PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE THERMAL ENGGINEERING & GAS DYNAMICS (ME404) NOTES PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE UNIT-1 STEAM GENERATOR DIFFERENT TYPES OF HIGH PRESSURE BOILER 1. LAMONT BOILER A forced circulation boiler was introduced in 1925 by LA Mont. The arrangement of water circulation and different components are shown in below fig. Modern high pressure water tube boiler steam boiler working on a forced circulation .The circulation of water is maintened by a centrifugal pump driven by a steam turbine. It generate 45 to 50 tons /hr of superheated steam at a pressure of 120 bar and at temperature of 500C The feed water from the hot well is goes through the economiser into the steam separating drum. The feed water is heated by flue gases. The circulating pump above the boiler pressure delivers feed water to the evaporator. The mixture of water and steam from these tubes enter into the boiler drum where steam is separated and is drawn off to the superheater. The superheated steam thus passes to the primemover (turbine) to run the engine. LIMITATIONS: Formation of bubbles on the inner surface of the water tubes. These bubbles reduce the heat flow and steam generation. PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE LAMONT BOILER PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE 2. BENSON BOILER In the lamount boiler, the main difficulty experienced is the formation and attachment of bubbles on the inner surface of the heating tubes . The attached bubbles on the inner surface of the heating tubes. The attached bubbles to the tube surface reduce the heat flow and steam generation as it offers high thermal resistance than water film. BENSON BOILER PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE Benson in 1922 argued that if the boiler pressure was raised to critical pressure 212bar, the steam and water have the same density and therefore the danger of bubbles formation can be easily eliminated . The boiler too makes use of forced circulation and uses oil as fuel. This boiler does not use any separating steam drum. The feed water after circulation through the economiser tubes flow through the radient parallel tube section to evaporate partly. The steam water mixture produced then moves to the transit evaporator section where this mixture is converted into the dry steam. The steam is now passed through the convective superheater and finally supplied to the prime mover (turbine) Generating capacity 150 tons/hr, temperature 650C, Maximum working pressure 500bar. LIMITATIONS: Salt formation in the evaporator tubes. So removing of deposite salt is required after required after each 400 working hours by special means. On evaporating tubes there is chance of corrosion. PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE 3. LOEFFLER BOILER In a benson boiler the major difficulty experienced is the deposition of salt and sediment on the inner surfaces of the water tubes. The deposition reduces the heat transfer and ultimately the generating capacity. This increases the danger of over heating of tubes. LOEFFLER BOILER This boiler also makes use of forced circulation. The high pressure feed pump draws water through the economiser and delivers it into the evaporating drum. The steam circulating pump draws saturated steam from the evaporating drum and passes it through radient and convective superheater where steam is heated to required temperature. PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE From the superheater about 30% of the superheated steam passes to the turbine, the remaining 65% passing through the water in the evaporating drum in order to evaporate feed water. Generating capacity 94.5 tonnes/hr and operating pressure at 140 bar. ADVANTAGES: The salt deposited in the evaporator drum can be easily brushed off by blowing off the water. There is no chance of corrosion. PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE PERFORMANCE OF BOILER: EVAPORATIVE CAPACITY: The evaporative capacity of a bolier may be expressed in terms of: Kg of steam/ hour Kg of steam/hr/m2 of heating surface Kg of steam/kg of fuel EQUIVALENT EVAPORATION: It is defined as the amount of water evaporated from water at 1000C to dry and saturated steam at 1000 C at normal atmospheric pressure (1.0132 bar). Heat required to evaporate Me kg of water = Me ( h – hf1) Where Me = MS / Mf Me = Equivalent mass MS = Mass of water evaporated into steam Mf = Mass of fuel h= Enthalpy of steam at pressure p bar hf1=Enthalpy of feed water at temperature t1 0C Equivalent evaporation from the definition: E = Me (h – hf1)/ 2257 PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE FACTOR OF EVAPORATION: It is defined as the ratio of heat received by 1kg of water under working condition to that heat received by 1kg of water evaporated from and at 1000C Fe = (h – hf1) / 2257 BOILER EFFICIENCY: It is the ratio of heat actually used to produce steam to the heat supplied by the furnace. Boiler efficiency = MS ( h – hf1) /( MS ×CV) Where CV= Calorific value of fuel KJ/Kg of fuel Numericals based on performance of Boiler: 1. A boiler produces 10 kg of steam per kg of fuel from feed water at 300C at 9 bar absolute pressure . What is equivalent evaporation from and at 1000C per kg of fuel and factor of evaporation , if the steam is 0.9 dry. Given: Me= 10 kg per kg of fuel Temperature of feed water= 300C Pressure of steam = 9bar X = 0.9 ( 90% steam dry) Formula used: PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE E = Me (h – hf1)/ 2257 Fe = (h – hf1) / 2257 SOL: From steam table At pressure base AT P= 9bar, hf = 742.64KJ/kg, hfg = 2029.5 KJ/kg , Using wet steam formula h = hf + hfg h= 742.64 + 2029.5×0.9 h= 2569.19 KJ/kg E = Me (h – hf1)/ 2257 Equivalent Evaporation E = 10 × (2569.19- 125.66)/ 2257 E= 10.83 Kg/kg of fuel Factor Of Evaporation Fe = (h – hf1) / 2257 Fe = (2569.19 – 125.66)/ 2257 = x= 0.9 PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE HEAT LOSSES IN BOILER PLANT SYSTEMATIC REPRESENTATIONS OF HEAT RELEASE (Per kg of fuel based) 1. Heat supplied by fuel Q Supplied= MS× CV KJ Where, Mf = Mass of fuel CV= Calorific value fuel 2. Heat utilized to generate steam Q1 = Me ( h- hf1) KJ h= Enthalpy of steam Where h = Enthalpy of steam hf1= Enthalpy of feed water KJ/Kg 3. Heat lost in dry flue per kg of fuel Q2 = MgCpg (Tg-Ta) Where Tg = Temperature of flue gases Ta = Temperature of air entering to combustion chamber or temperature of boiler room Cpg =Mean specific heat of dry flue gasses 4. Heat lost in moisture present in fuel. Assumed that the moisture is converted into superheated steam at atmospheric pressure (1.013bar) PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE Q3 = Mm (hsup – hb) Q3 = Mm (h +CP×(Tg-T) –hb) Where, Mm = Mass of moisture per kg of fuel hb = Enthalpy of water at boiler house t = Saturation temperature (100 OC ) 5. Heat lost due to incomplete combustion of fuel Q4 =(CO ×C× 24800)/ ( CO + CO) KJ/kg of fuel 6. Heat lost due to incomplete combustion of fuel Q5 = Mf1 × C.V Mf1 = Mass of unburnt fuel 7. Convection and Radiations Q6 = Q Supplied – ( Q1 + Q2 + Q3 + Q4 + Q5) PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE HEAT BALANCE SHEET Heat Supplied Heat supplied by fuel KJ Q Supplied= MS× CV Q Supplied TOTAL Heat Utilization KJ % 1.Heat supplied by fuel Q1 Q1/ Q Supplied×100 2.Heat utilized to generate steam Q2 Q2/ Q Supplied×100 3.Heat lost in dry flue per kg of fuel Q3 Q3/ Q Supplied×100 4.Heat lost in moisture present in fuel 5.Heat lost due to incomplete combustion of fuel Q4 Q4/ Q Supplied×100 Q5 Q5/ Q Supplied×100 6.Convection and Radiations Q6 Q6/ Q Supplied×100 TOTAL 100% PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE DRAUGHT The draught is one of the most essential systems of thermal power plant which supplies required quantity of air for combustion and removes the burnt products from the system. To move the air through the fuel bed and to produce a flow of hot gases through the boiler, economizer, preheater and chimney require a difference of pressure.This difference of pressure for to maintaining the constant flow of air and discharging the gases through the chimney to atmosphere is known as draught NATURAL DRAUGHT: The draught produced by the boiler chimney is known as natural draught.The natural draught is produced due to the difference in weight between the column of hot gases inside the chimney and the weight of equal column of cold air outside the chimney. PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE ARTIFICIAL DRAUGHT Natural draught is not sufficient for high rate of fuel burning and it becomes necessary to provide an artificial draught by some mechanical means. An artificial draught may be produced by steam jet, fan or blower accordingly it is known as mechanical draught. The artificial draught is of two types: 1. Forced draught 2. Induced draught FORCED DRAUGHT: In a forced draught system, a blower is installed near the base of the boiler. This draught system is known as positive draught system or forced draught system because the pressure of air throughout the system is above atmospheric pressure and air is forced to flow through the system. PROF. P. P. PANDIT 9425773517 THERMAL ENGG & GAS DYNAMICS MECHANICAL DEPARTMENT GEC GROUP OF COLLEGE FORCE DRAUGHT INDUCED DRAUGHT: In this system, the blower is located near the base of the chimney instead of near the grate. The air is sucked in the system by reducingthe pressure through the system below atmosphere.The action of the induced draught is similar to the action of the chimney. The draught produced is independent of the temperature of the hot gases therefore the gases may be discharged as cold as possibleafter recovering as much heat as possible in air-preheater and economiser. INDUCED DRAUGHT
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