BOILERS - 1

What is a Boiler?

A closed vessel in which steam is produced from water by combustion of fuel.

Working of a BoilerWater is pumped into the boiler at operating pressure

Heat of flue gases vaporises water to form steam

Steam formed is passed into steam space above the water space

This steam is always wet

Superheated steam is obtained by passing wet steam through super-heater, where it is dried and then superheated

To increase efficiency, water is passed through an economiser where its temp. is increased and then it is pumped into the boiler. This way less heat is required to heat the steam.

Steam GeneratorA combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat so made available to the fluid being heated and vapourised.

Components of a steam generator

Boiler drumSuper heaterEconomiserAir Preheater

Function of a BoilerControlled release of heat in the fuel.

Efficient transfer of heat to the feed water to produce steam at desired P, T and quality.

Steam to be supplied from boiler at constant P by maintaining the steam generation rate and steam flow rate equal.

Water must be contained safely.

Requirement of a Good BoilerCapable of producing maximum amount of steam with minimum fuel consumption.

It should occupy less space and be light in weight.

Capable of quick starting and should rapidly meet the fluctuation in load.

Safe in working.

Economical and should require very little attention during operation.

All parts should be easily accessible for inspection and repairs.

Components should be transported without difficulty.

Requirement of a Good BoilerInstallation should be simple.

Tubes should not accumulate soot or water deposits and should be sufficiently strong to allow for wear and corrosion.

Water and gas circuits should be such as to allow minimum fluid velocity for low frictional losses.

Should have as less joints as possible to avoid leaks, which may occur due to expansion and contraction.

Velocities should be high for high heat transfer rates with minimum pressure drop through the system.

13.Should confirm to safety regulations laid down in Boiler Act.

Selection of a BoilerOperating Pressure

Steam Generating Rate

Quality of steam required

Availability of floor space

Fuel to be used

Nature of the load on the boiler

Initial cost

Operating and maintenance cost

Availability of water

Accessibility for repair and inspection

Classification of BoilersPosition of flue gases and waterFire tube, water tube

Axis of shellHorizontal, vertical, inclined

Location of furnaceExternally fired, internally fired

Method of circulationNatural, Forced

Classification of Boilers5.MobilityStationary, Portable

UsagePackaged, Unpackaged

PressureHigh, Low

TubesSingle, Multi

Fire Tube Boilers

Fire Tube BoilerFurnace is located inside the Boiler Drum or shell.

Hot flue gases are fed through the tubes.

Tubes are surrounded by water.

Boiler shell contains water and steam under pressure.

Tube contains grate.

Air for combustion of fuel, flowing beneath the grate moves upwards through the fire bars.

Superheater tubes placed at exit of the flue gases.

Used generally for up to 10000 kg/hr and low pressures upto 20 bar

Advantages of Fire-Tube BoilerLow initial cost

Large water content provides safety in operation, during failure of feed water supply

Suitable for fluctuating loads

Space occupied

Disadvantages of Fire tube BoilersRate of evaporation is low and reaches operating pressures slowlySuitable only for low capacities.Shell size 2.4m, shell thickness 3 cm, stress considerations limit generation pressure to 20kg/cm2Lower efficiencyExplosion is very dangerous due to large water contentTransportation is difficult due to large shell sizeUseless for steam power plants due to slow rate of steam generationDifficult designLess accessible to repairs

Water Tube Boilers

Water Tube BoilersFurnace is outside the drum

Small parallel tubes contain water connected to drum with the header

Gases pass outside tubes containing water

Flow of water is downward

Steam returns to drum through uptake header

Advantages of Water tube boilersHigh pressures and capacityOperating temps and pressures reached faster due to smaller water to steam contentHot gases travel at right angles to water flow so heating surfaces are effectiveCombustion space is larger so complete combustion possible with better efficiencyThermal stresses low as all parts are at uniform temp due to quick circulationLess serious damage compared to others due to lesser water content.Furnace area can be alteredHigher EfficiencySuitable for power plantParts easily accessible for cleaningEasily transportedSimple design

Disadvantages of Water tube BoilersHigh initial cost

Impure and dirty water not suitable as a small deposit can cause explosion

Failure in feed water supply can cause boiler overheat

Comparision of Fire tube and water tube boilers

CriterionFire TubeWater tube

Flow of hot gases and waterGases in tubes surrounded by water outsideWater in tubes and gases flow outsideLocation of furnaceInternalExternalFloor area for given outputLarge SmallCapacity10000kg.hr50000kg/hrEvaporationSlowFast

Comparision of Fire tube and water tube boilers

CriterionFire TubeWater tube

Pressure range15 to 20 bar170 to 200 barEff80%92%SafetyLarge water content and low steam so better safetySmall water content and large steam generation. Needs expert attention.ExplosionLesser risk due to lower PHigher risk due to higher PApplicationNot suitable for large power plantSuitable

Comparision of Fire tube and water tube boilers

CriterionFire TubeWater tube

SkillLessMoreWater TreatmentNoYesConstructionDifficultSimpleShell DiameterMoreLessTransportationDifficultSimple

Boiler terms and detailsShellFurnaceGrateScaleBlowing OffRefractoryWater flow pathSteam flow pathGas flow pathWater spaceSteam spaceWater levelFittings/mountingsAccessories