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BASICS OF MECHANICAL ENGINEERING (ME-101 F)

Lectures onSTEAM GENRATION AND BOILERS

ByPriyanka SinghM.Tech (Applied Mechanics)Motilal Nehru National Institute of Technology, Allahabad

DefinitionsLatent Heat:Latent heat is the heat absorbed or releases during achange of state of matter.Latent Heat of Fusion:The specific latent heat of fusion is the heat needed tochange 1 kgof a solid at its melting point into a liquid, or vice versa,without a change in temperature.Latent Heat of Vaporization:The specific latent heat of vaporization is the heat needed tochange 1 kgof a liquid at its boiling point into vapour or vice versa,without a change in temperature. Steam:If water is heated beyond the boiling point, it vaporizes into steam, or water in the gaseous state. The properties of steam vary greatly depending on the pressure and temperature to which it is subject.

Heating Curve

Assume that unit mass of steam is generated from solid ice which is at-20C at atmospheric pressure (1.01325 bar).

Heating Curve - Latent HeatT1 is the melting point whereas T2 is the boiling point.

Regime P-Q: Initially ice is at point P, i.e. at -20C. Temperature of ice increases from -20C to 0C. Point Q represents 0C temperature condition.Regime Q-R: The ice starts melting in this regime. The heat supplied in this regime is called latent heat of fusion as it is responsible to change in phase not increasing the temperature. At point R total ice is convert into water.Regime R-S: At point R temperature of water is 0C. If any heat addition is there, the temperature of water will increase and it will continue till point S is reached where temperature become 100C. This heat addition is called sensible heat. Point S represents saturated liquid condition.Regime S-T: At point S water starts boiling and additional heat is responsible to convert the water into vapours (steam). This boiling is continue till whole water is convert into vapours. The heat supplied is called latent heat of vaporization. Point T shows the saturated steam condition.Regime T-U: Point T represents saturated steam condition. If any heat addition is there, volume of steam will increase very rapidly and it behave like gas. The difference between superheated temperature and saturation temperature is called degree of superheat.

From Q to R and S to T, the temperature remains constant because the heat supplied to the object is used to overcome the forces of attraction that hold the particles together.Heat obsorbs during Q-R is called the latent heat of fusion.Heat obsorbs during S-T is called the latent heat of vaporisation.

Wet Steam Quality and the Dryness FractionTo produce 100% dry steam in an boiler, and keep the steam dry throughout the piping system, is in general not possible. Droplets of water will escape from the boiler surface. Because of turbulence and splashing when bubbles of steam break through the water surface the steam space will contain a mixture of water droplets and steam.In addition heat loss in the pipes will condensate steam to droplets of water.Steam - produced in a boiler where the heat is supplied to the water and where the steam are in contact with the water surface of the boiler - will contain approximately 5% water by mass.Dryness fraction of Wet SteamIf the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95.Dryness fraction can be expressed as:x = ms/ (mw+ ms) (1)where x= dryness fractionmw= mass of water (kg)ms= mass of steam (kg)

Types of SteamIn the sections that follow, we will discuss the types of steam used in these applications.

Saturated Steam (Dry)Saturated steam occurs at temperatures and pressures where steam (gas) and water (liquid) can coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of condensation.Unsaturated Steam (Wet)This is the most common form of steam actually experienced by most power plants. When steam is generated using a boiler, it usually contains wetness from non-vaporized water molecules that are carried over into the distributed steam. Even the best boilers may discharge steam containing 3% to 5% wetness.Superheated SteamSuperheated steam is created by further heating wet or saturated steam beyond the saturated steam point. This yields steam that has a higher temperature and lower density than saturated steam at the same pressure. Superheated steam is mainly used in propulsion/drive applications such as turbines, and is not typically used for heat transfer applications.

Steam Generation TheoryWithin the boiler, fuel and air are force into the furnace by the burner. There, it burns to produce heat. From there, the heat (flue gases) travel throughout the boiler. The water absorbs the heat, and eventually absorb enough to change into a gaseous state - steam.To the left is the basic theoretical design of a modern boiler. Boiler makers have developed various designs to squeeze the most energy out of fuel and to maximized its transfer to the water.

Water enters the boiler, preheated, at the top. The hot water naturally circulates through the tubes down to the lower area where it is hot. The water heats up and flows back to the steam drum where the steam collects. Not all the water gets turn to steam, so the process starts again. Water keeps on circulating until it becomes steam.Meanwhile, the control system is taking the temperature of the steam drum, along with numerous other readings, to determine if it should keep the burner burning, or shut it down.As well, sensors control the amount of water entering the boiler, this water is know as feedwater. Feedwater is not your regular drinking water. It is treated with chemicals to neutralize various minerals in the water, which untreated, would cling to the tubes clogging or worst, rusting them. This would make the boiler expensive to operate because it would not be very efficient.

On the fire side of the boiler, carbon deposit resulting from improper combustion or impurities in the fuel can accumulate on the outer surface of the water tube. This creates an insulation which quickly decrease the energy transfer from the heat to the water. To remedy this problem the engineer will carry out soot blowing. At a specified time the engineer uses a long tool and insert it into the fire side of the boiler. This device, which looks like a lance, has a tip at the end which "blows" steam. This blowing action of the steam "scrubs" the outside of the water tubes, cleaning the carbon build up.Water tube boilers can have pressures from 7 bar to as high as 250 bar. The steam temperature's can vary between saturated steam, 100 degrees Celsius steam with particle of water, or be as high as 600 - 650 degrees Celsius, know as superheated steam or dry steam The performance of boiler is generally referred to as tons of steam produced in one hour. In water tube boilers that could be as low as 1.5 t/hr to as high as 2500 t/hr.

IntroductionBoiler is a device which is used to produce steam at high pressure. Steam is being used in thermal power plant, textile industries and for domestic uses during winter to heat the room.

Types of boilersHorizontal, Vertical and Inclined boilersFire tube and water tubeExternally fired and internally firedForced circulation and natural circulationHigh pressure and low pressureStationary and portable(moving)Single tube and multi tubeHorizontal, vertical or inclinedIf the axis of the boiler is horizontal, vertical or inclined then it is called horizontal, vertical or inclined boiler respectively.

BOILERS

Fire tube and water tubeIf 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, Sterling, Yarrow boiler etc.

Externally fired and internally firedThe boiler is known as externally fired if the fire is outside the shell.Examples: Babcock and Wilcox, SterlingThe boiler is known as internally fired if the furnace is located inside the boiler shell.Examples: Cochran, Lancashire

Forced circulation and natural circulationIn forced circulation type of boilers, the circulation of water is done by a forced pumpExamples: 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 pressureThe 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 portableStationary 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 tubeThe 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

Parts and terms in BoilerShellConsists of one or more steel plates bent into a cylindrical form and riveted or welded together. The shell ends are closed with end platesGrateIt is a platform in the furnace upon which fuel is burntFurnaceIt is the chamber formed by the space above the grate and below the boiler shell, in which combustion takes place. RefractoryInsulation material used for lining combustion chamberCombustion chamberPart of furnace where combustion of fuel takes place.SettingThe primary function of setting 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 chamberWater space and steam spaceThe volume of the shell that is occupied by the water is termed as water space while the entire shell volume less the water and tubes is called steam space.Water levelThe level at which water stands in the boiler is called water level.RefractoryInsulation material used for lining combustion chamber.

Boilers and Parts

FoamingFormation of steam bubbles on the surface of boiler water due to high surface tension of water.ScaleA deposit of medium due to extreme hardness occurring on the water heating surfaces of boiler because of an undesirable condition in the boiler water.Blowing offThe removal of mud and other impurities of water from the lowest part of the boiler. Accomplished with the help of blow off cock or valve.LaggingInsulation wrapped on the outside of the boiler shell or steam piping.PrimingItis the carryover of varying amounts of droplets of water in the steam (foam and mist), which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on the super heaters and in the turbines. Priming may be caused by improper construction of boiler, excessive ratings, or sudden fluctuations in steam demand. Priming is sometimes aggravated by impurities in the boiler-water.rate.

MountingsEquipment and devices, which are used for safety of boiler are called mountings, these are required parts for the successful operation of a boiler.Ex. Feed-check valve, safety Valve etc.

AccessoriesThe items which are used for increasing the boiler efficiency are called accessories.Ex. Super heaters, Steam separators etc.

BOILER MOUNTINGSImportant boiler mountings are as follows,

Pressure gauge

Safety valves

Fusible plug

Steam stop valve

Feed check valve

Blow off cock

Mud and man holes

Water level Indicator

PRESSURE GAUGE

Fitted in front of the boiler to record the steam pressure at which steam is generated in the boiler.

Two types of pressure gauges are being used in boiler operations.Diaphragm typeBourdon Tube

A bourdon pressure gauge in its simplest form consists of a simple elastic tube, one end of the tube is fixed and connected to the steam space in the boiler, other end is connected to a sector through a link.

Diaphragm Type

SAFETY VALVE

Safety valves are needed to blow off the steam when the pressure of the steam in the boiler exceeds the working pressure. It is placed on the top of the boiler.

Dead weight safety valveLever safety valveSpring loaded safety valveHigh steam and low water safety valve

Dead weight safety valve

Lever safety valve

Spring loaded safety valve

High steam and low water safety valve

FUSIBLE PLUG

To extinguish fire in the event of water level in the boiler shell falling below a certain specified limit. It is installed below boilers water level on the crown plate.

Fusible Plug

STEAM STOP VALVE

A valve is a device that regulates the flow of a fluid (gases , fluidized solids slurries or liquids) by opening or closing or partially obstructing various passageways

Function : to shut off or regulate the flow of steam from the boiler to the steam pipe or steam from the steam pipe to the engine

FEED CHECK VALVETo allow the feed water to pass in to the boiler

To prevent the back flow of water from the boiler in the event of the failure of the feed pump

BLOW OFF COCK To drain out water from the boiler for internal cleaning inspection or other purposes

WATER LEVEL INDICATORA: End plate of boilerH & J: Two ballsB & C: Hollow gun metal castingK: Drain cockD & E: CocksL: Guard glassF: Gauge glassM,N,P& R: Screwed capsG: Hollow metal columnX,Y: FlangesThe function of water level indicator is to show level of water present in the boiler.

BOILER ACCESSORIES

Accessories are the devices being used to increase the efficiency of the boiler. A large amount of heat is being carried out by the flue gases, this is wastage of useful energy, which can be recovered. Accessories are those equipment which recovers the wastage along with smoothing the operation to increase the utilization of energy as well as reducing the cost of operation. The waste recovery takes place by the help of flue gases, which has a large amount of heat.Accessories are not the mandatory parts or devices but being used for efficient operation.

Commonly used accessories are as follows,Steam Super heaterEconomizerAir preheaterSteam separatorFeed PumpInjector

STEAM SUPER HEATER

The function of a super heater is to increase the temperature of steam above its saturation point. That means it gives assurance of the quality of steam. During superheating pressure of steam remains same but the volume increases with its temperature, increasing the internal energy which in turns prove to increase in kinetic energy, resulting in:

Reduction of steam consumption of turbine.Reduction in losses due to condensation in steam pipes.Elimination of erosion of turbine bladesIncrease in efficiency.

There are two types of super heaters.Convective Super heaterRadiant Super heater

ECONOMIZER

In best way it is known as feed water heater, that refers heating of feed water, which is supplied to the boiler shell to get vaporized. It utilizes heat carried out but the waste furnace gases to heat the water before it enters boiler. By increasing the temperature of water, chilling of the boiler surface is prevented and then a less amount of sensible heat is required to achieve saturation temperature, it reduces then the input heat to the boiler and increasing efficiency.

There are two types of economizerIndependent type (not a part of boiler)Integrated type (a part of boiler)

AIR PREHEATER

The function of an air preheater is to heat the inlet air before it is sent to the furnace. It is placed after economizer, flue gases coming from economizer is being utilized to heat air. Preheated air accelerates combustion and increasing the amount of heat produced.

Degree of preheating depends uponType of fuelType of fuel burning equipmentRating of the boiler and furnace

Two types of preheaters commonly used,Recuperative type (Both the fluids pass simultaneously)Regenerative type (Fluids pass alternatively)

STEAM SEPARATOR

The basic work of steam separator is to ensure the quality of steam, steam from the boiler may be in the form of wet steam, or incase of regenerative cycle, where condensate from turbine is supplied back or being used by smaller capacity turbines to recover heat, steam must be in the wet format. Steam separator removes water particles.

There are three types of steam separatorImpact or baffle typeReverse current typeCentrifugal type

Baffle type steam separator

FEED PUMP

Feed pumps is the device required to supply water to the boiler. The quantity of feed water should be at least equal to the amount of steam delivered to the turbine or required space. For open cycle boiler in case of large plants, where there is no condenser or the amount of feedback water is less, pumps are inevitable.

There are two types of feed pumps,Reciprocating feed pump (Piston cylinder arrangement)Single actingDouble actingRotary or centrifugal feed pump

Duplex reciprocating pump

INJECTOR

The basic work of an injector is to feed water to the boiler on high pressure, it finds its application in such places where there is no space to install feed pumps. It works by the help of steam pressure in a way that the pressure of steam is being utilised to increase the kinetic energy of feed water.

AdvantagesLow initial costSimplicityCompactnessNo dynamic partsHigh thermal efficiency

DisadvantagesLow pumping efficiencyCant work for very hot steamIrregularity in the operation when steam pressure varies considerablyInjector

STUDY OF BOILER

Fire tube boiler (Cochran boiler)Water tube boiler (Babcock and Wilcox boiler)

FIRE TUBE BOILER (COCHRAN BOILER)IntroductionIts a multi tube internally fired, fire tube boiler, as an improvement of traditional vertical boiler, providing more heating space relatively.

ConstructionIt consists of a vertical cylindrical shell with a hemispherical top and hemispherical shaped furnace. Furnace has ash pit at the bottom end above which fire grate lies. Combustion chamber of the furnace widens to connect the flue passage pipes, lined with fir bricks and insulated with refractory materials on the shell side. Smoke box is fitted in the hinged door for easy access to tubes, and allow easy cleaning. A number of horizontal fire tubes of equal lengths and diameter connects the combustion chamber with smoke box. Furnace is surrounded by water on all the side except the opening for fire door and combustion chamber.

WATER TUBE BOILER(BABCOK & WILCOX BOILER)

This is a type of water tube boiler used when pressure exceeds 10 bar and capacity 7000 kg per hour.

ConstructionIt consists of a horizontal high pressure drum, from each end of it connections are made with an uptake and a down take header. Headers are joined to each other by large number of water tubes inclined at an angle of 15 degree to provide water circulation. Hand holes are provided for the maintenance of tubes. The entire assembly of water tubes is hung in a room made of masonry work, lined with fire bricks to resist thermal expansion. As super heater additional U-shaped tubes are arranged between drums and water tubes.Furnace is arranged below the uptake header. Baffles are provided across the water tubes to guide flue gases. A Chimney is provided for exit of gases and a damper at the opening of chimney to provide draught. Cleaning doors are provided to access the tubes for cleaning and removal of soot and various mountings for successful operation.

Advantage of Babcock and Wilcox boiler

Suitable for all types of fuels and hand stokers for firing.

Draught loss is small.

All components are accessible for inspection during operation.

Expansion and contraction has no harm on masonry work(construction).

Replacement of defective tubes are easy.

Applications of Cochran boiler and Babcock and Wilcox boiler

Cochran boiler finds its application in cranes, excavators and in small factories for general contract work where temporary supply of steam is required.

Babcock and Wilcox boiler finds its application in sugar mills and textile industries for power generations and processing works.

COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS

Particulars Fire-tube boilersWater-tube boilersPosition of water and hot gasesHot gases inside the tubes and water outside the tubesWater inside the tubes and hot gases outside the tubesMode of firingGenerally internally firedExternally firedOperation pressureLimited to 16 barCan go up to 100 barRate of steam productionLowerHigher

COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS

Particulars Fire-tube boilersWater-tube boilersSuitabilityNot suitable for large power plantsSuitable for large power plantsRisk on bursting Involves lesser risk of explosion due to lower pressureMore risk on bursting due to high pressureFloor areaFor a given power it occupies more floor areaFor a given power it occupies less floor areaConstructionDifficultSimple

COMPARISON OF FIRE TUBE AND WATER TUBE BOILERSParticularsFire-tube boilersWater-tube boilersTransportationDifficult SimpleShell diameterLarge for same powerSmall for same powerChances of explosionLess MoreTreatment of waterNot so necessaryMore necessaryAccessibility of various partsVarious parts not so easily accessible for cleaning, repair and inspectionMore accessible