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Basics of steam boilers section b


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Basics of steam boilers section b

  1. 1. BASICS OF MECHANICAL ENGINEERING (ME-101 F) Lectures on STEAM GENRATION AND BOILERS By Priyanka Singh M.Tech (Applied Mechanics) Motilal Nehru National Institute of Technology, Allahabad
  2. 2. Definitions Latent Heat: Latent heat is the heat absorbed or releases during a change of state of matter. Latent Heat of Fusion: The specific latent heat of fusion is the heat needed to change 1 kg of a solid at its melting point into a liquid, or vice versa, without a change in temperature. Latent Heat of Vaporization: The specific latent heat of vaporization is the heat needed to change 1 kg of a liquid at its boiling point into vapour or vice versa, without a change in temperature. Steam: If water is heated beyond the boiling point, it vaporizes into steam, or water in the gaseous state. The properties of steam vary greatly depending on the pressure and temperature to which it is subject.
  3. 3. Heating Curve Assume that unit mass of steam is generated from solid ice which is at- 20°C at atmospheric pressure (1.01325 bar).
  4. 4. Heating Curve - Latent Heat • T1 is the melting point whereas • T2 is the boiling point.
  5. 5. Regime P-Q: Initially ice is at point P, i.e. at -20°C. Temperature of ice increases from -20°C to 0°C. Point Q represents 0°C temperature condition. Regime Q-R: The ice starts melting in this regime. The heat supplied in this regime is called latent heat of fusion as it is responsible to change in phase not increasing the temperature. At point R total ice is convert into water. Regime R-S: At point R temperature of water is 0°C. If any heat addition is there, the temperature of water will increase and it will continue till point S is reached where temperature become 100°C. This heat addition is called sensible heat. Point S represents saturated liquid condition. Regime S-T: At point S water starts boiling and additional heat is responsible to convert the water into vapours (steam). This boiling is continue till whole water is convert into vapours. The heat supplied is called latent heat of vaporization. Point T shows the saturated steam condition. Regime T-U: Point T represents saturated steam condition. If any heat addition is there, volume of steam will increase very rapidly and it behave like gas. The difference between superheated temperature and saturation temperature is called degree of superheat.
  6. 6. • From Q to R and S to T, the temperature remains constant because the heat supplied to the object is used to overcome the forces of attraction that hold the particles together. • Heat obsorbs during Q-R is called the latent heat of fusion. • Heat obsorbs during S-T is called the latent heat of vaporisation.
  7. 7. Wet Steam Quality and the Dryness Fraction To produce 100% dry steam in an boiler, and keep the steam dry throughout the piping system, is in general not possible. Droplets of water will escape from the boiler surface. Because of turbulence and splashing when bubbles of steam break through the water surface the steam space will contain a mixture of water droplets and steam. In addition heat loss in the pipes will condensate steam to droplets of water. Steam - produced in a boiler where the heat is supplied to the water and where the steam are in contact with the water surface of the boiler - will contain approximately 5% water by mass. Dryness fraction of Wet Steam If the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a dryness fraction of 0.95. Dryness fraction can be expressed as: x = ms / (mw + ms) (1) where x= dryness fraction mw = mass of water (kg) ms = mass of steam (kg)
  8. 8. Types of Steam In the sections that follow, we will discuss the types of steam used in these applications.
  9. 9. Saturated Steam (Dry) Saturated steam occurs at temperatures and pressures where steam (gas) and water (liquid) can coexist. In other words, it occurs when the rate of water vaporization is equal to the rate of condensation. Unsaturated Steam (Wet) This is the most common form of steam actually experienced by most power plants. When steam is generated using a boiler, it usually contains wetness from non-vaporized water molecules that are carried over into the distributed steam. Even the best boilers may discharge steam containing 3% to 5% wetness. Superheated Steam Superheated steam is created by further heating wet or saturated steam beyond the saturated steam point. This yields steam that has a higher temperature and lower density than saturated steam at the same pressure. Superheated steam is mainly used in propulsion/drive applications such as turbines, and is not typically used for heat transfer applications.
  10. 10. Steam Generation Theory  Within the boiler, fuel and air are force into the furnace by the burner.  There, it burns to produce heat.  From there, the heat (flue gases) travel throughout the boiler.  The water absorbs the heat, and eventually absorb enough to change into a gaseous state - steam.  To the left is the basic theoretical design of a modern boiler.  Boiler makers have developed various designs to squeeze the most energy out of fuel and to maximized its transfer to the water.
  11. 11.  Water enters the boiler, preheated, at the top.  The hot water naturally circulates through the tubes down to the lower area where it is hot.  The water heats up and flows back to the steam drum where the steam collects.  Not all the water gets turn to steam, so the process starts again.  Water keeps on circulating until it becomes steam.  Meanwhile, the control system is taking the temperature of the steam drum, along with numerous other readings, to determine if it should keep the burner burning, or shut it down.  As well, sensors control the amount of water entering the boiler, this water is know as feedwater.  Feedwater is not your regular drinking water.  It is treated with chemicals to neutralize various minerals in the water, which untreated, would cling to the tubes clogging or worst, rusting them.  This would make the boiler expensive to operate because it would not be very efficient.
  12. 12.  On the fire side of the boiler, carbon deposit resulting from improper combustion or impurities in the fuel can accumulate on the outer surface of the water tube.  This creates an insulation which quickly decrease the energy transfer from the heat to the water.  To remedy this problem the engineer will carry out soot blowing. At a specified time the engineer uses a long tool and insert it into the fire side of the boiler.  This device, which looks like a lance, has a tip at the end which "blows" steam.  This blowing action of the steam "scrubs" the outside of the water tubes, cleaning the carbon build up.  Water tube boilers can have pressures from 7 bar to as high as 250 bar.  The steam temperature's can vary between saturated steam, 100 degrees Celsius steam with particle of water, or be as high as 600 - 650 degrees Celsius, know as superheated steam or dry steam  The performance of boiler is generally referred to as tons of steam produced in one hour.  In water tube boilers that could be as low as 1.5 t/hr to as high as 2500 t/hr.
  13. 13. • Introduction Boiler is a device which is used to produce steam at high pressure. Steam is being used in thermal power plant, textile industries and for domestic uses during winter to heat the room. • Types of boilers 1. Horizontal, Vertical and Inclined boilers 2. Fire tube and water tube 3. Externally fired and internally fired 4. Forced circulation and natural circulation 5. High pressure and low pressure 6. Stationary and portable(moving) 7. Single tube and multi tube • Horizontal, vertical or inclined If the axis of the boiler is horizontal, vertical or inclined then it is called horizontal, vertical or inclined boiler respectively. BOILERS
  14. 14. • Fire tube and water tube If hot gases are inside the tube and water is outside the tube, it is called fire-tube boiler. Examples: Cochran, Lancashire and locomotive boilers. If water is inside the tube and hot gases are outside the tube, it is called fire-tube boiler. Examples: Babcock and Wilcox, Sterling, Yarrow boiler etc. • Externally fired and internally fired The boiler is known as externally fired if the fire is outside the shell. Examples: Babcock and Wilcox, Sterling The boiler is known as internally fired if the furnace is located inside the boiler shell. Examples: Cochran, Lancashire • Forced circulation and natural circulation In forced circulation type of boilers, the circulation of water is done by a forced pump Examples: Velox, Lamont, Benson boiler In natural circulation type of boilers, circulation of water in the boiler takes place due to natural convection currents produced by the application of heat. Examples: Lancashire, Babcock and Wilcox.
  15. 15. • High pressure and low pressure The boilers which produce steam at pressures of 80 bar and above are called high pressure boilers. Examples: Babcock and Wilcox, Velox, Lamont, Benson boilers. The boilers which produce steam at pressure below 80 bar are called low pressure boilers. Examples: Cochran, Cornish, Lancashire and locomotive boilers. • Stationary and portable Stationary boilers are used for power plant-steam, for central station utility power plants, for plant process steam etc. Mobile or portable boilers include locomotive type, and other small unit for temporary use at sites. • Single tube and multi tube The fire tube boilers are classified as single tube or multi-tube boilers, depending upon whether the fire tube is one or more than one. Examples of single tube boilers are Cornish and simple vertical boiler
  16. 16. Parts and terms in Boiler • 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 • 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. • Refractory Insulation material used for lining combustion chamber • Combustion chamber Part of furnace where combustion of fuel takes place. • 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 • 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. • Water level The level at which water stands in the boiler is called water level. • Refractory Insulation material used for lining combustion chamber.
  17. 17. Boilers and Parts
  18. 18. • 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. • Lagging Insulation wrapped on the outside of the boiler shell or steam piping. • Priming It is the carryover of varying amounts of droplets of water in the steam (foam and mist), which lowers the energy efficiency of the steam and leads to the deposit of salt crystals on the super heaters and in the turbines. Priming may be caused by improper construction of boiler, excessive ratings, or sudden fluctuations in steam demand. Priming is sometimes aggravated by impurities in the boiler-water. rate.
  19. 19. • Mountings Equipment and devices, which are used for safety of boiler are called mountings, these are required parts for the successful operation of a boiler. Ex. Feed-check valve, safety Valve etc. • Accessories The items which are used for increasing the boiler efficiency are called accessories. Ex. Super heaters, Steam separators etc.
  20. 20. BOILER MOUNTINGS Important boiler mountings are as follows,  Pressure gauge  Safety valves  Fusible plug  Steam stop valve  Feed check valve  Blow off cock  Mud and man holes  Water level Indicator
  21. 21. PRESSURE GAUGE  Fitted in front of the boiler to record the steam pressure at which steam is generated in the boiler.  Two types of pressure gauges are being used in boiler operations. Diaphragm type Bourdon Tube A bourdon pressure gauge in its simplest form consists of a simple elastic tube, one end of the tube is fixed and connected to the steam space in the boiler, other end is connected to a sector through a link.
  22. 22. Diaphragm Type
  23. 23. SAFETY VALVE Safety valves are needed to blow off the steam when the pressure of the steam in the boiler exceeds the working pressure. It is placed on the top of the boiler.  Dead weight safety valve  Lever safety valve  Spring loaded safety valve  High steam and low water safety valve Dead weight safety valve Lever safety valve
  24. 24. Spring loaded safety valve High steam and low water safety valve
  25. 25. FUSIBLE PLUG To extinguish fire in the event of water level in the boiler shell falling below a certain specified limit. It is installed below boiler’s water level on the crown plate. Fusible Plug
  26. 26. 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
  27. 27. FEED CHECK VALVE • To allow the feed water to pass in to the boiler • To prevent the back flow of water from the boiler in the event of the failure of the feed pump
  28. 28. BLOW OFF COCK • To drain out water from the boiler for internal cleaning inspection or other purposes
  29. 29. WATER LEVEL INDICATOR A : End plate of boiler H & J : Two balls B & C : Hollow gun metal casting K : Drain cock D & E : Cocks L : Guard glass F : Gauge glass M,N,P& R: Screwed caps G : Hollow metal column X,Y : Flanges The function of water level indicator is to show level of water present in the boiler.
  30. 30. BOILER ACCESSORIES Accessories are the devices being used to increase the efficiency of the boiler. A large amount of heat is being carried out by the flue gases, this is wastage of useful energy, which can be recovered. Accessories are those equipment which recovers the wastage along with smoothing the operation to increase the utilization of energy as well as reducing the cost of operation. The waste recovery takes place by the help of flue gases, which has a large amount of heat. Accessories are not the mandatory parts or devices but being used for efficient operation. Commonly used accessories are as follows, • Steam Super heater • Economizer • Air preheater • Steam separator • Feed Pump • Injector
  31. 31. STEAM SUPER HEATER The function of a super heater is to increase the temperature of steam above its saturation point. That means it gives assurance of the quality of steam. During superheating pressure of steam remains same but the volume increases with its temperature, increasing the internal energy which in turns prove to increase in kinetic energy, resulting in: • Reduction of steam consumption of turbine. • Reduction in losses due to condensation in steam pipes. • Elimination of erosion of turbine blades • Increase in efficiency. There are two types of super heaters. 1. Convective Super heater 2. Radiant Super heater
  32. 32. ECONOMIZER In best way it is known as feed water heater, that refers heating of feed water, which is supplied to the boiler shell to get vaporized. It utilizes heat carried out but the waste furnace gases to heat the water before it enters boiler. By increasing the temperature of water, chilling of the boiler surface is prevented and then a less amount of sensible heat is required to achieve saturation temperature, it reduces then the input heat to the boiler and increasing efficiency. There are two types of economizer 1. Independent type (not a part of boiler) 2. Integrated type (a part of boiler)
  33. 33. AIR PREHEATER The function of an air preheater is to heat the inlet air before it is sent to the furnace. It is placed after economizer, flue gases coming from economizer is being utilized to heat air. Preheated air accelerates combustion and increasing the amount of heat produced. Degree of preheating depends upon 1. Type of fuel 2. Type of fuel burning equipment 3. Rating of the boiler and furnace Two types of preheaters commonly used, 1. Recuperative type (Both the fluids pass simultaneously) 2. Regenerative type (Fluids pass alternatively)
  34. 34. STEAM SEPARATOR The basic work of steam separator is to ensure the quality of steam, steam from the boiler may be in the form of wet steam, or incase of regenerative cycle, where condensate from turbine is supplied back or being used by smaller capacity turbines to recover heat, steam must be in the wet format. Steam separator removes water particles. There are three types of steam separator 1. Impact or baffle type 2. Reverse current type 3. Centrifugal type Baffle type steam separator
  35. 35. FEED PUMP Feed pumps is the device required to supply water to the boiler. The quantity of feed water should be at least equal to the amount of steam delivered to the turbine or required space. For open cycle boiler in case of large plants, where there is no condenser or the amount of feedback water is less, pumps are inevitable. There are two types of feed pumps, 1. Reciprocating feed pump (Piston cylinder arrangement) Single acting Double acting 2. Rotary or centrifugal feed pump Duplex reciprocating pump
  36. 36. INJECTOR The basic work of an injector is to feed water to the boiler on high pressure, it finds its application in such places where there is no space to install feed pumps. It works by the help of steam pressure in a way that the pressure of steam is being utilised to increase the kinetic energy of feed water. Advantages 1. Low initial cost 2. Simplicity 3. Compactness 4. No dynamic parts 5. High thermal efficiency Disadvantages 1. Low pumping efficiency 2. Can’t work for very hot steam 3. Irregularity in the operation when steam pressure varies considerably Injector
  37. 37. STUDY OF BOILER • Fire tube boiler (Cochran boiler) • Water tube boiler (Babcock and Wilcox boiler) FIRE TUBE BOILER (COCHRAN BOILER) Introduction It’s a multi tube internally fired, fire tube boiler, as an improvement of traditional vertical boiler, providing more heating space relatively. Construction It consists of a vertical cylindrical shell with a hemispherical top and hemispherical shaped furnace. Furnace has ash pit at the bottom end above which fire grate lies. Combustion chamber of the furnace widens to connect the flue passage pipes, lined with fir bricks and insulated with refractory materials on the shell side. Smoke box is fitted in the hinged door for easy access to tubes, and allow easy cleaning. A number of horizontal fire tubes of equal lengths and diameter connects the combustion chamber with smoke box. Furnace is surrounded by water on all the side except the opening for fire door and combustion chamber.
  38. 38. WATER TUBE BOILER (BABCOK & WILCOX BOILER) This is a type of water tube boiler used when pressure exceeds 10 bar and capacity 7000 kg per hour. Construction It consists of a horizontal high pressure drum, from each end of it connections are made with an uptake and a down take header. Headers are joined to each other by large number of water tubes inclined at an angle of 15 degree to provide water circulation. Hand holes are provided for the maintenance of tubes. The entire assembly of water tubes is hung in a room made of masonry work, lined with fire bricks to resist thermal expansion. As super heater additional U-shaped tubes are arranged between drums and water tubes. Furnace is arranged below the uptake header. Baffles are provided across the water tubes to guide flue gases. A Chimney is provided for exit of gases and a damper at the opening of chimney to provide draught. Cleaning doors are provided to access the tubes for cleaning and removal of soot and various mountings for successful operation.
  39. 39. Advantage of Babcock and Wilcox boiler  Suitable for all types of fuels and hand stokers for firing.  Draught loss is small.  All components are accessible for inspection during operation.  Expansion and contraction has no harm on masonry work(construction).  Replacement of defective tubes are easy. Applications of Cochran boiler and Babcock and Wilcox boiler  Cochran boiler finds its application in cranes, excavators and in small factories for general contract work where temporary supply of steam is required.  Babcock and Wilcox boiler finds its application in sugar mills and textile industries for power generations and processing works.
  40. 40. COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS Particulars Fire-tube boilers Water-tube boilers Position of water and hot gases Hot gases inside the tubes and water outside the tubes Water inside the tubes and hot gases outside the tubes Mode of firing Generally internally fired Externally fired Operation pressure Limited to 16 bar Can go up to 100 bar Rate of steam production Lower Higher
  41. 41. COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS Particulars Fire-tube boilers Water-tube boilers Suitability Not suitable for large power plants Suitable for large power plants Risk on bursting Involves lesser risk of explosion due to lower pressure More risk on bursting due to high pressure Floor area For a given power it occupies more floor area For a given power it occupies less floor area Construction Difficult Simple
  42. 42. COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS Particulars Fire-tube boilers Water-tube boilers Transportation Difficult Simple Shell diameter Large for same power Small for same power Chances of explosion Less More Treatment of water Not so necessary More necessary Accessibility of various parts Various parts not so easily accessible for cleaning, repair and inspection More accessible