessibleMore accessible

Requirement of skillRequirement of skill Require less skill for Require less skill for efficient and economic efficient and economic workingworking

Require more skill and Require more skill and careful attentioncareful attention

COCHRAN BOILERCOCHRAN BOILER

Cochran boiler

• Shell diameter: 2.75 mm• Height: 5.79 m• Working pressure: 6.5 bar• Steam capacity: 3500 kg/hr• Heating surface: 120 m3• Efficiency : 75%

WORKING OF AN COCHRAN WORKING OF AN COCHRAN BOILERBOILER

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

WORKING OF AN COCHRAN WORKING OF AN COCHRAN BOILERBOILER

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.

LANCASHIRE BOILERLANCASHIRE BOILER

•

Specifications

• Dia . Of shell: 2-3m• Length of shell: 7-9 m• Max. working pressure: 16 bar• Steam capacity: 9000 kg/hr• Efficiency: 50-70%

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).

WORKING OF LANCASHIRE BOILER

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.

CORNISH BOILERCORNISH BOILER

The capacity and working pressure of a Cornish boiler is low as compared to Lancashire boiler.

Specifications of cornish boiler

• No.of flue tubes: 1• Diameter of shell: 1.25-1.75m• Length of the shell: 4-7m• Pressure of steam: 10.5 bar• Steam capacity:6500 kg/hr

CORNISH BOILERCORNISH BOILER

It is a similar type of Lancashire boiler in all respects, except there is only one flue tube in Cornish boiler instead of two in Lancashire boiler.

The diameter of Cornish boiler is generally 1m to 2m and its length various from 5m to 7.5m.

The diameter of flue tube may be 0.6 times that of shell.

Locomotive boiler

Specifications of locomotive boiler

• Barrel diameter: 2.095m• Length of barrel: 5.206m• No of ordinary tubes:116• no of superheated tubes: 38• Working pressure: 14 bar• Efficiency: 70%

• Consists of cylindrical barrel with rectangular fire box at one end and smoke box at another end

• Hot gases generated due to burning of coal are deflected by an arch of fire bricks, so that walls of the fire box may be heated properly

• The heat of the hot gases is transmitted into water through the heating surfaces of fire tubes

Babcock and Wilcox boiler

Specifications of babcock boiler

• Diameter of drum:1.22-1.83m• Length :6.096-9.144m• Size of water tubes:7.62-10.16cm• Working pressure:40 bar• Steam capacity:40000 kg/hr• Efficiency:60-80%

Cont…

• It consists of a drum connected to a series of front end and rear end header by short riser tubes

• To these headers are connected a series of inclined (150 or more) water tubes

• A hand hole is provided in the header in front of each tube for cleaning and inspection of tubes

• Stirling boiler• The Stirling boiler is an early

form of water-tube boiler, used to generate steam in large land-based stationary plants.

• Stirling boilers are one of the larger arrangements for a water-tube boiler. They consist of a large brick-built chamber with a sinuous gas path through it, passing over near-vertical water-tubes that zigzag between multiple steam drums and mud drums.

Specification of stirling boiler

• Steam capacity:50000 kg/hr• Steam pressure:60 bar