Page 1 :
<{_ unit II <, , , , High Pressure Boilers, , , , , , , , , , , , , , Syllabus ;, , , , , , , , , , used to increases the efficiency of the plant. These are, developed because of rising cost of fuel and restrictions, on air pollution. Modern power generation plants, generally use high pressure boilers. These high pressure, boilers operates with a steam pressure above 140 bars, and steam generation capacity 30 to 650 tons/hour and, above with the temperature may be up to 540° to, GLO°C. It also, reduces drum size. The high-pressure, boilers are widely used for power generation in thermal, , , , , , power plants,, , High pressure boilers use both natural and forced, circulations, Forced circulation js achieved with the, help of centrifugal pumps. By using this forced, circulation, because of high velocity of water scale, formation in the tubes and boiler drums is reduced., , , , 2.1 High Prossuro Boilers - Classification., 2.2. Construction and principle of working of Lamont boiler, Benson boiler, Loeffler boiler, Velox boiler, Schmidt, Hartman boiler, Ramsin boiler., 2.3 Fluidized bed combustion boilers (FBC): principle, need, types, various arrangement, control system and, advantages over other boiler systems,, , 2.4 Comparison of various types of boilers., , 2.5 Indian Boller Regulation Act., , 2.6 Maintenance proceduro of major components of high pressure and FBC boilers., , 2.1.‘ Introduction to Boller ¢ Ina high-pressure boiler, if pressure on the feed water, , oO rises then saturation temperature of water increases and, , ¢ According to Indian Boiler (IBR) Regulation Act the latent heat of vaporization decreases., 2007 - Boiler is a closed pressure vessel in which steam | « The feed water in a boiler is indirectly heated to, is generated with a capacity exceeding 25 liters, gauge saturation of temperature in the economizer by using, pressure greater than or equal to 1 kgf/em*, and water is waste heat recovery from the exhaust gases which is, heated at 100°C or above. escaping to the chimney., , ¢ — In application where the 30 bar pressure and steam at | ¢ — Hence the heat required supplies to boiler is only latent, rate 30 tons/hour is require the fire tube boilers are heat of vaporization and superheat steam is obtained., preferred due to low cost, In the power plant it is Thus, for boiler operation at high pressure will require, necessary to produce the steam at high pressure, high less heat addition for steam generation., rate and high efficiency. So high Pressure Boilers are, , 2.1.1 Characteristics Features of High, , Pressure Boilers :, , Even though the moder boilers are termed as high, , pressure boilers, it does not mean them fully. Because high, pressure operation is only one of the characteristics of, modern boilers used for power generation., , a), , b), , High-pressure boilers have the following features :, , Method of water circulation : The force of water, circulation is increases and maintained with the help of, centrifugal pumps which forces the water through the, boiler plant. It increases the mean temperature of heat, addition and evaporation capacity., , Improved method of heating : The heat transfer rate, by radiation along with conduction and convection. The, total heat-receiving equipment is divided into several, parts, So they can easily be located in various zones of, the furnace for most efficient heat transfer to the water, , Scanned with CamScanner

Page 2 :
©), , 3), , ©), , b), , , , of water by mixing superheated steam for, Sigh heat wansfer rate., , 3) Increasing the combustion of air velocity over the, tube., , Type of mbing : The Large number of small diameter, , tubes ere used so the ratio of surface area to volume, , (sea density) increases. It helps in a high rate of the, , best tensfer to water flowing inside the tubes., , , , 2 Zg-zeg manner are used for water, orced circulation. The use of a short tube, = =n advantage to reduces the pressure loss and, keep the better control the quality of steam., , , , Pressurized combustion chamber : To improve the, rae of combustion and thus increase the heat release, rats, presswized air is forces in the furnace with the, , help of force drafi fan so a large amount of heat is, , , , produce in 2 small space., , Higher steam pressure and temperature : The steam, is generated at a high pressure in the range of 80 to, 300 bars and temperature of 450°C to 585°C the two, Supec-heaters are used in series. The use of such steam, ‘ry suitable for power generation. It increases the, thermal efficiency of the plant and reduces the moisture, contents in low-pressure stages of expansion in the, mxbine., , Compact size : The high rate of heat transfer inside the, boiler reduces the overall size of the boiler, and the, boiler becomes compact., , , , , , , , Intensive heating : The furnace temperature in highpressure boilers is high enough and therefore, 70% of, heat is trensferred to water by radiation. This is a faster, way of heat transfer., , High efficiency : These boilers have better firing, methods, monitoring, furnace conditions, contro] flue, gases and water velocity., , Once through construction : These are operating at, znd above the critical pressure so the water directly, flashes into steam in the tube itself. It eliminates the, need of a boiler drum., , 2-2 High Pressure Boilers, 2.1.2 Advantages of High Pressure Boilers ;, , The following advantages are listed :, , 1. Because of high velocities, tendency of scale formation, in the tubes is reduced., , 2. Small light weight tubes of better heating surface, arrangements can be used. This makes the unit compact, and reduces erection time and cost of boiler., , 3. Due to forced circulation, for heating of all the parts is, uniform, which avoids over heating and development, of thermal stresses., , 4. There is more flexibility in the arrangement of furnace,, tubes and other boiler components., , 5. The steam can be raised quickly to meet the variable, load requirements without the use of complicated, control devices., , The efficiency of the plant is increased by 40 tp 42%,, A very rapid start from cold is possible if an external, power supply is available. Hence the boiler can be used, for carrying peak loads or standby purposes with, hydraulic power stations. ., , 8. All the part are uniformly heated therefore danger of, overheating is reduces and thermal stress problem is, minimized ., , 9. They require less heat of vaporization., , 2.1.3. Classification of High Pressure, Boilers :, , The modem high pressure boilers are classified into, different types of as listed below :, , A) Type of high pressure supper critical boiler, Lamont Boiler, , Benson Boiler, , Loeffler Boiler, , Schmidt-Hartmann Boiler, , Velox Boiler,, , Supercritical Boiler., , Supercharge Boiler., , Fluidized bed combustion Boiler., , SNavNe LYNE, , B) High pressure boilers can be further classified, into, , 1. Natural circulation,, 2. Forced circulation and, , , , 3. Once through boilers, , , , Scanned with CamScanner

Page 3 :
Power Plant Engineering (MSBTE) 23 High Pressure Boilers, , 2.2. Lamont Boiler, , e A forced circulation boiler was first introduced in 1925 by Lamont. It is a forced circulation high pressure water tube, boiler. This is generally used in Europe and America., , ¢ = The feed water from hot well is supplied to a storage and separating drum (boiler) through the economizer., , © — The most of the sensible heat is supplied to the feed water passing through the economizer., , ht of steam evaporated. This water is circulated, , e¢ = Acentrifugal pump circulates the water equal to § to 10 times the wei, , , , parnted in the separator drum., , , , through the evaporator tubes and the part of the water evaporated is s, © = The steam separated in the boiler is further passed through the super heater as shown in Fig, 2.2.1 and finally supplied, to the prime mover., © = These boilers have been built to generate 45 to 50 tons of superheated steam at a pressure of 120 bar and at a, , temperature of 500°C., , Hot gases to chimney, 4 4, , , , , , , Air preheater (AP) ~, , Hotairto +; X)}<— Cold surrounding air, combustion chamber Ay - a, , Economiser (E) 7 Feed pump (FP), , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , NY Feed water, —, S N, Supetheater (SH) - NY, N) Evaporating drum (ED), Steam outlet, to turbine \, N, Sf, Convective yy, evaporating tubes (CET) = [S Combustion, N chamber (CC), Radiant evaporating ————}, tube (RET) j, N \ Steam circulating pump (CP), , \L“, N ANNAN Distributor header (DH), , Fig. 2.2.1 : Lamont Boiler, © The advantage of this boiler is that it is flexible, 2.2.1. Benson Boiler:, compact and small size of drum., , © But the disadvantage of this boiler is formation of | * The main difficulty experienced in the Lamont boiler is, , bubbles on inner surface of the tube reduces heat the formation and attachment of bubbles on the inner, , transfer and steam generation rate. as it offers high surfaces of the heating tubes., , thermal resistance than water film. * — The attached bubbles to the tube surfaces reduced the, ° This further increased the danger of overheating the heat flow and steam generation as it offers high thermal, , tubes due to salt deposition as it has high thermal ‘distance twin water film., , resistance., , , , Tech Knewtedg’, , Scanned with CamScanner

Page 4 :
The presence of steam bubbles in contact wit, , , , surface of thes seriously impairs heat transmission, from the flue gases to water., , It is the first boiler to be designed for openition above, critical pressure,, , By rising the boiler pressure to the critical pressure of, steam (225 ket ‘om®.), this difficulty is overcome, as, suggested by Mark Benson in 1922,, , ‘At the critical pressure water and steam have the same, density and no bubbles are formed. Further at critical, pressure the enthalpy of evaporation (Latent heat) is, zero. The water thus gets converted into steam directly, without passing through evaporation process., , The first modem high pressure drumless builer, developed by Benson was put into operation in 1927 in, west Germany power station., , Working Principle of Benson Boiler:, , This boiler has a unique characteristic of absence of, , steam separating drum. The entire process of heating, steam, generation and superheating is done in a single continuous, tube., , (a) Economiser :, , (b), , The feed water by means of the feed pump is circulated, through the economiser tubes., , Hot flue gases pass over the economiser tubes and the, feed water is preheated., , The feed water from the economiser flows into the, radiant evaporator with radiant parallel tube sections., , The radiant evaporator receives heat from the buming, fuel through radiation process and majority of water is, converted into steam in it., , But rising of water to a pressure of about, 221.2 bar will means large pump work and the, efficiency of plant may reduce., , Radiant evaporator :, , In this section most of the heat is transmitted and water, gets and remaining water is evaporated heated almost, to the critical temperature. (Water cooled walls from, the portion of section) absorbing the heat from the hot, gases by convection and radiation., , , , , , , Thess, , Convection supemhaater :, , , , stad steam is now, , seni through, , , , , , , , Jeater where the sthmatad sean,, , = iy, , orator, the, , convection evaporator ang, J in the peg, , The radiant ¢v, , the convection supesheater are all ammange, , , , , , of the flue gases., , Steam outlet :, , , , , , , The superheated steam is steam turing, through the steam outlet., , The uniform final steam temperature over the range of, output can be attained by, and convective super, Capacity :, , Capacity of Benson boiler is about 133000 ky hour ata, , es arrangement of tadiag, , , , , , ter component., , , , , pressure of 250 bar and at a temperanie of 650°C., (Efficiency may be improved by running the boiler ats, , , , pressure slightly lower than the critical pressure)., Supercritical pressure plant works at higher efficiency, of about 40%., , In supercritical boilers plant 65%% of heat is transferred, in superheating of steam and reheating of steam bled, from steam turbine at intermediate pressure., , Also to limit gas temperature a radiant super-heater is, used., , Salient Features of Benson Boiler :, , As there are no drums, the total weight of Bensoa, boiler is 20% less than other boilers. This also reduces, the cost of the boilers., , As no drums are required, the transfer of the Bensoa, parts is easy, Majority of the parts may be carried to tht, site without pre-assembly., , Since no drum is used, this is an once-through built, and the feed water entering at one end is discharged 8, , superheated steam at the other end,, , Circulating pump and down comers are dispensed with, , Teck, , Scanned with CamScanner

Page 5 :
Power Plant Engineering (MSBTE), , Exhaust, gases, , , , , , Cold, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Hot gases to Av prananter ‘air in, combustion <— 000 Blower, chamber, Water from,, well Gry, Gam, pump, Connective, evaporator, Superheated, steam to AXStarting, prime mover ve, Radient evaporator, LIVATTITFTTTOIFITT OOP AI, (a) Benson boiler, 3 ¢, 4, TK, , , , , (Superheater, region), , Condensing, temperature, , , , , , Entropy s, kj/kg’K —=, (b) T-S representation of steam, Fig. 2.2.2, Advantages of Benson Boiler :, , 1. As there are no drums, the total weight of Benson, boiler is 20% Jess than other boilers. This also reduces, the cost of boiler., , nv, , Natural circulation boilers require expansion joints but, these are not required for Benson as the pipes are, welded. The erection of Benson boiler is easier and, quicker as all the parts are welded at sites and, workshop job of tube expansion is all together avoided., , 3. The transfer of Benson’s parts is easy as no drums are, required and majority of the parts are carried to the site, without pre-assembly., , 2-5, , , , 6., , 2.2.2, , 1 Pressure Boilers, , , , The Benson boiler can be erected in a comparatively, smaller floor area. The space problem does not control, the size of Benson boiler used., , , , ‘The furnace walls of the boiler can be more efficiently, protected by using smaller diameter and closed pitched, tubes,, , ‘The super-heater in the Benson boiler is an integral part, of forced circulation system, therefore no special, starting arrangement for super-heater is required., , ‘The Benson boiler can be started very quickly because, of welded joints., , Loeffler Boiler :, , The difficulty encounter in Lamont boiler of, sedimentation and deposition of salt in tube is solved, by Loeffler boiler by preventing the flow of water into, the boiler tube., , This boiler works on the principle of evaporation of, feed water by means of superheated steam from superheater and hot gases from the furnace being primarily, used for superheating purposes. ., Fig. 2.3.3 shows a systematic representation of Loeffler, boiler. The boiler drum is constructed outside the, furnace setting. The high pressure feed pump feed, water through economizer and pickup some heat from, outgoing flue gases and discharge it into evaporating, drum., , , , The steam circulating pump draws saturated steam, from evaporating drum and passes it through radiant, and convective super heaters where steam is heated to, required temperature from super heater approximately, one-third of the superheated steam send to the prime, mover and 2/3 passes back to the evaporating drum to, evaporate feed water., , About 35% of the steam coming out from the superheater is supplied to the HP. steam turbine. The steam, coming out from HP. turbine is passed through reheater before supplying to LP.turbine as shown in, Fig.2.2.3., , The amount of steam generated in the evaporator drum, is equal to the steam tapped (65%) from the superheater., , , , WB ecttecstecyi, , peniecateass, , Scanned with CamScanner