This document is the preface and contents section of a book about steam generators written by Ganesh Kumar for Deutsche Babcock India. The preface states that the book aims to help design engineers and plant engineers understand boiler design and calculations by bringing together practical field design and theoretical curriculum. The contents section outlines 10 chapters that will cover topics like types of steam generators, steam and gases, furnaces, superheaters, drums, evaporators, air heaters, dust collectors, water chemistry, and boiler controls. Appendices will include charts, fuel analysis, steam tables, and other useful data.

1. Steam generator Ganesh kumar
A.GANESH KUMAR
DEUTSCHE BABCOCK, INDIA.
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 use………………………………………
1.3 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 boiler……………………………………………………………
1.8 Heat recovery steam generator………………………………………………
1.9 Practical guide lines for selection of boiler………………………………….
2.0 STEAM, GAS and AIR
2.1 Introduction……………………………………………………………………
2.2 Definitions for some commonly used terms………………………………
2.3 Steam………………………………………………………………………….
2.4 Fuel……………………………………………………………………………..
2.5 Gas and air…………………………………………………………………….
2.6 Some commonly used dimensionless numbers and their significance….
3.0 FURNACE
3.1 Introduction……………………………………………………………………
3.2 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 furnace………………………………
3.6 Classification of furnace…………………………………………………….
3.7 Modes of heat transfer in furnace…………………………………………
3.8 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 design………………………………………
4.3 Points to be noted while designing super heater…………………………
4.4 Classification of super heater……………………………………………….
4.5 Designing a super heater……………………………………………………
4.6 Overall heat transfer across bank of tubes……………………………….
4.7 Steam temperature control…………………………………………………
4.8 Pressure drop………………………………………………………………..

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5.0 DRUMS
5.1 Intruction……………………………………………………………………. Deleted: od
5.2 Optimal configuration of drums………………………………………………
5.3 Stubs and attachments in the steam drum/shell…………………………..
5.4 Maximum permissible uncompensated opening in drum…………………
5.5 Size of the drum………………………………………………………………
5.6 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 heater………………………………………………………
7.5 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 collector
Bag filter
Electro 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 treatment………………………………………………………
9.8 Practical guides for selecting water treatment plant……………………….
10.0 BOILER CONTROLS
10.1 Introduction……………………………………………………………………
10.2 Control philosophy……………………………………………………………
10.3 Drum level control…………………………………………………………….

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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 CHART
APPENDIX2 : PSYCHROMETRY CHART
APPENDIX3 : FUEL ANALYSIS
APPENDIX4 : STEAM TABLES
APPENDIX5 : POLLUTION NORMS IN VARIOUS INDIAN STATES
APPENDIX6 : USEFUL DATAS
APPENDIX7 : 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 main
power producing sources thermal plant produces 48215 MW (69%), hydro plant
produces 19300 MW (28%), nuclear plant produces 2033 MW (3%) as on 31st
March 1992. In the above power plants 72% of the generation is from thermal and
nuclear, 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 resources
suitable for power generation is in service. Except from gas turbines power the most
of 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 to
large electric power station utility boilers. In these large units pressure range from
100 kg/cm² to near critical pressures and steam is usually superheated to 550°C. In
India BHARAT HEAVY ELECTRICALS LTD (BHEL) is the pioneer in developing
the technology for combustion of high ash coal efficiently in atmospheric bubbling
fluidized bed. From where lot of industries in boiler manufacturing starts. Only after
the year 1990, India’s foreign policy was changed, various foreign steam generator
manufacture entered into Indian power market bringing various configuration and
competitiveness 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. Olden
day boiler construction were very much thicker shell plate and riveted constructions.
These boilers utilize huge amount of steel for smaller capacity. Followed this shell
and tube type boilers have been used and due to direct heating of the shell by
flames leads severe explosion causing major damages to life and property. For
safety need, after the Indian independence India framed Indian boiler regulations in
1950, similar to various other standards like ASME, BS, DIN, JIS followed world
wide. Till date IBR 1950 is governing the manufacturing and operation of boilers with
amendments then and there. Indian sugar industry uses very low pressure (15
kg/cm²) inefficient boilers during independence now developed to an operating
pressure of 65 kg/cm² and more of combined cycle power plant. If we analysis most
of the boilers erected in pre-independence period were imported boilers only and
now 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 year
1972, India’s first nuclear power plant was commissioned at Tarapore. This plant
was an pilot plant meant for both power and research work. This was made in
collaboration with then soviet republic of Russia. Now India has its own nuclear
technology 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 version
of boilers used in industry where a large flue tube was separated by a fixed grate
man 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, the
practice was merely to increase the number of boilers. This procedure eventually
proved to be uneconomical and larger maintenance. Afterwards, individual boilers
were build larger and larger size, however the size became such that furnace floor
area occupation was more. Therefore further research work have been developed in
this area and technologies such as pulverized coal fired furnace, circulated fluidized
bed furnace, pressurized circulated fluidized furnace (still under research stage)
were developed. These modern technologies have higher heat transfer coefficient in
furnace 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 the
fuel is burnt. Fuel feeding will be done manually or mechanically to have a sustained
flame. In this type burning will be done at higher excess air. Incoming air will be
used 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 construction
and arrangement. However the combustion phenomenon remains same.
Travelling grate
The travelling type is a continuous grate which slowly convey the burning fuel
through the furnace and discharge the ash to an ash pit. Grate speed is regulated
by the amount of ash discharging to ash pit ( 0 to 7m/hr)
Pulsating grate
The pulsating grate is non- continuous grate. The grate surface extends from the
rear 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 grate
Dumping grates are also a non-continuous type grate. The grate is split into
longitudinal 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 or
ash 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, the
actuation can be done either manually or pneumatic cylinder.
In dumping grate the grate sections should be designed in such a way that, while
dumping the ash part of grate surface not available for burning. In poorly designed
dumping grate there may be steam pressure. Therefore while sizing grate sections
care should be taken such that while dumping part of the grate, other fuel feeder and
remaining sections should able to take the full load.
Dumping grate is similar to fixed grates, it is best suitable for bagasse where the fuel
is of low calorific value and having high moisture content. Therefore air alone can
acts as a cooling medium. If we use coal the grate bar may not with stand higher
temperature and additional cooling by water tube is necessary. Travelling grate is
suitable for burning coal and lignite. As the grate rotates, the grate bar gets heated
and cooled by incoming air for the half of the cycle and remaining half of the cycle
grate bar cooled by the incoming air.
Spreader stoker
Mechanical spreader
The spreader stoker feeder takes fuel from the feeder hopper by either a small ram
or a rotating drum and delivers it into a spinning rotor. An adjustable trajectory plate
is located between the feed mechanism and the rotor. Adjusting the trajectory plate
fuel can be feed through out the entire length of the furnace.
Pneumatic spreader
In this rotor is replaced by high pressure air lines from Secondary air fan is used to
spread the fuel into the furnace. The fuel is carried into the furnace by means of
pneumatic system and the air flow adjustment makes the fuel to flow near or farther
of 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 the
flame will not come downwards to the incoming fuel, by this property combustion
can be controlled easily. Hence it is always better to bring the oil or gas train from
bottom of the burner.
A liquid or gas fuel has flowable property by nature and it has a lower ignition
temperature. When the fuel is forced to flow through the nozzle it will spread though
an predetermined length and burn completely from the point of entry to the firing
zone estimated. The fuel flow can be controlled by means of control valves.
CHARACTERISTICS OF OIL
In today’s climate of fluctuating international fuel prices and quality, the emphasis on
the ability of the boiler on low quality fuel oils has become more greater. In the
international market, the quality of the residual fuel oils is constantly getting poorer
due to the development of more sophisticated cracking methods and also our
indigenous crude production falls short of our requirements, about 15 million tons of
crude 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 finished
petroleum 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, asphaltenes
and metallic constituents is not possible.
Flash point
Flash point is important primarily from a fuel handling stand point. Too low a flash
point will cause fuel to be a fire hazard subject to flashing and possible continued
ignition and explosion. Petroleum products are classified as dangerous or non
dangerous for handling purposes based on flash point as given below.
Classification Flash point Petroleum
Product
Class A Below 23°C Naptha
Petrol
Solvent 1425
Hexane
Class B 23 to 64°C Kerosene
HSD
Class C 65 to 92°C LDO
Furnace oil
LSHS
Excluded Petroleum 93°C and above Tar
Pour Point
The pour point of the fuel gave an indication of the lowest temperature, above which
the fuel can be pumped. Additives may be used to lower the freezing temperature
of fuels. Such additives usually work by modifying the wax crystals so that they are
less likely to form a rigid structure. It is advisable to store and handle fuels around
10°C 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 systems
and the ease with which it can be atomized in a given type of burner. When the
temperature increases viscosity of fuel will reduce.

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The viscosity needed at burner tip for satisfactory atomization for various types of
burners are as follows.
Type of burner Viscosity at burner tip
In centi stokes
Low air pressure 15 to 24
Medium air pressure 21 to 44
High air pressure 29 to 48
Steam jet 29 to 37
Pressure jet less than 15
Metal Content
Sodium, Potassium, Vanadium, Magnesium, Iron, Silica etc. are some of the metallic
constituents present in fuel oil. Of the above metals, sodium and vanadium are the
most troublesome metals causing high temperature corrosion in boiler super heater
tubes and gas turbine blades. Much of the sodium is removed from the crude oil in
the desalting operation, which is normally applied in the refinery and additional
sodium 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 oil
prepared from these crude. No economical means for removal of vanadium from the
residual fuel oil is available. However certain additives like magnesium are available
to minimize the effect of vanadium.
Asphaltene content and Carbon residue
Asphaltenes are high molecular weight asphaltic material and it requires more
residence time for complete combustion. Asphaltenes as finely divided coke may be
discharged from the stack. Residual fuel oils may contain as much as 4%
asphaltenes.
Petroleum fuels have a tendency to form carbonaceous deposits. Carbon residue
figures for residual fuel oils from 1 to 16% by weight. This property is totally
dependent on the type of crude, refining techniques and the blending operations in
refinery.
Fuels with high carbon residue and asphaltenes requires large combustion chamber
and hence while designing the boiler for such fuel the volumetric loading has to be of
the order of 2 lakhs Kcal/m3hr

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OIL/GAS FIRING START UP LOGIC
MANUAL TRIP INTERLOCK CHECK
1.CHECK TRIP VALVES IN CLOSED POSITION
2 . CHECK WATER LEVEL IN DRUM
3. EMERGENCY PUSH BUTTON NOT OPERATED
CONTROL SUPPLY LAMP 4. CHECK FAN SUCTION DAMPER IN CLOSED
POSITION
5.CHECK FUEL PUMP/GAS TRAIN DELIVERY
VALVE IN CLOSED CONDITION
6. CHECK MANUAL ISOLATION VALVE IN START FD FAN
CONTROL POWER SUPPLY SELECTOR SWITCH POSITION.
IN GAS/OIL FIRING MODE
FAILED
DEENERGISE TR & PILOTVALVE
DEDUCT PILOT FLAME DEENERGISE TRANSFORMER
ENERGISE GAS/OIL SHUT OFF VALVE TO OPEN
YES AND VENT TO CLOSE
YES DEENERSISE PILOT GAS & RELESASE LOW FIRE POSITION
MAIN FLAME ESTABLISED
NO NO DEENERSISE PILOT GAS
CHECK
1.0PURGE COMPLETED 1.0 OIL/GAS MAIN SHUT OFF VALVE IN CLOSED POSITION
2.0ALL PURGE INTERLOCKS 2.0 RETURN OIL LINE SHUT OFF VALVE CLOSED POSI
ENERGISE IGNITION AGAIN CHECKED 3.0 AIR/ATOMISING STEAM LINE SHUT OFF VALVE CLSOED
TRANSFORMER & 3.0COMPUSTION AIR PR NOT LOW POSITION
PILOT GAS SHUTOFF VALVE 4.0 INSTRUMENT AIR PR NOT LOW 4.0 PILOT GAS/SCAVENGING LINE SHUT OFF VALVE IN CLOSED
5.0 COMBUSTION AIR DAMPER TO POSITION
LOW FIRE POSITION 5.0 FUEL GAS SHUT OFF VALVE I & II IN CLOSED POSITION
PRESS BURNER 6.0OIL/GAS AT REQUIRED PARAMETER PURGE 6.0 NO FLAME INSIDE FURNACE
START BUTTON 7.0 EMERGENCY PUSH BUTTON BUTTON ON 7.0 FUEL PUMP NOT RUNNING
NOT OPERATED 8.0 FURNACE PRESSURE NOT HIGH
8.0SCANNER COOLING AIR PR OK COMBUSTION AIR 9.0 DRUM LEVEL NOT HIGH HIGH & NOT LOW LOW
DAMPER TO LOW 10.0ALL TRIP PARAMETERS OK
AUTO GAS/OIL FIRING INTERLOCKS FIRE POSITION 11.0 FUEL GAS PRESSURE NOT HIGH & NOT LOW
PURGE 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 pipelines
and the pulverized coal behaves as if that of oil and hence the same can be easily
burnt in pulverized fuel burners. The heat release by the burners in very high and
un-burnt carbon is almost equal to zero. Hence efficiency achieved by pulverized
burners 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 coal
storage bunker located over the coal pulverization system. Coal via gravity feed is
delivered through a down spout pipe to the coal feeder. A coal shutoff gate is
provided 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 the
feeder discharge. A primary air fan delivers a controlled mixture of hot and cold air
to the crusher dryer to drive moisture in the coal facilitating pulverization the primary
air and crushed coal mixture is then fed to the coal pulverizer located below the
crusher dryer discharge. Selection of pulverizer has to be analyzed critically, since it
is one of the important equipment where the wear and tear is more. For the soft
lignite Beter wheel is preferable and for hard lignite, coal like fuels heavy pulveriser
of ball and hammer mill is preferable. The coal is pulverized to a fine powder and
conveyed 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. When
needed to fire the boiler the pulverized coal is then conveyed to the burners by an
exhaust fan. This method requires very special provisions to minimize risk of fire or
explosion. 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 sand
supported on a fine mesh or grid. The air initially will seek a path of least resistance
and pass upwards through the sand. With further increase in the velocity, the air
starts bubbling through the bed and particles attain a state of high turbulence. Under
such conditions bed assumes the appearance of a fluid and exhibits the properties
associated 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 and
fuel is injected continuously into the bed, the fuel will burn rapidly and attains a
uniform temperature due to effective mixing. This , in short is fluidized bed
combustion.
While it is essential that the temperature of bed should be equal to the ignition
temperature of fuel and it should never be allowed to approach ash fusion
temperature (1050° to 1150°C ) to avoid melting of ash. This is achieved by
extraction of heat from the bed by conductive and convective heat transfer through
tubes immersed in the bed.
If the velocity is too low fluidization will not occur, and if the gas velocity becomes too
high, the particles will be entrained in the gas stream and lost. Hence to sustain
stable 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 rate
over a small heat transfer area immersed in the bed.
2.0 Low combustion temperature of the order of 800 to 950°C facilitates burning of
fuel with low ash fusion temperature. Prevents Nox formation, reduces high
temperature corrosion and erosion and minimize accumulation of harmful
deposits 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 coals
like 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 is
possible.
7.0 Inherent high thermal storage characteristics can easily absorb fluctuation in fuel
feed rate.

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ATMOSPHERIC CIRCULATING FLUIDIZED BED COMBUSTION
TECHNOLOGY
Atmospheric circulating fluidized bed (ACFB) boiler is a devise used to generate
steam by burning solid fuels in a furnace operated under a velocity exceeding the
terminal velocity of bed material. I.e., solid particles are transported through the
furnace and gets collected in the cyclone at the end of furnace and again recycled
into furnace by means of pressure difference between fluidized bed and return
particle.
MECHANISM OF CIRCULATING FLUIDIZED COMBUSTION
The mechanism is similar to AFBC. However in AFBC the fluidization velocity is just
to make the particles in suspended condition. In ACFB boiler, special combination of
velocity 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 and
furnace above the secondary air injection is characteristic by Fast fluidized bed.
Most of the combustion and sulphur capture reaction takes place in the furnace
above secondary air level. This zone operates under fast fluidization. In CFB boiler
number of important features such as fuel flexibility, low Nox emission, high
combustion efficiency, effective lime stone utilization for sulphur capture and fewer
fuel feed points are mainly due to the result of this fast fluidization.
In fast fluidization heavier particles are drag down known as slip velocity between
gas and solid, formation and disintegration of particles agglomeration, excellent
mixing are major phenomenon of this regime.
CFB is suitable for
1.0 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 20
bar) is, reduction in steam generator size can be achieved and make possible the
development of a coal fired combined cycle power plant. The development of
pressurized fluidized bed combustion is still in research stage only. With help of
pressurized 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 turbine
is further cooled by the heat exchanger. The aim behind the development of
pressurized 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 for
the process requirement like baking coal for steel plant where high calorific coal is
required. Hence in post independence India coal fired boilers where flourished,
however due to the need of energy conservation and due to process parameter
requirements development of HRSG in recent periods is more. Moreover due to the
development of gas turbines with gaseous and liquid fuels, more GT are being
installed 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 maintaining
process parameter such as temperature and other is in the point of economic point
of view.
The process steam generator are generally referred by the term called waste heat
recovery boiler ( WHRB) where the gas contains heat in excess, this excess waste
heat has to be recovered or removed by any means so that the process parameter
can 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 Heat
recovery 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 oxide
concentration. In HRSG the gas quantity and inlet temperature is fixed and for
different 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 the
performance of the HRSG to be done.

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2.0 STEAM,GAS and AIR
2.1 INTRODUCTION
In steam generator water, steam, gas and air are the working fluids in this air and
gas have similar properties. Understanding the properties of gas and air are almost
one and the same. I have grouped steam and gas as one unit and water as a
separate unit just because understanding the behavior of steam and gas is more
important in design point of view where as knowledge of water is more important in
operational 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 virtue
of a temperature difference. The temperature difference is the potential and heat
transfer 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 the
work 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 state
of strain.
Kinetic energy ( ½ mv² = ½ x mass x velocity²)
Kinetic energy of a body is the energy possessed by it on account of its motion.
Enthalpy
Enthalpy is the quantity of heat that must be added to the fluid at zero degree
centigrade to the desired temperature and pressure. Enthalpy is defined as heat
within or heat content of the fluid.
Entropy
The word entropy is derived from a Greek word called ‘tropee’ which means
transformation. 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 and
solids have one specific heat. However gas have number of specific heats depends
on 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 degree
kelvin.
Integral constant pressure specificheat
It is the average heat required to rise the temperature between two temperature
difference 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 constant
volume to raise the temperature of one kilogram of substance through one degree
kelvin.
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 measured
at 0°C or 273.15 °K and pressure 1.01325 bar or 1.03 atm
STP condition is at Standard temperature and pressure i.e., the properties measured
at 25°C or 298.15°K and pressure 1.01325 bar or 1.03 atm.
Viscosity
Viscosity of a liquid is its property, due to the frictional resistance between the fluid
particles (cohesion between particles) or between fluid and the wall. Viscosity of
fluid controls the rate of flow.
Newton s Law of viscosity
The shear stress on a layer of a fluid is directly proportional to the rate of shear
strain. ( Velocity gradient )
τ α ν/l where τ is shear stress and ν is velocity , l is the distance or gap between
layers.
τ = µ ν/l where µ is the constant of proportionality and is known as absolute
viscosity or dynamic viscosity.
Kinematic viscosity is the ratio of absolute viscosity to density (µ/ρ)
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 cylinder
preserved at the international bureau of weights and measures at paris.

19. Steam generator Ganesh kumar
Meter
Meter is the length between two transverse lines en-grooved in platinum iridium bar
at 0°C. or The meter is the length equal to 1650763.73 vacuum wave length of the
orange light. ( λ = 605.8 mm of the Krypton 86 discharge lamp)
Second
Second is the duration of 9192631770 periods of the radiation corresponding to the
transition between two specified energy level of the Caesium –133 atom. Or
1/86400th 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 temperature
and pressure for steam. The required equation for derivation is given at appropriate
places.
For gaseous fuel,
Cp /R = f(T)
R = Cp – Cv
Cv = Cp - 1
R R
Specific enthalpy wrt NTP,
T
H ‘ = 1/T  Cp dT ( enthalpy with reference to 0°C)
RT  R
Tn
Specific enthalpy wrt STP
T
H* ‘ = 1/T  Cp dT + Hs ( enthalpy with reference to 25°C)
RT  R RT
Ts
Specific entropy,
T
S ‘ = So  Cp dT - ln(P/Pn) ( entropy with reference to 0°C)
R R R
Tn

20. Steam generator Ganesh kumar
Specific free enthalpy
G = H -S
RT RT R
The temperature dependent specific heat (Cp) can be represented by an equation of
4 th degree polynomial as shown below
Cp = a1 + a2T + a3T² + a4 T3 + a5T4 (for temperature from 273K to 1000K)
R
Cp = a9 + a10T + a11T² + a12 T3 + a13T4 (for temperature from 1001K to 5000K)
R
Integrating, and adding constant of integration we get
H = a1 + a2T + a3T² + a4T3 + a5 T4 + a8/T (for temperature from 273K to 1000K
RT 2 3 4 5
H* = a1 + a2T + a3T² + a4T3 + a5T4 + a6/T (for temperature from 273K to 1000K
RT 2 3 4 5
S = a1 ln T + a2T + a3T² + a4T3 + a5T4 + a7 – ln(P/Pn)
R 2 3 4
G = a1(1- ln T) - a2T - a3T² - 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 fine
easily with help of the properties of nitrogen and following constants.
Var Specific Dynamic Thermal Prandtl number
Heat Viscosity conductivity
Kj/kgK µPa.S W/mK
a1 0.8554535 -0.9124458E 1 -0.1083113E-1 0.492851
b1 0.2036005E-3 0.4564993E-2 0.5596822E-4 -0.1230046E-2
c1 0.4583082E-6 0.2198889E-4 0.7413502E-7 0.1662398E-5
d1 -0.279808E-9 -0.1891235E-7 -0.5901395E-10 -0.1052753E-8
e1 0.5634413E-13 0.5138895E-11 0.1961745E-13 0.2443111E-12
a2 -0.1002311 -0.4267768E1 -0.8035817E-2 -0.8820652E-2
b2 0.7661864E-3 0.4074274E-3 0.110672E-04 0.1855309E-3
c2 -0.9259622E-6 -0.5125357E-5 -0.8397255E-8 -0.3838084E-6
d2 0.5293496E-9 0.738556E-8 0.1130229E-10 0.3256168E-9
e2 -0.109357E-12 -0.343972E-11 -0.5731264E-14 -0.1005757E-12

21. Steam generator Ganesh kumar
Dynamic viscosity,
µg = µn + P1 XH2O + P2 XCO2
Where XH2O & XCO2 are Percentage of weight in flue gas
P1 = a1 + b1T + c 1T² + d1T3 + e1T4
P2 = a2 + b2T + c 2T² + d2T3 + e2T4 where T is temperature in °C
Thermal conductivity,
kg = kn + P1 XH2O + P2 XCO2
Where XH2O & XCO2 are Percentage of weight in flue gas
P1 = a1 + b1T + c 1T² + d1T3 + e1T4
P2 = a2 + b2T + c 2T² + d2T3 + e2T4 where T is temperature in °C
Prandtl number,
Prg = Prn + P1 XH2O + P2 XCO2
Where XH2O & XCO2 are Percentage of weight in flue gas
P1 = a1 + b1T + c 1T² + d1T3 + e1T4
P2 = a2 + b2T + c 2T² + d2T3 + e2T4 where T is temperature in °C
Pra = a + bT + cT² + dT3 + eT4
Specific heat,
Cpg = Cpn + P1 XH2O + P2 XCO2
Where XH2O & XCO2 are Percentage of weight in flue gas
P1 = a1 + b1T + c1T² + d1T3 + e1T4
P2 = a2 + b2T + c2T² + d2T3 + e2T4 where T is temperature in °C
Where 0 ≤XH2O ≤ 0.3 ,0 ≤ XCO2 ≤0.2 , 0 ≤ T ≤ 1200°C
Dynamic viscosity, thermal conductivity and Prandtl number of NITROGEN
Dynamic viscosity Thermal conductivity Prandtl number
µ Pa.s W/mK
a 0.1714237E02 0.2498583E-1 0.6901183
b 0.4636040E-01 0.6535367E-4 0.2417094E-05
c -0.2745836E-4 -0.7690843E-8 0.2771383E-7
d 0.1811235E-7 -0.1924248E-11 -0.3534575E-10
e -0.674497E-11 0.160998E-14 0.1717930E-13
f 0.1027747E-14 -0.2864430E-18 -0.2989654E-17

22. Steam generator Ganesh kumar
µn = a + bT + cT² + dT3 + eT4 + fT5
Kn = a + bT + cT² + dT3 + eT4 + fT5
Prn = a + bT + cT² + dT3 + eT4 + fT5
Cpn = a + bT + cT² + dT3 + eT4 + fT5 (for temp.273 K to 1000K)
And Cpn = a1 + b1T + c1T² + d1T3 + e1T4 + f1T5 (for temp. 1001K to 5000K)
273 K to 1000K 1001K to 5000K
a 0.3679321E1 ‘a1 0.2852903E1
b -0.1313559E-2 b1 0.1580411E-2
c 0.2615196E-5 c1 -0.6189378E-6
d -0.9629654E-9 d1 0.1119450E-9
e -0.9928002E-13 e1 -0.7607378E-14
f -0.9723991E3 f1 -0.8019835E3
2.3 STEAM
We can see in day to day life the process of boiling water to make steam. Steam is
water in the vapour or gaseous state. It is in visible, odorless, non-poisonous and
relatively non corrosive to boiler metals. Steam is uniquely adapted by its
advantageous properties for use in industrial process heating and power cycle.
Thermodynamically boiling is the result of heat addition to the water in a constant
pressure and constant temperature process. The heat which must be supplied to
change water into steam without raising its temperature is called the heat of
evaporation or vaporization and the boiling point of a liquid may be defined as the
temperature 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 words
temperature at which the partial pressure of vapour increases to make total pressure
above the liquid surface. This temperature is also known as the saturation
temperature.
EVAPORATION
Liquid exposed to air evaporate or vapourize. Evaporation is the process takes
place at the surface exposed to atmosphere. If there is any increase in ambient
temperature or increase of the liquid temperature evaporation rate becomes
increased. The reduction in pressure above the liquid surfaces accelerate the
evaporation 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 takes
place throughout the liquid column. A liquid will boil, when it’s saturated vapour
pressure exceeds the surrounding environment pressure acted upon the liquid.
Hence boiling point of a liquid will change depends on the pressure exerted by the
environment over the surface.

23. Steam generator Ganesh kumar
CONDENSATION
Condensation is the change in phase of vapour phase to it’s liquid phase. When
water vapour or steam comes in contact with cooler surfaces, it gives up the heat
and condenses to water. The heat released while changing from vapour phase to
liquid phase is called heat of condensation. In factories the steam released out of
the main steam line or process vents where we can see a remarkable phenomenon
of indication of dryness of steam. If the steam is dry, we can not visualize the steam
coming out of the vent but after some distance we can see a white cloud. This is
due to the condensation of steam which composed of small particles of water formed
when steam cooled in cooler atmosphere. In other case if the steam is wet, the
white 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 in
release of heat. The oxygen comes from the air, which is about 21% oxygen and
78% nitrogen by volume.
Most fuels contain carbon, hydrogen, and sometimes sulphur as the basic
composition of combustion materials. These three constituents’ reacts with oxygen
to produce carbon-di-oxide, water vapour, suphur di oxides gases respectively and
heat.
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 as
hydrocarbons(combination of hydrogen and carbon). When these burn, the final
products 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 the
quantity of heat required to raise the temperature of one pound of water one degree
fahrenheit. A kilocalorie is the quantity of heat needed to raise one kilogram of water
one degree celsius.
Experimental measurements have been made to determine the heat released by
perfect combustion of various fuels. The heat value is usually determined by
calorimeters. When a perfect mixture of a fuel and air originally at 15.6°C is ignited
and then cooled to 15.6°C the total heat released is termed the higher heating value
or Gross calorific value. There is also one more term called lower heating value or
the net calorific value it is the quantity of heat equal to gross calorific value minus the
heat absorbed by the latent heat of water moisture( inclusive of moisture generated
due to combustion of hydrogen present in the fuel) at 25°C.

24. Steam generator Ganesh kumar
Dulong’s 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)χ
Where χ is the latent heat of water vapour at reference temperature 25°C
=583.2 kcal/kg
Proximate Analysis
The general procedure for the analysis relating to proximate analysis is describe
below 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 at
108°C±2°C
ii) Volatile matter
The method for the determination of VM consists of heating a weighted quantity of
dried sample of coal at a temperature of 900°±10°C. for a period of seven minutes.
Oxidation has to be avoided as far as possible. VM is the loss in weight less by that
due to moisture. VM is the portion of the coal which, when heated in the absense of
air under prescribed conditions, is liberated as gases and vapour.
iii) Ash
In this determination, the coal sample is heated in air up to to 500°C for minutes from
500 to 815°C for a further 30 to 60 minutes and maintained at this temperature until
the sample weight becomes constant.
iv) Fixed carbon
Fixed 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 analysis
of 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. These
constituent 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/kg
in all states other than Assam B GCV exceeding 5600Kcal/kg but
Andhrapradesh,Meghalaya, not exceeding 6200Kcal/kg
Arunachalpradesh and Nagland C GCV exceeding 4940kcal/kg
not exceeding 5600Kcal/kg

25. Steam generator Ganesh kumar
D GCV exceeding 4200kcal/kg
not exceeding 4940Kcal/kg
E GCV exceeding 3360kcal/kg
not exceeding 4200Kcal/kg
F GCV exceeding 2400kcal/kg
not exceeding 3360Kcal/kg
G GCV exceeding 1300kcal/kg
not exceeding 2400Kcal/kg
2 Non coking coal, produced
Assam,Andhrapradesh,Meghalaya, Not graded
Arunachalpradesh 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 three
simple 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. These
laws are called ideal gas laws. These laws are applicable to gases, which do not
undergo changes in chemical complexity, when the temperature or pressure is
varied. I.e., in other words laws applicable to gases which do not undergo any
chemical reaction when subject to change in pressure or temperature.
GAS LAWS
Boyle’s law
Boyle’s law states that the pressure is inversely proportional to volume and the
product of pressure and volume is constant
PV =C
Charles law-I
Charles law states that at constant pressure, volume is directly proportional to
temperature.
V/T = C
Charles law-II
Charles law states that at constant volume, pressure is directly proportional to
temperature.
P/T = C

26. Steam generator Ganesh kumar
Absolute scale of temperature
This scale of temperature is based on Charles law. According to Charles law at
constant pressure, volume of given mass changes by 1/273 of its volume at 0°C for
every rise or fall in temperature by 1°C. if the volume of the gas at 0°C is Vo and its
volume at t°C,
Vt = V o + Vo x t = Vo (1 + t/273)
273
If t = -273°C, then volume is zero, the hypothetical temperature of –273°C at which
gas will have zero volume is known as absolute temperature or 0°K.
Avagadra s Law
Avagadra’ s law state that the volume occupied by any gas at normal temperature
and pressure is 22.41383 x 10-3 m3 per mol of gas. I.e., volume occupied by a kg mol
of gas is 22.41383 m3/kg mol.
GAS EQUATION
From Boyle’s 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/m²
Temperature = 273 K
Volume = 22.41383 x 10-3 m3
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).
Daltan s law
At a constant temperature, the total pressure exerted by a mixture of non- reacting
gases is equal to the sum of the partial pressure of each component gases of the
mixture. Thus the total pressure P of a mixture of r gases may be represented
mathematically as

27. Steam generator Ganesh kumar
r
Pt = Σ 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 pi can be
calculated using the expression pi = xi P
2.6 SOME COMMONLY USED DIMENSIONLESS NUMBERS AND
THEIR SIGNIFICANCE
NUMBER FORMULA SYMBOL DEFINITION & SIGNIFICANCE
Nusselt hd/k Nu Radio of temperature gradients by
conduction and convection at the
surface
-used for convection heat transfer
coefficient determination
Reynolds ρvd/µ Re Inertia force/viscous force
- used for forced convection and
friction factor
Prandtl Cpµ/k Pr Molecular diffusivity of momentum
Molecular diffusivity of heat
Grashof ρ²d3 gß∆T/µ² Gr Buoyancy force x Inertia force
Viscous force x viscous force
- used for natural convection
Biot hd/ks Bi Internal conduction resistance
Surface convection resistance
- used for fin temperature estimation
Peclet vdρCp/k Pe=RePr Heat transfer by convection
Heat transfer by conduction
Stanton h/Cpρv St=Nu/Pe Wall heat transfer rate
Heat transfer by convection
Euler ∆P/ρv² Eu Pressure force/Inertia force
- used to find pressure drop
Froude v²/gl Fr Inertia force/gravity force

28. Steam generator Ganesh kumar
Where v is velocity
‘ d is characteristic dimension
Cp is specific heat
ρ is density
g is acceleration due to gravity
h is convection heat transfer coefficient
µ is dynamic viscosity
ß is volumetric expansion coefficient
T is temperature
P 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 à 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 = 3.666kg
of carbon dioxide.
0.399kg of carbon in coal require = 0.39x2.666 = 1.064 kg of oxygen
H2 + 1/2O2 à H2O + heat
2 kg of hydrogen react with 16 kg of oxygen to produce 18 kg of moisture. I.e., one
kg 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 à 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 react
with 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

29. Steam generator Ganesh kumar
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 of
oxygen 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 required
for 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 kg
Moisture 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 humidity
and temperature 40°C.
For 40°C, 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.

30. Steam generator Ganesh kumar
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 is140°C and
ambient air temperature is 40°C. Ash content of the fuel is 42.3% and 20% of total
ash is collected in bed and 80% ash is carried in fly ash. As per lab report the loss on
ignition of ash samples collected in bed zone and fly ash zone is 0.1% by weight and
4.4%by weight. The boiler is operating at 20% Excess air and the dry kg/kg of gas
produced =5.91 and dry kg/kg of air required = 5.696. The moisture and hydrogen
present in the fuel is 6% and 2.7% respectively.
Basically following are the losses present in boiler,
1.0 Unburnt carbon loss
2.0 Sensible heat loss through ash
3.0 Moisture loss due to air
4.0 Moisture and combustion of hydrogen in fuel
5.0 Dry flue gas loss
6.0 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( 660.33–615.25)100/3200
=0.1043%
Note: The above implies that the water vapour at ambient temperature at partial
pressure exists in steam form and gets superheated at 140°C
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

31. Steam generator Ganesh kumar
= 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. The
hydrogen present in the fuel react with oxygen to form water. From combustion
equation 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. It
is 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 31000
kg/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 fan
Total 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/sec
FD fan head
Pressure head required for air flow sections like airheater, air ducts and grate are to
be calculated. Now in most of the practical applications the pressure drop works out
to 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)

32. Steam generator Ganesh kumar
= 84.09 KW
Motor selected = 84.09 x 1.1 = 92.5 KW (next nearest motor standard is 110 KW)
ID fan
Total 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 head
Pressure head required for gas flow sections like Furnace, Bank, Economiser, air
heater, gas ducts and dust collectors are to be calculated. Now in most of the
practical applications the pressure drop works out to be 230 mm WC and the same
can 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 boiler
application.
S.N Description Grate type AFBC CFBC OIL
fired
1 FD Fan Flow 15% 25% 25% 15%
Head 20% 25% 25% 20%
2 ID Fan Flow 25% 25% 25% 20%
Head 30% 25% 25% 20%
3 SA/PA/OF fan Flow 10% 25% 25% Not
Head 15% 25% 25% applicable
3.0 FURNACE
3.1 INTRODUCTION:
The design of furnace is considered as the vital part in the boiler. The furnace is the
zone experiencing a high temperature in boiler. The performance of the furnace
reflects 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

33. Steam generator Ganesh kumar
illustrated with following. If furnace outlet temperature (FOT) is high, then the next
zone 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 reflects
in the performance and cost of material. On the other hand if the furnace is over
sized the FOT will be lesser, to get the required steam temperature the super heater
heat transfer area to be increased. If the heat transfer area is increased it calls for
larger 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 ash
fusion temperature are also form as constraint over the furnace design. The type of
fuel 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 dumping
or 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 own
problems like high or low temperature corrosion and additives have to be used. For
coal ash fusion is the problem, since ash slag down deposits on the wall hindering
heat transfer to steam water mixture. Depends on property of coal, whether it can be
crushable to powdered form, pulverized firing or bubbling bed or cyclone furnace can
be decided.
When we go for oil or gas firing, we can have higher heat flux in the furnace because
of the higher emissivity of oil flame and relative cleanliness of walls compared to coal
firing. 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 is
normally designed for coal and performance for oil firing in that furnace will be
carried out. When a furnace designed for coal operated with oil, the higher furnace
absorption results in a lower furnace outlet temperature. Lower FOT means super
heater 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 be
increased by gas recirculation, otherwise when coal is fired FOT will be reduced by
some means of bed absorption (This is used in FLUIDISED BED COMBUSTION
techniques). Furnace size also governed by length of flame in gas or oil fired boiler
since 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 higher
rate of erosion due to carry over particles in flue gas. Normally a flue gas velocity of
6 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 and
there is a possibility of over heating. The boiling is the phenomenon, which governs
the rate of heat transfer from combustion to steam water mixture inside the tube. In
boiling when bubbles formed at tube wall hinders the heat transfer which cause

34. Steam generator Ganesh kumar
tubes over heating and tube failure. This sort of boiling occurs at nucleate boiling
stage. 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 of
steam generation. Hence CR is also directly related to dryness fraction of steam by
the expression CR = 1/x. which implies in one circulation 1/CR quantity of dry steam
was produced. Circulation number will be higher when the difference in density
between steam and water is more (i.e.) due to higher difference in density; steam
water mixture velocity will be more thereby overheating will be prevented. If the
proper circulation is not there, circulation in the boiler circuit is effected by means of
external agency (normally a circulation pump will be used). This type of circulation is
called 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.
H
E
A
T
F
L
U
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 process
is predominant in the region A (pool boiling). Formation of vapour bubbles at the
nuclei 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 the
heat flux is C. In this region the unstable film boiling manifests with an eventual
transition to a continuous vapour film. In the final region D film boiling becomes
stabilized. This phenomenon of stable film boiling is referred as “ LEINDENFROST
EFFECT”

35. Steam generator Ganesh kumar
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 frequently
burn out when the wall heat flux reaches Q maximum. Hence the design engineers
should have an idea of average heat flux to the tubes, how they vary around
periphery and fin tip temperature in case of membrane wall construction. Calculation
of 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 temperature
required 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 be
selected.
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 the
flame. Ash shall be removed dry from bottom. The fuel used has low heat
flux and high ash fusion temperature.
b) Wet bottom: Ash removed from bottom is of molten form. The fuel having
high heat flux low ash fusion temperature is used. The flue gas generated
here or clean and free from fly ash and hence erosion, fouling problems are
minimized.
ii) According to Type of combustion
a)Conventional firing
1) Travelling grate
2) Dumping grate
3) Pulsating grate
4) Step grate
5) Fixed grate

36. Steam generator Ganesh kumar
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 fans
are used so to maintain vacuum or zero pressure in furnace. There is no
leakage of combustion product in the atmosphere. In the atmospheric
pressure 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 gas
fired boilers Forced draft fan alone used. The furnace pressure will be of the
order of 100 to 150 mm a water column. The furnace has to be designed to
without 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. Natural
draft generated due to chimney itself used for the boiler draft. Very small
capacity 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 out
in three modes.
1.0 Conduction
2.0 Convection
3.0 Radiation
Conduction
Conduction refers to the transfer of heat between two bodies or two parts of the
same body through molecules, which are more or less stationary. Fourier law of
heat conduction states rate of heat flux is linearly proportional to temperature
gradient.
Q = --K dt/dx

37. Steam generator Ganesh kumar
Where,
Q rate of heat flux watts per sq.meter
K thermal conductivity (property of material)W/m°k
dt/dx temperature gradient in x –direction
Negative sign indicates heat flows from 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 plate
A1 outside cylinder surface
A2 inside cylinder surface
‘r cylinder radius
‘t temperature of surfaces
Convection
Convection is a process involving mass movement of fluids. When a temperature
difference produces a density difference which results in a mass movement.
Newton s law of cooling governs convection. In convection there is always a film
immediately adjacent to wall where temperature varies.
- kf A (tf - tw)
Q =
Where,
is film thickness
kf thermal conductivity of film
h = 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. Radiation
requires no medium between two bodies, irrespective of temperature the radiation
heat transfer takes place between each other. However the cooler body will receive
more heat then hot body. The rate at which energy is radiated by a black body at
temperature T( °K) is given by Stefan Boltzmann law.
4
Q= AT

38. Steam generator Ganesh kumar
Q rate of energy radiation in Watts
A Surface area radiating heat sq.m
–8 4
Stefan boltzmann constant = 5.67 x 10 Watt/sq.m K
–8 4
4.88 x 10 Kcal/sq.m hr K
3.8 HEAT TRANSFER IN FURNACE
Furnace heat transfer is a complex phenomenon, which can not be calculated by a
single 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, partly
due to luminous part of the flame and partly due to non-luminous gases. Overall
heat transfer coefficient in furnace is governed by three T’s temperature, turbulence
and time and calculated by two parts.
Hc - heat transfer coefficient by convection
Hr - 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. In
forced convection turbulence or laminar flow depends on mean velocity,
characteristic length L, density and viscosity. These variables are grouped together
in a dimensionless parameter called Reynolds number. Reynolds number is the
ratio between inertia force to viscous force.
Reynolds number = (mass x acceleration)/(shear stress x cross sectional area)
Mass = volume x density
Acceleration = velocity / time
Volume = cross sectional area x velocity
Shear stress = dynamic viscosity x velocity gradient(v / l)
Re = density x velocity x characteristic length
Dynamic 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 and
temperature difference, coefficient of volume of expansion. These variables are
grouped to form dimensionless numbers called Grashoff number and Prandl number.
Laminar or turbulence is identified with product of Grashoff number and prandl
number
9
When, Gr.Pr < 10 flow is laminar

39. Steam generator Ganesh kumar
9
Gr.Pr > 10 flow is turbulent.
DIMENSIONAL ANALYSIS FOR HEAT TRANSFER COEFFICIENT
The heat transfer coefficient may be evaluated from correlation developed by
dimensional analysis. In this method all the variables related to the phenomenon is
grouped by experience with help of basic fundamental units length, mass, time and
temperature.
The final equation arrived for
FORCED CONVECTION
h = f(L,U, ρ,µ,k,Cp) ,
where,
L characteristic length (meters)
U velocity (meters/second)
ρdensity ( kilogram/ cub.meter)
µ dynamic viscosity(kilogram/meter. Hour)
k thermal conductivity (watts/meter°kelvin)
Cp specific heat(watt/kilogram.°kelvin)
a b c d e f
Let h = B L U ρ µ k Cp , where B,a,b,c,d,e,f are constants
Expressing the variables in terms of their dimensions
-3 -1 a -1 b -3 c -1 -1 d -3 -1 e -2 -1 f
MT = B L .(LT ) .(ML ) .(ML T ) .(MLT ) .(L² T )
a+b-3c-d+e+2f –b-d-3e-2f c+d+e -e-f
= B.L .T .M .
0 = a + b –3c –d +e +2f
-3 = -b –d –3e –2f
1 =c+d+e
-1 = -e - f
The solution of the equation gives,
a = c-1, b =c, d = -c +f, e = 1-f
c-1 c c -c+f -1-f f
h = B. L .U . ρ .µ .k .Cp
by grouping the variables,
-1 c f
h/L k = B.(UL ρ / µ) . (µ. Cp /k)
c f
Nussultes number = B.(Reynolds number) .(Prandl number)
The constants B,c,f are evaluated from experimental data.

40. Steam generator Ganesh kumar
For turbulent flow inside tubes and fully developed flow the following equation
attributed to Mr.Dittus and Boelter,
0.8 n
Nu = 0.023 Re Pr where, n = 0.4 when the fluid is heated
n = 0.3 when the fluid is cooled.
For turbulent flow outside tubes
0.8 n
Nu = 0.037 Re Pr where, n = 0.4 when the fluid is heated
n = 0.3 when the fluid is cooled
FREE CONVECTION
Free convection depends on buoyancy force F, which is defined by,
Let a fluid at To with density ρo change to temperature T with density ρ then,
F = (ρo –ρ)g/P = ((ρo/ρ) – 1)g
Now,
ß coefficient of volume expansion
then, 1/ ρ = (1/ρo) + ß(To-T),
ρo = ρ (1 + ß T)
(ρo/ ρ ) – 1 = ß T
F = ßg T
For an ideal gas ß is inversely proportional to temperature,(i.e. dimensional number
for ß is -1 and F is -1 * LT-2 ie LT-2)
By dimensional analysis,
h = B.(Fa.Cpb.Lc. ρ d.µe.k f)
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[ (gß T)a . Cpb. L 3a-1. ρ2a. µ b-2a. k1-b)]
h = B[ (gß TL3 ρ2/ µ² )a . (µ.Cp/k)b] (k/L)
hL/k = B. Gra. Prb.

41. Steam generator Ganesh kumar
a b
Nu = B. Gr . Pr
By large number of experiments made on fluids it has been found that exponents a
a
and b are of the same value. So the expression reduce to Nu = B.(Gr.Pr)
HEAT TRANSFER BY RADIATION Hr
In furnace heat transfer is predominant by luminous and non-luminous radiation. A
general approximate expression may be written for furnace absorption using Stefan
boltzman law.
Q = A w [ g Tg4 – g TS
4
]
g = c c + w w -
emissivity pattern of tri atomic gases such as carbon di oxide and water vapour are
studied by Mr. Hottel and charts are available to predict gas emissivity as a function
of various gas temperature, partial pressure and beam length. I have also furnished
the expression form to find gas emissivity. When c and w are found from graph
c and w can be determined from the following expression or from graph.
Otherwise emissivity of gas can be directly found by the expression given in
equation1.
0.222 1 1
c = EXP _______________ -
P c *L +0.035 ln2.8 ln(p + 1.8)
1/3
0.23 1 2
w = 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 = c c + w w -
At wall temperature correction,
Pcw = P c (Tw/Tg) Pww = P w(Tw/Tg)
c = cw (Tg/Tw)0.65 w = ww (Tg/Tw)0.45

42. Steam generator Ganesh kumar
cw is a function of Pcw .L and wall temperature for this we have to see the emissivity
in graph
ww is a function of Pww .L and wall temperature for this we have to see the emissivity
in graph
pressure correction is same as gas emissivity factor.
= w = function of P w/(Pc + P w) , Pcw.L + Pww.L, and temperature of wall
The effect of absorptivty is negligible hence the same can be neglected and a
generalized form of Q = A w g [Tg4 –TS4] can be used.
Heat absorption by energy balance method,
Q = [ Wf . lower heat value – W g .gas exit enthalpy]
Where,
A effective projected area of heat transfer including wall opening
w wall emissivity
g gas emissivity
Stefan boltzman constant
Tg Flue gas temperature of mean theoretical flame temperature(adiabatic
temperature)
TS Furnace wall temperature (If calculated for outside heat transfer coefficient or
consider saturation temperature if calculated for over all heat transfer coefficient, the
difference will be of very minor).
W f Fuel burnt
Wg Flue gas produced
Gas emissivity g = 0.9( 1- e –k.L )………………………………………………1
The emissivity of flame is evaluated by
f = ( 1- e –k.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 fuels
0.62 for luminous and semi luminous flames of refinery gas fuel OR gas/oil
mixture
0.50 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 its
concentration. For non-luminous flame

43. Steam generator Ganesh kumar
K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + P w)
_______
(Pc +P w)L
For semi luminous flame, the ash particle size and concentration is taken in
calculation
K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + P w)
________ + 7µ(1/dm²TM²)1/2
(Pc +P w)L
dm mean effective diameter of ash particle in micron
dm 13 for coal ground in ball mills
16 for coal ground in medium or high speed mill
20 for coal milled in hammer mill.
µ - ash concentration in gm/Nm^3
TM – furnace mean temperature °k(Some authors will consider this as outlet
temperature, but it is convincing assumption that in furnace zone temperature will be
uniformly spread through out the furnace by radiation effect (spherical). Hence
considering mean temperature for calculating radiation heat transfer coefficient will
be more appropriate. You can appreciate a notable phenomenon of furnace
temperature depends on flame location inside the furnace, in case flame is located at
the center of furnace(like oil fired burners (refer example1)) mean temperature and
outlet temperature will be at the most equal and if flame is located at one end of the
furnace and radiation beam travels a larger distance of furnace(like AFBC boilers
assuming no free board combustion) the furnace temperature near flame will be
higher 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 cases
a wide variation would be occurred due to:
1.0 Combustion phenomenon itself
2.0 The flame does not fill the furnace fully. Unfilled portion are subject to only gas
radiation

44. Steam generator Ganesh kumar
3.0 The emissivity of radiation is far below the flame emissivity. Emissivity of gas
radiation 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 the
emissivity at appropriate places for AFBC, Dumping grate and fired boilers with
working of example.
The heat transfer by radiation is given as Q = A w g [ TM4 – TS4]. But mostly the
heat transfer will be of both convection and radiation occuring simultaneously and so
to put both process on a common basis, we may define a radiation heat transfer
coefficient by symbol Hr.
Qr = Hr. A. (TM – TS)
4 4
Hr = w g[TM -TS ]/(TM-TS )
While considering the total heat transfer by convection and radiation
Q = (Hc + Hr) A (TM –TS) 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 are
corrected by effectiveness factor. This depends on various manufacturers
experience on their steam generator.(Normally for oil fired boilers the value will be of
0.79 and gas fired boiler 0.67).
3.9 FURNACE CONSTRUCTION :
Basically three types of constructions are used
1.0 Plain tube construction with a refractory lined furnace
2.0 Tangent tube construction
3.0 Membrane wall construction.
Plain tube construction
FURNACE CHAMBER

45. Steam generator Ganesh kumar
REFRACTORY
The drawing shown gives complete idea of the above construction. Refractory lined
wall construction is out dated design since it calls for a lot of refractory work and flue
gas 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 tubes
is 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 continuous
longitudinal 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 the
tubes(pitch) are maintained in a such a way that the fin can be cooled by either of
the two side tubes and prevent warping of the panel. Water cooled furnaces not only
eliminated problem of rapid deterioration of refractory walls due to slag, but also
reduced fouling of convection heating surfaces to manageable extent, by lowering
the temperatures leaving the furnace. In addition to reducing furnace maintenance
and fouling of convection heating surfaces, water cooling also helped to generate
more steam. Consequently the boiler surface was reduced since additional steam
generating surface was available in water cooled furnace.
Ex.1.0 . Find the furnace outlet temperature for a fluidized bed boiler operating at 15
kg/cm^2(g) having furnace EPRS of 28.43 sq.m and having the following gas
parameters.

46. Steam generator Ganesh kumar
Flue gas produced 11016 kg/hr at a temperature of 900°C and partial water vapour
pressure 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 740°C
Flue gas properties at film temperature. (900+740 +200)/3 = 613.33°C
–5
Dynamic viscosity = 3.7392 x 10 kg/ms
Thermal conductivity = 0.065177 kcal/m hr.°c
Prandl 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 transfer
coefficient its effect will be negligible)
0.8 n
Nu = 0.037 Re Pr where n= 0.3 for cooling fluid
0.8 n
Hc/kL = 0.037 Re Pr
0.8
Hc = 0.037 x 0.396 x 1.1269 x 1.75 x 0.7152 0.3 x 0.06517/1.75
3.7392 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

47. Steam generator Ganesh kumar
flame emissivity f = 0.9 x (1- e –0.2904 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 745°C.
Ex 02. Evaluate the size of bed for a 10 tph boiler, operating at 14.5 ksc, satuated
steam from and at 100°C. Coal as a fuel. The efficiency of boiler is 80% and GCV of
coal as 3800 kcal/kg , Flue gas produced per kg of fuel is 6.802 kg/kg at 20% excess
air 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 can
be considered and above which water wall is located. Hence a water wall of size
3584 x 1680( 35 @ 112 pitch and 15 @ 112 pitch ) can be obtained.

48. Steam generator Ganesh kumar
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 100°C is the term used in boiler industry to specify the heat
capacity of boiler. This is value is assumed that water at 1kg/cm^2 100°C is given as
input and steam drawn at 1kg.cm^2 .(i.e. latent heat at 1kg/cm^2 pressure only
absorbed )
EX 03. Find the furnace outlet temperature of a 55Tph dumping grate bagasse fired
boiler operating at 42 kg/cm^2 and 420°C super heater outlet at furnace exit plane.
The effective projected area of furnace and superheater plane works out to be
212m^2 and 13.6m^2 respectively. Consider convection heat transfer coefficient
negligible and lower heating value of bagasse 1828 kcal/kg, 85% of air required
flows through air heater at a temperature of 170°C and 15% air for fuel distributor
and OFA at 40°C into the furnace. Fuel consumption 24209 kg/hr. 2% of gross heat
input 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/hr
2.0 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/kg°c 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 w g [ TM4 – TS4]
As it is a bagasse fired boiler volatile combustion is more TM will be equal to
temperature exit and w g is equal to 0.72.
Assuming 890°C as FOT
Saturation temperature 263°c .
Q1 = 212 x 0.72 x 4.88x10^-8 x ( 11634 – 5364)
= 13.01 x 10^6 kcal/hr.
superheater steam outlet 420°c

49. Steam generator Ganesh kumar
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/kg°C is specific heat of flue gas
Furnace outlet temperature = 890°C
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
112m² 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% excess
air. Air required 17.87 kg/sec at 27°C. Consider a radiation loss 0.33% and wall
emissivity 0.85, heat transfer effectiveness 0.79. adjacent radiation chamber extends
by 1.01 m length wise.
Total heat into Furnace at 27°C ambient.
Heat input by fuel = 1.16 x 41867 = 48.565 MW
Radiation loss = 48.565 x0.33/100 = 0.16 MW
Nett heat input = 48.405 MW
Heat absorbed by Furnace
4 4
Radiation coefficient Hr = w f[TM -TS ]/(TM-TS)
For oil fired boiler Tmean is equal to Tgas outlet
Wall emissivity = 0.85
Flame emissivity = 0.72 (1-e-kl)
Attenuation factor k = (1.6Tm/100)-0.5

50. Steam generator Ganesh kumar
Beam length, l = 3.4x5.74x3x6/2(5.74x3+5.74x6+3x6) = 2.52M
Assume gas outlet temperature,1285°c =1558°K
K =(1.6x1558/100)-0.5 = 1.9928
–1.9928 x2.52
Flame emissivity = 0.72x(1 –e ) = 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 = 749.5°C
Dynamic viscosity kg/m.s = 4.13276 x 10-5 kg/ms
Thermal conductivity = 0.072915 W/mK
Prandtl number = 0.711
Velocity = 19.03 /(0.345 x 3x6) = 3.064 m/s where 0.345 is density kg/m^3
0.8
0.3
Hc = 0.037 x 3.064 x5.74 x0.345 x 0.711 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 + 187.768)x 0.79 x 112 x(1558 - 487)
= 18.34MW
Heat absorbed by adjacent Radiation chamber
Heat absorbed by adjacent wall
= 5.67 x 10^-8 x 0.715 x(1-0.715) x 18 x 0.85 x(15584 –4874)
= 1.0316MW
Total heat lost by Gas
= 18.34 +1.0316 = 19.3716 MW

51. Steam generator Ganesh kumar
Heat balance (heat lost = heat gained)
19.3716 = (48.405 – 19.03 x Enthalphy of leaving gas)
Enthalphy of leaving gas = 1.526 Mj/kg For this gas temperature is 1255°C.
4.0 SUPER HEATER
4.1 INTRODUCTION:
The steam temperature above its saturation temperature corresponding to the
pressure is achieved by introducing super heater coils. In the modern industrial

52. Steam generator Ganesh kumar
world, it is expected to get the maximum attainable temperature for a pressure, since
the cycle efficiency depends on pressure and temperature. But nature restricts the
maximum temperature with material availability and metallurgical limitations.
Super heater is a critical section, where high metal working temperature involves,
since working fluid in super heater is hot steam when compared saturated water in
other regions. The super heater will work out approximately 5% of the cost of the
boiler and the material must be selected carefully. If improper selection of material
leads losses due to oxidation and improper sizing or thickness selection leads larger
reserve in thickness which is uneconomical and lesser thickness cause tube failure.
4.2 EFFECT OF FUEL ON SUPER HEATER DESIGN:
The mechanical arrangement of super heaters is governed by factors like furnace
design i.e. furnace outlet temperature which is explained in furnace chapter, fuel
characteristics, degree of super heat and manufacturer practice. The fuel
characteristics such as ash content, ash particle size, salts of alkali metals like
sodium and potassium which volatilize in the process of combustion and condenses
as a sticky substance at a temperature corresponding to super heater tubes. These
decide transverse pitching of super heater. Some light density fuels like husk burns
at the top resulting high heat flux input to super heater reducing heat transfer area
requirement itself.
4.3 POINTS TO BE NOTED WHILE DESIGNING SUPER HEATER
1.0 The super heater surface required to give the desired steam temperature.
2.0 The gas temperature zone in which the surface is to be located.
3.0 The steam temperature required
4.0 The type of steel and other material best suited to make the surface and its
supports.
5.0 The rate of steam flow through the tubes (mass velocity) which is limited by
permissible pressure drop, which will exert a dominant control over the tube
metal temperature.
6.0 The arrangement of surface to meet the characteristics of the fuels anticipated
with particular reference to the spacing of the tubes to prevent
a) Erosion
b) Accumulation of ash and slag or to provide space for the removal of such
formation in the early stage.
7.0 The mechanical design and type of super heater.
A change in any of the first six items will call for counter balancing change in all other
items.
4.4 CLASSIFICATION OF SUPER HEATERS
Super heaters are normally classified as Radiant and Convection super heaters.
Radiant super heaters are located in the radiant zone receiving energy directly from
the flame in the furnace. Convective super heaters do not receive furnace radiation.
A few super heaters receive energy partly from the flame are called semi radiant.
The other classification of super heaters depends on location arrangement and flow
pattern.
1 Radiant super heater located at radiant zone of boiler
2 Convection super heater located at the convection zone of boiler

53. Steam generator Ganesh kumar
3 In modern fluidized bed boilers, to achieve higher steam temperature a portion of
super heater located in the fluidized bed called bed super heaters.
Depends on arrangement
1 Horizontal 2. Vertical 3. Inline 4. Staggered
Depends on the flow pattern
1. Cross flow super heater
2. Parallel flow super heater
3. Counter flow super heater
Horizontal arrangement of super heaters have a advantage of easy drainage, which
is quite important in boilers where shut down will be for longer periods. Drainability
helps in non-accumulation of salts in water or steam inside the tubes.
In vertical arrangement the problem of expansion can be tackled easily than
horizontal arrangement.
Staggered bundles are difficult to clean but they offer a marginal improvement in
heat transfer coefficient. Inline arrangement provides lower gas side pressure drop.
Counter flow arrangement offers a slight improvement in log mean temperature
difference and there by decreasing surface area compared to parallel flow. The
parallel flow arrangement leads to cooler tubes. Metal temperature can be higher
with counter flow arrangement since both gas and steam temperature are higher at
steam exit portion.
4.5 DESIGNING A SUPER HEATER
Designing a super heater or any other heat transfer involves three steps one is the
thermal design, second is the mechanical design and other is the performance
calculation.
In a thermal design we have to perform the heat duty, heat transfer and surface area
required for the known thermal input and output parameters
In a mechanical design thickness and material with standing capacity are to be
checked.
Performance calculation involves confirmation of the designed equipment for various
loads.
4.6 OVERALL HEAT TRANSFER ACROSS BANK OF TUBES
According to the gentleman Mr. Colburn the following equation can be applied for
gas/air flowing normal to the bank of tubes
3
Nusselts number = 0.33 * (Reynolds number)0.6 * (Prandl number)0.
While calculating Reynolds number external tube diameter has to be used. This
expression can also expressed after introducing geometry factor. Geometry factor F
has to be taken from graph depends on transverse pitch to diameter ratio and
longitudinal pitch to diameter ratio
0.6 0.3
Nusselts number = 0.35* F * (Reynolds number) * (Prandl number)

54. Steam generator Ganesh kumar
According to another gentleman Mr. Grimson the outside heat transfer coefficient
was explained by the following equation
Arrangement Factor F
For Inline arrangement For staggered arrangement
ST/d SL/D 1.25 1.50 2.00 3.00 1.25 1.50 2.00 3.00
1.25 Re
2000 1.06 1.06 1.07 1.00 1.21 1.16 1.06 0.96
8000 1.04 1.05 1.03 0.98 1.11 0.99 0.92 0.95
20000 1.00 1.00 1.00 0.95 1.06 1.05 1.02 0.93
1.5 2000 0.95 0.95 1.03 1.03 1.17 1.15 1.08 1.02
8000 0.96 0.96 1.01 1.01 1.10 1.06 1.00 0.96
20000 0.95 0.95 1.00 0.98 1.04 1.02 0.98 0.94
2.00 2000 0.73 0.73 0.98 1.08 1.22 1.18 1.12 1.08
8000 0.83 0.83 1.00 1.02 1.12 1.10 1.04 1.02
20000 0.90 1.00 1.00 1.00 1.09 1.07 1.01 0.97
3.00 2000 0.66 0.66 0.95 1.00 1.26 1.26 1.16 1.13
8000 0.81 0.81 1.02 1.02 1.16 1.15 1.11 1.06
20000 0.91 0.91 1.01 1.00 1.14 1.13 1.10 1.02
Nusselts number = B * (Reynolds number) N
B and N are factors governed by the geometry, the values of B and N is given in the
table.
Many such persons worked on the heat transfer and gave various correlation for
certain pre defined condition and hence for practical purposes certain factors to be
considered for its accuracy. I have also tried to give those factors in worked
examples.
In the Colburn or Grimson equation correction factor for the heat transfer coefficient
for gas angle of attack on the tube has to be calculated into account.
Degree ° 90 80 70 60 50 40 30 20 10
Factor 1.0 1.0 0.98 0.94 0.88 0.78 0.67 0.52 0.42
ST/D 1.25 1.5 2.0 3.0
SL/D B N B N B N B N

55. Steam generator Ganesh kumar
STAGGE
1.25 0.518 0.556 0.505 0.544 0.519 0.556 0.522 0.562
1.5 0.451 0.568 0.46 0.562 0.452 0.568 0.488 0.568
2 0.404 0.572 0.416 0.568 0.482 0.556 0.449 0.57
3 0.31 0.592 0.356 0.58 0.44 0.562 0.421 0.574
INLINE
1.25 0.348 0.592 0.275 0.608 0.10 0.704 0.0633 0.752
1.5 0.367 0.586 0.25 0.62 0.101 0.702 0.0678 0.744
2 0.418 0.57 0.299 0.602 0.229 0.632 0.198 0.648
3 0.29 0.601 0.357 0.584 0.374 0.581 0.286 0.608
Correction factor for B corresponding to number of tubes deep
Number deep 1 2 3 4 5 6 7 8 9 10
Staggered 0.68 0.75 0.83 0.89 0.92 0.95 0.97 0.98 0.99 1.0
Inline 0.64 0.8 0.87 0.9 0.92 0.94 0.96 0.98 0.99 1.0
Simillarly Dittus & Boelter correlation gives heat transfer coefficient on inside tube.
0.8 0.3
Nusselts number = 0.023 * (Reynolds number) * (Prandl number)
Consider the super heater tubes of outer radius r1 & r2 respectively. Thermal
conductivity of material k. Cold fluid (steam) is flowing steadily inside the tube Tf1
and hot fluid Tf2 steadily outside the tube. Inner and outer wall temperature tw1 &
tw2. Heat transfer coefficient of steam and gas sides be h1 and h2. (h2 includes
convection heat transfer and non luminous heat transfer as explained in furnace
chapter)
STEAM ho Tf2
tw2 outside conve.heat transfer =3
tw1
Hot gas Conduction =2
hi inside conve. heat transfer =1
Tf1 Tube thickness’t’
Convection heat transfer phenomenon in super heater tube
ql = Q/l = hi *2 * *r1 [ tw1 – tf1]………………………………………….1
= 2* * k (tw2 – tw1)/ln(r2/r1)……………………………………..2
= ho * 2 * * r2 [ tf2 – tw2]…………………………………………3
1=2=3
the heat transferred at a given section at given time is equal.

56. Steam generator Ganesh kumar
tw1 – tf1 = ql / (hi*2* *r1)
tw2 – tw1= ql * ln(r1/r2)
2* *k
tf2 – tw2 = ql /(ho *2* *r2)
Adding the above equations we get,
ql 1 ln(r2/r1) 1
tf2 – tf1 = + +
2* hir1 k hor2
ql = 2* *(tf2 – tf1)
1 + ln(r2/r1) + 1
hir1 k hor2
Q/l = U [tf2 – tf1]
Divide by do
Q/( dol) = U[tf2 – tf1]/ do
U = 2*
do
1 + ln(r2/r1) + 1
hir1 k hor2
U = 1
do
1 + ln(r2/r1) + 1
2 hir1 2 k 2 hor2
1 do + d0 ln(d0/di) + 1
=
U hidi 2k ho
Introducing fouling factors in steam side and gas side we can get,

57. Steam generator Ganesh kumar
1 do + d0 ln(d0/di) + 1
= +ffo +ffi
U hidi 2k ho
In the above equation each unit refers to a resistance in heat transfer
1/ho is gas side heat transfer resistance.(this gives temperature drop in the film)
doln(do/di)/2k is metal resistance(this gives temperature drop in metal side)
do/hidi is steam side heat transfer resistance.(this gives temperature drop in steam
side film)
ffo is outside fouling resistance
ffi is inside fouling resistance
For extended surface following heat transfer coefficient equation can be applied.
1 At + At d0 ln(d0/di) + 1
= +ffo +ffi A t/Ai
U hiAi Aw 2k ho
Where, At is total surface area.
Ai is inside surface area.
Aw is average surface area.
is fin effectiveness
Q = U * A * [tf2 –tf1] for specific tube, while considering bundle of tubes log mean
temperature has to be used.
Q = U * A * lmtd.
METAL TEMPERATURE CALCULATION :
Q = U * do *(Tg – Ts) = hi * di * (Tw – Ts)
Where Tw is metal temperature °C
Metal temperature of bare tube can be estimated easily by calculating various
temperature drop across the resistance given above. This simple equation is
applicable for only bare tube surface. For extended surface fin temperature
calculation involves a detailed procedure which is discussed separately.
4.7 STEAM TEMPERATURE CONTROL

58. Steam generator Ganesh kumar
Super heater temperature depends on boiler working pressure. The degree of super
heat varies directly with respect to boiler working pressure. The various steam outlet
temperature corresponding to a pressure is fixed up by cycle efficiency and turbine
manufactures practice. Operating variables like flue gas temperature inlet, load,
excess air, fuel creates fluctuation in steam temperature leaving the final super
heater. If there is any fluctuation in steam temperature, there is a change in volume
of steam which will affect the turbine performance, since turbines are designed for
predetermined steam volume flow between stator and rotor and also the exhaust
steam temperature quality will varies with steam fluctuation which will affect
condenser performance. Hence it is mandatory to have a steam temperature
control.
In practice two types of attemperators (de-super heater) is widely used in boiler
industry, they are
1.0 Spray type
2.0 Surface type.
In the above two types, spray type attemperator gives a faster temperature control
compared to surface type. However quality water spraying into the super heater
should be taken care for boiler and turbine life. Attempeator temperature technique
is nothing but a simple energy balance. It consists of spraying water in mist form in
between stages, depends on the final steam temperature and pick up by individual
stages. The quantity of spray water varies with respect to load and it is controlled by
automatic control loops. Spray inside desuperheater accomplished by means of
nozzle arrangement.
Surface type attemperators are further divided into two types submerged type and
Shell and tube type. In submerged type the steam coils is submerged below the
drum water level. Part of the steam flows through the submerged coils and part of
the steam by-pass the coil. The flow of steam through the submerged coil is
regulated in such a way that the outlet steam temperature is of desired level after
mixing.
In shell and tube type attemperator super heater header itself modified into an
exchanger. Steam from super heater coils enter this intermediate header and leaves
to the second stage. In the super heater header the cooling water coils enter in both
the sides and leaves. The steam gets cooled when it contact with low temperature
cooling coils. In this steam temperature is adjusted by regulating the flow of cooling
water flow inside the coils. Higher the water flow steam cooling will be more.
The water which absorb heat usually mixed with feed water to avoid the heat loss.
SPRAY TYPE ATTEMPERATOR Water qty (M KG/HR & enthalpy

59. Steam generator Ganesh kumar
Hfw kcal/kg)
Steam flow(M2 kg/hr Steam flow (M1 kg/hr
& enthalpy H2) & enthalpy H1)
DE SUPER HEATER
SEC. SUPERHEATER PRI. SUPER HEATER
Energy balance,
M1* H1 + M * Hfw = M2H2……………………………………………1
Mass balance,
M1 + M = M2……………………………………………….2
From 1 & 2,
M1*H1 + M*Hfw = (M + M1) *H2
Quantity of water required spray,
M = M1 * { H1 H2}/{H2 Hfw}
Ex.01. Determine the spray quantity required for a 60TPH boiler having a primary
super heater steam outlet temp of 332°C and secondary super heater inlet steam to
be desuperheated to 316°c. The spray water temperature is 105°C.
Enthalpy of PSH = 721.7 kcal/kg
Enthalpy of SSH = 711.85 kcal/kg
Enthalpy of spray water = 105.5 kcal/kg.
Spray water required = (721.7 - 711.85)*60000 = 974.7 kg/hr.
( 711.85 –105.5)
Ex02. Estimate the heat transfer area required for surface type desuper heater
immersed in drum water operating at a pressure of 45 kg/cm²(g) and the steam at

60. Steam generator Ganesh kumar
the outlet of primary super heater is 380°C and the steam temperature at the inlet of
super heater is 350°C. the steam flow is 25000 kg/hr.
The heat transfer coefficient outside the tube to water is natural convection heat
transfer coefficient, governed by the equation
Nu = 0.54 ( Gr Pr) 0.25
‘d3ρ²gβ∆T Cpµ 0.25
‘hodo/k = 0.54 µ² k
Assume surface type desuper heater of tube diameter 38.1 x 3.25 mm
Properties of liquid at film temperature (saturation temperature at 45 kg/cm²(g))
Density of liquid kg/m3 = 787.5 kg/m3
Acceleration due fo gravity = 9.81 m/sec²
Dynamic viscosity = 0.000103067 kg/ms
Volumetri coefficient of expansion ß = 0.284 1/°C
Prandtl number = 0.83299
Thermal conductivity = 0.00061184 Kw/m°C
Specific heat Cp = 4.94512 kj/kg
Temperature difference between wall & water = 5° (assumed)
0.25
0.03813 x 787.5² x 9.81x0.281x 5 x 0.83299 x 0.61184
ho = 0.54 0.000103067² 0.0381
= 19497 W/m²°K
Inside heat transfer coefficient is governed by forced convection,
Nu = 0.023 Re0.8Pr0.4
Properties of steam at average temperature (380 +350)/2 = 365°C
Density of steam kg/m3 = 16.169 kg/m3
Dynamic viscosity kg/ms = 2.28175 x 10-5
Prandtl number = 0.9576
Thermal conductivity W/m°C = 5.945 x 10-2
Velocity inside the surface assume 25 m/sec
Reynolds number = 16.169 x 25 x 0.0316/2 .28175 x 10-5
= 559812
‘hi = 0.023 x 5598120.8 x 0.95760.4 x 0.0316/5.945 x 10-2

61. Steam generator Ganesh kumar
= 476.6 w/m²K
Metal resistance = d0 ln(d0/di) /2 Km
Metal conductivity = 49.5 w/mK
Metal resistance = 0.0381 x ln(0.0381/0.0316)
2 x 49.5
= 7.1988 x 10-5
Overall heat transfer coefficient
1/U = 1/ho + 1/hi + Rm + ffi + ffo
inside and outside fouling factor due to scale assume = 0.0002 m²/w°K
1/U = 1/19497 + 1/746 + 7.1988 x 10-5 + 0.0002
Overall heat transfer coefficient = 601.04 W/m²K
Heat transfer area required = Q/Ulmtd
= 25000( 3158.49 – 3083.08) x1000
3600x 601.04 x (( 380-350)/ln(123/93))
= 8.12 m²
Ex.03. Calculate gas outlet temperature for a super heater intended to raise steam
from 214°C to 258°C. steam flow 15.28 kg/sec, pressure 20 bar(a), tube size 38.1 x
3.25 mm thk and thermal conductivity 49.844 W/mK. The gas flow 19.03 kg/sec,
inlet temperature is 659°C and the pattern of flow is counter, furnace width is 3.06
meter and length 2.7meters, tube pitching width side 80mm and depth side 78mm,
inline arrangement, number of tubes in steam side path is 74. Heat transfer area of
super heater 88 sq.m. the super heater was enclosed in a water wall having EPRS
area of 21 sq.m and gas is flowing 90° to super hater tubes. Consider heat transfer
effectiveness of 82% and 71% for enclosure. Partial pressure of water =0.1158 bar
and carbon di oxide = 0.1249bar.
Steam inlet temperature = 214°C
Steam outlet temperature = 258°C
Average steam temperature = 236°C
Properties of steam
Density kg/cu.m = 9.328189
Dynamic viscosity kg/ms = 1.72232 x 10-5
Prandtl number = 1.083
Thermal conductivity W/mK = 4.346819 x 10-2

62. Steam generator Ganesh kumar
Steam velocity = 15.28 x 4 /(9.3281 x74x x 0.0316²)
= 28.225m/s
Forced convection inside heat transfer coefficient is
Hi = 0.023 x Re0.8 x Pr0.4 x k/di
0.8
9.328x0.0316x 28.255 0.4
= 0.023x x 1.083 x 4.346819 x10-2
-5
1.7223 x 10 0.0316
= 1152 W/sq.mK
Forced convection outside heat transfer coefficient is
Assume gas outlet temperature as 560°C
Film temperature = (average of gas temperature + average of steam temperature/2
=(659 +560 +214 + 258)/4
=422.75°C
Gas properties at film temperature
Density kg/cu.m = 0.503
Dynamic viscosity kg/ms = 3.1798 x 10-5
Prandtl number = 0.7126
Thermal conductivity W/mK = 0.05195
For inline arrangement
ST/d = 80/38.1 = 2.099; SL/d = 78/38.1 = 2.047
Arrangement factor fe = 1.18
Gas flow area = (3.06 x 2.7 – 37 x0.0381x2.7) = 4.4558sq.m
Gas velocity = 19.03/(0.503 x 4.4558) = 8.49 m/s
Reynolds number = 8.49 x0.503 x 0.0381/3.1798x10-5
= 5116.8
hc = 0.287 x fex Re0.6 xPr0.364 x k/d
= 0.287 x1.18x 5116.80.6 x 0.71260.364x0.05195/0.0381
= 68.58 W/sq.mK
Radiation heat transfer coefficient
Beam length = 3.4 x(0.08 x0.078x1-0.00114)/( x0.0381x1) = 0.144876m
Attenuation factor = (0.8 +1.6x0.1158)(1-0.38x(883/1000))(0.1158 + 0.1249)
((0.1158+0.1249)x0.14486)

63. Steam generator Ganesh kumar
= 0.8439
emissivity of gas = 0.9 x(1-e-0.8439x0.14486)
= 0.10358
hr = 5.67 x 10-8 x 0.85 x 0.10358x(8834 –5094)
(883 –509)
= 7.218 W/sqmK
total gas side heat transfer coefficient = hc + hr = 68.58 + 7.218 = 75.798 W/sqmK
1 do + d0 ln(d0/di) + 1
=
U hidi 2k ho
= 0.0381/(1152x0.0317) + 0.0381 ln(0.0381/0.0317) + 1/75.798
2 x 49.844
U = 69.898 W/sq.mK
Heat transferred to super heater tubes,
Q = U x effectiveness x Ax lmtd
= 69.898 x 0.82 x 88 x 372 = 1.876 MW.
Heat transferred to water wall encloser,
Neglecting metal resistance and internal conductance the convection heat transfer
coefficient be 68.58 w/sq.mK and non-luminous radiation heat transfer be
hr = 5.67 x 10-8 x 0.85 x 0.10358x(8834 –4874)
(883 –487)
= 6.9538 w/sqmK
ho = 68.58 + 6.9538 = 75.53 W/SqmK
Q = 75.53 x 0.71 x 21 x 393 = 0.4426 MW.
Total heat gained by super heater and encloser is = 1.8763 + 0.4426 = 2.319 MW.
Total heat lost by gas = Gas flow x (enthalpy difference)
= 19.03 x (752.63 – 629.46) /1000 = 2.34 MW.
Ex 05 Find the steam side pressure drop for a superheater, the steam flowing
through the super heater is 55000 kg/hr and the Outlet pressure of steam is 19

64. Steam generator Ganesh kumar
kg/cm²(a) and the number of parallel path is 74 nos. and coil developed length is
estimated as 11.64m, it has 3 nos. 180° bends and 2 nos. 45° bends. The steam
inlet condition is dry saturated and outlet condition is 255°C. the super heater tube
size is 38.1mm x 3.64mm including the positive tolerance of the tube.
Super heater steam flow kg/hr :55000
Steam flow/coil kg/sec:0.206
Steam parameter at inlet
Temperature °C : 209.84
Pressure kg/cm²(a): 19.34(assumed by trial and error)
Steam parameter at outlet
Temperature °C : 255
Pressure kg/cm²(a): 19
Average steam Parameter
Temperature °C : 232.42
Pressure Kg/cm²(a):19.17
Density kg/cu.m: 9.20739
Dynamic viscosity Kg/s.m : 1.71 x 10-5
Velocity through tubes = 0.206 x 4
9.20739x x(0.03082²)
=29.99 m/s
Reynolds number = vd/µ = 9.20739 x 29.99x 0.03082/1.71x 10-5
= 497679
Friction factor = f = (0.3964/Re0.3 ) + 0.0054
= (0.3964/4976790.3) +0.0054 =0.0131454
Pressure loss in straight length = flv²/2g d
=0.0131454 x 11.64 x 29.99²/(2x9.81x 0.03082) = 227.58 Mgc
= 227.58 x 9.20739 = 2090 kg / m² or mmWc
= 0.209 kg/cm².
Pressure loss in Bends,
For 180° bend = 0.47v²/2g
For 45° bend = 0.12v²/2g
Total bend loss = ( 3x0.47 + 2x 0.12) 29.99²x9.20739 /(2 x 9.81)
= 696.4 mmWc = 0.0696 kg/cm²
Pressure loss in entry and exit

65. Steam generator Ganesh kumar
Entry and Exit loss = 1.5 x 29.99² x 9.20739/(2 x 9.81)
= 633 mmWC = 0.0633 kg/cm²
Total pressure drop = 0.209+0.0696+0.0633 =0.3419 kg/cm²
5.0 DRUMS

66. Steam generator Ganesh kumar
5.1 INTRODUCTION
The recent development in boiler field involves, critical pressure boilers or once
through boilers in which drum is not necessary. But other than critical pressure
boilers, it involves two regimes of liquid and vapour phase where separation of
steam from liquid surface takes place in steam drum. The number of drums
required for a boiler depends on evaporator requirement, boiler pressure, stability,
manufacture experience on the type of configuration. Widely single drum boiler and
bidrum boiler are in use and in certain cases three or four drum design also available
but they are outdated now. For higher capacity and high pressure(more than 70
kg/cm^2(g) pressure) boilers are economical with single drum boilers and for
industrial process steam application where bidrum boiler works out to be economical.
In practice it was found that drum cost around 10% of the boiler pressure part cost.
5.2 OPTIMAL CONFIGURATION OF DRUMS
In world wide practice drums are designed in cylindrical shape with two dished end
with or without man hole at its end or cylindrical shell with tube sheet at its end or
cylindrical shell with water box but the uniqueness of the drum is cylindrical shape.
We consider the following three basic configuration for drum and let us analyze
about drum geometry.
1.0 why not drum be sphere?
2.0 why not drum be square?
3.0 why it is a cylinder?
Stress inside a sphere Sphere with internal diameter d
Y
X X
Y
Let the steam drum be a spherical shell of internal diameter ‘d’ and thickness ‘t’ and
subject to an internal pressure of intensity p
Bursting force P = p x projected area = p x x d²/4
Let 1 be the tensile stress induced in metal at section XX
Resisting force = 1x xdxt
Bursting force = Resisting force

67. Steam generator Ganesh kumar
px x d²/4 = 1x xdxt
1 = pd/4t
since any section taken in diagonal to a sphere is symmetrical, the stress at any
point in the metal of sphere will be equal and opposite and hence there will not be
any shear stress. The strain at any direction is given by
e1 = 1/E - 2/mE ( 1 = 2 )
e1 = 1/E(1-1/m) where E is Young’s modulus and 1/m is poisson ratio.
Hence
For the safety of shell thickness = t pd/4 1
If the shell has been riveted then factor e called efficiency factor to be introduced
i.e., ‘t pd/4 1e
Stress inside a square chamber
d
d
L
A square chamber has three planes of action and the stress and resisting force in all
the three direction to be checked for its stability
Let ‘d be the sides of the square section ,L be the length, t be the thickness and p
be the intensity of pressure
Let 1 be the tensile stress in direction xx. The force acting on xx be
= p x projected area
______
= p x d² + d² x L
_
Bursting force P = p x 2 d L…………………………………………..1
Area resisting this force = 2 x L x t
Resisting force = 1 x 2x L x t…………………………………………….2

68. Steam generator Ganesh kumar
Equating 1 and 2
1 = pd/ 2 t or t = pd/ 1
Similarly in zz direction 2 be the tensile stress
Bursting force P = p x d x L
Resisting force = 2x2xLxt
2 = pd/2t or t = pd/2 2
Similarly yy direction 3 be the tensile stress
Bursting force P = p x d²
Resisting force = 3 {(d +2t)² - d² } = 3x4xdxt (considering 4t² as
negligible)
3 = pd/4t or t = pd/4 3
Stress inside a cylinder
Z Y
X X p
p
Z Y
Let us consider the length and thickness of a cylinder be L and t whose mean
diameter is d and the internal pressure be p.
In this case section xx and zz experience same force and hence we have to
calculate force in xx and yy section only
Let 1 be the tensile stress in direction xx. The force acting on xx be
Bursting force = p x projected area = p x d x L
Resisting force = 1x2xtxL
Equating Bursting force and resisting force
= 1 = pd/2t or t = pd/2 1
This stress induced in circumferencial of the shell is called circumferencial stress of
hoops stress.
Similarly in longitudinal direction area resisting the pressure = x d²/4

69. Steam generator Ganesh kumar
Force acting on the end of the shell P = p x x d²/4
2 be the tensile stress on the section yy, equating the resistance offered by the
section yy to the total force on the end of the shell
2x xdxt = px x d²/4
2 = pd/4t or t = pd/4 2
This stress is longitudinal stress. Hence at any point in the metal of the shell there
are two principal stress namely hoop stress(pd/2t) and longitudinal stress(pd/4t)
acting perpendicular and parallel to the axis of shell respectively.
Greatest shear stress Qmax = ( 1 - 2)/2 = pd/8t
Circumferencial strain e1 = 1/E - 2/mE
Where E is the young’s modulus and 1/m is poisson ratio.
From the above equation minimum thickness required for a cylindrical drum can be
determined. Let be the permissible tensile stress for the material than for shell to
be safe major principal stress 1 should not exceed
From this pd/2t or t pd/2
Working pressure p 2x x t / d introducing ligament efficiency ‘e =pitch –hole dia
pitch
p = 2 x x e x t / d where d is the mean diameter.
As per IBR regulation 270, introducing addition of corrosion allowance to the
thickness the formula modified into,
W.P = 2 x x e x (t –c) where c is corrosion allowance 0.03 inches
(di + t - c) di is internal diameter
As per ASME PG27.1 , introducing addition of corrosion allowance to the thickness
the formula modified into,
W.P = 2 x x e x (yt –c) where c is corrosion allowance 0.03 inches
(do –2y( t - c)) do is outside diameter.
‘y is temperature correction coefficient.
Comparing the three geometries at their xx, yy and zz axis,

70. Steam generator Ganesh kumar
DESCRIPTION XX YY ZZ
SPHERE t =pd/4 t =pd/4 t =pd/4
SQUARE t =pd/ t =pd/4 t =pd/2
CYLINDER t =pd/2 t =pd/2 t =pd/4
For a same pressure, material, dimension the minimum thickness required for
sphere, cylinder and square is in the ratio of 1: 2 : 2.83 (thickness governing formula
shown in bold letters in the tabular column )
Sphere as a drum, there will be problem of having holding capacities and then
fabrication difficulties. Hence sphere can not be used in place of drum due to the
smaller volume.
For square when compared to circular drums it calls for 1.414 times greater
thickness in turn weight of the drum. Number of welds for making a square is more
where as a single fusion weld will do for circular drums. Square chamber has to be
placed in diagonal position for the lift of steam and therefore opening has to made in
the bends, which is very stress prone area. Practically speaking square sections are
inferior in strength compare to circular section. According to IBR regulation square
sections can be used as header and circular sections can be used for both drums
and header. The main advantage of square header is at one side openings for tubes
can be made and other opposite side openings for mechanical cleaning can be
made for straight inclined tubes. For this purpose some manufacturer prefer square
header, where inclined tubes are used.
5.3 STUBS AND ATTACHMENTS IN THE STEAM DRUM/SHELL
According to the IBR regulation 281, there are certain minimum requisite of
mountings, fittings and auxiliaries to be provided in pressure vessels for the safety of
the system. IBR regulation says that every boiler to be provided at least with
following
1.0 Two safety valves (minimum diameter 0.75 inches)
2.0 Two means of indicating water level gauge
3.0 A steam pressure gauge
4.0 A steam stop valve
5.0 A feed check valve
6.0 One feed apparatus(pump) when the heating surface exceeds 200 sq.ft and two
independent feed apparatus each such apparatus shall have a capacity of not

71. Steam generator Ganesh kumar
less than the maximum continuous rating of the boiler.
7.0 A blow down cock or valve
8.0 A fusible pug when boiler has internal furnace.
9.0 An attachment for inspector pressure gauge.
10.0 A man hole where the size and construction feasible.
In case of boilers fitted with integral super heaters an additional safety valve shall be
fitted at the end of the super heater outlet header.
There are also other attachments not in the IBR regulation such as steam separator,
air vent and pressure gauge stub more than one as desired by the manufacture or
user.
5.4 MAXIMUM PERMISSIBLE UNCOMPENSATED OPENING IN
DRUM
The opening in drum made for stubs, manhole, mud hole will weaken the plate which
calls for strengthening the plate by compensating plates. According to IBR the
maximum diameter for uncompensated opening is given by the following formula
_________
Maximum uncompensated opening = ½ (D +T)T + N
Where D is internal diameter of the drum
T is thickness of drum shell
N = 3 where E does not exceed 0.5
= 6 (1- E) in case exceeds 0.5, E is required thickness of seamless
un-pierced shell divided by thickness of shell(T).
Thickness of un-pierced seamless shell,
PD
= --------------- + 0.03 where e is 1 for un-pierced shell
2Fe -P
If the diameter of opening is less than the calculated value then it is not necessary to
give compensation for the drum. If it exceeds the limit compensation have to be
provided.
Area of compensation in Xaxis = dn x es where, dn nozzle internal diameter of
opening
5.5 SIZE OF THE DRUM
The steam drum must be large enough to accommodate drum internals, the
necessary steam surface for steam separation and water holding capacity. The
drum also provide sufficient space for change in water level that occur with change in
load. A sudden increase in steam demand cause a temporary pressure surge, until
firing rate is increased sufficiently for more steam generation. During this interval
due to lower pressure, steam volume throughout the boiler is increased (pressure

72. Steam generator Ganesh kumar
1/volume). This results in SWELL raise in water level in the drum. The raise in
water level depends on the rate at which heat and feed inputs can be changed to
meet the load demand. The higher the drum diameter better the control of raise in
water level
In GT HRSG during starting the steam generation cause sudden increase in drum
level and it is estimated that 70% of the evaporator water gets raised into the drum.
Hence the during start up the HRSGs are started with low water level and at once
the increased water level goes to high-high level blow down valve will be opened to
control the increase in water level. Normally blow down valve for HRSGs are sized
based on the same, by finding the 70% evaporator volume less the drum volume
between low to high-high. This difference in water has to be discharged with in 2 to 3
minutes.
STEAM SPACE
In the steam drum water and steam regions are clearly separated, the space above
the normal water level is called steam space. Steam space governs the steam
loading of the drum. Steam loading is the ratio between steam space and the
specific volume of the steam for pressure it is operating(M^3/m^3hr). Steam loading
for different pressures is fixed by the manufactures according to their practice or
experience. Steam loading is one of the important decision factor in deciding the
size of the drum. Steam loading ensures the velocity of separation in the inter phase
and steam carry over.
Let L be the length of steam drum and D be the internal diameter, r be the radius and
x be the distance from center line to normal water level.
o D
k
a b x nwl
‘h
c
L
Sin = ak/oa
= Sin-(x/r)
= (180 – 2 )
‘ab = 2 ac
= 2.r. Sin( /2)

73. Steam generator Ganesh kumar
The area of the arc oacb = x D² x
4x 360
The area of triangle oab =1/2 . ab . X
The area acba = area arc oacb – triangle oab
The steam space area in cylinder = D² - area acba
4
Steam space volume = ( D² /4 - area acba) (L +2l) + 2 volume of dished head
‘above normal water level
Volume of dished head for different shapes.
VOLUME UPTO NORMAL
TYPE FULL VOLUME WATER LEVEL
M^3 M^3
SEMI ELLIPSOIDAL . D3/24 .h².(1.5D –h)/12
HEMI SPHERICAL .D3/12 .h².(1.5D –h)/6
TORIS SPHERICAL
Ex 01 Estimate the shell thickness required for a single drum boiler, whose design
pressure is 73 kg/cm² and the internal diameter is 1300mm. The drum is located
outside the gas path.
Single drum the ligament efficiency is 1. However for practical estimation 0.95 is
considered.
As per IBR 270 regulation
Minimum thickness required T = PDi___ + C
2fE –P
= 73 x 1300__________ + 0.762
(2x 1230.4 x 0.95 –73)
= 42.66 mm(over this thinning allowance for rolling has to be considered)
Note. Stress value 1230.4 kg/cm² is calculated for the saturated temperature of
boiler design pressure. If the drum is exposed in gas path additional temperature

74. Steam generator Ganesh kumar
allowance of 28°C has to be added to saturated temperature and for this stress value
has to be considered.
Ex.02 Estimate the thickness required for the 2:1 semi ellipsoidal head as per IBR,
whose design pressure is 73 kg/cm² and the inside diameter is 1300mm. A opening
in the dished end of size 545 x 500 mm is made for man hole opening. The material
of construction is SA 516 Gr.70.
I 150
60
II 150
Inside 545x500 opening
Minimum required thickness for dished end calculated as per IBR 277 & 278
amendment 1995.
W.P = 2f(T-C)/(DK)
Where,
T = Minimum thickness
W.P = Maximum working pressure
D= Outside diameter = ( 1300+ 2xT)
‘f = Allowable Stress = 1230.4 kg/cm² (As per ASME section II part D 1995)
K= Shape factor
C= An additive thickness = 0.75mm
Reinforcement of large openings
‘d = Diameter of opening = 545 mm
A= Effective cross section of reinforcement in mm²(shaded portion)
T = Minimum thickness of dished head = 49 mm(assume)
Limit of Reinforcement
Tt = Actual thickness of the nozzle
L2 = d.Tt
_______
(544 x60) = 180.83 mm
Since the lenth of the nozzle projection inside the dished end is 150< 180.83, the
whole length is considered for compensation.
Area of reinforcement

75. Steam generator Ganesh kumar
I = 60 x150 x 2 =18000 mm²
II= 44.3 x 210 x2 = 18606 mm²
I + II = 18000+ 18606 =36606 mm²
Imaginary diameter = d –(A/T) = 545 - 36606/49
= -ve
Assuming di =1 Since the value is negative
Conditions
T/D < 0.1 di/D < 0.5
T/D = 49/1398
=0.035<0.1
di/D =1/1398 =0.0007<0.5 Both conditions are fullfilled.
‘di/ DT = 1/ ( 1398x49) = 0.0038
Ref. Fig 23-D The least valve of di/ DT =1
The same is considered for calculation
For Semi elliposidal head
H/D = 374/1398 = 0.267 (H is height of dished head for semi ellipsoidal ( (ID/4)+ T)
K = 1.1 from fig 23-D
Hence Minimum thickness = W.P x Dx K + C
2f
= 73 x 1398 x 1.1 + 0.75 = 47.08
2 x 1230.4
= 49 mm selected is sufficient. (Over the selected thickness dishing thinning
allowance has to be considered)
Ex 03 Estimate Manhole nozzle thickness for the above problem.
As Per IBR 338(a)
Thickness = P.D____ +C
2f + P
= 73 x 545_____ + 0 = 15.702
( 2x 1230.4 +73)
The nozzle thickness provided 60mm(Which is used in reinforcement calculation of
the above problem)

76. Steam generator Ganesh kumar
The provided thickness has to be cross checked, by compensation calculation.
As per IBR 279,
Inside diameter of nozzle dn = 545 –2x60 = 425
Actual nozzle thickness tn =60mm
Actual dished end thickness ts = 49 mm
Stress value for nozzle fn =1230.4 kg/cm²
Stress value for shell fs = 1230.4 kg/cm²
Equivalent thickness of nozzle(en)
= Pdn/(2f-P) + C
= 73 x 425______ + 0.75 =13.75
(2x 1230.4 –73)
Equivalent thickness of dished end (es)
= 47.08 mm
As per IBR three conditions,
‘b1 = minimum of 2.5ts or 2.5tn
‘b2= minimum of 2.5ts or 2.5tn of projection thickness
Ci = maximum of (ts+76) or dn/2
‘b1 = 2.5 x49 = 122.5 or 2.5 x60 =150
b2 = 2.5x49 = 122.5 or 2.5x60 = 150 or projection 150mm
Ci = 425/2 = 212.5 or (49 +76 =125)
The bolded values to be taken into account for calculation
Sectional area at X = dn xes
= 425x 47.08 = 20009 mm²
Sectional area at Y = 2(tn-en)b1(fn/fs) + 2tnb2(fn/fs) + 2(ts-es)Ci
=2(60-13.75)122.5(1230.4/1230.4) + 2x60x122.5x(1230.4/1230.4) +2(49 –
47.08)212.5
= 26574.25mm²
Area X < Area Y
Compensation shall be considered adequate when the sectional area X to be
compensated measured through the axis of the shell is less than the compensating
are Y. In other words area X should be less than Area Y for the design to be
satisfied, if X area is greater than area Y additional compensation pad to be
provided or thickness of nozzle to be increased.

77. Steam generator Ganesh kumar
In our case condition is satisfied and hence there is no need foe external
compensation pad.
Ex 04 Estimate the thickness required for end plate of thickness or closing the man
hole nozzle.
Man hole opening (max) = 545 – 2x60 = 425mm
Man hole opening (min) = 500 – 2x60 =380mm
As per IBR 342(b) ,
____
Minimum end plate thickness = {Opening dia x (PK/f)] + C
C= 1mm
For cylindrical ends
K= 0.19 for ends integral with or flanged and butt welded to the header.
= 0.28 for ends directly strength welded to the header
For rectangular ends,
Opening dia = Minimum width between the walls of rectangular header
K= 0.32 for ends integral with or flanged and butt welded to the header.
= 0.40 for ends directly strength welded to the header
In our case Ellipsoidal header is assumed as if circular header/nozzle and checked
for both major and minor axis.
_____________
= 425 x (73x0.19)/1230.4 +1 = 46.123 mm
Provided thickness = 65mm
Ex 05 Find maximum allowable uncompensated opening can be made in the above
drum.
As per IBR 187,
_________
Uncompensated Opening d = 0.5 (Di+T)T+N
Di = Internal diameter of the Shell
T = Thickness of shell
N = 3 where E does not exceed 0.5
= 3x (1-E) in other cases
0.5
E = The required thickness of a seamless unpierced Shell divide by Calculated
thickness
Di =1300mm
T = 42.66…………from problem 5.1
Ts = 73 x 1300____ + 0.762 = 40.505
(2x 1230.4 –73)
E= 40.505/42.66 =0.949

78. Steam generator Ganesh kumar
_______
N = 6x( (1-0.949) = 1.35
_____________________
Max. Uncompensated Opening size = 0.5 (1300+42.66)42.66 + 1.35
=119.66mm

79. Steam generator Ganesh kumar
6.0 EVAPORATOR AND ECONOMISER
6.1 INTRODUCTION
In steam generator boiling phenomenon takes place in furnace and evaporator
section only. Even though the terminology boiler is commonly used for whole steam
generator, boiler is applicable to the furnace and evaporator only. The other units
such as super heater, economiser, air heater etc., are only the heat exchanger used
to improve the cycle efficiency.
6.2 DIFFERENCE BETWEEN EVAPORATOR AND ECONOMISER
EVAPORATOR ECONOMISER
It is a part of the boiler circuit Economiser is a preboiler unit
and act as a heat recovery
unit.
Evaporator always precedes the economiser. Economiser follows the evap-
Since working fluid involved in an evaporator is orator and working medium in
boiler water and the temperature of the medium economiser is feed water.
is nearer to saturation temperature and it can not Temperature of feed water is
cool the flue gas beyond its working temperature. Well below its saturation temp.
corresponding to the working
pressure.
Log mean temperature prevailed in evaporator Log mean temperature in
Is less than economiser. Economiser is more due to the
Temperature gradient.
In evaporator both natural and forced circulation Economiser coils otherwise
can be provided which depends on design, called forced flow coil.
circulation ratio and manufacturer’s practice. Economiser inlet header is
connected to feed pump and
forced circulation is ensured.
Evaporator gets its working fluid (boiler water) Economiser gets its working
from steam drum. Fluid(feed water) from feed
tank and delivers into
steam drum.
The temperature difference between the saturation The temperature difference
temperature and gas temperature leaving the between the saturation
evaporator is called pinch point temperature and the water
leaving the economiser is
called approach point.

80. Steam generator Ganesh kumar
6.3 FIN EFFICIENCY
There are several types of fin types such as helical solid fins, serrated helical fins,
circular fins, rectangular (H type ) fins, peg pins, longitudinal fins are available due to
development in heat transfer study. Out of the various fin configuration longitudinal
fin configuration is the simplest fin configuration for estimating the fin efficiency, from
which correlation can be made to get other fin type fin efficiency.
a
hf
hb
hi l=b l l=0
hfo
At any cross section as in figure let Tc be the constant temperature of the hot fluid
everywhere surrounding the fin and let t be the temperature at any point in the fin
and variable. Let be the temperature difference driving heat from the fluid to the fin
at any point in its cross section. Then
= Tc – t
If l is the height of the fin varying from 0 to b
‘d /dl = -dt/dl
The heat within the fin which process through its cross section is
Q = ka d /dl…………………………………………….A
Differentiating the equation
‘dQ/dl = ka d² /dl²……………………………..1
Where a is the cross sectional area of the fin. This is equal to the heat which
passed into the fin through its sides from l=0 down to the darkened cross section.
If P is the perimeter of the fin, the area of the sides is Pdl and the film coefficient
from liquid to fin side, whether on fin surface or tube surface, is hf.
‘dQ = hf Pdl or dQ/dl = hf ……………………..2

81. Steam generator Ganesh kumar
Equation 1 and 2 are same and hence 1-2 =0
kad² /dl² -- hf =0
Rearranging,
d² /dl² - hf /ka =0
The direct solution of this equation is,
0.5 0.5
l(hfP/ka)
= C1e + C2e-l(hfP/ka) ……………………..3
_____
‘m = (hfP/ka)
The general soution is = C1eml + C2e-ml
Applying boundary layer concept,
At the outer edge of the fin l =0,
e = c1 + c2
Assume no heat enters from outside edge of the fin, then
‘d /dl =0 when =0 and C1 –C2 =0
C1 = C2 = e/2
Equation 3 becomes,
e = (eml+ e-ml)/2
= e cosh(ml)
Thus an expression has been obtained for the temperature difference between
constant fluid temperature and variable fin temperature in terms of the length of the
fin.
It is now necessary to obtain an expression for Q in terms of l
Differentiating the equation 2, we get
d²Q/dl² = hfPd /dl………………………………….4
/dl = ‘d²Q/dl² hfP
Substituting in equation A
Q = kad²Q/hfPdl²

82. Steam generator Ganesh kumar
d²Q/dl² = hfPQ/ka………………………………..5
d²Q/dl² - hfPQ/ka = 0
As before the solution is
0.5 0.5
l(hfP/ka)
Q = C3e + C4e-l(hfP/ka) ……………………..6
Q = C3eml + C4e-ml
At I =0,
C3 + C4 =0 , C3 = -C4 and dQ/dl =0
‘dQ/dl =hf e = mC3 –mC4 =0
C3 = hf e/2m and C4 = -hf e/2m
Q = hf e eml /2m - hf e eml /2m
Q = hf e /m(eml - eml )/2
Q = hf e /m sinh(ml)
The ratio of heat load Q to the temperature difference at the fin base is
Q/ = hf e sinh(ml)
m e cosh(ml)
Q/ = hfP tanh(ml) ……………………………………………………7
‘m
hf = Qm/ ptanh(ml)
Let hf is the heat transfer coefficient of fin and the bare tube and the heat absorbed
by fin through the heat transfer coefficient hf is getting transmitted into the tube by
means of base heat transfer coefficient hb. The ratio of heat transfer coefficient hb to
the heat transfer coefficient hf is termed as fin efficiency.
According to Fourier’s law of conduction,
hb = Q/ lP
η’ = hb/hf = Q P tanh(ml)
Qm lP

83. Steam generator Ganesh kumar
= tanh(ml)
ml
This equation applies only to the fin and not to the bare portion of the tube between
fins. The total heat removed from the annulus liquid and arriving at the tube inside
diameter is a composite of the heat transferred by the fins to the tube outside
diameter and that transferred directly to the bare tube surface. These may be
combined by means of the weighted efficiency.
If the heat tranferred through the bare tube surface at the tube outside diameter is
designated by Q0, then
Qo = hf Ao where A0 is the bare tube surface at the outside diameter exclusive of
the area beneath the bases of the fins. If there are N number of fins on the tube
l.P.Nis all of the fins surface. The total heat transfer at the outside diameter is given
by
Q = Qb + Qo = hb.l.P.N. + hf.Ao
=(hb.l.P.N.Ao/Ao + hf.l.P.N.Ao/P.l.N)
=(hb/Ao + hf /P.l.N)l.P.N.Ao.
=(l.P.N.tanh(ml) + Ao ) hf ………………………………………………..8
ml
Calling hfo the composite value of hf to both the fin and bare tube surfaces when
referred to the outside diameter of the tube, the fin effectiveness or weighted fin
efficiency is by definition η =hfo/hf. Combining equation 7 and 8
η =hfo/hf = l.P.N η’ + Ao
l.P.N +Ao
= (η’.At + Ao /(At + Ao))
inserting At –At,
=( At –At + Ao + Atη’) /(At+ Ao)
= (At+Ao -(1-η’)At)/(At+Ao)
Fin effectiveness η = 1- (1-η’)At/A where At is Area of fin surface
Ex.01. Design a feed water heater for a 10 tph boiler whose exhaust gas flow is
21355 kg/hr at an outlet temperature is 185°C and the desirable outlet temperature is
140°C. The feed water is available at 60°C.
I. Heat duty (gas side)
= 21355 ( 48.26 – 36.09)
=259890 kcal/hr

84. Steam generator Ganesh kumar
Where 48.26 kcal/kg enthalpy of gas at 185°C and 36.09 kcal/kg enthalpy of gas at
140°C.
II. Water outlet temperature to = 259890/10000 + 60
=85.98°C.
III. For the heat exchanger, gas is flowing inside the tube internal heat transfer
coefficient governs the overall heat transfer coefficient. The overall heat transfer
coefficient is around 85% of inside heat transfer coefficient.
Assume gas velocity inside the flue tubes 15 m/sec.
Flow area required to maintain the velocity in tubes
= 21355 (162.5 +273)/(3600x273x1.295x15) =0.48716m²
Tube selected 50.8 x 3.25
IV. Number of tubes = 0.48716 /( (0.0508 – 2x0.00325)² = 316 tubes.
V. Heat transfer coefficient
Gas properties at average temperature 162.5°C.
Thermal conductivity = 31.225 x 10-3 kcal/mhr°C
Kinematic viscosity = 28.577 x 10-6 m²/sec
Prandtl number = 0.6775
Reynolds number = vd/γ
= 15 x 0.0443 /28.577 x 10-6
= 23252.559
HTC = 0.023 Re0.8 Pr0.3 K/d
= 0.023 x (23252.559)0.8 x (0.6775)0.3 x (31.225 x10-3/0.0443)
= 39.2 kcal/m²hr°C
The overall heat transfer coefficient = 0.85 x39.2 =33.32 kcal/hrm²°C.
V Heat transfer area required = Q/(Ux Lmtd)
= 259890/(33.32 x89.15)
= 87.49m²
VI Heat transfer length

85. Steam generator Ganesh kumar
= 87.49/( x 0.0443) = 628 m
Number of tube required = 316
Tube length = 628/316 = 1.98 m
Considering tube length for welding to the tube sheet clearance 10 mm on both sides
Tube size chosen 2.0 m.
Ex.02. Calculate gas outlet temperature for a evaporator intended to generate
steam flow 15.28 kg/sec, pressure 20 bar(a), tube size 38.1 x 3.25 mm thk and
thermal conductivity 49.844 W/mK. The gas flow 19.03 kg/sec, inlet temperature is
1180°C and the pattern of tube arrangement furnace width is 3.06 meter and length
2.7meters, tube pitching width side 80mm and depth side 78mm, inline arrangement,
number of tubes in width side 36 and depth side 20. Heat transfer area of evaporator
246 sq.m. the evaportor was enclosed in a water wall having EPRS area of 26 sq.m
and gas is flowing 90° to evaporator tubes. Consider heat transfer effectiveness of
82% and 71% for enclosure. Partial pressure of water =0.1158 bar and carbon di
oxide = 0.1249bar.
Assume gas outlet temperature as 658°C
Heat loss by the gas = 19.03 *( 1418.1 – 745.94) = 12791.2 KW
Inside heat transfer coefficient = 14000 W/m²°C (assume)
Gas properties at average temperature = 919°C
Density kg/cu.m = 0.299
Dynamic viscosity kg/ms = 4.573 x 10-5
Prandtl number = 0.71
Thermal conductivity W/mK = 0.083
For inline arrangement
ST/d = 80/38.1 = 2.099; SL/d = 78/38.1 = 2.047
Arrangement factor fe = 0.98 (from tables)
Gas flow area = (3.06 x 2.7 – 36 x0.0381x2.7) = 4.558sq.m
Gas velocity = 19.03/(0.299 x 4.558) = 13.96 m/s
Reynolds number = 13.96 x0.299 x 0.0381/4.573x10-5
= 3478.5
hc = 0.287 x fex Re0.6 xPr0.364 x k/d
= 0.287 x0.98x 3478.50.6 x 0.710.364x0.083/0.0381
= 72.1 W/sq.mK
Radiation heat transfer coefficient
Beam length = 3.4 x(0.08 x0.078x1-0.00114)/( x0.0381x1) = 0.144876m

86. Steam generator Ganesh kumar
Attenuation factor = (0.8 +1.6x0.1158)(1-0.38x(1192/1000))(0.1158 + 0.1249)
((0.1158+0.1249)x0.14486)
= 0.6947
emissivity of gas = 0.9 x(1-e-0.69470.14486)
= 0.08617
hr = 5.67 x 10-8 x 0.85 x 0.08617x(11924 –4854)
(1192 –485)
= 11.53 W/sqmK
total gas side heat transfer coefficient = hc + hr =72.1+11.53 = 83.63 W/sqmK
1 do + d0 ln(d0/di) + 1
=
U hidi 2k ho
= 0.0381/(14000x0.0317) + 0.0381 ln(0.0381/0.0317) + 1/83.63
2 x 49.844
U = 82.55 W/sq.mK
Heat transferred to evaporator tubes,
Q = U x effectiveness x Ax lmtd
= 82.55 x 0.82 x 246 x 673 = 11206 KW.
Heat transferred to water wall encloser,
Neglecting metal resistance and internal conductance the convection heat transfer
coefficient be 82.55 w/sq.mK
Q = 82.55 x 0.71 x 26 x 673 = 1025 KW.
Total heat gained by evaporator and encloser is = 11206 + 1025 = 12231 KW.

87. Steam generator Ganesh kumar
7.0 AIRHEATER
7.1 INTRODUCTION
Air heater is a heat recovery unit, which is employed to recover heat from lower
temperature levels usually 240°C and below. Air heater is used in most of the
industrial and utility boilers. The construction of this equipment being a simple and
also a non pressure part. Gas temperature drop 240°c to 170°C can be effectively
achieved by means of air heater there by 4% boiler efficiency can be improved. In
cases of low capacity , low pressure boilers manufactures prefers to have an air
heater instead of economiser. This is to avoid problems arised due to economiser
like oxygen pitting, corrosion higher feed water temperature, necessity of having
deaerator more over economiser is a pressure part. These are avoided by means of
air pre heater
7.2 TYPES OF AIR HEATER
The basic classification of Air heater is based on their operating principle
1.0 Recuperative
2.0 Regenerative
Recuperative Heat Exchanger:
In recuperative heat exchanger heating and cooling medium are separated by
partition and the heat is transferred from one fluid to other by means of conduction
and convection and there is no moving part employed in recuperative heat
exchangers. According to the construction it is again sub divided into tubular or plate
type air heater.
Tubular air heater
This usually consists of large number of steel tubes of either welded or expanded
into the tube plate at the ends. Either gas or air may be designed to flow through the
tube. Gas through the tube normally requires higher size tube and vertical flow to
reduce fouling. Single or more passes on the gas side and multi pass to and fro on
the air side usually fits in with shorter tube length so as to facilitate maintenance o
surfaces due to corrosion and fouling. In some cases instead of using of boiler flue
gases separated external firing is used particularly during starting
Plate air heater
This comprise of parallel plates which provide alternate passage for gas and air. This
type is simple and compact to that of tubular type. The narrow passes between
plates make cleaning tedious but with shot cleaning method it is improved. But
replacement is a major task and nowadays it is not used in industry or utility boilers.
Regenerative Heat exchangers
In regenerative heat exchangers the heating fluid heat the closely packed matrix to
raise its temperature which is again placed in cooling medium to transfer the heat to

88. Steam generator Ganesh kumar
cooling fluid. In regenerative exchangers either matrix or the hoods are rotated to
achieve this. This is sub divided by the names of inventors into
i) Ljungstrom type
ii) Rothemuble type
It is beyond the scope of this chapter to define regenerative heater.
Recent technology involves usage of heat pipe in air heater.
A heat pipe is a device with a high thermal conductance. It is based on the large
latent heat of vapourisation/condensation of working fluid inside the heat pipe.
Hence all heat pipes are completely sealed one and the working fluid is the heat
transfer element between the hot and cold fluid passing through its ends separated
by a partition plate. In heat pipe heat is absorbed in the evaporator portion of the
heat pipe, and of vapourises the internal working fluid. The vapour is transferred to
the condenser portion on the other end where the fluid surrenders the heat to the
cold surrounding and condenses. This fluid is then returned to the evaporator and
the cycle is repeated. Heat pipe contains a working fluid specifically selected for
operating temperature range and hence heat pipes are arranged in modules
containing different working fluid for the range of temperature.
Types of heat pipes differ according to the method used to transport the working
fluid.
a) Thermo-syphon heat pipe.
The name itself indicates heat pipes uses gravity, buoyancy and vapour pressure
forces to transport the phase of the working fluid. It requires the heat source to be
below the heat sink that is evaporator must be below the condenser. The vapour
generated in the evaporator travels upward because of the buoyancy and expansion
forces. In the condenser the vapour is condensed to a liquid and turns to the
evaporator by gravity. The thermo syphon heat pipe can be made to function
efficiently with an orientation of approximately 10 to 90 degree from horizontal.
b) Capillary action heat pipe
The working fluid is transferred through capillary action and it does not require
gravity to return the condensate to the evaporator section. This type of heat pipes
contain an internal element known as wick, which spans the length of the pipe. The
wick may be made of gauze wire mesh or other materials that provide closely
spaced longitudinal paths for returning condensate. The path spacing is chosen
based on the working fluid to obtain high capillary forces and there are no orientation
restriction for this type of heat pipe.
There should not be gap between heat source and sink in the heat pipe. If a gap
exists then it can be spanned by an adiabatic(non heat transfer) section of heat pipe.
This adiabatic region is created by externally insulating the appropriate section of
heat pipe.

89. Steam generator Ganesh kumar
Toluene and naphthalene are commonly used heat transfer fluids in air heaters. The
major advantage of heat pipe design compared to conventional tubular heat
exchanger commonly used are that it is isothermal and can be built with better seals
to reduce leakage. Each pipe is fixed only at the center support plate separating the
hot flue gas and cold air. The ends of each pipe are free to expand and contract
independently. Since heat pipe has no hot and cold spot the possibility of cold end
corrosion is reduced.
A COMPARISON OF VARIOUS AIR HEATER TYPES
DESCRIPTION RECUPERATIVE REGENERATIVE HEAT PIPE
(TUBULAR) (ROTARY)
Size Large Compact Compact
Weight High Medium Medium
Corrosion Cold end corrosion Avoided Avoided
Will be there
Cleaning Water wash on load sootblowing/on load soot blowing
Water wash water wash
Pressure drop High Medium Medium
Cost of air heater Low High High
Maintenance Low High Periodic
Efficiency 65% 90% 90%
Suitability Industrial outdated Utility boilers
boilers
Duct routing Length required Simple and Simple and
More less costly less costly.
Cleaning of Air heater
Air heater are in low temperature operating region, it is subject to metal temperature
below dew point, which is primary cause of condensation of acid or moisture from
flue gas. Minimum metal temperature occurs at cold end where most fouling and
corrosion occurs. If the air heater is not cleaned at frequent intervals the fouling
increases and velocity increases in other tubes which cause severe erosion of high

90. Steam generator Ganesh kumar
velocity tubes and corrosion of fouled tubes. These problems can be eliminated by
frequent cleaning of air heater.
On load cleaning
Recuperative type proven practical method is by shot rain and regenerative type can
be cleaned by fixed or moving soot blowers. Heat pipe which uses finned tubes can
be cleaned by soot blowers.
Off load cleaning
If not provided with on load cleaning, air heater has to be cleaned at intervals during
shut down manually or mechanical methods. Large quantity of cold or warm water
has to be used for flushing the air heater since small quantity of water will actually do
harm by making deposits compact and hard. If deposit is severe soda ash solution
will assist in dissolving it.
Leakage
Leakage in air heater can be checked during operation by analyzing the flue gas for
carbon di oxide drop across the air heater. This leakage is through the tube leak
joint corrosion, erosion holes etc., in case of tubular air heater. While through seals
in case of regenerative air heater.
7.3 ADVANTAGES OF AIR HEATER
In addition to increase in boiler efficiency the other advantages that may result are
1.0 Stability of combustion is improved by use of hot air.
2.0 Intensified and improved combustion which result in faster load variation and
fluctuation.
3.0 Permitting to burn poor quality coal.
4.0 High heat input to the furnace and hence high heat flux and high heat transfer
rate.
5.0 Less unburnt fuel particle in flue gas, thus combustion and boiler efficiency
further improved.
6.0 In case of FBC and pulverized coal combustion hot air can be used for drying
the coal as well as for transporting the coal into furnace.
7.0 This being a non pressure part shut downing of unit is not required due to
corrosion of heat transfer surface or failure.
8.0 High temperature of inlet cause reduction in carbon mono oxide.

91. Steam generator Ganesh kumar
7.4 HEAT TRANSFER IN AIR HEATER
The operation region involves low temperatures, where heat transfer takes place
predominantly through convection and little conduction. Since the thickness of tube
is very less conduction heat transfer can be neglected.
Convection heat transfer takes place in two modes
1) Gas to metal heat transfer
2) metal to air heat transfer.
As the air heater is located in cold end of heat transfer side, non-luminous radiation
will be very negligible and hence the same can be eliminated. (Above 400°C non
luminous radiation heat transfer will be negligible).
The over all heat transfer coefficient can be estimated as per the derivation given in
the super heater chapter. In that equation neglecting fouling resistance and metal
resistance we will get
U = ho x hidi/do
(ho + hidi/do )
7.5 PRACTICAL GUIDE LINES FOR DESIGNING AIR HEATER.
1.0 Gas or air velocity inside tube has to be limited to maximum of 15 to 18
meters/sec and out side tube has to be limited to 6 to 7 meters/sec beyond
which hysterisis will occur.
2.0 The air or gas passes over the tubes, where the lesser number of tubes deep
arranged this is to avoid higher pressure drop.
3.0 The unit height of tube should not exceed 3.5 meters to avoid vibration problem
and preferable height be 2 to 3m between tube sheets.
4.0 In FBC and conventional boilers it is desirable to heat the air not more than
180°c, in order to have a better cooling for grate or Distributed plate.
5.0 Normally a tube diameter 50.8 mm or 63.5 mm will be used.
Ex 01 Design tubular air heater for 200 TPH steam generator having following
parameter,
Gas flow kg/hr = 272160
Air flow kg/hr =213156
Air inlet temperature = 26.67°c
Gas Inlet temperature =271.3°c
Gas Outlet temperature =160°c
Let the air heater be cross type and staggered arrangement and the boundary
condition of air heater be

92. Steam generator Ganesh kumar
Flue gas inlet temperature °c Tgi
Flue gas outlet temperature °c Tgo
Air inlet temperature °cTai
Air outlet temperature °cTao
Mass of gas flow kg/hr mg
Mass of air flow kg/hr ma
Heat duty Ql = mg (Hgi– Hgo) kcal/hr
= 272160 ( 75.93-44.1244) = 8.656 x106 kcal/hr
Air outlet temperature = {Ql/(cpo *ma)} + Tai
=8.656 x106 + 26.67 = 194.47
0.242x213156
Assuming optimal velocity of 15m/sec
Gas flow area = flue gas flow * (Tgav+273)
3600* 273 * 1.218*15
= 272160 x ( 215.65 +273)/(3600x273x1.218x15) = 7.4065 m²
Assume 63.5 x 2.06 mm tube size
Number of tubes = GFA * 4/( * d²)
= 7.4065 x4 / x 0.05938² = 2675
Arrangement 88 x 30 =2640 tubes (velocity works out to be 15.198m/s)
Height of air heater tube may be considered as h
‘h = 5.5 m ( Intermediate support to be considered)
Air flow area = h(num. of tubes in air path * pitch – num. of tubes in air path * od)
= 5.5 (88x 0.080 – 88 x0.0635) = 7.986 m²
Air velocity = ma * (Taav +273)
3600 *273 * AFA* density of air
= 213156 x( 110.335 +273)/ (3600 x 273x 7.986 x 1.293)
= 8.051 m/s
Properties of gas at mean gas temperature,
Specific heat = 0.2856 kcal/kg°C
Th.conductivity = 0.0383 W/m°K =0.0329 kcal/mhr°C
Viscosity = 2.39 x 10-5 kg/ msec
Density of gas = 0.6804 kg/m3

93. Steam generator Ganesh kumar
Properties of air at mean air temperature,
Specific heat = 0.241 kcal/kg°C
Th.conductivity = 0.03248 W/m°K = 0.0279 kcal/mhr°C
Viscosity = 2.2216 x 10-5 kg/ msec
Density = 0.9208 kg/m3
Gas side heat transfer coefficient,
Nu = 0.023 Re0.8 Pr0.3
Reynolds number = 15.198 x 0.05942 x 0.6804/2.39 x 10-5
=25711
Prandtl number =0.2856 x 2.39 x 10-5 x3600/0.0329 =0.7469
‘hg = 0.023 x 257110.8 x 0.74690.3 x 0.0329/0.05942
= 39.36 kcal/hrm²°C
Air side heat transfer coefficient,
Nu = 0.3 Re0.6
Reynolds number = 8.016 x 0.0635 x 0.9208 /2.2216 x 10-5
= 21097
‘ha = 0.3 x 21097 0.6 x 0.0279/0.0635 = 51.81 kcal/hrm²°C
1/U = do/dihg + 1/ha + do ln(do/di)
2Km
1/U = 0.0635 + 1/51.81 + 0.0635 x Ln(0.0635/0.05942)
0.05942 x 39.36 2 x 49.14
U = 21.46 kcal/hrm²°C
Considering an effectiveness factor 0.8
The Overall heat transfer coefficient is 21.46x0.8 = 17.168 kcal/m²hr°C
Assuming Counter flow
Lmtd = (160-26.67)- (271.3 – 194.47)
Ln[(160.-26.67)/(271.3 –194.47)]
= 102.49°
As the flow is not perfectly counter flow, correction factor in lmtd of 0.95 has to be
made.
Corrected Lmtd = 0.95x 102.49 = 97.36 °
Heat transferred = U A Lmtd

94. Steam generator Ganesh kumar
Heat transfer area arranged in two blocks
A = 8.656 x 106/17.168 x 97.36
Required heat transfer area = 5178.65 m².
Provided heat transfer area in two blocks = 2x x0.0635x5.5x88x30 = 5793 m²
Gas side Pressure drop,
‘f = 0.184/Re0.2
f = 0.184/ 257110.2
= 0.02414
Pressure drop = 0.02414x5.5/0.05942 x 15.198² x 0.68/(2x9.81)
=17.89 mmWc
For two Passes = 17.89 x2 = 35.78 mmWc
Entry and exit loss = 1.5 x 2 x 15.198² x 0.68/(2x9.81) = 24.016 mmWc
For two passes gas side
Total pressure drop = 35.78 + 24.016 = 59.796 mmWC
Air side Pressure drop,
‘f = 1.632/Re0.15
f = 1.632/ 210970.15
= 0.3665
Number deep = 30 nos/pass
Pressure drop = 0.3665 x 30 x 2 x 8.051² x 0.9208/(2x9.81) =66.89 mmWC

95. Steam generator Ganesh kumar
8.0 DUST COLLECTOR
8.1 INTRODUCTION
The advent of scientific and technological revolution has brought tremendous
benefits to mankind and also given rise to many problems related to our
environment. Rapid industrialization as associated with scientific exploitation of
natural resources and this has resulted in contamination of three major constituents
of our environment – air, water and soil.
Combustion process is the main source of elements that pollute the atmosphere. A
500MW thermal power plant produces 2000 tones of ash every day, it also produces
about 50 MT of sulphur di oxide (considering sulphur content in coal as 0.5%).
8.2 EFFECTS OF AIR POLLUTION
The consequences of air pollution have to be gauged from the factors like
1.0 First the amount of substances polluting the atmosphere are eventually
accumulating
2.0 The distribution of pollutants is not uniform and in certain places, their
concentration is in-admissibly high
3.0 Very small concentrations of certain substances can be highly dangerous.
4.0 The damages caused by air pollution is enormous. It involves losses in all the
spheres of the national economy.
PARTICULATE EMISSION
Smog is closely linked to air pollution. Many cities in the world, particularly in
London catastrophe of 1952, roughly 4000 people are known to have lost their life
due to smog. The phenomenon of smog is a mixture of smoke and fog, has been
related to temperature inversion, the heavier cold air saturated with industrial and
transport emissions remains near the grounds compressed under a dome off light
warm air. Hence it is necessary for industry to let the particulate emission to a
maximum height possible. Pollution control board framed minimum height required
for letting out the smoke in atmosphere.
CARBON MONO OXIDE
The unburnt carbon mono oxide in the presence of oxygen react and form carbon di
oxide. This carbon mono oxide loss reduce efficiency of power cycle and also it
affects human population causing branchits problem.
CARBON DI OXIDE
The effects of air pollution cannot be completed without a reference to the green
house effect. The dramatic increase in the carbon di oxide level in today’s
atmosphere is 27% higher than the year 1850. Scientists believe that the growing
burden of carbon di oxide and other gases are the causes for change in earth’s

96. Steam generator Ganesh kumar
climate. ( Recent ELINO effect increase in pacific temperature cause shift in
monsoons) Carbon di oxide allows most of the solar radiation to penetrate the
atmosphere but prevents part of the heat re-radiated by land and water bodies from
escaping into space. It is estimated that the earth’s mean temperature could rise 1.5
to 4.5°C by the middle of next century if green house gases continue to increase at
current rate. This impact could be even greater at the poles. The rise in global
temperature could result in shift in rain fall patterns, melting of the polar ice caps and
raising of ocean levels.
The wide ranging effects of atmospheric pollution under lines the need for controlling
the industrial and other emissions and heaving clean air in our atmosphere. The
cost of providing clean air may be more, but the cost of polluted air is much more.
8.3 AIR QUALITY STANDARDS
Apart from particulate matter the other major air pollutants are gaseous pollutants. In
general, gaseous pollution is classified into two categories, namely inorganic and
organic vapours. In the former class there are sulphur di oxide, nitrogen oxides,
hydrogen sulphite, hydrogen chloride, hydrogen fluride, carbon mono oxide and
ammonia. In the latter are the hydrocarbons, mercaptans, keytones and esters.
As per recent BIS standard the following limits have been recommended. The
ambient air standards as propounded by EPA(Environmental protection agency) of
USA and adopted by BIS are shown in the following table.
INDUSTRIAL RESIDENCIAL SENSITIVE
ENVIRONMENT ENVIRONMENT AREA
POLLUTANTS PPM PPM PPM
PARTICULATE 350 200 100
SULPHUR DI
OXIDE 120 80 30
CARBON
MONO OXIDE 5000 2000 1000
NITROGEN
OXIDE 80 30 12
8.4 AIR POLLUTION CONTROL DEVICES
Air pollution control equipment is broadly classified into two categories- one controls
particulate matter and the other controls gaseous emissions. Various consideration
are required to be made while selecting a control device for an industry. Mostly
industries face problem with particulate matter collection and gaseous emission has
to be controlled by proper pre treatment and maintaining certain design parameters
such as dew point. For particulate matters, some of the major factors are collection
efficiency of the device, initial cost operating and maintenance cost, space required

97. Steam generator Ganesh kumar
arrangement, material of construction. Important factors considered in this
connection are as follows.
1.0 Particulate characteristics such as particle size range, shape, density and
physical, chemical properties such as agglomeration tendencies, corrosiveness,
hydroscopic tendencies, stickiness, inflammability, toxicity, electrical
conductivity and others.
2.0 Gas characteristics such as temperature, pressure, humidity density, dew points
of condensable components viscosity, electrical conductivity etc.,
3.0 Process factors such as volumetric gas emission rate, particulate concentration,
variability of material flow rates, collection efficiency allowable pressure drop,
product quality requirement etc.
4.0 Operational factors such as floor space, pressure, temperature, corrosion etc.
In boiler applications bag filter, ESP, Wet scrubber and centrifugal mechanical dust
collector are quite widely used for fly ash removal. So let us try to understand how
these systems are operating and the merits and demerits of these systems.
COLLECTION PRESSURE POWER
EFFICIENCY IN DROP IN CONSUMED
TYPE PERCENTAGE MM WC BHP PER100
WEIGHT M^3/MIN
ELECTROSTATIC 80 TO 99.5% 2 TO 20 0.2 TO 0.5
PRECIPITATOR
FABRIC FILTER 95 TO 99.5% 50 TO 150 0.8 TO 2.6
CYCLONE 50 T0 95% 25 TO 100 0.4 TO 2.2
SEPARATOR
WET SCRUBBER 75 TO 99% 50 TO 400 0.8 TO 6
CENTRIFUGAL CYCLONE DUST COLLECTOR
It is the cheapest and most effective dust collector. In the multi cyclone device there
is a nest of cyclones in parallel having one header and dust hopper. Here the
centrifugal action removes the particulate from the gas stream. The larger the
particles, the more easily or they likely to be collected due to their proportionate
higher ratio of centrifugal force imparted to the particle of given mass. Dust
collection depends on the radius through which the gas is turned, the smaller the
radius the higher the centrifugal force. Centrifugal action throw the heavy particles to
the side of the cyclone, where the dust can slide down to a hopper at the bottom.
The cyclone works with two vortex. The cyclone possesses a high separation factor

98. Steam generator Ganesh kumar
given as the ratio of the radial velocity in the cyclones to the stokes velocity in the
setting chamber.
Large separation factor requires high tangetical velocities and small diameters, both
of which result in a large pressure drop. In India the industrial application of both
large and small diameter cyclones are very common. The multi cyclone dust
collector can effectively remove dust particle of diameter above 50 microns and
having a particle density of 600 kg/m^3. So it is not always possible to meet the
stringent emission levels to PCB norms, it becomes essential to provide either a bag
house or ESP.
BAG HOUSE
Basically, bag house is a large metal box divided into two chambers of plenums, one
for dirty air and one for clean air. Rows of cloth bags form a partion or interface
between the plenums. A polluted gas stream is ducted into the dirty air plenum, and
is exhausted into the atmosphere through a stack. Almost 100% of the particulate
matter in the process effluent can be filtered out by bags if the system is designed,
operated and maintained properly but bag house will not be suitable for fuels like
bagasse where burning particles may enter into bag house causing burning of bags.
When a new bag house is first started up with factory fresh bags, some stack
emissions are usually visible. This is because the filtering medium (which is the
bags made of fabric called Ryton and bag size is of 5m length and 0.3m diameter) is
porous and allows a certain amount of very fine particulate matter to pass through
the interstices between the fibers. After a short period of operation, a dust cake
builds upon the surface of the bags and become actual filtering medium. The bags,
in effect act primarily as a matrix to support the dust cake.
The dust cake is desirable only upto a point, when that point is reached the bag must
be cleaned properly other wise the pressure drop through the filter system will
continue to increase. At high pressure drops, particles of dirt can be forced into the
bag filters causing bags to become blinded. When this happens, air flow is restricted
and the bags may have to be replaced or removed and cleaned.
Practical guides for proper maintenance of bag filter.
Maintenance personnel must learn to recognize the cause of the difficulty, and to
mend it either by in planed action or by contact with the manufacture. High pressure
drop across the system is a symptom for which there are many possible causes,
example, 1. Difficulties with the bag cleaning mechanism 2. Low compressed air
3. Loose bag tension are usually available the reasons for high pressure drop and
corrective action to be taken appropriately. I have furnished below some of the
check points to be taken care while doing routine bag house inspection.
----------------------------------------------------------------------------------------------------------------
COMPONENTS CHECKLIST
----------------------------------------------------------------------------------------------------------------
Bags Worn, damaged bags, condensation on bags,

99. Steam generator Ganesh kumar
improper bag tension, loose, damaged or
improper bag connections.
Differencial pressure Steadiness of pressure drop(should be read
daily)
Dust removal system Worn bearings, loose mountings, deformed
parts, worn or loose drive mechanism, proper
lubrication.
Bag house structure Loose bolts, cracks in welds, corrosion, holes
(housing, hopper)
Solenoids, pulsing valves(reverse Proper operation
Pulse system)
Compressed air system Proper operation, lubrication of compressor,
(reverse pulse, plenum pulse) leaks in header, piping. Proper mounting.
Dampers valves Proper operation and synchronization, leaking
(Reverse flow, plenum pulse) cylinders, bad air connections, proper
lubrication, damaged.
Doors Worn, loose, damaged or missing seals, proper
tight closing.
----------------------------------------------------------------------------------------------------------------
ELECTROSTATIC PRECIPITATOR
An electrostatic precipitator (ESP) consists very basically of a precipitator chamber
and an electrical unit. The precipitator chamber includes discharge and collection
electrodes, an electronic cleaning system, gas distribution devices, and a precipitator
shell and hopper. The electrical unit is made up of a power supply, high voltage
transformers, rectifiers and precipitator bus sections.
ESP is physical process by which a particulate suspended in a gas stream is
charged electrically and then, in the influence of an electrical field, is separated and
removed from gas stream. The system that does this consists of positively charged
collecting plate in close proximity to a negative charged emitting electrode. A high
voltage charge is imposed on the electrode and the grounded collection surface.
The dust particles pass between the electrode and grounded collection surface. The
dust particles pass between the electrodes, where negatively charged and diverted
to the positively charged collecting plates.
Periodically, the collected particles must be removed from the collecting surface.
This is done by vibrating (usually by rapping) and or water washing the surface of the
collection plates to dislodge the dust. This dislodged dust drops into a dust removal
system and is collected for disposal.
Practical problems faced in ESP

100. Steam generator Ganesh kumar
Discharge electrode failure is the primary cause of operational breakdown. After this
are rapper malfunctions, insulator failure, shorts caused by dust build up, hopper
plugging and transformer- rectifier failures. Most of these problems occur when
preventive measures are not used. For example discharge electrode failures can be
reduced if the hoppers are properly discharged and cleaned to prevent grounding out
and burning off the discharge electrodes.
Fly ash build up on the collecting plates should normally be about 1/8 to ¼ inch thick.
If the build up exceeds this thickness, the intensity of the plate rappers should be
increased. If the collecting plates are clean, this may be an indication of high gas
velocity or low operating voltage.
Collecting plates should be checked for proper alignment and spacing. Hangers and
spacers at the top and bottom should be adjusted so that they do not bind the plates
or prevent proper rapping. It is necessary to check for corrosion.
Hoppers should be checked periodically to be sure they empty properly and to
inspect for corrosion, which is likely to be most severe at points where at points
where dust builds up. The heating system and insulation on the hoppers are
checked to prevent condensation.
Insulator compartments and housings must be checked frequently, leakage of
corrosive gases from the precipitators into this area can cause dirt deposit that result
in breakdown of electrical insulators.
9.0 WATER CHEMISTRY
9.1 INTRODUCTION
All system obeys law of conservation, energy can be neither created nor destroyed
but energy can be converted from one form to another. In the system of power
cycle, water is basically most important raw material which acts as energy
transferring element.
H2O↓(w) + Heat à H2O ↑(s)
As the raw water contains dissolved solids, suspended matters like mud, clay,
calcium and magnesium salts free minerals and dissolved gases like oxygen and
carbon di oxide, hydrogen sulphite etc., it is required to be treated before it is fed into
the boiler to prevent boiler damages and to get required steam purity.
Most of the boiler shut downs are arise due to uncontrolled water treatment, an
unnoticed boiler shut down runs in loss of money to company. Therefore it is
necessary to have a controlled water quality for the boiler.
Although the manufacturers supplies and recommends water quality, it is upto the
plant personnel to meet the required water quality. It is not possible to suggest one
type of water treatment for all the industry as composition of different water supply

101. Steam generator Ganesh kumar
vary greatly from place to place and from time to time. Therefore selecting water
treatment plant starts with analyzing the raw water. The water should be analyzed
periodically, if the supply or availability is seasonal, the treatment plant selected is
based on the worst analysis made by the study conducted in series for minimum of
one year. In high pressure boiler plant, engineer should not hesitate to stop the
boiler if water treatment plant fails or water quality is poor.
9.2 NAMES OF WATER FLOWING IN THE POWER PLANT CYCLE
a) Raw water - Water as received from the source of supply
b) Treated water - Any treatment added.
c) Softened water - Hardness removed water
d) Condensate - Condensed steam
e) Demineralised
water or
DM water - All ionisable solids removed by ION – Exchange
treatment
f) Deaerated
water - Oxygen content removed in deaerator
g) Feed water - Any mixture of the above to pass through feed pump.
h) Boiler water - Water present in a boiler when steaming.
i) Make up water - Added to make up for losses.
SOURCES OF WATER
Surface water: Ponds, Lakes, Reservoirs, Streams and rivers are the sources of
surface water.
Ground water: Wells, Mines and Springs are sources of ground water.
9.3 MAJOR IMPURITIES IN WATER
In water the major dissolved impurities are bi-carbonate, sulphates, chlorides,
nitrates of calcium and magnesium. Minerals have two components Radial or acidic
component and metallic component. In the water analysis as there are various solids
dissolved in various percentage, the concentration of dissolved salts impurities are
reported in CaCo3 terms only.
Temporary hardness are due to calcium, magnesium bi carbonate salts. These does
not cause hard scales and these can be removed by boiling the water. Calcium and
magnesium bi carbonates are broken down by moderate heating(100°C) into
relatively insoluble mono carbonate and carbon di oxide as given in the reaction.

102. Steam generator Ganesh kumar
The less soluble salt like calcium carbonate precipitate and settle in the boiler water
which can be removed by blow down.
Permanent hardness are due to sulphates, phosphates of calcium and magnesium
salts. These form sticky hard scales and the amount of dissolved salts in water is
the measure of hardness. Among all, calcium is the principal scale forming agent
and particularly calcium sulphate. Solubility of Ca SO4 decreases with increase in
water temperature.
9.4 EFFECTS OF VARIOUS IMPURITIES IN BOILER WATER
a) Acidity
Boiler water should be always alkaline in nature. The alkalinity is to be maintained
for the following reasons.
1.0 To keep magnetite layer (Iron oxide) uniform, which protects the metal and
minimize corrosion and to promote formation of magnetite layer as and when
impaired.
2.0 To keep sludge in floating condition.
3.0 To keep silica in soluble condition preventing precipitation and carry over.
4.0 To neutralize any acid generated in the system.
Highly acidic water dissolves metal and mild acidic water hastens pitting and oxygen
corrosion.
b) Alkalinity
Alkalinity should also be maintained as per recommended value based on drum
pressure ratings. Higher values result in foaming leading to carry over and also
caustic embrittlement. In highly concentrated alkali the protective film of magnetic is
dissolved forming a mixture of ferrite and hypo ferrite irons. But at low concentration
the layer of magnetite is porous. In the presence of pororus deposit hydroxyl ions
tend to concentrate between metal and deposit leading to a typical corrosion pattern
called caustic gauging.
c) Hardness
Most of the calcium and magnesium salts present in the water contribute hardness.
Ex: calcium carbonate, magnesium silicate etc., there salts form sludge and calcium
sulphate, calcium silicate etc., form scales on the evaporating surface. Hardness is
measured in terms of mg/lit or ppm as CaCo3.
Alkaline hardness
Hardness caused by the presence of bicarbonate, carbonate, hydroxide of calcium
and magnesium (temporary hardness). Alkalinity is the concentration of alkaline
salts. Bicarbonate alkalinity due to bicarbonate salts, caustic alkalinity due to
hydroxide salts (P or phenophthaline alkalinity).

103. Steam generator Ganesh kumar
M- alkalinity is the total alkalinity which is a combined hardness of bicarbonate and
hydroxide salts. Hence in an analysis P never exceeds M alkalinity.(if P= M then
alkalinity is due to hydroxide, if P=0 then alkalinity is due to bicarbonate and
carbonate)
Total hardness(H)
Total hardness is the concentration of calcium and magnesium salts. If total
alkalinity (M) exceeds total hardness(H) then the hardness due to bicarbonate,
carbonate and hydroxide only the value(M-H gives sodium bicarbonate present in
water)
Total alkalinity (M) less than total hardness (H) then the value (H – M then gives
carbonate hardness) ie. Remaining is non alkaline hardness.(chlorides, nitrates,
sulphate)
If total hardness is equal to total alkalinity the only carbonate hardness is present.
d) Oxygen
Water at room temperature always contain oxygen in dissolved form of air. Oxygen
promotes corrosion. The rate of wastage and thinning of tubes in highly acidic water
is enhanced heavily by the presence of oxygen. Oxygen is released from water on
heating.
e) Total dissolved solids (TDS) mg/lit of CaCo3
All the salts dissolved in the water (sodium, calcium, magnesium ) are together
accounted against total dissolved solids. The effects of the total dissolved solids in
feed water depends on the type of salt and the type of boiler. The effect are also
complex due to mutual action between the various salts. Generally, presence of
chlorides, iron salts accelerate corrosion. Presence of other salts increase scale
formation, priming (sweeping action of suspended solids towards steam outlet) and
foaming ( Persistent bubble formation). Priming and foaming cause carryover of
solids and heavy fluctuation in water level. A high TDS in the boiler water will also
increase steam contamination. 0.1 mg/lit is the limit to TDS in steam used for TG
sets. Approximately value of TDS in mg/lit can be obtained by measuring the
electrical conductivity in micro siemens/cm. It is found that TDS will be
approximately half that of conductivity.
Conductivity
Pure water is a poor conductor of electricity, but water contain ionized impurities
such as salts, acids, alkali is conductive and it can be used to define purity of water.
The conductivity produced by the presence of given concentration of impurity
depends upon nature of impurity and temperature of water. It is measured in terms
of micro siemens per cm because the numerical value of the same is found
proportional to the concentration of dissolved solids in mg/lit or ppm.
f) Turbidity (NTU)

104. Steam generator Ganesh kumar
Turbidity is an indicator of undissolved solids. Turbid water may induce erosion.
The undissolved solids also impair circulation of water on the heating surface which
in turn can reduce the cooling action of the water. The undissolved solids cause
consequent over heating of the surface. In practice suspended undissolved solids
are expressed in ppm and micro organic undissolved solids are expressed in NTU.
g) Dissolved carbon di oxide.
Dissolved carbon di oxide appears in the water in the form of carbonic acid. The
bicarbonate salts present in the water are often considered as bound carbon di oxide
in the water. Carbon di oxide being a gas separated out of water on heating. The
carbon di oxide released from water gets carried away along with steam and when
steam condenses the carbon di oxide redissolves in the condensate forming
carbonic acid. This acid corrodes pipe lines and other user equipment.
h) PH value.
It is not a constituent or impurity. The concentration of hydrogen ions produced by
various chemicals dissolved in water.
+ _
H2O à H + OH
In pure water hydrogen ion concentration will be 0.0000001 gm/lit. of water or 10-7 of
hydrogen ions per liter. This is balanced by hydroxyl ions(same amount). If HCl or
any acids dissolved in water it gives additional hydrogen ions causing increase in
hydrogen ions 10-7 à 10-6 à 10-5 ie water becomes acidic. Hence for PH value of
7 water is neutral and PH less than 7 is acidic greater than 7 in alkaline. The
corrosion rate is foud to be very slow when PH range is 9.5 to 10.5 in boiler water.
The protective magnetite (a corrosion product) is not distubed under the above
regimes.
Tube failure.
1) Hydrogen damage/ Embrittlement
Hydrogen damage is mainly due to low PH environment. Presence of carbonic acid,
improper deaeration, condenser leakage. Hydrogen produced in the boiler water
diffuses through underlying metal producing decarburisation and inter granular micro
fissuring of the structure. Brittle fracture occurs in the tubes, the failure will be of
thick edged fracture.
2) Ductile gauging / pitting attack.
Ductile corrosion is more probable when boiler water contains highly soluble alkaline
treatment chemicals such as sodium hydroxide. PH VS PHOSPHATE curve has to
be followed to limit free hydroxide. In this the metal is gradually eaten away and
when tube is insufficient to with stand the pressure, tube fails by pinhole or burst. It
is predominantly seen in low sloped tubes such as roof tube, bed coils, idle
compartment with zero flow are attacked by this.

105. Steam generator Ganesh kumar
j) Iron
Iron will form sludge like layer and prevent heat transfer. Iron content in water will be
removed by oxidizing ferrous iron to ferric iron by air and catalyst, then filtered and in
some cases special iron removing technique/ equipment will be employed.
k) Organic matter
The soluble organic matter present in natural water are either fulvic or humic acid
material ( Fulvic fairly recent organic decay, Humic – old organic decay). Main
source of organic matter in the boiler water is oil, grease with in the boiler, pipeline,
pumps etc. It is difficult to predict the effects of organic matter in the water as it
depends on their nature. Oil and grease induce priming and foaming consequently
water level fluctuations and carry over.
Methods to get rid of the offending impurities in the water and to ensure the
quality requirement is given below.
Impurities Action to be taken Method Equipment
Hardness Softening Base exchange Softner
Acidity/PH Dosing Phosphate & Chemical
Caustic dosing doser
Oxygen Scavenger/deaeration Sodium sulphite Deaearator
Hydrazine chemical
Deaeration doser
TDS Reduce Blow down DM plant
Demineralize
Turbidity Remove Filteration Clarifier/
Sand filter
Organics Preventing Boilout Filteration
Silica Reduce Demineralize DM plant
9.5 NEED FOR WATER TREATMENT

106. Steam generator Ganesh kumar
High pressure boilers are normally designed close to the limiting conditions of heat
transfer, tube metal temperature, circulation etc., to make the units compact and
economical. Modern steam turbine rated for high capacities call for stringent steam
quality to avoid damages. Without strict control over the impurities in steam cause,
deposits over turbine blades and nozzles reducing the output. Thus successful
operation of high pressure boiler and turbine unit requires a through understanding
of all aspects of water treatment. In short the following are the reasons for need of
treatment.
1.0 In an effort to optimize capacity and efficiency, deposit of any sort on tubes are
virtually intolerable.
2.0 Optimization of tubes thickness can not even a moderate level of corrosion tgo
take place.
3.0 Blow down requirements are being cut to have energy conservation.
4.0 High steam purity requirement calls stringent quality. Foreign particles like oil,
organic matter, iron and copper ions contaminate the purity.
TDS in steam = ( TDS in boiler water/ sodium in boiler water) x sodium in steam
Steam purity is determined either by conductivity method or sodium method.
9.6 EXTERNAL WATER TREATMENT
CHLORINATION AND CLARIFICATION
Removal of suspended solids or flocullants can be achieved by sedimentation
coagulants or salts of aluminium or iron added. Salts react with alkalinity to produce
precipitate that attract and absorb fine suspended solid and organic matter to form
large floc particle which settle easily. This process is clarification and done in
clarifier.
Filter alum
Al 2(SO4)3 + 3 Ca(HCO3)2 à 3 CaSO4 + 2 Al(OH)3 + 6 CO2↑
Fe2(SO4)3 + 3 Ca(HCO3)2 à 3 CaSO4 + 2 Fe(OH)3 + 6 CO2↑
Coagulation is the process by which the positively charged co-agulants attract the
negatively charged fine suspended matter and repulsive force is reduced. For
effective coagulation temperature and PH conditions to be maintained properly.
Floculation is the aggregation of particles under the influence of agitation. Chlorine
addition to destroy organic matter because organic matter tends to keep some iron
and manganese in solution and aeration is preceded in the case of iron and
manganese presence. Aeration leads to oxidation of iron and manganese and
removal of carbon di oxide.
FILTERATION

107. Steam generator Ganesh kumar
Removal of coagulant is carried by fiteration.
Two type of filteration are
01. Gravity filter.
02. Pressurized filter.
Filter bed are of stone gravel or coarse anthracite. Anthracite is non silcieous and
does not add silica to water. Gravity filter are open type RCC tank and treatment is
carried 5 to 6 meter per hour and used for treatment of high amount of water.
ION EXCHANGE
Demineralization use of both cation and anion exchange for removal of dissolved
solids. Ion exchange resins are hard spherical beads of 0.3 mm to 1.2mm diameter.
It swells when it condact with water. Resin is to be kept moist and supplied in wet
form. When resin gets dried the resin will be cracked and lose its ion exchange
properties. Hence storing for long period is not advisable. In ion exchange
SAC,WAC,SBA, WBA,MB and degasser are arrangement in different combination to
get the desired water quality based on the raw water analysis.
Cat ion exchange and its regeneration.
Ca HCO3 HCO3 Ca Ca
Mg NO3 + HR à HNO3 + Mg R + HCl à HR + Mg Cl
Na SO4 H2SO4 Na Na
Anion exchange and its regeneration
HCO3 CO3 CO3
HNO3 + ROH à R NO3 + NaOH à ROH + Na NO3
H2SO4 SO4 SO4
REVERSE OSMOSIS
Recent times due to the fast industrialization all natural resources are more polluted
which results in water level TDS increase to 1000 and above ppm of CaCO3. To
use thes high TDS water for boiler, TDS level has to be brought down and more over
chemically treating this high TDS water will be highly uneconomical. Osmosis is the
technique where in two solutions of low and high TDS water are separated by semi
permiable membrane results movement of ions from high concentration to low
concentration zone. Reverse osmosis is the reversal of osmosis process by
applying external force where in movement of ions will be from low concentration to
high concentration zone.
9.7 INTERNAL WATER TREATMENT
LOW PRESSURE DOSING

108. Steam generator Ganesh kumar
After deaeration, deaerated water gets collected in deaerated storage tank. This
deaerated water contains traces of oxygen due to the in complete mechanical/
thermal deaeration. These traces of oxygen is scavenged by chemical deaeration.
Chemicals like sodium sulphite or hydrazine is dosed to the suction header offered
pump by LP dosing pump. Residual hydrazine of 0.02 ppm has to be maintained in
feed water.
Hydrazine reaction
N2H4 + O2 à N2 ↑ + H2O
One ppm of hydrazine react with one ppm of oxygen
Sodium sulphite reaction
2Na2SO3 + O2 à 2Na2SO4
Na2SO3 + H2O + HEAT à 2NaOH + SO2 at 900 to 1000 psi
Hence it is not recommended in high pressure boilers. It also can not be used with
co-ordinated phosphate treatment since it affects balance of sodium and phosphate.
Ex.02. Calculate the quantity of hydrazine required for the steam generator of
capacity 65000 kg/hr. after deaerator. The oxygen percentage after deaerator is 0.02
ppm and the desired level of oxygen in feed water is 0.007 ppm.
The oxygen to be scavenged = 0.02 –0.007 = 0.013 ppm
Minimum hydrazine required = 0.013 ppm
Practically 2 times of required hydrazine will be dosed hence = 0.026 ppm
Residual hydrazine to be present in water = 0.02 0 ppm
Total hydrazine required = 0.046 ppm or mg/kg
Hydrazine for 55.6 kg/sec of water = 0.046 x 65000 = 2990 mg/hr at 100% conc.
Hydrazine available is at 60% conc. Therefore the required hydrazine of commercial
grade = 4983 mg/hr.
HIGH PRESSURE DOSING
Scale formation is limited by converting hardness salts to a free flowing sludge. Hp
dosing is done by carbonate control or phosphate control by addition of sodium

109. Steam generator Ganesh kumar
carbonate or tri sodium phosphate respectively. Other chemicals such as sodium
hydroxide or calcium oxy phosphate can also be added. Best result of calcium
carbonate , magnesium hydroxide, calcium silicate, magnesium silicate will be
available as free flowing sludge where caustic alkalinity is 10 to 15% of the dissolved
solids. OH alkalinity leads to prevention of magnesium phosphate as bad scale by
converting magnesium to magnesium hydroxide. If there is no effective means of
blow down is available it is better not to concentrate on internal treatment. Tri
sodium phosphate is done in such a way that boiler blow down contain residual
phosphate(PO4) of 7 ppm.( I.e., 4 ppm of Na3PO4.12H2O contain 1PPM PO4, hence
28PPM tri sodium phosphate required, 0.028 kg tri sodium phosphate per 1000 kg
of water blow down.)
Ex.03. Calculate the tri sodium phosphate quantity required for the above specified
boiler in example 01. Consider blow down of 1%.
Blow down quantity = 0.01 x 65000 = 650 kg/hr
0.028 kg of tri sodium phosphate required for 1000 kg of blow down
therefore required tri sodium phosphate = 0.028 x 650/1000 = 0.0182 kg/hr.
BLOWDOWN
In spite of all treatment, boiler water will contain dissolved solids. In order to keep
the level of total dissolved solids in boiler within the limits and to remove any sludge,
loose scales and corrosion products, a certain quantity of boiler water should be
regularly drained. This process is known as blow down. The blow down can be
intermittent say once a shift or continuous. The quantity of the water to be blow
down will depend on the dissolved solids entering the boiler through the feed water
and the maximum tolerable levels of these salts in the boiler water. While
determining the dissolved solids content in the feed water it is necessary to takes
into account not only the original dissolved solids but also the dissolved solids added
in the form of dosing.
Continuous blow down
Continuous blow down is a continuous removal of boiler water controlled by a
specially designed adjustable valve or by an orifice plate. The installation of heat
recovery equipment in continuous system may be economically justified. Suspended
solids may block or erode the adjustable valve or orifice plate and continuous blow
down is there fore limited to the control of dissolved solids. Additional manual blow
down is necessary to control suspended solids and prevent the build up of sludge.

110. Steam generator Ganesh kumar
When boiler water from high pressure is suddenly reduced to low pressure blow
down water, water enthalpy will drop and steaming will occur. In utility boilers
recovering this blow down flash steam proves economical. The quantity of flash
steam generated is the product of ratio between difference in boiler water enthalpy
and blow down water enthalpy to latent heat of blow down water and blow down
quantity.
Intermittent blow down
Intermittent blow down may be effected by specially designed values either operated
manually or automatically controlled by timers, feed water flow and conductivity. It
should be noted that the most effective intermittent blow down is achieved by
frequent full open operation for short intervals rather than small extended infrequent
operation. It is not practicable to recover waste heat from infrequent intermittent
blow down.
Ex.04 Find the blow down quantity and flash steam produced for a 160tph boiler
operating at a pressure of 120 bar(a) and the permissible silica content in feed water
and boiler water be 0.01ppm and 0.8ppm respectively. The flash steam blow down
vessel is maintained at 4.5 bar(a).
Blow down quantity
Continuous blow down = S x 100
( C –S)
= 0.01 x 100/(0.8 –0.01) = 1.26 %
Blow down quantity = 0.0126 x 160 = 2.025 tph
Flash steam calculation
Enthalpy of boiler water leaving the drum = 1491.2 kj/kg
At 120 bar(a)
Enthalpy of water in blow down tank at = 623.14 kj/kg
4.5 bar(a)
Enthalpy of flashed steam in blow down = 2743.55 kj/kg
Tank at 4.5 bar(a)
Latent heat of flashed steam at 4.5 bar(a) = 2120.41 kj/kg
Flash steam percentage = enthalpy of boiler water – enthalpy of blow down water
Latent heat of blow down water
= (1491.2 – 623.14)x 100/2120.41 = 40.94%

111. Steam generator Ganesh kumar
Flash steam quantity = 2.025 x 0.4094 =0.829 tph.
10.0 BOILER CONTROLS
10.1 INTRODUCTION
In modern industries automation is carried by means of micro processor chips (µp)
made of silicon, the semi conductor which controls all the functions. The basic
building blocks of integrated chips are of diodes, gate circuit. Today’s boiler are
designed with high heat flux and very high capacity, safety and automation is
becoming prime important. Automation of boiler is done primarily by relay system
and microprocessor system. Relay technology is the oldest one, even now adapted
for small industrial boilers for the basic boiler inter locks like trips and alarms. Most
of the automation is carried by microprocessor based control.
10.2 CONTROL PHILOSOPHY
The control in boiler is required for the following critical items
1.0 Boiler water level has to be maintained always in the system in order to prevent
starvation boiler and tube failure due to over heating.
2.0 The steam temperature at the outlet of superheater has to be controlled within

112. Steam generator Ganesh kumar
the limits other wise higher metal temperature leads metallurgical problem and
also it affects turbine and condenser performance due to volumetric change and
dryness fraction shift.
3.0 The draft system has to be controlled according to the design of the boiler, i.e. if
the boiler furnace designed for negative pressure and if subjected to positive
pressure due failure of draft equipment back firing will take place, Which is
hazardous to boiler and the people.
4.0 In the view of boiler performance, the combustion of fuel takes place completely
with minimum excess air. Hence air flow control with respect to fuel has to be
done and fuel flow with respect to steam demand has to be achieved.
5.0 Considering acid dew point corrosion to metals, the metal temperature at the
back end of economiser tubes or air heater tubes has to be achieved. For this
cooling medium temperature inlet temperature at exhaust gas exit temperature
above the dew point has to be maintained.
6.0 If there is any tripping of rotating equipment like fans, pumps due failure of the
equipment or power the boiler has to be tripped automatically. For this interlock
arrangement has to be made. Further to this, if equipment trips due to its own
defect the stand by equipment has to be started automatically.
In the above point numbers one to five involves variables with complex non linear
and linear relation ship this is grouped as PID controls and point number six is
interlocks with change of two states (on or off) is called on/off control.
Instrumentation
In boiler controls, as explained earlier the instrument can be broadly classified into
field instruments and panel instruments. The field instrument comprises mainly of
Sensors, Transmitters, Actuators etc.,
10.7 FIELD INSTRUMENTS
Sensors
Sensors which has to sense the mechanical process parameters like pressure,
temperature, flow etc., In boiler application the following are the major process
parameters to be converted into electrical signal for measuring and controlling
purposes.
1.0 Flow
2.0 Pressure
3.0 Temperature
4.0 Level
5.0 Flame
6.0 Position of actuators

113. Steam generator Ganesh kumar
Transmitters
The sensed or measured process parameter variable has to be transmitted from the
field to control panel. For this transmitters converts the process parameter into
electrical current of 4 to 20 mA which will be transmitted through 2 core, copper
cables.,
1.0 FLOW
The flow measurement is made based on several principle, some of the commonly
used instruments are, differential pressure flowmeter, area flowmeter, positive
displacement flowmeter, eletro magnetic flowmeter, ultrasonic flowmeter. The type
of flow meter selection is based on accuracy, magnitude of flow, type of fluid being
used like liquid or gas, properties of fluid like temperature, pressure, viscosity etc.,
a Differential pressure flowmeter
This is also called constriction flowmeter. The flow sensors either orifice or nozzle or
venturi etc., this type of flowmeters are widely used in industries for various
application.
2.0 PRESSURE
Pressure is applied to the two sides of the twin diaphragm capsule. The pressure is
transmitted from the twin diaphragm to the sensing diaphragm through the sealing
liquid. Two fixed electrodes are placed symetrically on the left and right side of the
insulator and electrical capacitance is formed between these electrodes and sensing
diaphragm.
3.0 TEMPERATURE
a) Sebeck's effect
When junctions formed between two dissimilar metals kept in two different
temperature, an electromotive force is induced in the system. It is called Sebeck's
effect.
.
4.0 LEVEL
In boiler drum level maintenance is one of the critical parameter for safe boiler
operation.
5.0 FLAME
Flame or light is detected by photo electric cells. Presense of UV or IR rays is
detected by the Photo cells and produce an emf. This signal is used in burner
scanner and fire alarm application.
6.0 POSITION FEED BACK

114. Steam generator Ganesh kumar
The position of an actuator has to be constantly feed backed to the remote center.
In order to have a control over the process parameters. The actuator movement can
be either of linear or circular.
b) Inductive Sensors
The actuator movement is transmitted to the rotating shaft of the positioner. The
cam is attached to the shaft is scanned by an inductive non contact sensors. The
angle of rotation is then converted to an electrical signal, as shown in the diagram.
The main advantage of this type of sensors is there is no frictional contact between
rotating part and sensor. The sensor senses the varying gap due to the cam
movement and produces varying current level by induction. Sensors are supplied
with 24 V DC for generation of 4 to 20 mA.
(e.g.) The control valve actuators used in flow control and pressure control uses this
type of inductive principle.
10.8 PANEL INSTRUMENTS
The signal given from field instrument is processed in panel and necessary control
signal will be generated in the panel and the same is send to field for control action.
Due to the industrial development various control techniques and automation has
been adapted in industry. The following Instrumentation system hierarchy gives the
basic concept behind the same.
Management
Distribution Concentration
Monitor and Operation
Control and Safety

115. Steam generator Ganesh kumar
10.3 DRUM LEVEL CONTROL
F Steam
I
E
L Drum LT PT FT TT
D
4 to 20 mA signal
P
A Σ √
N
E PV
L SP internal Σ
LIC
I CV Σ
N SV
S
T FIC
R
M PV
E I/P √
N
T
F FCV
I
E Air supply FT
L
D Drum Feed water
Three element drum level control loop have one or more drum level, feed water flow
and steam flow transmitters for achieving level control. The controller receives input
from the above three main transmitters and depends on the level set point the
processed output of the controller will actuate the control valve in feed line either to
close or open as desired by the controller.

116. Steam generator Ganesh kumar
10.4 SUPER HEATER STEAM TEMPERATURE CONTROL
F
I super heater steam
E TE thermocouple
L 4 to 20 ‘mV
D signal
P
A Controller mA signal
N TIC
E
I/P
Air supply
F SCV
I
E
L Air supply
D
Spray water line to DSH
10.5 FURNACE DRAFT CONTROL
F
I Furnace pressure
E PT
L 4 to 20 ‘mA
D signal
P
A Controller mA signal
N PIC
E
L
I/P
Air supply
F Power cylider
I

117. Steam generator Ganesh kumar
E
L Air supply
D ID fan damper
In super heater steam temperature control the measured variable steam temperature
is compared with set point or desired temperature in the controller and accordingly
the spray control valve is regulated. Similar to the temperature control, in draft
control furnace pressure is measured by means of pressure transmitter and
controlled by regulating the ID fan damper.