steam generator

Steam generator Ganesh kumar A.GANESH KUMAR DEUTSCHE BABCOCK, INDIA. For internal circulation only. All rights reserved by author.

PDF created with pdfFactory trial version www.pdffactory.com

http://www.pdffactory.com

Steam generator Ganesh kumar

DEDICATED TO MY COLLEGE AND MY PROFESSORS.



Steam generator Ganesh kumar 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)



Steam generator Ganesh kumar 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………………………………………………………………..



Steam generator Ganesh kumar 5.0 DRUMS 5.1 Introduction……………………………………………………………………. 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…………………………………………………………….



Steam generator Ganesh kumar 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



Steam generator Ganesh kumar 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.



Steam generator Ganesh kumar 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.



Steam generator Ganesh kumar 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



Steam generator Ganesh kumar 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.



Steam generator Ganesh kumar 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




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 FD FAN ON LAMP 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 AUTO PURGE INTERLOCKS 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 POSITION 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



Steam generator Ganesh kumar 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.



Steam generator Ganesh kumar 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.



Steam generator Ganesh kumar 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.,



Steam generator Ganesh kumar 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.



Steam generator Ganesh kumar 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)



Steam generator Ganesh kumar 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.



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



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 + a5T4 + 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 Heat Kj/kgK

Dynamic Viscosity µPa.S

Thermal conductivity W/mK

Prandtl number

a1 b1 c1 d1 e1

0.8554535 0.2036005E-3 0.4583082E-6 -0.279808E-9 0.5634413E-13

-0.9124458E 1 0.4564993E-2 0.2198889E-4 -0.1891235E-7 0.5138895E-11

-0.1083113E-1 0.5596822E-4 0.7413502E-7 -0.5901395E-10 0.1961745E-13

0.492851 -0.1230046E-2 0.1662398E-5 -0.1052753E-8 0.2443111E-12

a2 b2 c2 d2 e2

-0.1002311 0.7661864E-3 -0.9259622E-6 0.5293496E-9 -0.109357E-12

-0.4267768E1 0.4074274E-3 -0.5125357E-5 0.738556E-8 -0.343972E-11

-0.8035817E-2 0.110672E-04 -0.8397255E-8 0.1130229E-10 -0.5731264E-14

-0.8820652E-2 0.1855309E-3 -0.3838084E-6 0.3256168E-9 -0.1005757E-12



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 + c1T² + d1T3 + e1T4 P2 = a2 + b2T + c2T² + 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 + c1T² + d1T3 + e1T4 P2 = a2 + b2T + c2T² + 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 + c1T² + d1T3 + e1T4 P2 = a2 + b2T + c2T² + 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 µ Pa.s

Thermal conductivity W/mK

Prandtl number

a b c d e f

0.1714237E02 0.4636040E-01 -0.2745836E-4 0.1811235E-7 -0.674497E-11 0.1027747E-14

0.2498583E-1 0.6535367E-4 -0.7690843E-8 -0.1924248E-11 0.160998E-14 -0.2864430E-18

0.6901183 0.2417094E-05 0.2771383E-7 -0.3534575E-10 0.1717930E-13 -0.2989654E-17



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 b c d e f

0.3679321E1 -0.1313559E-2 0.2615196E-5 -0.9629654E-9 -0.9928002E-13 -0.9723991E3

‘a1 b1 c1 d1 e1 f1

0.2852903E1 0.1580411E-2 -0.6189378E-6 0.1119450E-9 -0.7607378E-14 -0.8019835E3

2.3 STEAM We can see in day to day life the process of boiling water to make steam. Steam 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.



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.



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



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



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 = Vo + 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



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



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



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.



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



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)



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

Head 15% 20%

25% 25%

25% 25%

15% 20%

2 ID Fan Flow Head

25% 30%

25% 25%

25% 25%

20% 20%

3 SA/PA/OF fan Flow Head

10% 15%

25% 25%

25% 25%

Not 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



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



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”



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




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



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 = α Where, α is film thickness kf thermal conductivity of film h = kf / α 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. Q = σ A T4



Steam generator Ganesh kumar Q rate of energy radiation in Watts A Surface area radiating heat sq.m σ Stefan boltzmann constant = 5.67 x 10 –8 Watt/sq.m K4 4.88 x 10 –8 Kcal/sq.m hr K4

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 When, Gr.Pr < 10 9 flow is laminar



Steam generator Ganesh kumar Gr.Pr > 10 9 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) Let h = B La Ub ρ c µd ke Cpf , where B,a,b,c,d,e,f are constants Expressing the variables in terms of their dimensions MT-3Θ-1 = B La.(LT-1)b.(ML-3)c.(ML-1T-1)d.(MLT-3Θ-1)e.(L² T-2Θ-1)f

= B.L a+b-3c-d+e+2f. T –b-d-3e-2f. M c+d+e. Θ-e-f 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 h = B. Lc-1.Uc. ρ c.µ -c+f.k -1-f .Cp f by grouping the variables, h/L-1k = B.(UL ρ / µ)c. (µ. Cp /k)f Nussultes number = B.(Reynolds number)c.(Prandl number)f The constants B,c,f are evaluated from experimental data.



Steam generator Ganesh kumar For turbulent flow inside tubes and fully developed flow the following equation attributed to Mr.Dittus and Boelter, Nu = 0.023 Re 0.8 Pr n where, n = 0.4 when the fluid is heated n = 0.3 when the fluid is cooled. For turbulent flow outside tubes Nu = 0.037 Re 0.8 Pr n 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 Δ T) (ρo/ ρ ) – 1 = ß Δ T F = ßg Δ T For an ideal gas ß is inversely proportional to temperature,(i.e. dimensional number for ß is Θ-1 and F is Θ-1 * LT-2*Θ ie LT-2) By dimensional analysis, h = B.(Fa.Cpb.Lc. ρ d.µe.kf) MT-3 Θ-1 = B[ (LT-2)a.(L2 T-2Θ-1)b. Lc.(ML-3)d.(ML-1T-1)e.(MLT-3Θ-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 Δ T)a . Cpb. L 3a-1. ρ2a. µ b-2a. k1-b)] h = B[ (gß Δ TL3 ρ2/ µ² )a . (µ.Cp/k)b] (k/L) hL/k = B. Gra. Prb.



Steam generator Ganesh kumar Nu = B. Gra. Prb

By large number of experiments made on fluids it has been found that exponents a and b are of the same value. So the expression reduce to Nu = B.(Gr.Pr)a

HEAT TRANSFER BY RADIATION Hr In furnace heat transfer is predominant by luminous and non-luminous radiation. A general approximate expression may be written for furnace absorption using Stefan boltzman law. Q = A εw σ [εg Tg

4 – αg TS4]

εg = ηc εc + ηw εw - Δ ε 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 εc and εw are found from graph ηc and ηw 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 ηc = EXP _______________ - √ Pc *L +0.035 ln2.8 ln(p + 1.8) 1/3 0.23 1 2 ηw = 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 ηw and ηc are pressure correction factor for gas pressure absorptive of gasses can be determined at wall temperature. αg = ηc αc + ηw α w - Δ α At wall temperature correction, Pcw = Pc (Tw/Tg) Pww = Pw(Tw/Tg) αc = εcw (Tg/Tw)0.65 αw = εww (Tg/Tw)0.45



Steam generator Ganesh kumar εcw is a function of Pcw .L and wall temperature for this we have to see the emissivity in graph εww 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. Δ α = Δ εw = function of Pw/(Pc + Pw) , Pcw.L + Pww.L, and temperature of wall The effect of absorptivty is negligible hence the same can be neglected and a generalized form of Q = A εw εg σ [Tg

4 –TS4] can be used.

Heat absorption by energy balance method, Q = [ Wf . lower heat value – Wg .gas exit enthalpy] Where, A effective projected area of heat transfer including wall opening εw wall emissivity εg gas emissivity σ 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). Wf Fuel burnt Wg Flue gas produced Gas emissivity εg = 0.9( 1- e –k.L )………………………………………………1 The emissivity of flame is evaluated by εf = Ω ( 1- e –k.L ) where Ω is the characteristic flame filling volume. Ω = 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



Steam generator Ganesh kumar K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + Pw) _______ √(Pc +Pw)L For semi luminous flame, the ash particle size and concentration is taken in calculation K = (0.8 +1.6 Pw).(1-0.38 TM/1000)(Pc + Pw) ________ + 7µ(1/dm²TM²)1/2 √(Pc +Pw)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



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 εw εg σ [ TM

4 – 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) Hr = σ εwεg[TM

4-TS4]/(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



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.



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 Dynamic viscosity = 3.7392 x 10 –5 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) Nu = 0.037 Re 0.8 Pr n where n= 0.3 for cooling fluid Hc/kL = 0.037 Re 0.8 Pr n

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



Steam generator Ganesh kumar flame emissivity εf = 0.9 x (1- e –0.2904 x 1.2709) = 0.2778 Wall emissivity εw = 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.



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 εw εg σ [ TM

4 – TS4]

As it is a bagasse fired boiler volatile combustion is more TM will be equal to temperature exit and εw εg 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



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 Radiation coefficient Hr = σ εwεf[TM

4-TS4]/(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



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 Flame emissivity = 0.72x(1 –e –1.9928 x2.52) = 0.715 Hr = 5.67 x 10^-8 x0.85x0.715x(15584 –4874)/(1558 –487) = 187.768 W/mK4 Nu = 0.037 x Re0.8 xPr0.3 Gas properties at film temperature (1285+ 214)/2 = 749.5°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 Hc = 0.037 x 3.064 x5.74 x0.345 x 0.7110.3 x 0.072915 4.13276x10^-5 5.74 = 5.769 W/mK Heat absorbed by Furnace Q = (Hc +Hr)x effectiveness A Lmtd =(5.769 + 187.768)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



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



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



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 Nusselts number = 0.33 * (Reynolds number)0.6 * (Prandl number)0.3

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 Nusselts number = 0.35* F * (Reynolds number)0.6 * (Prandl number)0.3



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




STAGGE 1.25 1.5 2 3

0.518 0.451 0.404 0.31

0.556 0.568 0.572 0.592

0.505 0.46 0.416 0.356

0.544 0.562 0.568 0.58

0.519 0.452 0.482 0.44

0.556 0.568 0.556 0.562

0.522 0.488 0.449 0.421

0.562 0.568 0.57 0.574

INLINE 1.25 1.5 2 3

0.348 0.367 0.418 0.29

0.592 0.586 0.57 0.601

0.275 0.25 0.299 0.357

0.608 0.62 0.602 0.584

0.10 0.101 0.229 0.374

0.704 0.702 0.632 0.581

0.0633 0.0678 0.198 0.286

0.752 0.744 0.648 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. Nusselts number = 0.023 * (Reynolds number)0.8 * (Prandl number)0.3

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 * π *r1 [ tw1 – tf1]………………………………………….1 = 2* π * k (tw2 – tw1)/ln(r2/r1)……………………………………..2 = ho * 2 * π * r2 [ tf2 – tw2]…………………………………………3 1 = 2 = 3 the heat transferred at a given section at given time is equal.



Steam generator Ganesh kumar tw1 – tf1 = ql / (hi*2*π*r1) tw2 – tw1= ql * ln(r1/r2) 2*π*k tf2 – tw2 = ql /(ho *2*π*r2) Adding the above equations we get, ql 1 ln(r2/r1) 1 tf2 – tf1 = + + 2*π hir1 k hor2 ql = 2* π *(tf2 – tf1) 1 + ln(r2/r1) + 1 hir1 k hor2 Q/l = U [tf2 – tf1] Divide by πdo Q/(πdol) = U[tf2 – tf1]/πdo U = 2* π πdo 1 + ln(r2/r1) + 1 hir1 k hor2 U = 1 πdo 1 + ln(r2/r1) + 1 2πhir1 2πk 2πhor2 1 do + d0 ln(d0/di) + 1

= U hidi 2k ho Introducing fouling factors in steam side and gas side we can get,



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 At/Ai U hiAi Aw 2k η ho Where, At is total surface area. Ai is inside surface area. Aw is average surface area. η 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



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



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



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



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



Steam generator Ganesh kumar Steam velocity = 15.28 x 4 /(9.3281 x74xπ 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 1.7223 x 10-5 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)/(πx0.0381x1) = 0.144876m Attenuation factor = (0.8 +1.6x0.1158)(1-0.38x(883/1000))(0.1158 + 0.1249) √((0.1158+0.1249)x0.14486)



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



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 πx(0.03082²) =29.99 m/s Reynolds number = ρvd/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



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



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 π x d²/4 Let σ1 be the tensile stress induced in metal at section XX Resisting force = σ1 x π x d x t Bursting force = Resisting force



Steam generator Ganesh kumar p x π x d²/4 = σ1 x π x d x t σ1 = 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 = σ1/E - σ2/mE (σ1 = σ2 ) e1 = σ1/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/4σ1 If the shell has been riveted then factor e called efficiency factor to be introduced i.e., ‘t ≥ pd/4σ1e 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 σ1 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 = σ1 x 2x L x t…………………………………………….2



Steam generator Ganesh kumar Equating 1 and 2 σ1 = pd/√2 t or t = pd/√2σ1 Similarly in zz direction σ2 be the tensile stress Bursting force P = p x d x L Resisting force = σ2 x 2 x L x t σ2 = pd/2t or t = pd/2σ2 Similarly yy direction σ3 be the tensile stress Bursting force P = p x d² Resisting force = σ3 {(d +2t)² - d² } = σ3 x 4 x d x t (considering 4t² as negligible) σ3 = pd/4t or t = pd/4σ3 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 σ1 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 = σ1 x 2 x t x L Equating Bursting force and resisting force = σ1 = pd/2t or t = pd/2σ1 This stress induced in circumferencial of the shell is called circumferencial stress of hoops stress. Similarly in longitudinal direction area resisting the pressure = π x d²/4



Steam generator Ganesh kumar Force acting on the end of the shell P = p x π x d²/4 ‘σ2 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 σ2 x π x d x t = p x π x d²/4 σ2 = pd/4t or t = pd/4σ2 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 = (σ1 - σ2)/2 = pd/8t Circumferencial strain e1 = σ1/E - σ2/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 σ be the permissible tensile stress for the material than for shell to be safe major principal stress σ1 should not exceed σ From this σ ≥ pd/2t or t ≥ pd/2σ Working pressure p α 2 x σ x t / d introducing ligament efficiency ‘e =pitch –hole dia pitch p = 2 x σ 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 σ 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 σ 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,




DESCRIPTION

XX

YY

ZZ

SPHERE

t =pd/4σ

t =pd/4σ

t =pd/4σ

SQUARE

t =pd/√2σ

t =pd/4σ

t =pd/2σ

CYLINDER

t =pd/2σ

t =pd/2σ

t =pd/4σ

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



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 α



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 α a b x nwl ‘h c L Sin α = ak/oa ‘α = Sin-(x/r) Θ = (180 – 2α) ‘ab = 2 ac = 2.r. Sin(Θ/2)



Steam generator Ganesh kumar The area of the arc oacb = π x D² xΘ 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 = πD² - area acba 4 Steam space volume = ( πD² /4 - area acba) (L +2l) + 2 volume of dished head ‘above normal water level Volume of dished head for different shapes.

TYPE

FULL VOLUME

M^3

VOLUME UPTO NORMAL WATER LEVEL

M^3

SEMI ELLIPSOIDAL

π. D3/24

π.h².(1.5D –h)/12

HEMI SPHERICAL

π.D3/12

π.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



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



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)



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.



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



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 6.0 EVAPORATOR AND ECONOMISER



Steam generator Ganesh kumar 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.



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 Θ be the temperature difference driving heat from the fluid to the fin at any point in its cross section. Then Θ = Tc – t If l is the height of the fin varying from 0 to b ‘dΘ/dl = -dt/dl The heat within the fin which process through its cross section is Q = ka dΘ/dl…………………………………………….A Differentiating the equation ‘dQ/dl = ka d²Θ/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 = hfΘPdl or dQ/dl = hfPΘ ……………………..2 Equation 1 and 2 are same and hence 1-2 =0



Steam generator Ganesh kumar kad²Θ/dl² -- hfPΘ = 0 Rearranging, d²Θ/dl² - hfPΘ/ka =0 The direct solution of this equation is, 0.5 0.5

Θ = C1el(hfP/ka) + C2e-l(hfP/ka) ……………………..3 _____ ‘m = √(hfP/ka) The general soution is Θ = C1eml + C2e-ml Applying boundary layer concept, At the outer edge of the fin l =0, Θe = c1 + c2 Assume no heat enters from outside edge of the fin, then ‘dΘ/dl =0 when Θ =0 and C1 –C2 =0 C1 = C2 = Θe/2 Equation 3 becomes, Θ/Θe = (eml+ e-ml)/2 Θ = Θe 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 = hfPdΘ/dl………………………………….4 dΘ/dl = ‘d²Q/dl² hfP Substituting in equation A Q = kad²Q/hfPdl² d²Q/dl² = hfPQ/ka………………………………..5



Steam generator Ganesh kumar d²Q/dl² - hfPQ/ka = 0 As before the solution is 0.5 0.5

Q = C3el(hfP/ka) + C4e-l(hfP/ka) ……………………..6 Q = C3eml + C4e-ml At I =0, C3 + C4 =0 , C3 = -C4 and dQ/dl =0 ‘dQ/dl =hfPΘe = mC3 –mC4 =0 C3 = hfPΘe/2m and C4 = -hfPΘe/2m Q = hfPΘe eml /2m - hfPΘe eml /2m Q = hfPΘe /m(eml - eml )/2 Q = hfPΘe /m sinh(ml) The ratio of heat load Q to the temperature difference Θ at the fin base is Q/Θ = hfPΘe sinh(ml) m Θe cosh(ml) Q/Θ = hfP tanh(ml) ……………………………………………………7 ‘m hf = Qm/Θptanh(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/ΘlP η’ = hb/hf = Q Θ P tanh(ml) Qm ΘlP = tanh(ml)



Steam generator Ganesh kumar 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 AoΘ 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.Θ + hf.Ao.Θ =(hb.l.P.N.Ao/Ao + hf.l.P.N.Ao/P.l.N)Θ =(hb/Ao + hf /P.l.N)l.P.N.Ao.Θ =(l.P.N.tanh(ml) + Ao ) hfΘ………………………………………………..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 Where 48.26 kcal/kg enthalpy of gas at 185°C and 36.09 kcal/kg enthalpy of gas at 140°C.



Steam generator Ganesh kumar 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 /(π(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 = 87.49/(πx 0.0443) = 628 m



Steam generator Ganesh kumar 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)/(πx0.0381x1) = 0.144876m Attenuation factor = (0.8 +1.6x0.1158)(1-0.38x(1192/1000))(0.1158 + 0.1249)



Steam generator Ganesh kumar √((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. 7.0 AIRHEATER



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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. 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



Steam generator Ganesh kumar 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 velocity tubes and corrosion of fouled tubes. These problems can be eliminated by frequent cleaning of air heater. On load cleaning



Steam generator Ganesh kumar 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. 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.



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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/(π * d²) = 7.4065 x4 /πx 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 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



Steam generator Ganesh kumar 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 Heat transfer area arranged in two blocks A = 8.656 x 106/17.168 x 97.36



Steam generator Ganesh kumar Required heat transfer area = 5178.65 m². Provided heat transfer area in two blocks = 2xπx0.0635x5.5x88x30 = 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 8.0 DUST COLLECTOR 8.1 INTRODUCTION



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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.

POLLUTANTS

INDUSTRIAL ENVIRONMENT PPM

RESIDENCIAL ENVIRONMENT PPM

SENSITIVE AREA 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 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,



Steam generator Ganesh kumar 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.

TYPE

COLLECTION EFFICIENCY IN PERCENTAGE WEIGHT

PRESSURE DROP IN MM WC

POWER CONSUMED BHP PER100 M^3/MIN

ELECTROSTATIC PRECIPITATOR

80 TO 99.5%

2 TO 20

0.2 TO 0.5

FABRIC FILTER

95 TO 99.5%

50 TO 150

0.8 TO 2.6

CYCLONE SEPARATOR

50 T0 95%

25 TO 100

0.4 TO 2.2

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 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



Steam generator Ganesh kumar 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, improper bag tension, loose, damaged or improper bag connections. Differencial pressure Steadiness of pressure drop(should be read daily)



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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. The less soluble salt like calcium carbonate precipitate and settle in the boiler water which can be removed by blow down.



Steam generator Ganesh kumar 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). 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)



Steam generator Ganesh kumar 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) 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



Steam generator Ganesh kumar 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.



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 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,



Steam generator Ganesh kumar 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 Al2(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 Removal of coagulant is carried by fiteration. Two type of filteration are



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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. 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 generator Ganesh kumar 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% Flash steam quantity = 2.025 x 0.4094 =0.829 tph.



Steam generator Ganesh kumar 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 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.



Steam generator Ganesh kumar 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 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



Steam generator Ganesh kumar 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 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.



Steam generator Ganesh kumar 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 10.3 DRUM LEVEL CONTROL



Steam generator Ganesh kumar 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. 10.4 SUPER HEATER STEAM TEMPERATURE CONTROL



Steam generator Ganesh kumar 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 E L Air supply D ID fan damper



Steam generator Ganesh kumar 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.



steam generator

Documents