THERMAL ENGINEERING & GAS DYNAMICS ME-6002

Transcription

1 SAGAR INSTITUTE OF RESEARCH & TECHNOLOGY, BHOPAL DEPARTMENT OF MECHANICAL ENGINEERING THERMAL ENGINEERING & GAS DYNAMICS ME

2 UNIT I STEAM GENERATOR

3 SYLLABUS Steam generators: classification, conventional boilers, high-pressure boilers-lamont, Benson, Loeffler and Velox steam generators, performance and rating of boilers, equivalent evaporation, boiler efficiency, heat balance sheet, combustion in boilers, super critical boilers, fuel and ash handling, boiler draught, overview of boiler codes. Engineering, SIRT Bhopal 3

4 STEAM GENERATORS Boiler is an important equipment of the most industrial facilities and power plants. Boiler is a closed pressure vessel used to produce high pressure or low pressure steam or to produce hot water for industrial or domestic use. 4

5 CLASSIFICATIONS Classification of boilers The steam boilers are classified according to the following conditions:- 1. According to the relative position of water and hot gases a. Fire tube boiler : Fire tube boilers are those boilers in which fire or flue gases are inside of the tubes and water is on shell side. Fire Tube Boilers are low capacity low pressure boilers usually used for heating, process or to produce hot water. [Cochran Boiler] b. Water tube boiler : The arrangement in the water tube boiler water is that the water is contained inside the tubes while the hot combustion gas is flows on other side of tubes. Water tubes boilers produce high pressure steam which is used for power generation purposes. Water tube boilers are more efficient and less dangerous than fire tube boilers. [Babcock Wilcox Boiler] 5

6 CLASSIFICATIONS 2. According to the axis of shell. a. If the axis of boiler is horizontal, the boiler is called horizontal Boiler [Lancashire Boiler]. b. b. If the axis is vertical then it is called vertical Boiler. [Cochran Boiler] c. If the axis is inclined then it is called inclined. 6

7 CLASSIFICATIONS 3. According to the position of FURNACE a. Internally fired boiler:- In case of internally fired boilers, the furnace is located inside the boiler shell. [ all fire tube boilers] [Cochran Boilers] b. Externally fired boiler: The boiler is known as externally fired if the fire is outside the shell. [ all water tube boilers] [Babcock and Wilcox Boilers] 7

8 CLASSIFICATIONS 4. According to the pressure developed a. The boiler which produce steam at pressure below 80 bar are called low pressure boiler. ex. Cochran[Pressure less than 80 bar] b. The boilers which produce steam at pressure of 80 bar and above are called high pressure boiler. ex. Velox High pressure boiler [Pressure greater than 80 bar] 5. According to the method of water circulation a. In natural circulation type of boiler the circulation of water in boiler takes place due to natural convention. ex. Lancashire, Babcock and Wilcox. [all low pressure boilers] [ Cochran Boiler] b. Forced circulation In forced circulation type of boilers the circulation of water is done by forced pumps. [all high pressure boilers] ex. Velox, Lamont Boiler 8

9 CLASSIFICATIONS 6. According to the use of the boiler a. Stationary boiler [Cochran Boiler] b. Mobile boiler [Locomotive Boiler] 7. According to the number of drums a. Single Drum b. Multi Drum 8. According to the nature of draught a. Natural Draught b. Forced Draught 9

10 BOILER MOUNTINGS and ACCESSORIES BOILER MOUNTINGS are the components generally mounted on the surface of the boiler to have safety during operation. These are the essential parts of the boiler, without which the boiler operation is not possible. The following are the important mountings of the boiler: 1. Safety valves [a]. Dead weight safety valve [b] Spring loaded safety valve [c] Lever safety 2. Water level indicator 3. Pressure Gauge 4. Steam stop valve 5. Feed check valve 6. Blow off cock 7. Manhole 8. Fusible plug 10

11 BOILER MOUNTINGS SAFETY VALVE: The function of the safety valve is to permit the steam in the boiler to escape to atmosphere when pressure in the s in the boiler drum raises above the working pressure. The safety valve operates in the principle that a valve is pressed against its seat through some agency such as strut, screw or spring by external weight or force, when the steam force due to boiler pressure acting under the valve exceeds the external force, the valve gets lifted off its seat and some of the steam rushes out until normal pressure is restored again. The commonly used safety valves are given below: i) Dead weight safety valve ii) Lever safety valve iii) Spring loaded safety valve iv) High steam-low water safety valve 11

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13 BOILER MOUNTINGS Water level indicator: the function of the water level indicator is to show the level of water in the boiler. 13

14 BOILER MOUNTINGS FUSIBLE PLUG: The function of the fusible plug is to extinguish the fire in the event of the boiler shell failing below a certain specified limit. We know that when the water on heating transforms into steam, the level of water in the boiler falls down. If the water is not replenished and the steam generation continues then the parts, which have been uncovered by water uncovered by water may get overheated and subsequently are melted. To safeguard against this eventuality we use fusible plug. The fusible plug is inserted at the box crown or cover the combustion chamber at the lowest permissible water level. 14

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16 BOILER MOUNTINGS PRESSURE GAUGE: Each boiler has to be provided with a pressure gauge, which record the pressure at which the steam is being generated in the boiler. 16

17 BOILER MOUNTINGS BLOW OFF COCK: The blow of cock serves to drain out the water from the boiler periodically for any one of the following reasons: 1) To discharge mud, scale and other impurities which settle down at the bottom of the boiler? 2) To empty the boiler for internal cleaning and inspection. 3) To lower the water level rapidly if the level becomes too high. The unit is fitted at the lowest portion of the boiler. It may be mounted directly to the boiler shell or through an boiler elbow pipe, which is fitted to the boiler shell. 17

18 BOILER MOUNTINGS FEED CHECK VALVE: The feed check valve has the following two functions to perform: 1. To allow the feed water to pass into the boiler. 2. To prevent the back flow of water from the boiler in the events of the failure of the feed pump. 18

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21 BOILER MOUNTINGS STOP VALVE: The function of the steam stop valve is to shut off or regulate the flow of steam from the boiler to the steam pipe or from the steam pipe to the engine. When used for the former purpose, it is called junction valve. Usually the junction valve means a regulating valve of larger size and a stop valve refers to a regulating valve of smaller size. The junction valve is mounted on the highest part of the steam space of the boiler and is connected to the steam pipe, which carries the steam to the engine. 21

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23 BOILER MOUNTINGS MAN HOLES: These are door to allow men to enter inside the boiler for the inspection and repair. 23

24 BOILER MOUNTINGS and ACCESSORIES Boiler Accessories: These are those devices which are installed with a boiler and its neighbouring area to increase the efficiency of the boiler. These are not the essential part of the boiler and thus without installing these devices, the boiler operation can be accomplished though at a lower efficiency. The following are the important accessories of the boiler are: 1. Economiser 2. Air pre-heater 3. Super heater 4. Feed pumps 24

25 BOILER ACCESSORIES AIR PREHEATER: Air heater or air pre-heater is waste heat recovery device in which the air on its way to the furnace is raised in temperature by utilizing the heat of the exhaust gases. Air pre-heater are classified into the following two categories. Recuperative Air heater Regenerative Air heater 25

26 BOILER ACCESSORIES 26

27 BOILER ACCESSORIES ECONOMISER: The economizer is a device, which serves to recover some of the heat being carried by exhaust flue gases. The heat thus recovered is utilized in raised temperature in feed water being supplied to the boiler. If the water is raised and thus there is a saving in the consumption of fuel. The economizer unit is installed in the path of the flue gases between the boiler and the chimney. 27

28 BOILER ACCESSORIES 28

29 BOILER ACCESSORIES 29

30 BOILER ACCESSORIES SUPER HEATER:- The steam generated by a simple boiler in generally wet or at the most dry saturated. Steam super heater is a surface heat exchanger in which the wet steam is first dried at the same temperature and pressure and then raised to temperature above the saturation temperature at constant pressure. Heat of flue gasses utilized in super heating the steam and as the super heater is placed in the path of the flue gasses, Since superheating result in the increased efficiency and economy of the steam plant. 30

31 BOILER ACCESSORIES 31

32 BOILER ACCESSORIES FEED WATER EQUIPMENT: The pressure inside a steaming boiler is high and so the feed water has to be raised in pressure before its entry can be affected in the boiler. Feed pump is a device which raised the pressure of water and forces it into the boiler. 32

33 CLASSIFICATION OF BOILERS The boiler may be classified as : 1. Horizontal, vertical or inclined 2. Fire tube & water tube 3. Externally fired & internally fired 4. Forced circulation & internally fired 5. High pressure & low pressure 6. Forced circulation & natural circulation 7. Stationary & portable 8. Single tube & multi tube boilers Horizontal, vertical or inclined If the axis of boiler is horizontal, the boiler is called horizontal. If the axis is vertical then it is called vertical. If the axis is inclined then it is called inclined. Advantage of horizontal boiler: a. it should be repaired easily. b. occupies less floor area Fire tube & water tube boiler In the fire tube boiler the hot gases are inside the tubes & the water surrounds in the tube. ex. Cochran, locomotive etc. In the water tube boiler the water is inside the tube 33

34 & the hot gases are surround them. ex. Stirling Externally & internally fired The boiler is known as externally fired if the fire is outside the shell. ex. Babcock & wilcox In case of internally fired boilers, the furnace is located inside the boiler shell. ex: cochran, lancashire etc Forced circulation & natural circulation In forced circulation type of boilers the circulation of water is done by forced pumps. ex. Velox,lamont. In natural circulation type of boiler the circulation of water in boiler takes place due to natural convention. ex. Lancashire, babcock. 34

35 5. High pressure & low pressure boilers The boilers which produce steam at pressure of 80 bar and above are called high pressure boiler. ex. Velox The boiler which produce steam at pressure below 80 bar are called low pressure boiler. ex. cochran Single tube multi-tube boilers The fire tube boilers are classified as single tube & multi tube boilers, depending upon the fire tube is one more than one. Ex: cornish boiler 12. Boiler mainly classified as: a. fire tube b. water tube 13. a). FIRE TUBE BOILERS 14. b). WATER TUBE BOILERS 15. Comparison between Fire-tube & water-tube boilers S no. Particulars Fire tube boilers Water tube boilers 1. Mode of firing Internally fired Externally fired 2. Rate of steam production lower Higher 3. construction Difficult Simple 4. transportation Difficult Simple 5. Treatment of water Not so necessary More necessary 6. Operating pressure Limited to 16 bar Under high pressure as 100 bar 7. Floor area More floor area Less floor area 8. Shell diameter Large for same power Small same power 9. explosion Less More 10. Risk of bursting lesser More risk 16. TYPES OF FIRE TUBE BOILERS There are mainly five types of fire tube boilers : 1). Simple vertical boiler 2). Cochran boiler 3.) Cornish boiler 4). Lancashire boiler 5). Locomotive boilers 35

36 1). SIMPLE VERTICAL BOILER It consist of a cylindrical shell, the greater portion of which is full of water & remaining is the steam space. At the bottom of the fire box is grate on which fuel is burnt and the ash from it falls in the ash pit. A simple vertical boiler is self-contained & can be easily transported ). COCHRAN BOILER It is one of the best types of vertical multi-tubular boiler, and has a number of horizontal fir tubes. Cochran boiler consist of a cylindrical shell with a dome shaped top where the space is provided for steam ). LANCASHIRE BOILER This boiler is reliable, has simplicity of design, ease of operation & less operating & maintenance costs. It is commonly used in sugar- mills & textile industries where along with the power system & steam for the process work is also needed ). LOCOMOTIVE BOILERS 21. About Locomotive boilers : Locomotive boiler is a horizontal fire tube type mobile boiler. The main requirement of this boiler is that it should produce steam at a very high rate. Therefore, this boiler requires a large amount of heating surface and large grate area to burn coal at a rapid rate. In order to provide the large heating surface area, a large number of fire tubes are setup and heat transfer rate is increased by creating strong draught by means of steam jet. ADVANTAGE : a. Large rate of steam generation per square meter of heating surface. To some extent this is due to the vibration caused by the motion. b. It is very compact. c. The pressure of the steam is limited to about 20 bar. 36

37 CONVENTIONAL BOILERS 37

38 LANCASHIRE BOILER 38

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41 LANCASHIRE BOILER Lancashire is a horizontal, stationary fire tube boiler. This boiler is invented by Sir William Fairbairn in the year The flue gases flows through the fire tube, situated inside the boiler shell, so it is a fire tube boiler. This boiler generate low pressure steam. It a internally fired boiler because the furnace is placed inside the boiler. 41

42 LANCASHIRE BOILER Principle of Lancashire Boiler: This boiler works on the basic principle of heat ex-changer. It is basically a shell and tube type heat ex-changer in which the flue gases flow through the tubes and the water flows through shell. The heat is transfer from flue gases to the water through convection. It is a natural circulation boiler which uses natural current to flow the water inside the boiler. 42

43 LANCASHIRE BOILER Construction: This boiler is similar a shell and tube type heat ex-changer. It consist a large drum of diameter up to 4-6 meter and length up to 9-10 meter. This drum consist two fire tube of diameter up to 40% of the diameter of shell. The water drum is placed over the bricks works. Three spaces create between the drum and the bricks, one is at bottom and two are in sides as shown in figure. Flue gases passes through the fire tubes and side and bottom space. The water level inside the drum is always above the side channels of flue gases, so more heat transfer to the water. The drum is half filled with water and the upper half space for steam. The Furnace is located at one end of the fire tubes inside the boiler. The low brick is situated at the grates (space where fuel burns) which does not allow to un-burned fuel and ash to flow in fire tubes. The boiler also consist other necessary mountings and accessories like economizer, super heater, safety valve, pressure gauge, water gauge, etc. to perform better. 43

44 LANCASHIRE BOILER Working: The Lancashire boiler is a shell and tube type heat ex-changer. The fuel is burn at the grate. The water is pumped into the shell through the economizer which increases the temperature of water. Now the shell is half filled with water. The fire tube is fully immersed into the water. The fuel is charged at the grate which produces flue gases. These flue gases first passes through the fire tube from one end to another. This fire tubes transfer 80-90% of total heat to the water. The backward flue gases passes from the bottom passage where it transfer 8-10% heat to water. The remaining flue gases passes from the side passage where it transfer 6-8% of heat to water. The brick is the lower conductor of heat, so work as heat insulator. The steam produces in drum shell it taken out from the upper side where it flows through super heater if required. So the steam produce is taken by out for process work. 44

45 Advantages: LANCASHIRE BOILER 1. This boiler is easy to clean and inspect. 2. It is more reliable and can generate large amount of steam. 3. It required less maintenance. 4. This boiler is a natural circulation boiler so lower electricity consumption than other boilers. 5. It can easily operate. 6. It can easily meet with load requirement. 7. Lancashire boiler has high thermal efficiency about 80-90%. Disadvantages: 1. This boiler required more floor space. 2. This boiler has leakage problem. 3. It requires more time to generate steam. 4. It cannot generate high pressure steam if required. 5. Grates are situated at the inlet of fire tube, which has small diameter. So the grate area is limited in this boiler. 45

46 BABCOCK AND WILCOX BOILER 46

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48 BABCOCK AND WILCOX BOILER Babcock and Wilcox boiler is a natural circulation, externally fired medium pressure, stationary horizontal water tube boiler in which water is flows in the inclined tubes. The furnace is located outside of the drum. This boiler is invented by Stephen Wilcox and George Herman Babcock in the year So this is named as Babcock and Wilcox. 48

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50 BABCOCK AND WILCOX BOILER Main Parts: Babcock and Wilcox is a stationary water tube boiler. It has inclined tubes which are inclined at 10 to 15 degree, which are inclined to maintain natural circulation of water from drum to tubes and steam from tubes to drum. The main parts of this boiler are as follow. 1. Steam separator drum: This drum is situated upside of the boiler. It is larger diameter drum in which water and steam placed together. The one half of the drum is filled with water and the other half is remaining for steam. 2. Water tubes: Water tubes are situated bottom side of the drum. Water flows from the drum to the tubes. 3. Uptake header: Steam separator drum and water tubes are connected by the two tubes. One is known as uptake header and the otherone is known as down take header. The steam from the water tubes to the drum flow by the uptake header. 4. Down take header: The water flows form the drum to the water tubes through down take header. When the steam flows by uptake header to the drum, at the same time water flows from drum to the water tubes by down take header which maintains the flow of water. 50

51 BABCOCK AND WILCOX BOILER 5. Grate: The place in the furnace, where the fuel is placed and burn known as grate. 6. Furnace: The furnace is the place where the fuel burns. This is situated at the down side of the water tubes. When the fuel burns, the flue gases generate. This gases flow upper side and passes through water tube, which heat the water and convert it into steam. 7. Super heater: Super heater is situated upper side of the water tube. One end of super heater is connected to the drum and other end is for process work. Steam flows from the drum to the super heater, where it heated by the flue gases and send for the process work. 8. Baffles: Baffles are provided between the water tubes. The main function of baffles is to divert the flue gases, so it flows more than one time through the tube and more heat is transfer. Other mounting like safety valve, blow off valve, fuel door, inspection door, mud box, feed valve, stop valve, pressure gauge etc. are also provided for safely working of boiler. 51

52 BABCOCK AND WILCOX BOILER Advantages: 1. The steam generation rate is higher about 20 ton per hour at pressure 10 to 20 bars. 2. The tubes can be replaced easily. 3. The boiler can expand and contract freely. 4. It is easy to repair maintenance and cleaning. 5. It drought loss is low compared to other boiler. 6. The overall efficiency of this boiler is high. 52

53 Working: BABCOCK AND WILCOX BOILER This is a high pressure, natural circulate, water tube boiler. The working of this boiler is as follow:- First water is filled in the water drum through feed pump. The drum is half filled with water and the upper half is for steam. First flue is fired at the grate. The flue gases generate by burning of fuel. These gases start flowing from one end to another end of boiler. The flue gases passes by the water tubes and exchange heat with water. The baffles are provided in the way, which deflects the flue gases before escaping from the chimney. Due to this deflection, the flue gases pass more than one time through water tubes, which cause more heat transfer. The water flows from Richard the Victor, drum Department to the of Mechnical water tube through down take header. 53

54 BABCOCK AND WILCOX BOILER The water tube nearer to the furnace heated more than the other, so the density of water decrease in this tube which causes the flow of steam from tube to drum through uptake header. At the same time the water from the drum enters into the tubes through down take header. The circulation of water from drum to tubes and again tubes to drum is natural, due to density difference. The steam separates in the drum at the upper half. This is saturated steam. This steam sends to the super heater through steam pipe. The steam is heated again by the flue gases in the super heater and taken out for process work. The flue gases send to the atmosphere from the super heater. This process repeat until sufficient amount of steam generates. This boiler can generate 20 ton steam per hour. 54

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56 COCHRAN BOILER Cochran boiler is a modified form of fire tube boiler which has various fire tubes which exchange heat to water through convection. It I It is a simple fire tube boiler which has multiple fire tubes. These tubes increase the heat transfer area of boiler. Water passes over the tubes. The heat is transfer from fire tubes to water through convections used in small power plant and industries. 56

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58 COCHRAN BOILER Main parts of Cochran boiler: 1. Shell: The main body of the boiler is known as shell. It has vertical shall which has hemispherical top head. This hemispherical top gives higher volume to area ratio which increases steam capacity. 2. Grate: The area where flue is placed known as grate. 3. Combustion chamber: It is at the bottom side of the boiler. The combustion chamber is connected to the furnace. 4. Fire tubes: There are various fire tubes which one end is connected to the furnace and other by chimney. 58

59 COCHRAN BOILER 5. Fire hole: The small hole is provided at the bottom of combustion chamber to place fuel is known as fire hole. 6. Furnace: It works as a mediator of fire tubes and combustion chamber. It is also has dome shaped top. The main advantage of this shape is that it deflects back the un-burned charge and flue gases to the combustion chamber and furnace. 7. Chimney: The flue gases leave atmosphere through chimney. 8. Fire brick lining: The brick lining outside the combustion chamber prevent the overheating of it. 59

60 COCHRAN BOILER Along these parts Cochran boiler equipped with following mountings and accessories. 1. Safety valve 2. Pressure gauge 3. Water level indicator 4. Stop valve 5. Blow off cock 60

61 COCHRAN BOILER Working of Cochran boiler: Cochran boiler works as same like other fire tube boilers. It can be understand by following points. Coal is placed at the grate through fire hole. Air is entering into the combustion chamber through atmosphere. Fuel is sparked through fire hole Flue gases start flowing into the hemispherical dome shaped combustion chamber. This flue gases further moves into the fire pipes. Heat is exchange from flue gases to the water into the fire tubes. The steam produce collected into the upper side of the shell and taken out by when required pressure generated. The flue gases now send to the chimney through fire box where it leaves to the atmosphere. 61

62 Advantages and disadvantages of Cochran boiler Advantages: 1. Low floor area required. 2. Low initialization cost. 3. It is easy to operate. 4. It is easy to transport from one place to another. 5. It has higher volume to area ratio. Disadvantages: 1. Low steam generation rate. 2. Limited pressure handles capacity. 3. It is difficult to inspect and maintenance. 62

63 LOCOMOTIVE BOILER 63

64 LOCOMOTIVE BOILER Locomotive boiler is a horizontal drum axis, multi-tubular, natural circulation, artificial draft, forced circulation, mobile, medium pressure, solid fuel fired fire tube boiler with internally fired furnace. It is used in railway locomotive engines and in marine. It is a mobile boiler and has high steam generation rate. 64

65 1. Fire hole: It is a hole provided at the rear end of the boiler. The solid fuel is inserted and ignited into the furnace through this hole. 2. Fire box: It is a box in which the burning of the fuel takes place. 3. Grate: Grate is a platform on which the solid fuel is kept and burnt. 4. Fire brick arch: It is a brick arch placed inclined over the grate. It prevents the entry of the ash, dust and burnt fuel particles into the fire tubes. It provides a way to the hot flue gases to travel a definite path before entering into the fire tubes of the boiler. 5. Boiler tubes: They are the fire tubes through which the hot flue gases passes and exchange the heat with surrounding water. 65

66 6. Smoke box: According to its name, it is a box in which the smoke of the burnt fuel after passing through the fire tubes gets collected. From there it is exhausted in the environment by the chimney. 7. Blast pipe: It is pipe provided above the steam engine. The exhaust steam passes through this blast pipe. It is used to create the artificial draft that pushes the smoke out through the chimney and creates suction for the hot flue gases. The suction created allows the hot flue gases to move forward through the fire tubes. 8. Steam pipe: It is a pipe through which the steam passes. We have two steam pipes; one is main steam pipe present in between the superheater header and dome. And second one is that which connects the superheater exit end to the steam engine. 9. Superheater: It superheats the steam to the desired temperature before entering into the cylinder of the steam engine. 10. Super heater element pipes: These are the pipes of superheater through which the steam travels and gets superheated. 11. Dome: It is present at the top and contains the regulator for regulating the steam produced through the steam pipe. 12. Regulator valve: It is a valve that regulates the steam through main steam pipe for superheating. 13. Safety valve: It is used to maintain the safe working steam pressure in locomotive boiler. It blows off steam when the pressure of the steam increases above safety level and prevents blasting of the boiler. 14. Superheater header: It is the head of the superheater which accepts the steam form the steam pipe. 15. Chimney: It is used to throw out the exhaust smoke and gases to the environment. The length of the chimney is very small in this boiler. 66

67 Working In locomotive boiler, first the solid fuel (coal) is inserted on the grate and is ignited from the fire hole. The burning of the fuel starts and it creates hot flue gases. A fire brick arch is provided that makes the flow of hot flue gasses to a definite path before entering into the long tubes (fire tubes). It also prevents the entry of burnt solid fuel particles into the fire tubes. The hot flue gases passes through the long fire tubes and heats the water surrounding them. Due to the heating the water gets converted into saturated steam and gets collected at the top. The saturated steam from the dome enters into the main steam pipe through the regulator valve. The steam travels in the main steam pipe and reaches to the superheater header. Form header, the steam enters into super heater element pipes. Here it is superheated and then the superheated steam enters into the steam pipe of the smoke box. The steam form the superheater goes to the cylinder containing piston. The superheated steam made the piston moves within the cylinder. The piston is connected to the wheels of the steam engine and the wheels start rotating. The exhaust steam from the cylinder enters into the blast pipe. The burnt gases and smoke after passing through the fire tubes enters into the smoke box. The exhaust steam coming out from the blast pipe pushes the smoke out of the boiler through the chimney. Here the smoke cannot escapes out form the boiler by its own, so artificial draft is created by exhaust steam coming out from the steam engine. This artificial draft created pushes the smoke out of the smoke box and creates suction for the hot flue gases. 67

68 Advantages It is portable. It is capable of meeting sudden and fluctuating demands of steam. It is cost effective boiler. It has high steam generation rate. It is compact in size and its operation is easy. Disadvantages It faces the problems of corrosion and scale formation. Unable to work under heavy load conditions because of overheating problems. Some of its water space are difficult to clean. Application Locomotive boilers are mostly used in railways and marines. The efficiency of this boiler is very less. It cannot work in heavy load conditions because this leads to the overheating of the boiler and finally gets damage. They are also used in traction engines, steam rollers, in portable steam engines and some other steam road vehicles. 68

69 HIGH PRESSURE BOILER- LAMONT Lamont boiler is the first forced convection boiler which is introduced in the year A LaMont boiler is a type of forced circulation water-tube boiler in which the boiler water is circulated through an external pump through long closely spaced tubes of small diameter. The mechanical pump is employed in order to have an adequate and positive circulation in steam and hot water boilers. A water circulating pump is used in the boiler to circulate water inside the boiler. This pump is driven by the steam turbine which uses steam of boiler. It is used in power plant industries to generate electricity. 69

70 Principle: This boiler works on basic principle of forced convection. If the water is circulate by a pump inside the tube, the heat transfer rate from gas to the water is increases. It is the basic principle of it. 70

71 Construction: This boiler is the first force circulation boiler. This boiler consist various part which are as follow. Economizer: Economizer use to preheat the water by using remaining heat of the combustion gases. It increases the boiler efficiency. The feed water first supplied to the economizer before entering to the boiler. Centrifugal pump: The Lamont boiler is a force convection boiler. So a centrifugal pump is used to circulate water inside the boiler. This pump is driven by a steam turbine. The steam for the turbine is taken by the boiler. Evaporator tube: The evaporator tube or can say water tubes are situated at furnace wall which increase the heating surface of boiler. This is also at the up side and down side of the furnace and other equipment. The main function of these tubes to evaporate water into steam. This also cools down the furnace wall. Grate: The space in the furnace where the fuel is burn is called grate. It is bottom side of furnace. Furnace: In the Lamont boiler vertical furnace is used. The main function of Furnace is to burn the fuel. Super heater: The steam generated by the evaporator tube is saturated steam. If it directly used in steam turbine can cause the corrosion. So the saturated steam sends to the super heater where it can increase the temperature of steam. Water steam separator drum: The steam separator is situated outside from the boiler. The mixture of water and steam from the evaporator tube send to the steam separator where it separate the steam and send it to super heater. The remaining water again sends to the economizer. Air preheater: It s main function to preheat air before entering into furnace. 71

72 Working: Lamont boiler is a forced circulation, internally fired water tube boiler. The fuel is burn inside the boiler and the water is circulating by a centrifugal pump through evaporator tubes. The working of this boiler is as follow. A feed pump forces the water into the economizer where the temperature of water increases. This water forced into the evaporator tube by using a centrifugal pump driven by steam turbine. Water passes times into the evaporator tube. The mixture of saturated steam and water is formed inside the tube. This mixture sends to the steam separator drum which is outside the boiler. Steam from the separator sends to the super heater, where the saturated steam converts into superheated steam. The water again sends to the economizer where it again passes by the evaporator tubes. The air from the air preheater enter into the furnace where fuel burn. The flue gases first heat the evaporator tube then passes by the super heater. These gases from the super heater again use to preheat the air into air preheater before exhaust into atmosphere. This working pressure of this boiler is above 170 bar and have the steam generation capacity of about kg/hour at temperature 773 K. 72

73 Advantages: 1. It is a high pressure boiler. 2. It is flexible in design. 3. This boiler can be reassembled into natural circulation boiler. 4. It can be easily started. 5. It has high steam generation capacity of about 50 tonnes/ hour. 6. This boiler has higher heat transfer rate 73

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77 BENSON BOILER 77

78 Benson boiler is also known as super critical steam generator which is developed by Mark Benson in the year This boiler can generate high pressure steam, which is further used in production of electricity and other industrial processes. It is a water tube boiler. In the early stage water tube boiler used to generate steam at the pressure up to 10MPa, which is known as sub critical boiler. Two big problem occur in sub-critical boiler. 78

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81 Construction 1. Air Preheater: It preheats the air before entering into the furnace. The preheated air increases the burning efficiency of the fuel. 2. Economiser: It heats the water to a certain temperature. 3. Radiant Superheater: It is super heater which heats the water with radiation produced by the burnt fuel. It raises the temperature to supercritical temperature. 4. Convection Evaporator: It evaporates the superheated water and converts them into steam. It does so by the convection mode of heat transfer to the water from the hot flue gases. 5. Convection Superheater: It superheats the steam to the desired temperature (nearly 650 degree Celsius). 6. Furnace: It is the place where the fuel is burnt. 7. Feed Pump: It is used to supply the water inside the boiler at supercritical pressure of 225 bars. 81

82 Working Principle It works on the principle that the pressure of the water is increased to the supercritical pressure (i.e. above critical pressure of 225 bar). When the pressure of water is increased to the super critical level, the latent heat of water becomes Zero and due to this, it directly changes into steam without boiling. And this prevents the formation of bubbles at tube surface. Working In Benson Boiler, the feed pump increases the pressure of the water to the supercritical pressure and then it enters into the economiser. From economiser, the water the water passes to the radiant heater. Here the water receives the heat through radiation and partly gets converted into steam. The temperature raises almost to the supercritical temperature. After that mixture of steam and water enters into convective evaporator where it is completely converted into steam and may superheated to some degree. Finally it is passed through the superheater to obtained the desired superheated steam. This superheated steam is then used by turbines or engine to produce the electricity. 82

83 Advantages The various advantages of the boiler are It is a drum less boiler and hence the weight of this type of boiler is 20 % less as compared with other types of boiler. It is light in weight. Occupy smaller floor area for its erection. Explosion hazard is almost negligible because of use of smaller diameter tubes. It can be started very easily within 15 minutes. It avoids bubble formation due to the super critical pressure of water. Transportation is easy. This boiler may achieve thermal efficiency upto 90 %. Application This supercritical boiler is used in different industries to generate steam for the production of electricity or mechanical power. The average operating pressure, temperature and capacity of benson boiler is 650 degree Celsius, 250 bar and 135 tonnes/h. 83

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85 Loeffler Boiler 85

86 LOEFLLER BOILER Loeffler Boiler is a forced circulation, high pressure, and water tube boiler with internally fired furnace. In this boiler, the 2/3 of superheated steam is used to evaporate the water in the evaporating drum and remaining 1/3 of the steam from the superheater is used by the turbine. A steam circulating pump is used to circulate the steam into the boiler. 86

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89 Working Principle Its main working principle is to evaporate the feed water by the use of superheated steam from the superheater. 2/3 of the total steam generated by the superheater is made to flow into the evaporator drum. The superheated steam changes the feed water into saturated steam. And 1/3 part of the superheated steam is used by the turbine to do work. 89

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92 1. Economiser: It preheats the feed water before entering into the evaporating drum. 2. Evaporating drum: It is placed at lower portion of the boiler. It contains the mixing nozzle. Here the feed water is converted into saturated steam with the help of the superheated steam from the superheater. 3. Steam circulating pump: It is present in between the evaporating drum and radiant superheater. It circulates the steam into the boiler. 4. Radiant super heater: It superheats the steam with the help of radiations produced by the burnt fuel in the boiler. 5. Convective superheater: Convective superheater superheats the steam to the desired temperature of about 500 degree C. 6. Mixing nozzle: It is present inside the evaporating drum. It mixes the steam from the superheater with feed water and evaporates them. 92

93 Working In loeffler boiler, the feed pump forces the water to enter into the economiser. The economiser preheats the feed water and then it is passed to the evaporator drum. The evaporator drum has nozzles. The 2/3 of the superheated steam from the superheater enters into the nozzles of the evaporator drum and the nozzles mix this superheated steam with the feed water. This changes the feed water into saturated steam. This saturated steam is then drawn from the evaporating drum by a steam circulating pump and allows it to passes through the radiant superheater. The radiant superheater superheats the saturated steam with help of radiation energy produce from the burning of the fuel. The radiant superheater tubes are placed in furnace. The heat transfer to the water takes place through the radiation produced. After the radiant superheater, the steam is passed to the convective superheater. The convective superheater is placed in the path of hot flue gases. It superheats the steam coming from the radiant superheater to a temperature of about 500 degree C. This superheated steam is than flows to the turbine and evaporating drum. In this boiler, the 2/3 part of the superheated steam is used to evaporate the feed water into the evaporating drum and remaining 1/3 part flows to the turbine. Loeffler boiler has the capacity Richard Victor, to produce Department of 100 Mechnical tonnes/ h of steam at a temperature of 500 degree Engineering, C and pressure SIRT Bhopal of 140 bar. 93

94 Advantages It can use salt water for the steam generation. The problem of deposition of sediments and scale in the boiler tubes are eliminated. It is compact in size. 94

95 VELOX BOILER 95

96 VELOX BOILER Velox boiler is a forced circulation water tube boiler. It is mostly used in gas turbine. In this boiler, the velocity of flue gases is greater than the velocity of sound, which causes more heat transfer from gas to the water, which increases the steam generation rate. Due to this, it is most important boiler. 96

97 Principle: When the velocity of the gas is greater than the speed of sound, its heat transfer rate is also increases. So more heat is transferred from gas to water as compare when the heat transfer at the subsonic speed. This is the basic principle of it. This boiler can increase the heat transfer rate or can say steam generation rate without increasing boiler size. This is why; Velox boiler is most successful boiler in the gas turbine industries. 97

98 Construction: Velox boiler is a water tube forced circulation boiler. It has a gas turbine driven air compressor, which compresses the air. This compressed air enters into the vertical combustion chamber, as result, high rate of heat release from the fuel, which increases the flue gases velocity up to the sound velocity. This is a force circulation boiler, so pump is used to circulate water inside the boiler. This boiler also consist water and fire tube to maintain the flow of gas and water inside the boiler. This boiler also consists other necessary mounting and accessories like economizer, super heater, blow off valve, safety valve etc. 98

99 Working: The Velox boiler works as a basic heat exchanger. The working of this boiler is as follow. The air is compressed by air compressor driven by gas a turbine driven. This compressed air passes from the combustion chamber, where more heat release by the fuel which increase the velocity of the flue gases up to sound velocity. From the bottom of combustion chamber, this flue gases pass from the fire tubes. These fire tubes surrounded by the evaporator water tubes. The water from the economizer passes from the evaporator tube force by a circulating pump. This water passes time from the evaporator tube at very high speed. Due to this high speed circulation, heat is transfer from the gases to the water at very high rate. The mixture of water and steam is formed which further passes from the water and steam separator. The steam from the steam separator passes to the super heater and further for process work. The remaining water in the steam separator again passes from the evaporator tube. The flue gases from the fire tubes send to the super heater tubes, where it increases the steam temperature. The gas from the super heater sends to the turbine where it rotates the gas turbine and then passes from the economizer. 99

100 Advantages: 1. This boiler has high heat transfer rate. 2. It has great flexibility. 3. It is compact in design. 4. It is easy to control. It is fully automatic. 5. It has great thermal efficiency of about 90-95%. 100

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104 A supercritical steam generator is a type of boiler that operates at supercritical pressure, frequently used in the production of electric power. In contrast to a subcritical boiler in which bubbles can form, a supercritical steam generator operates at pressures above the critical pressure 3,200 psi or 22 MPa. Therefore, liquid water immediately becomes steam. Water passes below the critical point as it does work in a high pressure turbine and enters the generator's condenser, resulting in slightly less fuel use and therefore less greenhouse gasproduction. Technically, the term "boiler" should not be used for a supercritical pressure steam generator as no "boiling" actually occurs in the device. 104

105 In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. The most prominent example is the liquid-vapor critical point, the end point of the pressure-temperature curve that designates conditions under which a liquid and its vapor can coexist. At higher temperatures, the gas cannot be liquefied by pressure alone. At the critical point, defined by a critical temperature T c and a critical pressure p c, phase boundaries vanish. Other examples include the liquid liquid critical points in mixtures. 105

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107 DRAUGHT DRAUGHT may be defined as the small pressure difference which causes a flow of gas to take place. In case of boiler the function of the draught is to force air to the fire and through a boiler furnace and flue, and to discharge the products of combustion to atmosphere via stack or chimney. 107

108 Classification of draught Draught Natural Artificial Steam jet Mechanical (fan draught) Induced Forced 108

109 Functions of Chimney 1. It produces the draught whereby the air and gas are forced through the fuel bed, furnace, boiler passes and settings. The air which carries the oxygen necessary for the proper combustion of the fuel, is thereby furnished to the fuel bed. 2. It carries the products of combustion to such a height before discharging them that they will not pollute the surroundings. 109

110 Total static draught is the total pressure difference which results owing to the difference in the weights of the column of the hot flue gas inside the chimney and a column of the outside air of the same area and height. 110

111 Estimating the height of Chimney diameter and height 111

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114 Tutorial Q1. Calculate the draught in mm of water column produced by a chimney 30 metre high when the average temperature of hot gases is 225 C and the temperature outside air is 20 C. the quantity of air supplied is 18 kg/kg of the fuel. (13.7 mm of water or Pa) Q2. Estimate the height of a chimney to produce a static draught of 20 mm of water if the mean temperature of flue gases in the chimney is 250 C and the temperature of outside air is 20 C. the densities of atmospheric air and the flue gases at NTP are and 1.34 kg/cu metre respectively. (39.6 metres) Q3. A boiler is equipped with a chimney 25 metre high. The temperature of outside air is 25 C and the average temperature of flue gases in the chimney is 320 C. if the boiler is supplied with 20kg of air per kg of fuel burnt, calculate (a) the theoretical draught created in cm of water and (b) the velocity of flue gases in the chimney, if 60% of the draught is lost in friction at the grate and passages. (1.4 cm of water; metre/sec) 114

115 Condition for maximum discharge through a chimney The chimney draught is most effective when the maximum mass of hot gases is discharged in a given time, and it will be shown that this occurs when the absolute temperature of the chimney gases bears a certain relation to the absolute temperature of the outside air. The theoretical velocity, V, of the gases produced by a total static draught is given by the equation 2 V =2gh' h'=height of a coulmn of flue gases corresponding to the draught pressure 115

116 Q. Calculate the static draught produced by a chimney of 35 metre height when the mean gas temperature is such as to cause the mass of these gases discharged in a given time to be maximum. The temperature of atmospheric air is 25 C. 116

117 PERFORMANCE EVALUATION of BOILERS 117

118 T0 = Absolute Zero Temperature = 273 K ρ0 = Density of air at absolute temperature = kg m3 Tg = Temperature of hot gas ρg = Density of hot gas Now from PV = m R T P = ρ R T ρ T = consta nt o Density of air at temp. Tg ρ0t0 = ρair Tg ρair hot = ρ0 T0 T 118

119 o m = mass of air kg of fuel = m a m c = Ai r Fuel Ratio Total mass of Hot air kg of fuel = m a+ m c m c = m g = m

120 PERFORMANCE EVALUATION OF BOILERS The different boilers may have different evaporation rate of water at different feed water temperature, different boiler pressure, different saturation temperature and produce different kind of steam (wet, dry or superheated). Hence under such different conditions, it is very difficult to compare boiler performance. Comparison between any two boilers is possible only when they use same fuel, have the same feed water temperature and working pressure. 120

121 The feed water temperature adopted is 100 C and the working pressure bar pressure of the atmosphere above sea level. Under these conditions, evaporation of 1 kg of water requires the latent heat of 1 kg of steam= kj/kg. this quantity is known as Standard Evaporation Unit. 121

122 Equivalent Evaporation A certain amount of heat H 1 kj is required for raising a given quantity of steam, m Kg. The same quantity of heat applied to feed water at standard temperature 100 C produces a mass m e of steam at the standard pressure and temperature adopted. This quantity of steam m e is said to be equivalent to the quantity of steam m actually raised and is known as the equivalent evaporation. 122

123 The specific enthalpy of evaporation at a pressure of bar corresponding to saturation temperature of 100 C is kj/kg 123

124 1 Equivalent evaporation, H m[( h 1 f h1 ) x hfg ] me = m mass of water actually evaporated h latent heat of steam at boiler pressure in kj fg h specific enthalpy of feed water at the generation temp[erature, kj x= dryness fraction m equivalent evaporation from and at 100 C. e H Heat required to produce 1 kg of steam

125 Boiler efficiency Boiler efficiency heat actually required producing steam in a certain period heat liberated in furnace during the same period = m [( h h ) x h ] 1 f 1 m C fg 125

126 Tutorial Q1. A boiler working at a pressure of 14 bar evaporates 8.6 kg of water per kg of coal fired from the feed water entering at 39 C. The steam at the boiler stop valve is 0.92 dry. Determine the equivalent evaporation from and at 100 C. Also determine the thermal efficiency of the boiler if the calorific value of the coal is kj/kg. Q2. A boiler produces 8 kg of steam per kg of coal at a pressure of 1 MPa absolute from a feed water temperature of 40 C. Calculate the equivalent evaporation assuming dryness fraction of steam to be Also, find out the efficiency of boiler if the calorific value of coal used is 32000kJ/kg. Q3. In a boiler trial, 1200 kg of coal is consumed in 24 hours of duration. If the boiler generates kg of dry and saturated steam at a pressure of 0.8 Mpa from a feed water of 35 C, calculate the (a) equivalent evaporation/kg of coal and (b) efficiency of boiler. Take CV of fuel as 30000kJ/kg, specific heat of water=4.18 kj/kgk. 126