Study of Boilers for Mechanical Engineering

It's study on boilers

1. ME 4101: Introduction to Mechanical Engineering Steam Generators

2. Steam generator-Introduction The primary function of a steam generator is to generate steam under pressure, but modern steam generators are also required to do the following: Ensure generation of exceptionally high-purity steam by eliminating all impurities from saturated steam. Raise the degree of superheat of supplied steam as specified and maintain the same temperature over a defined range of load. In large power stations after partial expansion in the turbine steam is returned to the steam generator for further superheating and then transmitted to the turbine for complete expansion. This new degree of superheat is called “reheat” and should also be maintained constant over a defined range of load. While executing the above duties, a steam generator must utilize the heat of combustion of fuel as efficiently as possible.

3. Circulation In drum-type subcritical boilers, water flows from the drum through downcomers to the bottom of the furnace, then moves up through risers or evaporator tubes and returns to the drum. The figure shows that the water in the downcomers does not receive any heat while risers, which form furnace walls, absorb the heat of combustion of the fuel. As a result, the water inside the riser tubes is heated and gradually steam bubbles form in the water. This mixture of water and steam in the riser tubes has a much lower density than the density of water alone in the downcomers.

4. Circulation . Working principal: https://www.youtube.com/watch?v=nL-J5tT1E1k A density difference is thus generated which results in a static head difference causing the thermo-siphon effect and provides the driving force for a downward flow in downcomers and an upward flow in risers. Since the flow of fluid is generated by density difference alone, this is known as natural circulation in boilers. For maintaining the continuous circulation of fluid, the fluid flow must overcome friction losses in downcomer and riser tubes, headers, bends, etc. One way of overcoming resistance due to friction is by increasing heat input to the furnace

5. . Forced/assisted circulation Boilers In the earlier sections, the density difference between the saturated liquid and saturated vapor starts diminishing at 18 MPa or higher fluid pressure, thus it is difficult to maintain the natural circulation of fluid flow in boiler tubes. In such cases, fluid flow is ensured with the help of forced/assisted circulation using pumps. The forced/assisted circulation applies equally in both supercritical and subcritical ranges .

6. Design As discussed earlier, the chemical energy in fuel is converted to heat energy by combustion, and low-temperature water in the steam generator receives this heat of combustion and becomes high-energy steam for use in the steam turbine. The salient features of this heat-transfer process encompasses the following circuits, systems, and/ or areas: Water and Steam Circuit Furnace Fuel-burning System Draft System Heat Recovery System

7. Furnace In the furnace of a steam generator combustion of fuel takes place with atmospheric air to release heat. Hence, it is essential to ensure the complete combustion of fuel under all operating conditions to harness the maximum heat potential of the fuel under combustion. The furnace in a modern boiler is a large, water-cooled chamber in which fuel and air are mixed and burned in suspension. Its purpose is to achieve the most efficient transfer of heat energy from the fuel to the working fluid; water/steam. The furnace houses the firing equipment and confines the combustion process. A typical furnace has two entry and two exit ports. These are: (1) fuel inlet, (2) combustion air inlet, (3) products of combustion (flue gas) outlet and (4) a disposal port, usually in the form of an ash hopper.

8. Furnace The combustion chamber in modern boilers is constructed entirely of water-cooled tubes which are formed into four walls and surround the furnace. Factors that must be taken into account when designing a combustion chamber of a furnace are : Capacity of the boiler type of circulatory system type of firing range of fuels to be burnt

9. Boiler terms Shell : Consists of one or more steel plates bent into a cylindrical form and riveted or welded together. The shell ends are closed with end plates Setting : The primary function of setting is to confine heat to the boiler and form a passage for gases. It is made of brick work and may form the wall of the furnace and combustion chamber Grate : it is a platform in the furnace upon which fuel is burnt Furnace : it is the chamber formed by the space above the grate and below the boiler shell, in which combustion takes place. Water space and steam space : the volume of the shell that is occupied by the water is termed as water space while the entire shell volume less the water and tubes is called steam space Mountings : The items which are used for safety of boiler are called mountings Accessories : The items which are used for increasing the boiler efficiency are called accessories Water level : The level at which water stands in the boiler is called water level

10. Refractory : insulation material used for lining combustion chamber Lagging : Insulation wrapped on the outside of the boiler shell or steam piping Foaming : Formation of steam bubbles on the surface of boiler water due to high surface tension of water Scale : A deposit of medium due to extreme hardness occurring on the water heating surfaces of boiler because of an undesirable condition in the boiler water Blowing off : The removal of mud and other impurities of water from the lowest part of the boiler. Accomplished with the help of blow off cock or valve

11. STEAM BOILERS, MOUNTINGS & ACCESSORIES Classification of Boilers: Boilers may be classified according to the following- 1. Relative position of Hot gases and Water Fire tube boiler: The hot gases passes through the tubes that are surrounded by water. Fire tube boilers are also known by certain common names- Horizontal return tubular Locomotive fire box Scotch marine and, Vertical tubular https://www.youtube.com/watch?v=I2ZifNtUJkU

12. STEAM BOILERS, MOUNTINGS & ACCESSORIES b) Water tube Boiler: The water passes through the tubes and the hot gases produced by combustion of fuel, flow outside. This type of Boilers designated by the following common names: Babcock and Wilcox Boiler (straight but inclined tubes which connect the headers). Stirling Boiler (multitubular boiler having bent tubes that connect boiler drums to headers).

13. STEAM BOILERS, MOUNTINGS & ACCESSORIES 2. Method of firing a) Internally fired Boilers: The furnace is provided inside the boiler shell and is completely surrounded by water cooled surfaces. This method of firing is used in: Lancashire Boilers Locomotive Boilers and Scotch Boilers

14. STEAM BOILERS, MOUNTINGS & ACCESSORIES i) b) Externally fired Boilers: The furnace is provided outside/ under the boiler. It has an advantage that its furnace is simple to construct and can easily be enlarged, as and when required. This method of firing is used in Babcock and Wilcox Boiler. 3) Pressure of Steam: High pressure Boilers: Boilers producing steam 80 bar and above are called High pressure boilers. E.g. Babcock and Wilcox Boiler Lamont Boilers Velox Boilers and Benson Boilers etc.

15. STEAM BOILERS, MOUNTINGS & ACCESSORIES ii) Low pressure Boilers: Boilers producing steam Lower than 80 bar are called Low pressure boilers. E.g. Cochran Boilers Cornish Boiler Lancashire Boiler Locomotive Boiler

16. STEAM BOILERS, MOUNTINGS & ACCESSORIES 4. Method of circulation of water: Natural circulation method Circulatio n se t up by convectio n curren t or by gravity. Forced circulation method Circulation set up for high pressure steam through pumps.

17. STEAM BOILERS, MOUNTINGS & ACCESSORIES 5. Nature of service to be performed: Land Boilers: Boilers which are used with stationary plants Portable Boilers: Boilers which can be readily dismentaled and easily carried out from one site to another. Mobile Boilers: Boiler s whic h ar e fitted on mobil e carriage s ar e called Mobile Boilers. E.g. marine and locomotive boilers.

18. STEAM BOILERS, MOUNTINGS & ACCESSORIES 6. Once through Boilers: The boilers in which no circulation of water takes place i.e . th e feed wate r leave s th e tub e a s stea m e.g. Benson Boilers. Position and No. of Drums: Singl e or multidrum s ma y be positioned longitudinally or crosswise. Design of gas passages: Single pass Return pass Multipass

19. STEAM BOILERS, MOUNTINGS & ACCESSORIES 9. Nature of Draught: Natural Draught: when the fuel burns in the furnace of the boiler, with the circulation of air, the draught is named as Natural Draught. Artificial Draught: When the air is forced by means of forced fan, the draught is named as Artificial Draught.

20. STEAM BOILERS, MOUNTINGS & ACCESSORIES 10.Heat Source: Combustion of solid, liquid or gaseous fuel. Electrical and nuclear energy. Hot waste gases of other chemical reactions. 11. Fluid Used: Steam Boilers- use water as a fluid. Mercury Boilers- use mercury as a fluid. Other Boilers- use special chemicals as a fluid.

21. STEAM BOILERS, MOUNTINGS & ACCESSORIES 12. Material of construction of Boiler Shell: Cast Iron Boilers: Low pressure heating Boilers. Steel Boilers: Low pressure heating Boilers. Coppe r and Stainles s stee l Boilers : Miniature Boilers.

22. Difference between the boiler mountings and accessories:

23. Boiler accessories

24. Boiler accessories Feed pumps : Used to deliver feed water to the boiler. It is desirable that the quantity of water supplied should be at least equal to that evaporated and supplied to the engine Two types of which are commonly used as feed pumps are (1) reciprocating pump (2) rotary pump

25. Injector Function of injector is to feed water into the boiler It is commonly employed for vertical and locomotive boilers and does not find its application in large capacity high pressure boilers Also used where the space is not available for the installation of feed pump

26. Economizer The exhaust from the boilers is generally in the temperature range of 200°C – 250°C, so there are a huge amount of losses from the boiler if any heat recovery devices are not installed after it. If the exhaust gases which are leaving the Boiler at such high temperature is made to pass through the Economizer in order to provide the required sensible heat to the water by increasing its temperature, it will reduce the heat load on the boiler to the greater extent.

27. ECONOMISER Generally there are two types of economisers: condensing and non-condensing. Condensing economisers are designed to accommodate the corrosive liquids which contain moisture in them. There are two types : heat exchanger type and spray type. Non-condensing economisers are the most common economisers. They are generally of fin type which located in the flue gas ducting at the exit so that they can gain more heat which is being wasted anyways.

28. Air Pre-heater The function of the air pre-heater is to increase the temperature of air before it enters the furnace. It is placed after the economizer. Flue gases pass through the economizer and then to the air preheater Degree o f preheating depend s on Type of fuel Type of fuel burning equipment, and Rating a t whic h the boiler an d furnace ar e operated

29. Super heater The superheater raises the temperature of saturated steam without increasing its pressure. It consists of a small bundle of tubes. It is set in the path of the hot flue gas of the furnace. Saturated steam passes inside the heater tubes, and hot flue gases pass outside the tubes. Thus the transfer of heat takes place in saturated steam from hot flue gases and raises its temperature without increasing the pressure of steam.

30. Advantage s o f supe r heated steam Steam consumption o f the engine o r turbine i s reduced Erosion of turbine blade is eliminated Efficiency of the steam plant is increased Losses due to condensation in the cylinders and the steam pipes are reduced.

31. Steam separator The function o f a stea m separato r i s to remov e the entrained water particles from the steam conveyed to the steam engine or turbine. It is installed as close to the steam engine as possible on the main steam pipe from the boiler

32. Boiler mountings Pressure gauge Fusibl e plug Stea m sto p valve Fee d chec k valve Blow off cock Mu d an d ma n holes

33. BOILER MOUNTINGS 1. WATER LEVEL INDICATOR It is an main fitting in the boiler, Water level indicator indicates the water level inside the boiler. It is a safety device upon which safe working of the boiler depends. Figure 3.9 shows the water level indicator. It consist s o f a thick hard glas s tube. The ends are connected with two hollow blocks that are connected with boiler end plate such that one end of the blocks is connected with steam space an d othe r end with wate r storage . Parallel to the glass tube, there is another metallic tube. In case, the gauge glass breaks, the rush of steam and water will push the steel balls thereby blocking the passage to the position as shown by black color stop and prevents water and steam to come out of the boiler shell or drum. The broken glass tube is replaced immediatel y to function i t a s usual.

34. Pressure gauge The function of the pressure gauge is to measure the pressure of steam inside the boiler . The gauge is usually mounted on the front top of the shell or drum such that it should be clearly visible to the operator. (C). The pressure gauge shown in fig is bourdon pressure gauge. It consists of a circular spring tube A. One end of the bourdon tube is closed and connected to a link L and the other end is sequared to a Hollow block B. The link L connects the closed end of the tube to the toothed sector C which is hinged at O. The toothed sector gears with pinion D which carries a pointer P. The pointer moves on a dial graduated in pressure units.

35. Fusible plug The main purpose of a fusible plug is to extinguish the fire in the furnace of the boiler when the water level in the boiler falls below an unsafe level. Thus, explosion is avoided which may take place due to overheating of the tubes and shell. It is generally fitted over the crown of the furnace or over the combustion chamber. This is shown in Fig. 3.12. Plug s A an d B ar e separate d by fusible metal. In normal case, when the boiler contains sufficient water, the plug also remains covered by water. But when the water level falls, the plug is exposed to steam. Because the steam temperature is larger than the water temperature, the fusible metal melts. Plug B drops down and Thus the steam space gets communicated to fire box and extinguishes the fire.

36. By removing the gun metal plug and copper plug the Fusible plug can be put i n position agai n by insertin g the fusible meta l usually lea d o r metal alloy

37. Safety valve

38. Steam stop valve A valve is a device that regulates the flow of a fluid (gases , fluidized solids slurries or liquids) by opening or closing or partially obstructing various passageways Function : to shut off or regulate the flow of steam from the boiler to the steam pipe or steam from the steam pipe to the engine

39. Feed check valve When the level of water in the boiler falls, it is brought back to the specified level by supplying the additional water called feed water. The pressure inside the boiler will be high therefore the pressure of the feed water has to be raised by a pump before it is fed into the boiler. The feed water under high pressure is fed into the boiler through the feed check valve. The function of a feed check valve is to control the flow of water from the feed pump to the boiler and to prevent the backflow of water from the boiler to the pump when the pump pressure is less than the pressure or when the feed pump ceases to work. Evidently feed check valve is placed at the boiler end of the delivery pipe of the feed pump.

40. Blo w off cock The function of a blow-off valve is to periodically remove the sediments deposited at the bottom of the boiler while the boiler is in operation and to empty the boiler while it is being cleaned or inspected. When the blow-off valve is opened the water which is under the pressure of steam, rushes out with tremendous velocity thus carrying out the sediments along with it.

41. MANHOLE This is provided at suitable position on the boiler shell so that the man can enter into boiler shell for inspection, maintenance and repairs. This hole is usually made in elliptical shape of the size convenient for a man to enter through this hole. The opening is closed by steam tight cover.

42. ANTIPRIMING PIPE Function: The steam in contact with the water surface always contains water particles. A steam scrubber also known as steam drier is used in a boiler to remove the water particles contained in steam. Construction and operation: Refer Fig. 29.12. Anti-priming is a cast iron box which is fitted under the mounting block on which the steam stop valve is to be bolted. When the steam with water particles passed between closely fitted corrugated plates or the perforation made in the upper half of the anti-priming pipe, the heavier water particles separate out and are collected at the bottom of the anti-priming pipe. The water thus collected is later on drained to the boiler through the small holes ‘O’ provided at the bottom of the anti-priming pipe.

43. FIRE TUBE BOILER

44. Cochran Boiler Working Principal: https://www.youtube.com/watch?v=hVcqqaEulVA

45. • • Simple vertical, portable, natural circulation, low pressure boiler Suitable for small plants require small quantity of steam. Siz e = 1 m Dia . x 2 m high (evaporation 20kg/hr.) • • • Siz e = 3 m Dia . x 6 m high (evaporation 3000kg/hr.) Heating surface= 10 to 25 times of grate area Stea m pressure= upto 20 bar Efficiency = 70 to 75%

46. FIRE TUBE BOILER LANCASHIRE BOILER Working Principal: https://www.youtube.com/watch?v=xMAymzTl2gQ

47. LANCASHIRE BOILER Internally fired, horizontal, natural draft and natural circulation type boiler. Diameter of tubes is 0.4 times the diameter of shell. Each flue tube has its own furnace with grate of about 2 met. Length. Dampers path of flue gases flow and ar e placed i n the th e to gas air regulate inflow. Openings ar e mad e i n the boiler for inspection.

48. Working of Lancashire Boiler Lancashire boiler is a shell and tube type, heat exchanger. The fuel is burned at the grate. The water is pumped into the shell through the economizer which increases the temperature of the water. Now the shell is filled with water. The fire tube is fully immersed in the water. The fuel is charged at the grate produces exhaust gases. These flue gases first pass through the fire tubes from one end to another. These fire tubes transfer 80% to 90% of the heat to the water. The backward flue gases pass from the bottom passage where it transfers 8-10 % heat to water. The remaining flue gases pass from the side passage where it transfers 6-8% of the heat to the water. The brick is the lower conductor of heat, so works as a heat insulator. The steam produces in the drum shell is taken out from the upper side where it flows through the superheater if required. So the steam generated is taken out for process work.

49. (Capacity and Utility) Made to withstand working pressure up to 20bar. Evaporative capacity 8000 kg/hr. Heating surface: grate area 24:30 Shell size ranges from (2m diameter x 6m length) to (3m diameter x 10m length) Widely used in sugar mills, chemical plants, power systems, process works. Lancashire boilers are employed to operate steam turbines, locomotives, marines etc. Lancashire boiler has a high thermal efficiency of about 80-90%.

50. LANCASHIRE BOILER Disadvantages S.No Advantages Simplicity of design Good steaming quality Can burn coal of inferior quality Easy to clean and inspect 5 Less maintenance and operating cost Maximum working pressure is limited to 20 bar. More floor area is required due to brick work setting. Cracks in setting due to large temperature difference inside and out side. Restricted grate area due to furnace being inside More time taken for developing steam pressure due to large water capacity. 6 Stand against sudden heavy demand for considerable time.

51. LOCOMOTIVE BOILER Working Principal: https://www.youtube.com/watch?v=9mhYnQGZJuM

52. 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 an internally fired furnace. It is used in railway locomotive engines and in marine. It is a mobile boiler an d has a high stea m generatio n rate. There are three main parts in this boiler including a horizontal cylindrical boiler baller, a smokebox, and a double-walled firebox.

53. LOCOMOTIVE BOILER (Utility/ Application) • • The locomotive boiler is so designed that it is capable of meeting sudden and fluctuating demand of steam, which may be imposed due to variation in power and speed. Uses in railways, road rollers etc. Also used in agricultural fields, saw mill plants and stationary power services where semi- portability is desired.

54. LOCOMOTIVE BOILER Advantages Disadvantages Larg e rate to70kg/sec) of evaporation per sq . met. heating surface. (55 Incapable of meetin g very high O f overloads because of danger of being damaged due to overheat. Freedo m from brickwork, Max. steam pressure is limited to chimney and special foundations, 20 bar. which reduces the cost of installation. Reasonable low cost and Leakage occurs frequently at the compactness makes it ideal for place where the tube joins the portable unit. plate. Large flat surface needs bracing Accumulation of mud particles in water lags cause scale formation.

55. WATER TUBE BOILERS

56. BABCOCK & WILCOX BOILERS Working Principal: https://www.youtube.com/watch?v=ae_QmSRhD5w

57. BABCOCK & WILCOX BOILERS (Constructional features) It consist of welded steel high pressure drum mounted at the top. Drum is connected with uptake header an d dow n take header. Water tubes connected to the headers are inclined at 15 to the horizontal. Water tubes are straight and 10cm diameter expended into the bored holes of header. Serpentine from of header provides complete heating surface to the flue gases. Furnace is arranged below the uptake header.

58. BABCOCK & WILCOX BOILERS (Constructional features) Unit is provided with chain grate stroker. Speed of chain is adjusted with respect to the complete combustion of coal. Deflectors to flue gases are provided in the form of baffles. Mud Box: To collect the sedimentation in water. Super heater tubes: To enhance the super heated steam. Vents provided: safety valve, pressure gauge, water level indicator, fusible plug an d feed chec k valve. Water tube and drum assembly hung on steel girder frame called slings.

59. BABCOCK & WILCOX BOILERS (Capacity and utility) Evaporative capacity ranges from 20000 to 40000 kg/hr Operativ e pressure range s from 11.5 to 17.5 bar. Steam formed from such boilers are primarily used to run steam turbines and generate electric power. (Salient Aspects) ✔ ✔ ✔ ✔ Capability to cope with high peak loads which are generally needed in thermal power stations. Inspection of the boiler can be carried even when the boiler is in operation. Draught loss is minimum. Replacement o f defectiv e tubes can be mad e easily.

60. Disadvantages High maintenance cost. It is not much suitable for impure and sedimentary water. In the case of impure and sedimentary water, the scale may deposit in the tubes and this leads to overheating and bursting of tubes. That’s why water treatment is must before feeding into the boiler. A continuous supply of feed water is required for working. In the case, if feed wate r i s not continuousl y supplie d even for a shor t period o f time, the boiler gets overheated. The water level must be carefully watched during the operatio n o f the Babcoc k an d Wilco x boiler.

61. STIRLING BENT- TUBE BOILER Working Principal: https://www.youtube.com/watch?v=Rv5X8KEvRWQ

62. STIRLING BENT- TUBE BOILER 1. a) Drums are interlinked to each other with bent water tubes for the following reasons: To allow free expansion and contraction of the tubes. Tube replacement become easier. b) c) Flexibilit y i n desig n wit h regard s to location of drums. d) Tubes ca n enter the drum s in approximately radial direction. Mu d dru m i s usually 10 to 25cm large r in diamete r than a stea m drum. Entire unit is independent of brick work. High stea m pressures (60ba r an d 45 c). Evaporation capacity up to 50000kg/hr.

63. COMPARISION Between WATER TUBE & FIRE TUBE BOILERS WATER TUBE FIRE TUBE Water passes through water Hot gases passes through tubes. flues. Water content: steam capacity low Water content: Steam capacity high Complexity in design requires Simple & rigid construction quick examination by skilled hence greater reliability & low hands. operating cost. Operating pressure up to 200 bar. Pressure ranges from 17.5 bar to 24.5 bar Evaporatio n rate ranges from 20,000 to 50,000kg/hr. Evaporation rate 900kg/hr. Increased heating surface area. Low heating surface area. Low water to steam ratio Large water to steam ratio Bigger in size, suitable for large power plants Smaller in size, used only for small power plants Transportation and installation Transportation and installation is easy due to handling of is difficult due to large size of dismentaled parts shell. Externally fired boilers, furnace size can be varied. Internally fired boilers, furnace size can not be varied. Requires more floor area Requires less floor area

64. ESSENTIALS OF A GOOD BOILER 1. Heat generation capability should be at: Required pressure Required quality Fas t speed Minimum fuel consumption 2. Economic : Low initial cost Low installation cost Low operating cost Low maintenance cost