steam genaration

7/28/2019 Steam Genaration

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Steam generator Ganesh kumar

A.GANESH K UMARDEUTSCHE BABCOCK, INDI A.

For internal circulation only. All rights reserved by author.


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DEDICATED TO MY COLLEGE AND MY PROFESSORS.


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PREFACE

Dear friends,

This book was prepared in view of giving assistance to design

engineers entering into the boiler field and to plant engineers whom

I have met always in desire to know the ABC of the boiler design

and related calculations. I have made an attempt in bringing close

relation of practical field design and theoretical syllabus of

curriculum. Engineering students, who always wonder how the

theory studying in curriculum will help them in real life of business.

For them this book will give an inspiration.

I have designed this book in two parts. First, the basic theory of

working fluid in the steam plant cycle. This will be the basic

foundation for development of boiler science. Secondly the main

components of steam generator and its design. Also you can find

various useful data for ready reference at the end of this book.

(A.GANESH KUMAR)


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CONTENTS

PREFACE.

1.0 TYPES OF STEAM GENERATORS

1.1 Introduction.1.2 History of steam generation and use1.3 Shell and tube boiler.1.4 Conventional grate type boiler.1.5 Oil/gas fired boiler.1.6 Pulverized fuel boiler.1.7 Fluidized bed boiler1.8 Heat recovery steam generator1.9 Practical guide lines for selection of boiler.

2.0 STEAM, GAS and AIR

2.1 Introduction2.2 Definitions for some commonly used terms2.3 Steam.2.4 Fuel..2.5 Gas and air.2.6 Some commonly used dimensionless numbers and their significance.

3.0 FURNACE

3.1 Introduction3.2 Effect of fuel on furnace..3.3 Forced or Natural Circulation.3.4 Heat flux to furnace walls...3.5 Points to be noted while designing furnace3.6 Classification of furnace.3.7 Modes of heat transfer in furnace3.8 Heat transfer in furnace.3.9 Furnace construction.3.10 Practical guides for designing fluidized bed, conventional

and oil/gas fired furnace..

4.0 SUPERHEATER

4.1 Introduction..4.2 Effect of fuel on super heater design4.3 Points to be noted while designing super heater4.4 Classification of super heater.4.5 Designing a super heater4.6 Overall heat transfer across bank of tubes.4.7 Steam temperature control4.8 Pressure drop..


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5.0 DRUMS

5.1 Intruction.5.2 Optimal configuration of drums5.3 Stubs and attachments in the steam drum/shell..5.4 Maximum permissible uncompensated opening in drum5.5 Size of the drum5.6 Drum internals..

6.0 EVAPORATOR AND ECONOMISER

6.1 Introduction.6.2 Difference between evaporator and economiser..6.3 Fin efficiency

7.0 AIRHEATER

7.1 Introduction.7.2 Types of air heater.7.3 Advantages of air heater..7.4 Heat transfer in air heater7.5 Practical guide lines for designing airheater.

8.0 DUST COLLECTOR

8.1 Introduction.8.2 Effects of air pollution

8.3 Air quality standards..8.4 Air pollution control devices.Centrifugal cyclone dust collectorBag filterElectro static precipitator

9.0 WATER CHEMISTRY

9.1 Introduction.9.2 Names of water flowing in the power plant cycle..9.3 Major impurities in water..9.4 Effects of various impurities in boiler water..9.5 Need for water treatment.

9.6 External water treatment..9.7 Internal water treatment9.8 Practical guides for selecting water treatment plant.

10.0 BOILER CONTROLS

10.1 Introduction10.2 Control philosophy10.3 Drum level control.

Deleted: od


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10.4 Super heater steam temperature control..10.5 Furnace draft control.

10.6 Combustion control...10.7 Field instruments..10.8 Panel instruments

APPENDIX 1 : MOLLIEAR CHARTAPPENDIX2 : PSYCHROMETRY CHARTAPPENDIX3 : FUEL ANALYSISAPPENDIX4 : STEAM TABLESAPPENDIX5 : POLLUTION NORMS IN VARIOUS INDIAN STATESAPPENDIX6 : USEFUL DATASAPPENDIX7 : UNIT CONVERSION TABLE


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1.0 TYPES OF STEAM GENERATOR

1.1 INTRODUCTION

Indian power demand is met mainly from thermal, hydro and nuclear power. Non-conventional energy power production is very much negligible. Out of the mainpower producing sources thermal plant produces 48215 MW (69%), hydro plantproduces 19300 MW (28%), nuclear plant produces 2033 MW (3%) as on 31

st

March 1992. In the above power plants 72% of the generation is from thermal andnuclear, where steam generation is one of the main activity. In the years to come,the demand of electricity is going on increasing and already most of water resourcessuitable for power generation is in service. Except from gas turbines power the mostof new electric capacity has to be met by utilizing steam.

Steam boiler today range in size from those to dry the process material 500 kg/hr tolarge electric power station utility boilers. In these large units pressure range from100 kg/cm to near critical pressures and steam is usually superheated to 550C. InIndia BHARAT HEAVY ELECTRICALS LTD (BHEL) is the pioneer in developingthe technology for combustion of high ash coal efficiently in atmospheric bubblingfluidized bed. From where lot of industries in boiler manufacturing starts. Only afterthe year 1990, Indias foreign policy was changed, various foreign steam generatormanufacture entered into Indian power market bringing various configuration andcompetitiveness in the market.

1.2 HISTORY OF STEAM GENERATION AND USE

The most common source of steam at the beginning of the 18th

century was the shell

boiler. Little more than a kettle filled with water and heated from the bottom. Oldenday boiler construction were very much thicker shell plate and riveted constructions.These boilers utilize huge amount of steel for smaller capacity. Followed this shelland tube type boilers have been used and due to direct heating of the shell byflames leads severe explosion causing major damages to life and property. Forsafety need, after the Indian independence India framed Indian boiler regulations in1950, similar to various other standards like ASME, BS, DIN, JIS followed worldwide. Till date IBR 1950 is governing the manufacturing and operation of boilers withamendments then and there. Indian sugar industry uses very low pressure (15kg/cm) inefficient boilers during independence now developed to an operatingpressure of 65 kg/cm and more of combined cycle power plant. If we analysis mostof the boilers erected in pre-independence period were imported boilers only andnow steam generators were manufactured in India to the world standards on budget,

delivery and performance. In power industry India made a break through in the year1972, Indias first nuclear power plant was commissioned at Tarapore. This plantwas an pilot plant meant for both power and research work. This was made incollaboration with then soviet republic of Russia. Now India has its own nucleartechnology for designing nuclear power plant. Even though there is a development,Indian industry has to go a long way in power sectors.


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1.3 SHELL AND TUBE BOILER

Steam was originally used to provide heat to the industrial process like drying,boiling. In small industry the people are not taken care in fuel consumption point,they have generated steam in crude manner. Shell and tube boilers are old versionof boilers used in industry where a large flue tube was separated by a fixed grateman power is used to throw husk and shells into the grate and firing was done.

In early days, as individual electric generating stations increased in capacity, thepractice was merely to increase the number of boilers. This procedure eventuallyproved to be uneconomical and larger maintenance. Afterwards, individual boilerswere build larger and larger size, however the size became such that furnace floorarea occupation was more. Therefore further research work have been developed inthis area and technologies such as pulverized coal fired furnace, circulated fluidizedbed furnace, pressurized circulated fluidized furnace (still under research stage)

were developed. These modern technologies have higher heat transfer coefficient infurnace and allow higher volumetric combustion rates.

1.4 CONVENTIONAL GRATE TYPE BOILERS

TECHNOLOGY

This is the oldest method of firing fuel. Fuel will be spread over the grate, where thefuel is burnt. Fuel feeding will be done manually or mechanically to have a sustainedflame. In this type burning will be done at higher excess air. Incoming air will beused for cooling the grate.

Types of grate

Common types of grate that are used for fuel are fixed grate, pulsating grate,dumping grate, travelling grate. Each type of grate differ slightly in their constructionand arrangement. However the combustion phenomenon remains same.

Travelling grateThe travelling type is a continuous grate which slowly convey the burning fuelthrough the furnace and discharge the ash to an ash pit. Grate speed is regulatedby the amount of ash discharging to ash pit ( 0 to 7m/hr)

Pulsating grateThe pulsating grate is non- continuous grate. The grate surface extends from therear of furnace to ash pit. Here the grate will be given a racking motion at pre

determined frequency depending on the fuel/ash bed depth.

Dumping grateDumping grates are also a non-continuous type grate. The grate is split intolongitudinal sections, one for each feeder. Fuel is distributed on the grate and burns.When ash depth gets to a depth where air can not diffuse it , the grates are tilted orash is dumped into the hopper in the following manner.


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Alternating fuel feeding is stopped and grate is tilted by lever arrangement, theactuation can be done either manually or pneumatic cylinder.

In dumping grate the grate sections should be designed in such a way that, whiledumping the ash part of grate surface not available for burning. In poorly designeddumping grate there may be steam pressure. Therefore while sizing grate sectionscare should be taken such that while dumping part of the grate, other fuel feeder andremaining sections should able to take the full load.

Dumping grate is similar to fixed grates, it is best suitable for bagasse where the fuelis of low calorific value and having high moisture content. Therefore air alone canacts as a cooling medium. If we use coal the grate bar may not with stand highertemperature and additional cooling by water tube is necessary. Travelling grate issuitable for burning coal and lignite. As the grate rotates, the grate bar gets heatedand cooled by incoming air for the half of the cycle and remaining half of the cyclegrate bar cooled by the incoming air.

Spreader stokerMechanical spreaderThe spreader stoker feeder takes fuel from the feeder hopper by either a small ramor a rotating drum and delivers it into a spinning rotor. An adjustable trajectory plateis located between the feed mechanism and the rotor. Adjusting the trajectory platefuel can be feed through out the entire length of the furnace.Pneumatic spreaderIn this rotor is replaced by high pressure air lines from Secondary air fan is used tospread the fuel into the furnace. The fuel is carried into the furnace by means ofpneumatic system and the air flow adjustment makes the fuel to flow near or fartherof the furnace.

1.5 OIL/GAS FIRED BOILERS

TECHNOLOGY

Flame has a tendency to burn upward only. This forms the basic concept of burner.Whenever fresh fuel enters into the ignition zone it starts burning upwards and theflame will not come downwards to the incoming fuel, by this property combustioncan be controlled easily. Hence it is always better to bring the oil or gas train frombottom of the burner.A liquid or gas fuel has flowable property by nature and it has a lower ignitiontemperature. When the fuel is forced to flow through the nozzle it will spread thoughan predetermined length and burn completely from the point of entry to the firingzone estimated. The fuel flow can be controlled by means of control valves.

CHARACTERISTICS OF OIL

In todays climate of fluctuating international fuel prices and quality, the emphasis onthe ability of the boiler on low quality fuel oils has become more greater. In theinternational market, the quality of the residual fuel oils is constantly getting poorerdue to the development of more sophisticated cracking methods and also ourindigenous crude production falls short of our requirements, about 15 million tons ofcrude is imported from outside sources. These outside sources are many, our


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refineries handle a variety of crude. Since the inherent properties of the finishedpetroleum products are directly dependent on the parent crude, one can imagine the

petroleum involved in producing residual fuel oil within narrow limits of specifications,especially with respect to specified characteristics like carbon residue, asphaltenesand metallic constituents is not possible.

Flash point

Flash point is important primarily from a fuel handling stand point. Too low a flashpoint will cause fuel to be a fire hazard subject to flashing and possible continuedignition and explosion. Petroleum products are classified as dangerous or nondangerous for handling purposes based on flash point as given below.

Classification Flash point PetroleumProduct

Class A Below 23C NapthaPetrolSolvent 1425Hexane

Class B 23 to 64C KeroseneHSD

Class C 65 to 92C LDOFurnace oilLSHS

Excluded Petroleum 93C and above Tar

Pour Point

The pour point of the fuel gave an indication of the lowest temperature, above whichthe fuel can be pumped. Additives may be used to lower the freezing temperatureof fuels. Such additives usually work by modifying the wax crystals so that they areless likely to form a rigid structure. It is advisable to store and handle fuels around10C above the expected pour point.

Viscosity

Viscosity is one of the most important heavy fuel oil characteristics for industrial and

commercial use, it is indicative of the rate at which the oil will flow in fuel systemsand the ease with which it can be atomized in a given type of burner. When thetemperature increases viscosity of fuel will reduce.


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The viscosity needed at burner tip for satisfactory atomization for various types ofburners are as follows.

Type of burner Viscosity at burner tipIn centi stokes

Low air pressure 15 to 24Medium air pressure 21 to 44High air pressure 29 to 48Steam jet 29 to 37Pressure jet less than 15

Metal Content

Sodium, Potassium, Vanadium, Magnesium, Iron, Silica etc. are some of the metallicconstituents present in fuel oil. Of the above metals, sodium and vanadium are themost troublesome metals causing high temperature corrosion in boiler super heatertubes and gas turbine blades. Much of the sodium is removed from the crude oil inthe desalting operation, which is normally applied in the refinery and additionalsodium can be removed from the finished fuel oil by water washing and centrifuging.

Vanadium is found in certain crude oils and is largely concentrated in fuel oilprepared from these crude. No economical means for removal of vanadium from theresidual fuel oil is available. However certain additives like magnesium are availableto minimize the effect of vanadium.

Asphaltene content and Carbon residue

Asphaltenes are high molecular weight asphaltic material and it requires moreresidence time for complete combustion. Asphaltenes as finely divided coke may bedischarged from the stack. Residual fuel oils may contain as much as 4%asphaltenes.

Petroleum fuels have a tendency to form carbonaceous deposits. Carbon residuefigures for residual fuel oils from 1 to 16% by weight. This property is totallydependent on the type of crude, refining techniques and the blending operations inrefinery.

Fuels with high carbon residue and asphaltenes requires large combustion chamberand hence while designing the boiler for such fuel the volumetric loading has to be of

the order of 2 lakhs Kcal/m3

hr


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OIL/GAS FIRING START UP LOGIC

MANUAL TRIP INTERLOCK1.CHECK TRIP VALVES IN CLO

2 . CHECK WATER LEVEL IN DR3. EMERGENCY PUSH BUTTON

CONTROL SUPPLY LAMP 4. CHECK FAN SUCTION DAMPER IPOSITION5.CHECK FUEL PUMP/GAS TRAIN D

VALVE IN CLOSED CONDITION6. CHECK MANUAL ISOLATION VALVE

CONTROL POWER SUPPLY SELECTOR SWITCH POSITION.IN GAS/OIL FIRING MODE

FAILEDDEENERGISE TR & PILOTVALVE

DEDUCT PILOT FLAME DEENERGISE TRANSFORMERENERGISE GAS/OIL SHUT OFF VALVE TO OPEN

YES AND VENT TO CLOSE

YES DEENERSISE PILOT GAS & RELESASE LOW FIREMAIN FLAME ESTABLISED

NO NO DEENERSISE PILOT GAS

1.0PURGE COMPLETED 1.0 OIL/GAS MAIN SHUT O

2.0ALL PURGE INTERLOCKS 2.0 RETURN OIL LINE SHUENERGISE IGNITION AGAIN CHECKED 3.0 AIR/ATOMISING STEATRANSFORMER & 3.0COMPUSTION AIR PR NOT LOW POSITIONPILOT GAS SHUTOFF VALVE 4.0 INSTRUMENT AIR PR NOT LOW 4.0 PILOT GAS/SCAVENG

5.0 COMBUSTION AIR DAMPER TO POSITIONLOW FIRE POSITION 5.0 FUEL GAS SHUT OFF

PRESS BURNER 6.0OIL/GAS AT REQUIRED PARAMETER PURGE 6.0 NO FLAME INSIDE FUSTART BUTTON 7.0 EMERGENCY PUSH BUTTON BUTTON ON 7.0 FUEL PUMP NOT RUN

NOT OPERATED 8.0 FURNACE PRESSURE8.0SCANNER COOLING AIR PR OK COMBUSTION AIR 9.0 DRUM LEVEL NOT HI

DAMPER TO LOW 10.0ALL TRIP PARAMETEAUTO GAS/OIL FIRING INTERLOCKS FIRE POSITION 11.0 FUEL GAS PRESSURPURGE COMPLETED PURGE IN PROGRESS LAMP ON


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1.6 PULVERIZED FUEL BOILERS

TECHNOLOGY

When coal is powdered to micron size it can be conveyed easily by air in pipelinesand the pulverized coal behaves as if that of oil and hence the same can be easilyburnt in pulverized fuel burners. The heat release by the burners in very high andun-burnt carbon is almost equal to zero. Hence efficiency achieved by pulverizedburners is much more than any type of coal combustion.

MECHANISM OF PULVERIZED FUEL BURNING

There are two systems of pulverized firing 1.0 direct firing 2.0 indirect firing.

In the direct firing system, raw coal from the storage area is loaded on a conveyor

and fed to a coal crusher. A second conveyor system loads coal into the coalstorage bunker located over the coal pulverization system. Coal via gravity feed isdelivered through a down spout pipe to the coal feeder. A coal shutoff gate isprovided prior to the coal feeder inlet to allow emptying the system down stream.The coal feeder meters the coal to the crusher dryer located directly below thefeeder discharge. A primary air fan delivers a controlled mixture of hot and cold airto the crusher dryer to drive moisture in the coal facilitating pulverization the primaryair and crushed coal mixture is then fed to the coal pulverizer located below thecrusher dryer discharge. Selection of pulverizer has to be analyzed critically, since itis one of the important equipment where the wear and tear is more. For the softlignite Beter wheel is preferable and for hard lignite, coal like fuels heavy pulveriserof ball and hammer mill is preferable. The coal is pulverized to a fine powder andconveyed through coal pipes to the burners. Primary air is the coal pipe

transportation medium.

The indirect firing system utilizes basically the same coal flow path to the pulverizer.After the classification of pulverized coal, it is delivered to a coal storage bin. Whenneeded to fire the boiler the pulverized coal is then conveyed to the burners by anexhaust fan. This method requires very special provisions to minimize risk of fire orexplosion. Of the two systems, the direct firing is more common.Neyveli lignite power corporation has pulverized boiler of direct firing system.

1.7 FLUIDIZED BED BOILERS

ATMOSPHERIC FLUIDIZED BED COMBUSTION

TECHNOLOGY

When air or gas is passed through an inert bed of solid particles such as sandsupported on a fine mesh or grid. The air initially will seek a path of least resistanceand pass upwards through the sand. With further increase in the velocity, the airstarts bubbling through the bed and particles attain a state of high turbulence. Undersuch conditions bed assumes the appearance of a fluid and exhibits the propertiesassociated with a fluid and hence the name fluidized bed.


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MECHANISM OF FLUIDIZED BED COMBUSTION

If the sand, in a fluidized state is heated to the ignition temperature of the fuel andfuel is injected continuously into the bed, the fuel will burn rapidly and attains auniform temperature due to effective mixing. This , in short is fluidized bedcombustion.While it is essential that the temperature of bed should be equal to the ignitiontemperature of fuel and it should never be allowed to approach ash fusiontemperature (1050 to 1150C ) to avoid melting of ash. This is achieved byextraction of heat from the bed by conductive and convective heat transfer throughtubes immersed in the bed.

If the velocity is too low fluidization will not occur, and if the gas velocity becomes toohigh, the particles will be entrained in the gas stream and lost. Hence to sustainstable operation of the bed, it must be ensured that gas velocity is maintained

between minimum fluidization and particle entrainment velocity.

Advantages of FBC.

1.0 Considerable reduction in boiler size is possible due to high heat transfer rateover a small heat transfer area immersed in the bed.

2.0 Low combustion temperature of the order of 800 to 950C facilitates burning offuel with low ash fusion temperature. Prevents Nox formation, reduces hightemperature corrosion and erosion and minimize accumulation of harmfuldeposits due to low volatilization of alkali components.

3.0 High sulphur coals can be burnt efficiently without generation of Sox by feeding

lime stone continuously with fuel.

4.0 The units can be designed to burn a variety of fuels including low grade coalslike floatation slimes and washery rejects.

5.0 High turbulence of the bed facilitates quick start up and shut down.

6.0 Full automation of start up and operation using simple reliable equipment ispossible.

7.0 Inherent high thermal storage characteristics can easily absorb fluctuation in fuelfeed rate.


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ATMOSPHERIC CIRCULATING FLUIDIZED BED COMBUSTION

TECHNOLOGY

Atmospheric circulating fluidized bed (ACFB) boiler is a devise used to generatesteam by burning solid fuels in a furnace operated under a velocity exceeding theterminal velocity of bed material. I.e., solid particles are transported through thefurnace and gets collected in the cyclone at the end of furnace and again recycledinto furnace by means of pressure difference between fluidized bed and returnparticle.MECHANISM OF CIRCULATING FLUIDIZED COMBUSTION

The mechanism is similar to AFBC. However in AFBC the fluidization velocity is justto make the particles in suspended condition. In ACFB boiler, special combination ofvelocity by primary air and secondary air, re-circulation rate, size of solids, and

geometry of furnace, give rise a special hydrodynamic condition known as fast bed.

Furnace below secondary air injection is characteristic by bubbling fluidized bed andfurnace above the secondary air injection is characteristic by Fast fluidized bed.Most of the combustion and sulphur capture reaction takes place in the furnaceabove secondary air level. This zone operates under fast fluidization. In CFB boilernumber of important features such as fuel flexibility, low Nox emission, highcombustion efficiency, effective lime stone utilization for sulphur capture and fewerfuel feed points are mainly due to the result of this fast fluidization.

In fast fluidization heavier particles are drag down known as slip velocity betweengas and solid, formation and disintegration of particles agglomeration, excellentmixing are major phenomenon of this regime.

CFB is suitable for1.0 Capacity of the boiler is large to medium.2.0 The boiler is required to fire a low grade fuel or highly fluctuating fuel quality.3.0 Sox and Nox control is important.

PRESSURIZED FLUIDIZED BED COMBUSTION

The advantage of operating fluidized combustion at the elevated pressure ( about 20bar) is, reduction in steam generator size can be achieved and make possible thedevelopment of a coal fired combined cycle power plant. The development ofpressurized fluidized bed combustion is still in research stage only. With help ofpressurized hot gas coming out of the furnace is cleaned primarily by a cyclone like

CFBC boiler and the gas is expanded in a turbine and the exhaust gas from turbineis further cooled by the heat exchanger. The aim behind the development ofpressurized fluidized bed are:

1.0 To develop steam generator of smaller size for the higher capacity.

2.0 To reduce the cost of generation of power per MW.

3.0 To develop turbines which make use of solid fuels such as coal, lignite etc.,


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1.8 HEAT RECOVERY STEAM GENERATOR

In India, coal availability is 97% of the requirement and we are importing coal only forthe process requirement like baking coal for steel plant where high calorific coal isrequired. Hence in post independence India coal fired boilers where flourished,however due to the need of energy conservation and due to process parameterrequirements development of HRSG in recent periods is more. Moreover due to thedevelopment of gas turbines with gaseous and liquid fuels, more GT are beinginstalled due to their lower gestation period and higher efficiency than Rankine cycle.

As explained earlier HRSG can be classified into two types, one is for maintainingprocess parameter such as temperature and other is in the point of economic pointof view.

The process steam generator are generally referred by the term called waste heatrecovery boiler ( WHRB) where the gas contains heat in excess, this excess wasteheat has to be recovered or removed by any means so that the process parametercan be maintained. ( e.g. Sulphuric acid plant, hydrogen plant, sponge iron plant,Kiln exhaust etc.,)

The steam generator stands behind the gas turbine are usually referred as Heatrecovery steam generator.

The HRSG or WHRB the design greatly vary with respect to the size of the plant,the gas flow, gas volumetric analysis, dust concentration and sulphur di oxideconcentration. In HRSG the gas quantity and inlet temperature is fixed and fordifferent load the variation of heat will not be proportional and hence at part loads the

heat absorbed at different zones will vary widely and hence for different loads theperformance of the HRSG to be done.


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2.0 STEAM,GAS and A IR

2.1 INTRODUCTION

In steam generator water, steam, gas and air are the working fluids in this air andgas have similar properties. Understanding the properties of gas and air are almostone and the same. I have grouped steam and gas as one unit and water as aseparate unit just because understanding the behavior of steam and gas is moreimportant in design point of view where as knowledge of water is more important inoperational point of view.

2.2 DEFINITIONS FOR SOME COMMONLY USED TERMS

Heat

Heat is defined as the form of energy that is transferred across a boundary by virtueof a temperature difference. The temperature difference is the potential and heattransfer is the flux. In other words heat is the cause and temperature is the effect.

Energy

Energy of a body is its capacity to do work and is measured by the amount of thework that it can perform.

Potential Energy( mgh = mass x gravitational force x datum level)Potential energy of a body is the energy it possesses by virtue of its position or stateof strain.

Kinetic energy ( mv = x mass x velocity)

Kinetic energy of a body is the energy possessed by it on account of its motion.

EnthalpyEnthalpy is the quantity of heat that must be added to the fluid at zero degreecentigrade to the desired temperature and pressure. Enthalpy is defined as heatwithin or heat content of the fluid.

Entropy

The word entropy is derived from a Greek word called tropee which meanstransformation. The unit of entropy is Joules/kelvin.

Specific heat

Specific heat of a substance is defined as the amount of heat required to raise the

temperature of one kilogram of substance through one degree kelvin. All liquids andsolids have one specific heat. However gas have number of specific heats dependson the condition with which it is heated.

Cp = f(T)


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Specific heat at constant pressure.Specific heat of a substance is defined as the amount of heat required at constant

pressure to raise the temperature of one kilogram of substance through one degreekelvin.

Integral constant pressure specificheatIt is the average heat required to rise the temperature between two temperaturedifference t1 and t2 i.e., Cp = ( H2 H1)/(t2 t1)

H = f(Cp/T)

Specific heat at constant volume.Specific heat of a substance is defined as the amount of heat required at constantvolume to raise the temperature of one kilogram of substance through one degreekelvin.

NTP and STP condition

It is customary to specify the gas or steam properties at NTP or STP condition,NTP condition is at Normal temperature and pressure, i.e., the properties measuredat 0C or 273.15 K and pressure 1.01325 bar or 1.03 atmSTP condition is at Standard temperature and pressure i.e., the properties measuredat 25C or 298.15K and pressure 1.01325 bar or 1.03 atm.

Viscosity

Viscosity of a liquid is its property, due to the frictional resistance between the fluidparticles (cohesion between particles) or between fluid and the wall. Viscosity offluid controls the rate of flow.

Newtons Law of viscosityThe shear stress on a layer of a fluid is directly proportional to the rate of shear

strain. ( Velocity gradient )

/l where is shear stress and is velocity , l is the distance or gap betweenlayers.

= /l where is the constant of proportionality and is known as absoluteviscosity or dynamic viscosity.

Kinematic viscosity is the ratio of absolute viscosity to density (/)

Thermal conductivity

Thermal conductivity is the property of substance, that its ability to conduct heat and

expressed in W/mK.

Kilogram

Kilogram is the mass of one international prototype made of platinum iridium cylinderpreserved at the international bureau of weights and measures at paris.


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Meter

Meter is the length between two transverse lines en-grooved in platinum iridium bar

at 0C. or The meter is the length equal to 1650763.73 vacuum wave length of theorange light. ( = 605.8 mm of the Krypton 86 discharge lamp)

Second

Second is the duration of 9192631770 periods of the radiation corresponding to thetransition between two specified energy level of the Caesium 133 atom. Or1/86400th part of mean solar day.

Specific volume

Specific volume is the volume occupied per kg of steam or water or fluid.Specific volume is the inverse of density.

For heat and mass transfer calculations, we have to know the above properties.

The properties where mainly depends on the temperature for gases and temperatureand pressure for steam. The required equation for derivation is given at appropriateplaces.

For gaseous fuel,

Cp /R = f(T)

R = Cp Cv

Cv = Cp - 1R R

Specific enthalpy wrt NTP,T

H = 1/T Cp dT ( enthalpy with reference to 0C)RT R

Tn

Specific enthalpy wrt STPT

H* = 1/T Cp dT + Hs ( enthalpy with reference to 25C)RT R RT

TsSpecific entropy,

TS = So Cp dT - ln(P/Pn) ( entropy with reference to 0C)R R R

Tn


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Specific free enthalpy

G = H - SRT RT R

The temperature dependent specific heat (Cp) can be represented by an equation of4 th degree polynomial as shown below

Cp = a1 + a2T + a3T + a4 T3

+ a5T4

(for temperature from 273K to 1000K)R

Cp = a9 + a10T + a11T + a12 T3

+ a13T4

(for temperature from 1001K to 5000K)RIntegrating, and adding constant of integration we get

H = a1 + a2T + a3T + a4T

3

+ a5T

4

+ a8/T (for temperature from 273K to 1000KRT 2 3 4 5

H* = a1 + a2T + a3T + a4T3 + a5T

4 + a6/T (for temperature from 273K to 1000KRT 2 3 4 5

S = a1 ln T + a2T + a3T + a4T3

+ a5T4

+ a7 ln(P/Pn)R 2 3 4

G = a1(1- ln T) - a2T - a3T - a4T3

- a5T4

+ a6 -a7 + ln(P/Pn)RT 2 6 12 20 T

Dynamic viscosity , thermal conductivity and prandtl number

Dynamic viscosity, thermal conductivity and prandtl number of a flue gas can be fineeasily with help of the properties of nitrogen and following constants.

Var SpecificHeatKj/kgK

DynamicViscosityPa.S

ThermalconductivityW/mK

Prandtl number

a1b1c1d1

e1

0.85545350.2036005E-30.4583082E-6-0.279808E-9

0.5634413E-13

-0.9124458E 10.4564993E-20.2198889E-4-0.1891235E-7

0.5138895E-11

-0.1083113E-10.5596822E-40.7413502E-7-0.5901395E-10

0.1961745E-13

0.492851-0.1230046E-20.1662398E-5-0.1052753E-8

0.2443111E-12

a2b2c2d2e2

-0.10023110.7661864E-3-0.9259622E-60.5293496E-9-0.109357E-12

-0.4267768E10.4074274E-3-0.5125357E-50.738556E-8-0.343972E-11

-0.8035817E-20.110672E-04-0.8397255E-80.1130229E-10-0.5731264E-14

-0.8820652E-20.1855309E-3-0.3838084E-60.3256168E-9-0.1005757E-12


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Dynamic viscosity,

g = n + P1 XH2O + P2 XCO2

Where XH2O & XCO2 are Percentage of weight in flue gasP1 = a1 + b1T + c1T + d1T

3 + e1T4

P2 = a2 + b2T + c2T + d2T3

+ e2T4

where T is temperature in C

Thermal conductivity,

kg = kn + P1 XH2O + P2 XCO2


3 + e1T4

P2 = a2 + b2T + c2T + d2T

3

+ e2T

4


Prandtl number,

Prg = Prn + P1 XH2O + P2 XCO2


3+ e1T

4

P2 = a2 + b2T + c2T + d2T3

+ e2T4

where T is temperature in CPra = a + bT + cT + dT

3+ eT

4

Specific heat,

Cpg = Cpn + P1 XH2O + P2 XCO2


3 + e1T4

P2 = a2 + b2T + c2T + d2T3

+ e2T4


Where 0 XH2O 0.3 ,0 XCO2 0.2 , 0 T 1200C

Dynamic viscosity, thermal conductivity and Prandtl number of NITROGEN

Dynamic viscosity Pa.s

Thermal conductivityW/mK

Prandtl number

a

bcdef

0.1714237E02

0.4636040E-01-0.2745836E-40.1811235E-7-0.674497E-110.1027747E-14

0.2498583E-1

0.6535367E-4-0.7690843E-8-0.1924248E-110.160998E-14-0.2864430E-18

0.6901183

0.2417094E-050.2771383E-7-0.3534575E-100.1717930E-13-0.2989654E-17


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n = a + bT + cT + dT3

+ eT4

+ fT5

Kn = a + bT + cT + dT3

+ eT4

+ fT5

Prn = a + bT + cT + dT

3

+ eT

4

+ fT

5

Cpn = a + bT + cT + dT3

+ eT4

+ fT5

(for temp.273 K to 1000K)

And Cpn = a1 + b1T + c1T + d1T3

+ e1T4

+ f1T5(for temp. 1001K to 5000K)

273 K to 1000K 1001K to 5000K

abcdef

0.3679321E1-0.1313559E-20.2615196E-5-0.9629654E-9-0.9928002E-13-0.9723991E3

a1b1c1d1e1f1

0.2852903E10.1580411E-2-0.6189378E-60.1119450E-9-0.7607378E-14-0.8019835E3

2.3 STEAM

We can see in day to day life the process of boiling water to make steam. Steam iswater in the vapour or gaseous state. It is in visible, odorless, non-poisonous andrelatively non corrosive to boiler metals. Steam is uniquely adapted by itsadvantageous properties for use in industrial process heating and power cycle.Thermodynamically boiling is the result of heat addition to the water in a constantpressure and constant temperature process. The heat which must be supplied tochange water into steam without raising its temperature is called the heat ofevaporation or vaporization and the boiling point of a liquid may be defined as thetemperature at which its vapour pressure(pressure exerted due to the vapour of the

liquid) is equal to the total pressure above its free surface. In other wordstemperature at which the partial pressure of vapour increases to make total pressureabove the liquid surface. This temperature is also known as the saturationtemperature.

EVAPORATION

Liquid exposed to air evaporate or vapourize. Evaporation is the process takesplace at the surface exposed to atmosphere. If there is any increase in ambienttemperature or increase of the liquid temperature evaporation rate becomesincreased. The reduction in pressure above the liquid surfaces accelerate theevaporation rate. Evaporation will be there at all temperature and pressure,unsaturated surrounding environment also one of the factor increases the

evaporation rate.

BOILING

Boiling is the phenomenon takes place at boiling point of the liquid. Boiling takesplace throughout the liquid column. A liquid will boil, when its saturated vapourpressure exceeds the surrounding environment pressure acted upon the liquid.Hence boiling point of a liquid will change depends on the pressure exerted by theenvironment over the surface.


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CONDENSATION

Condensation is the change in phase of vapour phase to its liquid phase. Whenwater vapour or steam comes in contact with cooler surfaces, it gives up the heatand condenses to water. The heat released while changing from vapour phase toliquid phase is called heat of condensation. In factories the steam released out ofthe main steam line or process vents where we can see a remarkable phenomenonof indication of dryness of steam. If the steam is dry, we can not visualize the steamcoming out of the vent but after some distance we can see a white cloud. This isdue to the condensation of steam which composed of small particles of water formedwhen steam cooled in cooler atmosphere. In other case if the steam is wet, thewhite smoke cloud is directly released from the vents.

2.4 FUEL

Combustion

Combustion or burning, is a rapid combination of oxygen with a fuel resulting inrelease of heat. The oxygen comes from the air, which is about 21% oxygen and78% nitrogen by volume.

Most fuels contain carbon, hydrogen, and sometimes sulphur as the basiccomposition of combustion materials. These three constituents reacts with oxygento produce carbon-di-oxide, water vapour, suphur di oxides gases respectively andheat.

Carbon, hydrogen and sulphur are found exists in direct form in most of the solid and

liquid fuels and in gaseous fuels the combustion matter is found ashydrocarbons(combination of hydrogen and carbon). When these burn, the finalproducts are carbon di oxide and water vapour unless there is a shortage of oxygen,in which case the products may contain carbon mono oxide, unburnt hydrocarbons,and free carbon.

Heat value of fuel

Quantities of heat are measured in BTU, kiloCalories, or joules. A BTU is thequantity of heat required to raise the temperature of one pound of water one degreefahrenheit. A kilocalorie is the quantity of heat needed to raise one kilogram of waterone degree celsius.

Experimental measurements have been made to determine the heat released byperfect combustion of various fuels. The heat value is usually determined bycalorimeters. When a perfect mixture of a fuel and air originally at 15.6C is ignitedand then cooled to 15.6C the total heat released is termed the higher heating valueor Gross calorific value. There is also one more term called lower heating value orthe net calorific value it is the quantity of heat equal to gross calorific value minus theheat absorbed by the latent heat of water moisture( inclusive of moisture generateddue to combustion of hydrogen present in the fuel) at 25C.


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Dulongs formula is used to find Calorific value of the fuel

HHV(kj/kg) =338.21C% +1442.43(H-O/8)% + 94.18S%

Relation between HHV and LHV

LHV = HHV (%H2O + %H2x8.94)Where is the latent heat of water vapour at reference temperature 25C=583.2 kcal/kg

Proximate Analysis

The general procedure for the analysis relating to proximate analysis is describebelow as per IS 1350(partI). For full details, the original standard may be referred to

i) Moisture

The moisture in the coal is determined by drying the known weight of the coal at108C2C

ii) Volatile matterThe method for the determination of VM consists of heating a weighted quantity of

dried sample of coal at a temperature of 90010C. for a period of seven minutes.Oxidation has to be avoided as far as possible. VM is the loss in weight less by thatdue to moisture. VM is the portion of the coal which, when heated in the absense ofair under prescribed conditions, is liberated as gases and vapour.

iii) AshIn this determination, the coal sample is heated in air up to to 500C for minutes from500 to 815C for a further 30 to 60 minutes and maintained at this temperature until

the sample weight becomes constant.

iv) Fixed carbonFixed carbon is determined by deducting the moisture. VM and ash from 100

Ultimate analysis

The ultimate analysis of fuel gives the constituent elements namely carbon,hydrogen,nitrogen, sulphur , hydrocarbons, nitrogen etc., For the ultimate analysisof the coal sample is burnt in a current of oxygen. As a result the carbon, hydrogen,sulphur oxidized to water, carbon di oxide and sulphur di oxide respectively. Theseconstituent are absorbed solvents to estimate the percentage of C,H2,S,N etc.,

The classification of Indian coal on the basis of proximate analysis.S.n Description Grade Specification

1 Non coking coal, produced A GCV exceeding 6200kcal/kgin all states other than Assam B GCV exceeding 5600Kcal/kg butAndhrapradesh,Meghalaya, not exceeding 6200Kcal/kgArunachalpradesh and Nagland C GCV exceeding 4940kcal/kg

not exceeding 5600Kcal/kg


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D GCV exceeding 4200kcal/kg

not exceeding 4940Kcal/kgE GCV exceeding 3360kcal/kgnot exceeding 4200Kcal/kg

F GCV exceeding 2400kcal/kgnot exceeding 3360Kcal/kg

G GCV exceeding 1300kcal/kgnot exceeding 2400Kcal/kg

2 Non coking coal, producedAssam,Andhrapradesh,Meghalaya, Not gradedArunachalpradesh and Nagland

3. Coking coal Steel GrI Ash content not exceeding 15%Steel GrII Ash content 15% to 18%

Washery GrI Ash content 18% to 21%Washery GrII Ash content 21% to 24%Washery GrIII Ash content 24% to 28%

2.5 GAS and AIR

IDEAL GAS OR PERFECT GAS

At low pressure and high temperature, all gases have been found to obey threesimple laws. These laws relate the volume of gas to the pressure and temperature.

All gases, which obey these laws, are called ideal gases or perfect gases. Theselaws are called ideal gas laws. These laws are applicable to gases, which do notundergo changes in chemical complexity, when the temperature or pressure isvaried. I.e., in other words laws applicable to gases which do not undergo anychemical reaction when subject to change in pressure or temperature.

GAS LAWS

Boyles lawBoyles law states that the pressure is inversely proportional to volume and theproduct of pressure and volume is constant

PV =CCharles law-I

Charles law states that at constant pressure, volume is directly proportional totemperature.

V/T = C

Charles law-IICharles law states that at constant volume, pressure is directly proportional totemperature.

P/T = C


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Absolute scale of temperature

This scale of temperature is based on Charles law. According to Charles law atconstant pressure, volume of given mass changes by 1/273 of its volume at 0C forevery rise or fall in temperature by 1C. if the volume of the gas at 0C is Vo and itsvolume at tC,

Vt = Vo + Vo x t = Vo (1 + t/273)273

If t = -273C, then volume is zero, the hypothetical temperature of 273C at whichgas will have zero volume is known as absolute temperature or 0K.

Avagadras Law

Avagadra s law state that the volume occupied by any gas at normal temperatureand pressure is 22.41383 x 10-3 m3 per mol of gas. I.e., volume occupied by a kg molof gas is 22.41383 m

3/kg mol.

GAS EQUATION

From Boyles law PV = nRoT

Where, Ro is UNIVERSAL GAS CONSTANT

n = m/M = Weight of gas in kg at NTP

Molecular weight of the gas in kg

At normal temperature and pressure

Pressure = 1.01325 x 105

N/mTemperature = 273 KVolume = 22.41383 x 10

-3m

3

n = 1 mole

Ro= PV/nT = 1.01325 x 105

x22.41383 x10-3

/(1 x273) = 8.314 Nm mol-1

K-1

= 8.314 joules /mol K

Gas constant R = Universal gas constant (Ro) / molecular weight (M).

Daltans law

At a constant temperature, the total pressure exerted by a mixture of non- reactinggases is equal to the sum of the partial pressure of each component gases of themixture. Thus the total pressure P of a mixture of r gases may be representedmathematically as


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r

Pt = pI where pi is the partial pressure of each components gas of the mixture.i =1

If P and the molar composition (% volume) of the mixture are known p i can becalculated using the expression pi = xi P

2.6 SOME COMMONLY USED DIMENSIONLESS NUMBERS ANDTHEIR SIGNIFICANCE

NUMBER FORMULA SYMBOL DEFINITION & SIGNIFICANCE

Nusselt hd/k Nu Radio of temperature gradients byconduction and convection at thesurface-used for convection heat transfercoefficient determination

Reynolds vd/ Re Inertia force/viscous force- used for forced convection andfriction factor

Prandtl Cp/k Pr Molecular diffusivity of momentumMolecular diffusivity of heat

Grashof d3 gT/ Gr Buoyancy force x Inertia forceViscous force x viscous force- used for natural convection

Biot hd/ks Bi Internal conduction resistanceSurface convection resistance- used for fin temperature estimation

Peclet vdCp/k Pe=RePr Heat transfer by convectionHeat transfer by conduction

Stanton h/Cpv St=Nu/Pe Wall heat transfer rateHeat transfer by convection

Euler P/v Eu Pressure force/Inertia force- used to find pressure drop

Froude v/gl Fr Inertia force/gravity force


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Where v is velocity

d is characteristic dimensionCp is specific heat

is densityg is acceleration due to gravity

h is convection heat transfer coefficient

is dynamic viscosity is volumetric expansion coefficientT is temperatureP is pressure

Ex.01. Estimate the air and flue gas produced per kg of the following coal analysis.

Ultimate analysis: Carbon = 39.9%, Hydrogen = 2.48% , Sulphur = 0.38 %, Nitrogen= 0.67%, Oxygen = 6.76 %, Moisture =8% and Ash = 42%. The analysis is based

on weight basis. Consider 4% carbon loss in combustion of AFBC system.

AIR REQUIREMENT CALCULATION

Amount of oxygen required for burning coal

C + O2 CO2 + heat

12 kg of carbon react with 32 kg of oxygen to produce 44 kg of carbon di oxide. I.e.,one kg of carbon required 32/12 = 2.666 kg of oxygen and produce 44/12 = 3.666kgof carbon dioxide.

0.399kg of carbon in coal require = 0.39x2.666 = 1.064 kg of oxygen

H2 + 1/2O2 H2O + heat

2 kg of hydrogen react with 16 kg of oxygen to produce 18 kg of moisture. I.e., onekg of hydrogen requires 16/2 = 8 kg of oxygen and produce 18/2 = 9 kg of moisture.

0.0248 kg of hydrogen in coal requires = 0.0248x8 = 0.1984 kg of oxygen

S + O2 SO2 + heat

32 kg of sulphur require 32 kg of oxygen to produce 64 kg of sulphur di oxide. I.e.,one kg of sulphur require one kg of oxygen and produce 64/32 = 2 kg of sulphur di

oxide.

0.0038 kg of sulphur in coal require =0.0038 x 1 = 0.0038 kg

the other composition like nitrogen, argon(if present) is inert gas and it will not reactwith oxygen. Moisture is in saturated form and it does not require oxygen.

The total oxygen required = 1.064 + 0.1984 +0.0038 = 1.2662 kg


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The oxygen present in fuel = 0.0676 kg

Net oxygen required = 1.2662 0.0676 = 1.1986 kg

Air contains 23.15 % oxygen by weight and hence the air required for 1.1986 kg ofoxygen is = 1.1986/0.2315 = 5.176 kg of dry air.

Amount of wet air required considering 60% Relative humidity = 5.176 x 1.013 =5.244 kg.

Coal requires 20% excess air for combustion in AFBC system hence wet air requiredfor burning per kg of fuel = 5.244 x 1.2 = 6.292 kg.

FLUE GAS GENERATION ESTIMATION

Carbon di oxide produced = (0.399 0.0188) x 3.666 = 1.3915 kgMoisture produced = (0.0248 x 9 ) = 0.2232 kg.Moisture in fuel = 0.08 kg.Moisture in air = 0.013 x 6.212 = 0.0807 kg.

Total moisture in flue gas = 0.3839 kg

Sulphur di oxide produced = 0.0038 x 2 = 0.0076 kg.

Nitrogen in air = 6.212 x 0.7685 = 4.7739 kg.Nitrogen in fuel = 0.0067 kg.

Total nitrogen in the fuel = 4.7739 + 0.0067 = 4.7806 kg.

Excess oxygen in gas = (6.212 5.176)x0.2315 = 0.2398 kg.

Total Flue gas produced

Per kg of fuel = 1.391 + 0.3839 + 0.0076 + 4.7806 + 0.2398 = 6.803 kg.

Ex.02 Find the weight of water present in atmospheric air at 60% relative humidityand temperature 40C.

For 40C, the saturation pressure of water is = 0.075226 atm (from steam tables)

At 60% RH the partial pressure of water vapour is 0.6 x 0.075226

=0.045135 atm

Weight of moisture present in air = 0.622 x Pw/(1.035 Pw)

= 0.622 x 0.045135(1.035 0.045135)

= 0.02836 kg/kg.


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Ex03. Estimate the efficiency of a boiler firing with coal as a fuel having GCV of

3200 kcal/kg. Furnace is Fluidized bed boiler. Apply ASME PTC 4.1 indirect method

to calculate the efficiency. Flue gas temperature leaving the boiler is140C andambient air temperature is 40C. Ash content of the fuel is 42.3% and 20% of totalash is collected in bed and 80% ash is carried in fly ash. As per lab report the loss onignition of ash samples collected in bed zone and fly ash zone is 0.1% by weight and4.4%by weight. The boiler is operating at 20% Excess air and the dry kg/kg of gasproduced =5.91 and dry kg/kg of air required = 5.696. The moisture and hydrogenpresent in the fuel is 6% and 2.7% respectively.

Basically following are the losses present in boiler,1.0 Unburnt carbon loss2.0 Sensible heat loss through ash3.0 Moisture loss due to air4.0 Moisture and combustion of hydrogen in fuel

5.0 Dry flue gas loss6.0 Radiation loss.

Unburnt Carbon loss =4%

Sensible heat loss in ash,

Flyash = %Flyash x% of ash qty x sp.heat (Tgo Tamb) x100/GCV

= 0.8 x 0.423 x0.22(140-40) 100/3200=0.233%

Bed ash

= 0.2x0.423x0.22(900-40)100/3200=0.5%

Sensible heat loss due to ash = 0.233+ 0.5 =0.733%

Heat loss due to moisture in air

= kg/kg of moist in air x kg/kg of dry air( Enthalpy of steam at Tgo in 0.013ata Enthalpy of steam at Tamb in 0.013 ata)

= 0.013 x 5.696 x( 660.33615.25)100/3200=0.1043%

Note: The above implies that the water vapour at ambient temperature at partialpressure exists in steam form and gets superheated at 140C

Heat loss due to moisture in fuel and combustion of hydrogen,

=(%of moisture in fuel + % of hydrogen x8.94)(Enthalpy of steam Tamb)100/3200

= (0.06 + 0.027x8.94)(658.37 40)100/3200


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= 5.824%

Note: The above implies that the water moisture present in fuel is in liquid form,during combustion it will absorb latent heat and superheat from combustion. Thehydrogen present in the fuel react with oxygen to form water. From combustionequation of hydrogen it is found that 1 kg of hydrogen form 8.94 kg of water.

Dry flue gas loss,

= kg/kg of dry flue gas x (Enthalpy of gas at Tgo Air enthalpy at Tamb)x100/3200

=Kg/kg of dry flue gas x Spheat (Tgo Tamb)100/3200

=5.91 x 0.24 x(140 40)100/3200 = 4.433%

Radiation loss,

From ABMA Chart the loss is estimated as =0.5%

Note: In the indirect method Blow down losses will not be considered into account. Itis assumed the boiler is operated under zero present blow down.

Ex07 Estimate the FD and ID fan flow and power required for a bagasse fired

dumping grate boiler, whose bagasse consumption at 100% MCR capacity is 31000kg/hr and the boiler is operating at 35% excess air. The fuel air requirement is 3.909

kg/kg of fuel and gas generation is 4.873 kg/kg.

FD fanTotal air requirement = 31000 x 3.909 = 121179 kg/hr.

Fan design flow with 15% margin = 121179 x 1.15/(3600 x1.128)

= 34.31 m3/secFD fan head

Pressure head required for air flow sections like airheater, air ducts and grate are tobe calculated. Now in most of the practical applications the pressure drop works outto be 165 mm WC and the same can be assumed for this calculation.

FD fan head with margin = 165 x 1.2 = 200mmWc

FD fan power required.

= flow x head/102 x efficiency

= 34. 31 x 200 / (102 x 0.8)


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= 84.09 KW

Motor selected = 84.09 x 1.1 = 92.5 KW (next nearest motor standard is 110 KW)

ID fanTotal gas produced = 31000 x 4.873 = 151063 kg/hr.

Fan design flow with 25% margin = 151063 x 1.25 x (273 +140)/(3600 x1.295x273)

= 61.27 m3/sec

ID fan headPressure head required for gas flow sections like Furnace, Bank, Economiser, airheater, gas ducts and dust collectors are to be calculated. Now in most of thepractical applications the pressure drop works out to be 230 mm WC and the samecan be assumed for this calculation.

ID fan head with margin = 230 x 1.3 = 300mmWc

ID fan power required.

= flow x head/102 x efficiency

= 61.27 x 300 / (102 x 0.8)

= 225 KW

Motor selected = 225 x 1.1 = 247.7 KW (next nearest motor standard is 250 KW)

Table showing percentage margin on flow and head required for different boilerapplication.S.N Description Grate type AFBC CFBC OIL

fired

1 FD Fan FlowHead

15%20%

25%25%

25%25%

15%20%

2 ID Fan FlowHead

25%30%

25%25%

25%25%

20%20%

3 SA/PA/OF fan FlowHead

10%15%

25%25%

25%25%

Notapplicable

3.0 FURNACE

3.1 INTRODUCTION:

The design of furnace is considered as the vital part in the boiler. The furnace is thezone experiencing a high temperature in boiler. The performance of the furnacereflects or has an impact over other parts behind it such as super heater, evaporator,and air heaters. For instant, how the furnace design affects super heater can be


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illustrated with following. If furnace outlet temperature (FOT) is high, then the nextzone is super heater it gets high amount of heat input naturally the metal

temperature is high and the steam temperature also increased, which in turn reflectsin the performance and cost of material. On the other hand if the furnace is oversized the FOT will be lesser, to get the required steam temperature the super heaterheat transfer area to be increased. If the heat transfer area is increased it calls forlarger space and cost wise it becomes uneconomical.

3.2 EFFECT OF FUEL ON FURNACE DESIGN:

The type of fuel, form of fuel, heat content and the properties of the fuel such as ashfusion temperature are also form as constraint over the furnace design. The type offuel whether solid or liquid or gas and quantity decides how efficiently we can burn.Whether we can have a burner (for liquid & gases), solids bubbling bed or dumpingor travelling grate. When the fuel is some thing like bagasse (fibrous and long strand

structure) it can be burnt well in dumping or travelling grate.

A gaseous fuel offers fewer problems since it is clean. Fuel oil brings its ownproblems like high or low temperature corrosion and additives have to be used. Forcoal ash fusion is the problem, since ash slag down deposits on the wall hinderingheat transfer to steam water mixture. Depends on property of coal, whether it can becrushable to powdered form, pulverized firing or bubbling bed or cyclone furnace canbe decided.

When we go for oil or gas firing, we can have higher heat flux in the furnace becauseof the higher emissivity of oil flame and relative cleanliness of walls compared to coalfiring. There by size of furnace will be smaller for oil or gas fired steam generators.The volume of the furnace for oil fired boilers will be 60 to 65 percentage of

pulverized fuel firing. However, if a furnace designed for both coal and oil it isnormally designed for coal and performance for oil firing in that furnace will becarried out. When a furnace designed for coal operated with oil, the higher furnaceabsorption results in a lower furnace outlet temperature. Lower FOT means superheater pick up in super heater will be less and steam outlet temperature will be less.This is avoided by several techniques out of which, when oil is fired FOT will beincreased by gas recirculation, otherwise when coal is fired FOT will be reduced bysome means of bed absorption (This is used in FLUIDISED BED COMBUSTIONtechniques). Furnace size also governed by length of flame in gas or oil fired boilersince the flame should not impinge on the water walls and cause overheating.Likewise in coal fired boilers flue gas velocity should be optimized to prevent higherrate of erosion due to carry over particles in flue gas. Normally a flue gas velocity of6 to 8 meters per sec was allowed for coal fired boilers and 12 to 15 meters per sec

was allowed for bagasse fired boilers.

3.3 FORCED OR NATURAL CIRCULATION:

Water wall is receiving radiation from flames and are exposed to high heat flux andthere is a possibility of over heating. The boiling is the phenomenon, which governsthe rate of heat transfer from combustion to steam water mixture inside the tube. Inboiling when bubbles formed at tube wall hinders the heat transfer which cause


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tubes over heating and tube failure. This sort of boiling occurs at nucleate boilingstage. Therefore proper circulation must be ensured to cool all tube. Circulation

ratio (CR) is the ratio between mass of water circulated inside the boiler to rate ofsteam generation. Hence CR is also directly related to dryness fraction of steam bythe expression CR = 1/x. which implies in one circulation 1/CR quantity of dry steamwas produced. Circulation number will be higher when the difference in densitybetween steam and water is more (i.e.) due to higher difference in density; steamwater mixture velocity will be more thereby overheating will be prevented. If theproper circulation is not there, circulation in the boiler circuit is effected by means ofexternal agency (normally a circulation pump will be used). This type of circulation iscalled Forced or controlled circulation.

3.4 HEATFLUX TO FURNACE WALLS:

Boiling phenomenon can be represented by a log-log plot of heat flux Vs surface

temp-bulk temperature as shown

Q max.

HEAT

FLU

X

A B C D

SURFACE TEMP

The different regimes of boiling indicated by the letters A, B, C, D. Absence of

bubble formation and the influence of natural convection on the heat transfer processis predominant in the region A (pool boiling). Formation of vapour bubbles at thenuclei with resulting agitation of liquid by the bubble characteristics at the region B(nucleate boiling). The most important perhaps the critical region with respect to theheat flux is C. In this region the unstable film boiling manifests with an eventualtransition to a continuous vapour film. In the final region D film boiling becomesstabilized. This phenomenon of stable film boiling is referred as LEINDENFROSTEFFECT


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In the regime of boiling the maximum wall heat flux is observed in region C. Many

experimentalists refer this state of maximum wall heat flux as BURN OUT FLUX.The reason being when the wall is heated electrically, the heating element frequentlyburn out when the wall heat flux reaches Q maximum. Hence the design engineersshould have an idea of average heat flux to the tubes, how they vary aroundperiphery and fin tip temperature in case of membrane wall construction. Calculationof fin temperature was discussed in latter part of this chapter.

3.5 POINTS TO BE NOTED WHILE DESIGNING FURNACE

1.0 Optimal heat transfer area to reduce the gas temperature to a temperaturerequired from the point of super heater.

2.0 Sufficient height to ensure adequate circulation in the water walls

3.0 Fins in the wall to be properly cooled, accordingly the pitch of water wall to beselected.

4.0 Flames should not impinge on water wall

5.0 Proper provision should be there to remove ash generated.

6.0 Optimal furnace outlet temperature.

7.0 Sufficient residence time inside the furnace for complete combustion

3.6 CLASSIFICATION OF FURNACE

i) According to ash removal

a) Dry bottom: It consists of water walls or refractory walls enclosing theflame. Ash shall be removed dry from bottom. The fuel used has low heatflux and high ash fusion temperature.

b) Wet bottom: Ash removed from bottom is of molten form. The fuel havinghigh heat flux low ash fusion temperature is used. The flue gas generatedhere or clean and free from fly ash and hence erosion, fouling problems areminimized.

ii) According to Type of combustion

a)Conventional fi ring1) Travelling grate2) Dumping grate3) Pulsating grate4) Step grate5) Fixed grate


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b)Bubbling Fluidized bed combustion

c)Circulated Fluidized bed combustion

d)Pulverized fuel combustion

e) Cyclone furnace.

iii) According to draft system

a) Balance draft: In balanced draft both Forced draft and Induced draft fansare used so to maintain vacuum or zero pressure in furnace. There is noleakage of combustion product in the atmosphere. In the atmosphericpressure air leaks into furnace. This type of draft system is widely adapted in

industries.

b) Forced draft or pressurized draft: Considering economic aspect in oil or gasfired boilers Forced draft fan alone used. The furnace pressure will be of theorder of 100 to 150 mm a water column. The furnace has to be designed towithout leakage. Otherwise combustion product will leak into atmosphere.

c) Induced draft: Induced draft fan is used for sucking the flue gas generated.The furnace pressure will be maintained below atmospheric pressure.

d) Natural draft: There is no draft fan will be provided for this system. Naturaldraft generated due to chimney itself used for the boiler draft. Very smallcapacity steam generators will be of this type.

3.7 MODES OF HEAT TRANSFER

In general heat transfer from higher temperature to lower temperature is carried outin three modes.

1.0 Conduction2.0 Convection3.0 Radiation

Conduct ion

Conduction refers to the transfer of heat between two bodies or two parts of thesame body through molecules, which are more or less stationary. Fourier law of

heat conduction states rate of heat flux is linearly proportional to temperaturegradient.

Q = --K dt/dx


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Where,Q rate of heat flux watts per sq.meter

K thermal conductivity (property of material)W/mkdt/dx temperature gradient in x directionNegative sign indicates heat flows f rom high temperature to low temperature.

Heat transfer by conduction in plate and cylinder

Plate Q = k.A. (t1 - t2) watts

X

Cylinder Q =k.(A2-A1).(t1

-t2)

(r2- r1) ln(A2/A1)

where,A area of plateA1 outside cylinder surfaceA2 inside cylinder surfacer cylinder radiust temperature of surfaces

Convect ion

Convection is a process involving mass movement of fluids. When a temperaturedifference produces a density difference which results in a mass movement.Newtons law of cooling governs convection. In convection there is always a filmimmediately adjacent to wall where temperature varies.

- kfA (tf - tw)Q =

Where, is film thicknesskf thermal conductivity of filmh = kf/ heat transfer coefficient (kcal/ sq.m hr C or W/sq.m C)

Radiation

All bodies radiate heat. This phenomenon is identical to emission of light. Radiationrequires no medium between two bodies, irrespective of temperature the radiationheat transfer takes place between each other. However the cooler body will receivemore heat then hot body. The rate at which energy is radiated by a black body attemperature T( K) is given by Stefan Boltzmann law.

Q = A T4


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Q rate of energy radiation in Watts

A Surface area radiating heat sq.m

Stefan boltzmann constant = 5.67 x 108

Watt/sq.m K4

4.88 x 108

Kcal/sq.m hr K4

3.8 HEAT TRANSFER IN FURNACE

Furnace heat transfer is a complex phenomenon, which can not be calculated by asingle formula. It is the combination of above said three modes of heat transfer.However in a boiler furnace heat transfer is predominantly due to radiation, partlydue to luminous part of the flame and partly due to non-luminous gases. Overall

heat transfer coefficient in furnace is governed by three Ts temperature, turbulenceand time and calculated by two parts.

Hc - heat transfer coefficient by convectionHr - heat transfer coefficient by radiation.

HEAT TRANSFER COEFFICIENT BY CONVECTION (Hc)

Heat transfer by convection may carry out in turbulent or laminar flow of the fluid. Inforced convection turbulence or laminar flow depends on mean velocity,characteristic length L, density and viscosity. These variables are grouped togetherin a dimensionless parameter called Reynolds number. Reynolds number is theratio between inertia force to viscous force.

Reynolds number = (mass x acceleration)/(shear stress x cross sectional area)

Mass = volume x densityAcceleration = velocity / time

Volume = cross sectional area x velocityShear stress = dynamic viscosity x velocity gradient(v / l)

Re = density x velocity x characteristic lengthDynamic viscosity.

When Re > 2100 then flow is turbulence< 2100 then flow is laminar. In practical case the flow is most often

turbulent only.In free convection turbulence or laminar flow depends on the buoyancy force andtemperature difference, coefficient of volume of expansion. These variables aregrouped to form dimensionless numbers called Grashoff number and Prandl number.Laminar or turbulence is identified with product of Grashoff number and prandlnumber

When, Gr.Pr < 109

flow is laminar


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Gr.Pr > 109

flow is turbulent.

DIMENSIONAL ANALYSIS FOR HEAT TRANSFER COEFFICIENT

The heat transfer coefficient may be evaluated from correlation developed bydimensional analysis. In this method all the variables related to the phenomenon isgrouped by experience with help of basic fundamental units length, mass, time andtemperature.

The final equation arrived for

FORCED CONVECTION

h = f(L,U, ,,k,Cp) ,where,L characteristic length (meters)U velocity (meters/second)

density (kilogram/ cub.meter) dynamic viscosity(kilogram/meter. Hour)k thermal conductivity (watts/meterkelvin)Cp specific heat(watt/kilogram.kelvin)

Let h = B La

Ub

c

d

ke

Cpf

, where B,a,b,c,d,e,f are constants

Expressing the variables in terms of their dimensions

MT-3

-1= B L

a.(LT

-1)b.(ML

-3)c.(ML

-1T

-1)d.(MLT

-3

-1)e.(L T

-2

-1)f

= B.L a+b-3c-d+e+2f. Tb-d-3e-2f. M c+d+e. -e-f

0 = a + b 3c d +e +2f-3 = -b d 3e 2f1 = c + d + e

-1 = -e - f

The solution of the equation gives,

a = c-1, b =c, d = -c +f, e = 1-f

h = B. Lc-1

.Uc. c. -c+f.k -1-f.Cp f

by grouping the variables,

h/L-1

k = B.(UL / )c. (. Cp /k)

f

Nussultes number = B.(Reynolds number)c.(Prandl number)

f

The constants B,c,f are evaluated from experimental data.


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For turbulent flow inside tubes and fully developed flow the following equationattributed to Mr.Dittus and Boelter,

Nu = 0.023 Re0.8

Prn

where, n = 0.4 when the fluid is heated

n = 0.3 when the fluid is cooled.

For turbulent flow outside tubes

Nu = 0.037 Re0.8

Prn

where, n = 0.4 when the fluid is heated

n = 0.3 when the fluid is cooledFREE CONVECTION

Free convection depends on buoyancy force F, which is defined by,

Let a fluid at To with density o change to temperature T with density then,

F = (o )g/P = ((o/) 1)gNow, coefficient of volume expansion

then, 1/ = (1/o) + (To-T),o = (1 + T)

(o/ ) 1 = T

F = g T

For an ideal gas is inversely proportional to temperature,(i.e. dimensional numberfor is -1 and F is -1 * LT-2 ie LT-2)

By dimensional analysis,h = B.(Fa.Cpb.Lc. d.e.kf)

MT-3-1 = B[ (LT-2)a.(L2 T-2-1)b. Lc.(ML-3)d.(ML-1T-1)e.(MLT-3-1)f]

1 = d + e+ f= a + 2b + c 3d e + f-3 = -2a 2b-e-3f-1 = -b-f

solving this equation.c = 3a 1,d = 2a , e = b 2a, f = 1- b

h = B[ (g T)a . Cpb. L 3a-1. 2a. b-2a. k1-b)]

h = B[ (g TL3 2/ )a . (.Cp/k)b] (k/L)

hL/k = B. Gra. Pr

b.


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Nu = B. Gra. Pr

b

By large number of experiments made on fluids it has been found that exponents a

and b are of the same value. So the expression reduce to Nu = B.(Gr.Pr)a

HEAT TRANSFER BY RADIATION Hr

In furnace heat transfer is predominant by luminous and non-luminous radiation. Ageneral approximate expression may be written for furnace absorption using Stefanboltzman law.

Q = A w [g Tg4

g TS4]

g = c c + ww -

emissivity pattern of tri atomic gases such as carbon di oxide and water vapour arestudied by Mr. Hottel and charts are available to predict gas emissivity as a functionof various gas temperature, partial pressure and beam length. I have also furnishedthe expression form to find gas emissivity. When c and w are found from graphc and w can be determined from the following expression or from graph.Otherwise emissivity of gas can be directly found by the expression given inequation1.

0.222 1 1c = EXP _______________ - Pc *L +0.035 ln2.8 ln(p + 1.8)

1/30.23 1 2w = EXP 0.842 -

(0.23 +Pw*L 0.75 0.5+Pw+p

where p is gas pressure in bar(a)L is beam length meter

w and c are pressure correction factor for gas pressure

absorptive of gasses can be determined at wall temperature.

g = cc + w w -

At wall temperature correction,

Pcw = Pc (Tw/Tg) Pww = Pw(Tw/Tg)

c = cw (Tg/Tw)0.65 w = ww (Tg/Tw)

0.45


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cw is a function of Pcw .L and wall temperature for this we have to see the emissivityin graph

ww is a function of Pww .L and wall temperature for this we have to see the emissivityin graph

pressure correction is same as gas emissivity factor. = w = function of Pw/(Pc + Pw) , Pcw.L + Pww.L, and temperature of wall

The effect of absorptivty is negligible hence the same can be neglected and ageneralized form of Q = A wg [Tg

4TS

4] can be used.

Heat absorption by energy balance method,

Q = [ Wf. lower heat value Wg .gas exit enthalpy]

Where,A effective projected area of heat transfer including wall openingw wall emissivityg gas emissivity Stefan boltzman constantTg Flue gas temperature of mean theoretical flame temperature(adiabatictemperature)TS Furnace wall temperature (If calculated for outside heat transfer coefficient orconsider saturation temperature if calculated for over all heat transfer coefficient, thedifference will be of very minor).WfFuel burntWg Flue gas produced

Gas emissivity g = 0.9( 1- ek.L

)1

The emissivity of flame is evaluated by

f = ( 1- ek.L

)

where is the characteristic flame filling volume.

= 1.0 for non luminous flame(practical 0.9) of solid fuels.0.90 for luminous and semi luminous flame of coal .lignite & husk(AFBC )0.85 for luminous and semi luminous flame of bagasse (conventional firing)0.72 for luminous and semi luminous sooty flame of liquid fuels0.62 for luminous and semi luminous flames of refinery gas fuel OR gas/oil

mixture0.50 for luminous and semi luminous flames of natural gas

L beam length meters = 3.4* volume/surface area.For cuboid furnace chamber and bundle of tubes.

K attenuation factor, which depends on fuel type and presence of ash and itsconcentration. For non-luminous flame


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K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + Pw)

_______ (Pc +Pw)L

For semi luminous flame, the ash particle size and concentration is taken incalculation

K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + Pw)________ + 7(1/dmTM)

1/2

(Pc +Pw)L

dm mean effective diameter of ash particle in microndm 13 for coal ground in ball mills

16 for coal ground in medium or high speed mill

20 for coal milled in hammer mill.

- ash concentration in gm/Nm^3

TM furnace mean temperature k(Some authors will consider this as outlettemperature, but it is convincing assumption that in furnace zone temperature will beuniformly spread through out the furnace by radiation effect (spherical). Henceconsidering mean temperature for calculating radiation heat transfer coefficient willbe more appropriate. You can appreciate a notable phenomenon of furnacetemperature depends on flame location inside the furnace, in case flame is located atthe center of furnace(like oil fired burners (refer example1)) mean temperature andoutlet temperature will be at the most equal and if flame is located at one end of thefurnace and radiation beam travels a larger distance of furnace(like AFBC boilers

assuming no free board combustion) the furnace temperature near flame will behigher and it gradually degrees at the furnace exit.

For luminous oil or gas flame

K = (1.6 TM/1000) 0.5

Pw and Pc are partial pressure of water vapour and carbon di oxide

Above equations give only Theoretical values for flame emissivity. In practical casesa wide variation would be occurred due to:

1.0 Combustion phenomenon itself2.0 The flame does not fill the furnace fully. Unfilled portion are subject to only gas

radiation


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3.0 The emissivity of radiation is far below the flame emissivity. Emissivity of gasradiation may be in the range 0.15 to 0.3. Therefore overall emissivity of flame

reduces. Hence emissivity changes with respect to location.

Due to the above fact I have tried to give the practical values and graphs for theemissivity at appropriate places for AFBC, Dumping grate and fired boilers withworking of example.

The heat transfer by radiation is given as Q = A wg [ TM4

TS4]. But mostly the

heat transfer will be of both convection and radiation occuring simultaneously and soto put both process on a common basis, we may define a radiation heat transfercoefficient by symbol Hr.

Qr = Hr. A. (TM TS)

Hr = wg[TM4

-TS4

]/(TM-TS)

While considering the total heat transfer by convection and radiation

Q = (Hc + Hr) A (TMTS) for fired furnace where gas throughout furnace is same.

Q = (Hc + Hr) A Lmtd for AFBC and Radiation chambers.

By this equations we can get theoretical Hr value but in practice these values arecorrected by effectiveness factor. This depends on various manufacturersexperience on their steam generator.(Normally for oil fired boilers the value will be of0.79 and gas fired boiler 0.67).

3.9 FURNACE CONSTRUCTION :

Basically three types of constructions are used1.0 Plain tube construction with a refractory lined furnace2.0 Tangent tube construction3.0 Membrane wall construction.

Plain tube construction

FURNACE CHAMBER


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REFRACTORY

The drawing shown gives complete idea of the above construction. Refractory linedwall construction is out dated design since it calls for a lot of refractory work and fluegas leaks are heavy and it can not with stand positive furnace pressure.

Tangent tube construction

FURNACE CHAMBER

REFRACTORY

Tangent tube is a improvement of refractory lined. Here requirement of boiler tubesis comparatively more and also refractory structure is not eliminated.

Membrane wall construction

In industries widely used boiler furnace construction is of membrane wall

construction type. In this design the tubes are joined by welding a continuouslongitudinal strip forming a solid panel, which can be as large as transportable.Panels can be welded together on site to form the furnace. The gap between thetubes(pitch) are maintained in a such a way that the fin can be cooled by either ofthe two side tubes and prevent warping of the panel. Water cooled furnaces not onlyeliminated problem of rapid deterioration of refractory walls due to slag, but alsoreduced fouling of convection heating surfaces to manageable extent, by loweringthe temperatures leaving the furnace. In addition to reducing furnace maintenanceand fouling of convection heating surfaces, water cooling also helped to generatemore steam. Consequently the boiler surface was reduced since additional steamgenerating surface was available in water cooled furnace.

Ex.1.0 . Find the furnace outlet temperature for a fluidized bed boiler operating at 15kg/cm^2(g) having furnace EPRS of 28.43 sq.m and having the following gasparameters.


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Flue gas produced 11016 kg/hr at a temperature of 900C and partial water vapourpressure 0.15 ata , partial carbon di oxide pressure 0.14 ata .

The furnace size is 2.424 x 2.828m and height of 1.75meters.

Assume FOT 740C

Flue gas properties at film temperature. (900+740 +200)/3 = 613.33C

Dynamic viscosity = 3.7392 x 105

kg/ms

Thermal conductivity = 0.065177 kcal/m hr.cPrandl number = 0.7152

Flue gas velocity at outlet = 11016 x (613.33 +273)3600 x 273 x 1.286 x 2.424 x 2.828

= 1.1269 meter/sec.

Convection heat transfer coefficient at gas side(Hc ) =(As steam side heat transfer coefficient is very high, in over all heat transfercoefficient its effect will be negligible)

Nu = 0.037 Re0.8

Prn

where n= 0.3 for cooling fluid

Hc/kL = 0.037 Re0.8

Prn

0.8

Hc = 0.037 x 0.396 x 1.1269 x 1.75 x 0.7152

0.3

x 0.06517/1.753.7392 x 10-5

= 3.56 kcal/m^2 hr.C

Radiation heat transfer coefficient (Hr)

Beam length = 3.4 x(w x d x l)/2(l.w +l.d + w.d )

Substituting w= 2.424,d = 2.828, l =1.75

L = 1.2709 m

For non luminous flame attenuation factor

K = (0.8 + 1.6x 0.15) x(1-0.00038x(820+273)) x (0.14 +0.15)_________________

(0.14 +0.15)1.2709= 0.2904


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flame emissivity f = 0.9 x (1- e0.2904 x 1.2709

)

= 0.2778

Wall emissivity w = 0.9 (practically adopted for fluidized bed boilers)

Radiation heat transfer coefficient Hr

= 4.88 x 10-8

x 0.2778 x 0.9 x [(820+273)4(200 + 273)

4]

[820 200]

= 27.1 kcal/hr m^2 K

Total heat transfer coefficient Hc + Hr = 3.56 + 27.1 =30.66 kcal/hr m^2 K

Heat transferred Qg = U A (lmtd)

= 30.66 x 28.43 x[(900 - 740)/ln(700/540)]

= 537419 kcal/hr.

Heat lost by gas QL = Wg ( Hi Ho)

= 11016 (257.3 207.45)= 549147 kcal/hr

Qg not equal to QL try with 745C.

Ex 02. Evaluate the size of bed for a 10 tph boiler, operating at 14.5 ksc, satuatedsteam from and at 100C. Coal as a fuel. The efficiency of boiler is 80% and GCV ofcoal as 3800 kcal/kg , Flue gas produced per kg of fuel is 6.802 kg/kg at 20% excessair operation.

Heat output = 10000 x 540 = 5400000 kcal/hr.

Heat input = 5400000/0.8 = 6750000 kcal/hr.

Fuel input = 6750000/3800 =1776.3 kg/hr.

Flue gas produced = 1776.3 x 6.802 = 12082.4 kg/hr.

Bed area = (Flue gas qty x bed temp)/(velocity x density of gases)

= 12082.4 x (900 +273)/(3600 x 273 x 1.295 x 2.8)

= 3.977 m^2.

Bed size arrived = 3200 x1250 mm x mm a refractory wall thickness of 370 mm canbe considered and above which water wall is located. Hence a water wall of size3584 x 1680( 35 @ 112 pitch and 15 @ 112 pitch ) can be obtained.


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The sizing of bed area and water wall size is an art rather than a scientific approach

a better configuration has to be arrived on the basis of experience.

Note: From and at 100C is the term used in boiler industry to specify the heatcapacity of boiler. This is value is assumed that water at 1kg/cm^2 100C is given asinput and steam drawn at 1kg.cm^2 .(i.e. latent heat at 1kg/cm^2 pressure onlyabsorbed )

EX 03. Find the furnace outlet temperature of a 55Tph dumping grate bagasse firedboiler operating at 42 kg/cm^2 and 420C super heater outlet at furnace exit plane.The effective projected area of furnace and superheater plane works out to be212m^2 and 13.6m^2 respectively. Consider convection heat transfer coefficientnegligible and lower heating value of bagasse 1828 kcal/kg, 85% of air requiredflows through air heater at a temperature of 170C and 15% air for fuel distributor

and OFA at 40C into the furnace. Fuel consumption 24209 kg/hr. 2% of gross heatinput goes as carbon loss and 1% goes as radiation loss.

FURNACE HEAT INPUT

1.0 Fuel heat input = 24209 x 1828 = 44.254 x 10^6 kcal/hr2.0 Air heat input = 0.85 x 24209 x 3.909 x 0.24 x 170 +

0.15 x 24209 x 3.909 x 0.24 x 40=3.418 x 10^6 kcal/hr

where,3.909 is air required for burning one kg of bagasse at 35% excess air.0.24 kcal/kgc specific heat of air.

3.0 Un burnt carbon loss = 0.02 x 24209 x2272 = 1.1 x 10^6 kcal/hr

4.0 Radiation loss = 0.01 x 24209 x2272 = 0.55 x 10^6 kcal/hr

Where 2272 kcl/kg is GCV of fuel.

NET FURNACE HEAT INPUT = 1+2 3 4

= 46.072 X 10^6 KCAL/HR

applying stefan boltzman law,Q = A wg [ TM

4 TS

4]

As it is a bagasse fired boiler volatile combustion is more TM will be equal totemperature exit and wg is equal to 0.72.

Assuming 890C as FOTSaturation temperature 263c .

Q1 = 212 x 0.72 x 4.88x10^-8 x ( 11634

5364)

= 13.01 x 10^6 kcal/hr.superheater steam outlet 420c


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Q2 = 13.6 x 0.72 x 4.88 x 10^-8 x( 11634

6934)

= 0.764 x 10^6 kcal/hr.

Total heat absorbed by surface = Q1 + Q2 = 13.77x 10^6 kcal/hr.

Heat lost by gas,

Q = ( furnace heat input gas flow x outlet enthalphy)

= (46.072 x 10^6 - 24209 x4.873 x 890 x 0.3076 )

= 13.776 x 10^6 kcal/hr.

where 4.873 is kg of flue gas produced per kg of bagasse

0.3076 kcal/kgC is specific heat of flue gas

Furnace outlet temperature = 890C

Radiation heat pick up contribution to raise steam temperature,(it is assumed that 70% of heat absorbed will go to steam temperature raise)

= 0.7 x 0.76 x10^6/55000 = 9.67 kcal/kg

Ex.04. Estimate FOT for the furnace operating at 20.66 bara, having EPRS area

112m and size 5.74 x 3 x 6 m. firing LDO as fuel having LCV of 41867 kj/kg and

fuel consumption 1.16 kg/sec and flue gas generated 19.03 kg/sec at 10% excessair. Air required 17.87 kg/sec at 27C. Consider a radiation loss 0.33% and wallemissivity 0.85, heat transfer effectiveness 0.79. adjacent radiation chamber extendsby 1.01 m length wise.

Total heat into Furnace at 27C ambient.

Heat input by fuel = 1.16 x 41867 = 48.565 MWRadiation loss = 48.565 x0.33/100 = 0.16 MW

Nett heat input = 48.405 MW

Heat absorbed by Furnace

Radiation coefficient Hr = wf[TM4-TS

4]/(TM-TS)

For oil fired boiler Tmean is equal to Tgas outletWall emissivity = 0.85Flame emissivity = 0.72 (1-e-kl)Attenuation factor k = (1.6Tm/100)-0.5


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Beam length, l = 3.4x5.74x3x6/2(5.74x3+5.74x6+3x6) = 2.52M

Assume gas outlet temperature,1285c =1558K

K =(1.6x1558/100)-0.5 = 1.9928

Flame emissivity = 0.72x(1 e1.9928 x2.52

) = 0.715

Hr = 5.67 x 10^-8 x0.85x0.715x(15584

4874)/(1558 487)

= 187.768 W/mK4

Nu = 0.037 x Re0.8 xPr0.3

Gas properties at film temperature (1285+ 214)/2 = 749.5C

Dynamic viscosity kg/m.s = 4.13276 x 10-5 kg/msThermal conductivity = 0.072915 W/mKPrandtl number = 0.711

Velocity = 19.03 /(0.345 x 3x6) = 3.064 m/s where 0.345 is density kg/m^3

0.8

Hc = 0.037 x 3.064 x5.74 x0.345 x 0.7110.3

x 0.072915

4.13276x10^-5 5.74

= 5.769 W/mK

Heat absorbed by Furnace

Q = (Hc +Hr)x effectiveness A Lmtd

=(5.769 + 187.76

steam genaration

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