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STEAM GENERATORS.pptx

The document discusses different types of steam generators or boilers. It begins with an introduction to boilers, then describes how boilers are classified. It provides details on the key differences between fire-tube and water-tube boilers, including their construction, operation, applications and advantages/disadvantages. The document also discusses boiler terms, selection criteria for boilers, essential characteristics of good boilers, and provides details on specific types of fire-tube and water-tube boilers.

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NAME : MALIK RAFIYAREYAZ CLASS: BE 5TH SEM BRANCH: MECHANICAL ROLLNO: GCET-303-19 (191105040) SUBJECT: APPLIED THERMODYNAMICS
STEAM GENERATORS
CONTENTS  INTRODUCTION  CLASSFICATION OF BOILERS  COMPARISION BETWEEN ‘FIRE-TUBE’ AND ‘WATER-TUBE’ BOILERS  SELECTION OF A BOILER  ESSENTIALS OF A GOOD STEAM BOILER  BOILER TERMS  FIRE TUBE BOILERS  WATER TUBE BOILERS  BOILER MOUNTINGS  BOILER ACCESSORIES  DRAUGHT  PERFORMANCE OF STEAM GENERATORS
INTRODUCTION A steam generator, popularly known as a boiler, is a closed vessel made of high-quality steel in which steam is generated from water by the application of heat. The water receives heat from the hot gases formed by burning fuel through the heating surface of the boiler. Steam is mainly required for power generation, process heating, and space heating purposes.
CLASSFICATION OF BOILERS
DIFFERENCE BETWEEN FIRE TUBE AND WATERTUBE BOILERS FIRE TUBE BOILER WATER TUBE BOILER Hot flue gases flow inside the tube and the water outsides the tubes. Water flows inside the turbine and hot flue gases outside the tube. These boilers are generally internally fired. These boilers are generally extrernally fired. The boiler pressure limited to 20 bar. The boiler pressure is limited to up to 100 bar. The fire-tube boiler has a lower rate of steam production. A higher rate of steam production. Not suitable for larger power plants. Suitable for larger power plants. Involves lesser risk of explosion due to low pressure. The risk of the explosion is higher due to high boiler pressure.
DIFFERENCE BETWEEN FIRE TUBE AND WATERTUBE BOILERS Difficult in transportation. Simple in transportation. They require less skill to operate, as compare to the water tube boiler. They required a skilled operator. They are difficult to repair and cleaning as they are internally fired. They are easy to repair and clean as they are externally fired. They required a large shell diameter. Because the fire tube situated inside the shell. They required a small shell diameter. The efficiency of the fire tube boiler is less as compared to the water tube boiler. The efficiency of the water tube boiler is more. The maintenance of this boiler is costly. It requires regular inspection. They are easy to maintain as they are externally fired. Ex: Cornish Boiler, Lancashire Boiler. Ex: Babcock and Wilcox Boiler.
DIFFERENCE BETWEEN FIRE TUBE AND WATERTUBE BOILERS
SELECTION OF A BOILER  High Pressure Vessel Mass  High Water Volume  High Efficiency  Quick Startup Times  Low Footprint  Low Heat Flux and High Heating Surface Area  Ease of Clean Out  Effective Turndown  Ease of Maintenance  Large Steam Disengagement Area
ESSENTIALS OF A GOOD BOILER The essentials of a good boiler :-  The boiler should be reliable and produce the required amount of steam per hour.  The maintenance of the boiler should be easy and cheap.  It should be efficient. It should have minimum fuel consumption and less initial cost.  It should start quickly.  It should be light in weight, occupy less space and it should be able to meet the variable demand of steam.  The joints should be leak proof. The joints should be accessible for inspection and these should be away from flame.  The mud and other deposits should not collect on heated surface since this would affect the heat transfer rates and consequently the steam generation rates.  The refractory material used should be minimum.  The design of boiler allow maximum fluid velocity in order to increase the heat transfer rates between water and flue gases. It should have minimum friction losses.  It should comply with safety regulations according to Indian Boiler Act 1923.
BOILER TERMS  Shell:- Shell is the main container that contains water for steam usually made up of Steel plates bent into a cylindrical shape. The plates may be welded or reverted together.  Furnace:- Furnace is a firebox and is the space above the grate and below boiler shell suitable area of furnace improves the efficiency of combustion.  Grate:- It is the platform upon which the fuel burnt. Generally, the grate is made up of cast iron bars evenly.  Water flow path:- It is the path followed by the water in the boiler.  Gas flow path:- It is the path followed by the flue gases of combustion. They may be water tubes or fire tube boilers. The gas flow path should be so arranged that it transferred maximum heat to the water to produce steam. This much affects the efficiency of the boiler.  Steam path:- Generally steam path is at the top of the shell to avoid water particles being carried with the steam. To do so, steam separators are used in case of large capacity and high-pressure boilers.
BOILER TERMS  Fitting ( Mountings):- The valves and gauge which are necessary for the safety of the boiler referred to as mountings. Without mountings, a boiler cannot function safely. Example:- Water level indicator, Pressure gauge, Fusible plug, Safety valve, Blow off cock, etc.  Accessories:- The equipment which improves the efficiency of the boiler is known as accessories. Example:-  Economizer and air preheater are used to extract maximum heat from the flue gases.  Super heater used to raise the temperature of steam above the saturation temperature.  Blowing off:- The process of removing the impurities floating on the water surface level is termed as blowing off. Some special device is used to blowing off impurities.  Settings:- Settings are the walls of the Furnace and combustion chamber. These are made up of fire bricks. They form the passage through which the gases pass.  Water space and steam:- The volume of the shell, which occupies the water is known as water space. The level at which the water stands in the boiler shell is known as the water level. The remaining volume of Shell that not occupied by water is called steam space
FIRE TUBE BOILERS These are one of the most fundamental and old designed boilers. These boilers are very famous in the 18th century, and especially applicable for train engines. In this kind of boiler, combustion of heat, as well as gases, flows through the pipe enclosed by water. These boilers may be high-pressure or low- pressure boilers. The measurement of these boilers can be done always with their external diameter. Generally, these boilers are intended for pressures up toward a highest of 250 psi & around 750 horsepower These are one of the most fundamental and old designed boilers. These boilers are very famous in the 18th century, and especially applicable for train engines. In this kind of boiler, combustion of heat, as well as gases, flows through the pipe enclosed by water. These boilers may be high-pressure or low- pressure boilers. The measurement of these boilers can be done always with their external diameter. Generally, these boilers are intended for pressures up toward a highest of 250 psi & around 750 horsepower
FIRE TUBE BOILERS Construction and Working Principle of Fire Tube Boiler The construction of a water tube boiler can be equipped with cylindrical shell, vertical, firebox in the base, space for water in the center segment, and space for steam in the higher segment. A fire-grate is located at the fire-box base as well as coal is fired-up on the firebox. For burnt coal, an ash pit is placed at the base of the grate for gathering the ash from the burnt coal, and sometimes it can be detached. One or many cross-tubes are flanged to the space of water that is placed within the box to raise the outside area of heating for improving the water flow. A small chimney is associated with the pinnacle of the firebox for releasing the waste outlets at some larger height. The cleaning of the boiler can be done by the hand holes as well as manholes of the tubes & shell of the boiler.
Construction and Working Principle of Fire Tube Boiler  These boilers include a water level indicator; a pressure gauge, steam stop tap security tap, & a manhole like mountings for providing security as well as simplicity of working. The Fuel burns on the grate of the firebox in the boiler and the resultant hot-flue gases are permitted to flow in the region of cross tubes.  The water nearby the cylindrical type firebox also gets heat through radiation as well as convection, therefore steam will be generated. The flow of water in the boiler will be based on the difference of density within water that is formed with the difference of temperature. FIRE TUBE BOILERS
FIRE TUBE BOILERS Types of Fire Tube Boiler  The different types of this boiler include the following. 1) Cornish Fire Tube Boiler The first Cornish boiler was approved by an engineer namely “Trevithick a Cornish”. This kind of boiler includes a plane cylindrical shell, as well as a tiny flue pipe holding the heating system, flows through it. 2) Lancashire Fire Tube Boiler The construction of the Lancashire boiler is related to Cornish boiler but as a substitute of one flue pipe, two flue pipes are utilized.
FIRE TUBE BOILERS 3) Locomotive Fire Tube Boiler The locomotive boiler is an inactive boiler which is used in train engines. This type of boiler is capable while producing steam as well as it is solid. The design of the locomotive boiler is horizontal multi tubular. The main advantage of using this type of boiler is low cost for construction, installation and steam capacity is high. 4) Vertical Fire Tube Boiler The vertical boiler is a simple boiler, and it comprises of a cylindrical shell enclosed a larger section of water and rest of section will be occupied with steam. It contains cross tubes and a furnace at the bottom of the boiler. The combustion gases after heating the water are allowed to escape into the atmosphere.
FIRE TUBE BOILERS 5) Cochran Fire Tube Boiler The Cochran boiler is a vertical type multi-tubular boiler, and it includes several horizontal fire tubes. The heating system is one part of the construction and it is perfect. 6) Scotch Marine Fire Tube Boiler The Scotch marine boiler is very popular boilers used for high vapor capacities at high forces. This type of boiler includes a huge number of tiny diameter tubes for providing the benefit of the high-heating region of the surface. The boilers are fired up internally and leave from the boiler using a chimney to the environment.
WATER TUBE BOILERS A water tube boiler can be defined as a Steam boiler in which the flow of water in the tubes, as well as hot gases, enclose the tubes. Not like fire tube boilers, this boiler attains high-pressures, as well as high-steam capabilities, can be achieved. This is because of condensed tangential pressure on tubes which is known as hoop stress.
Construction and Working Principle of Fire Tube Boiler: The working principle of water tube boiler is thermal siphoning (circulation of natural water). Basically, this type of boiler includes two drums namely steam, lower or mud drum. Two drums are associated via two tubes such as down comer and riser. At first, the water is supplied into the steam type drum with the help of a water pump. Whenever the fuel is burned, then hot gases will be generated that are permitted to supply in the shell part of the boiler. The hot gases which are produced by the fuel will replace heat by the water; the water gets changed into steam. Because, the water temperature increases, the concentration will increase automatically. Clearly, the concentration of steam will be lesser than the water. Thus in the steam drum, the water, as well as steam, gets divided obviously due to variation in concentration. Here the traveling of steam will be upward because of low concentration as well as water will travel downward because of high concentration. WATER TUBE BOILERS
WATER TUBE BOILERS Construction and Working Principle of Fire Tube Boiler: The flow of hot water at the base of the steam drum will be supplied into mud drum via downcomer tube as well as to heat the water in the mud type drum. Whenever cooler supply water is initiated into steam type drum due to the high concentration of cool water, and it moves down in the downcomer type tube near mud drum. When transferring hot water from the mud type drum to steam type drum using riser tubes then it consequences in normal water circulation in the water tube type boiler. As additional steam is generated in the boiler then the force in the drum enhances & the water supply into the drum reduces which tend for decreasing the flow of steam. In the same way, when the production rate of steam reduces the force in the water- tube boiler reduces, & the water supply into the boiler enhances which tends to enhance the production rate. Like this, the water tube boiler controls the production of steam.
WATER TUBE BOILERS Types of Water Tube Boilers The types of water tube boilers include the following. 1) Simple Vertical Boiler This is one type of water tube boiler. In this type of boiler, the axis of direction is perpendicular with respect to the position. The major components of this boiler include ash pit, grate, feed check tap, fire hole, firebox, cross box, hand hole, fusible plug, water gauge, cylindrical shell, steam space, manhole, pressure gauge, steam stop tap, safety tap, chimney. In this type of boiler, using the fire hoke fuel is added into the grate which destroys by fire to generate the warm gases. Ash pit is for collecting the ash which is converted from the fuel. Hot gases increase high and supply their heat toward the water within the cross box, then moves out using the chimney.
2) Stirling Boiler The Stirling boiler is one type of water tube boiler, used for generating steam (50,000 kg steam/hour and 60 kgf /cm2 pressure) in the large area of the stationary plant. This type of boiler consists of 3 steam drums as well as 2 mud drums. The steam drums are located on the top section of the boiler whereas mud drums are located on the base of the arrangement. The steam drums and mud drums are connected through bent tube banks. When the tubes are turned then the mechanical pressures due to pipes extension throughout heating cannot influence the system. The two drums as well as tubes are designed of steel which will support the total system. The arrangement of the Stirling boiler is enclosed with brickwork. Here, the arrangement of bricks will avoid the heat dissipation in the surroundings WATER TUBE BOILERS
3) Babcock and Wilcox Boilers This is a horizontal straight water tube boiler; it has a steam drum which is made of steel. The two ends of the drum are associated with a series of two end headers with short riser pipes. These are disposed at 15o0 angle to the horizontal axis of the steam drum. The disposed of the arrangement of tubes assists in the supply of water, and the water level in the drum is indicated with a water level indicator. The fire door in the water boiler will be located at the bottom and the fuel will be supplied through this door and burns in a grate. The burning fuel will generate the hot gases that are forced in the grate to supply upward with baffle plates. At the bottom of the water boiler, mud collector is located for removing the mud particles using blow down a cock. Therefore, a nonstop water circulation from the steam drum toward the water tubes is maintained by convective currents because of the difference in concentration and it is called as normal circulation WATER TUBE BOILERS
BOILER MOUNTINGS  Regular accidents and boiler break down can happen due to improper maintenance and cleaning. Without boiler mounting one can damage boiler and its surrounding with one mistake followed by a series of events. Without high lift safety valve the shell could explode resulting into great loss of life and machinery; similarly a shell could collapse under vacuum if air vent is not present during cooling down. During cargo operation there is a increased demand for steam and so the mountings help operate the boiler with more and ease and safety. Boiler mountings include:
1 ) Safety Valves Safety valves are fitted on every boiler to avoid over pressurizing. Normally three safety valves are fitted on the boiler with one on the super heater and rest two on steam drum. In no condition these valves be less than two in number and must lift at a pressure 3% above boiler working pressure irrespective of boiler types. A steam valve is made up of cast iron body with two independent valves fitted on the valve seat. These valves are connected to a lever by means of a pivot held tight to its position by spring. The spring force keep the valve sit shut on the valve seat under normal condition. When the upward pressure exceeds the downward spring force; the valve is lifted and excess steam is released to the atmosphere. 2 ) Vent Valve Vent valve is installed on the boiler shell to vent air from steam drum during starting of boiler. These vent valves also comes handy during boiler shut down as it let fresh air to enter the boiler drum avoiding its collapse under pressure. A vent valve can also be used to release / dump moist steam at start. BOILER MOUNTINGS
3 ) Fusible Plug A fusible plug is the threaded gunmetal cylinder with conical plug and tappet hole drilled into it. This This hole is then filled with an alloy of low melting point such as tin. The plug can be of either fire actuated or steam actuated type fitted over the combustion chamber. Under normal condition one side of the plug is exposed to extreme temperature while the other submerged under water; keeping it cool. This low melting point alloy can not melt away till submerged and so remain intact even under extreme condition. In event of water level reach below a safe limit and plug tip is exposed to steam; the tin alloy will melt exposing combustion chamber with steam. As steam is not effective coolant and convection medium the tin alloy can’t transfer heat to the steam leading it to melt away. This sudden injection of steam into the furnace will stop the combustion protecting boiler from any damage. 4) Steam Stop valve A steam stop valve is connected to the boiler to stop and regulate steam flow from boiler to the distribution lines. Main steam stop valve on boiler is kept shut to avoid back-flow of steam to the boiler. The flange of the steam stop valve is bolted on top of the steam drum. Valve main body is made of cast iron while the valve seat is made from gun metal. The spindle on one end is connected to the valve while the other end to the handle wheel passing through ( yoke / Gland nut ) and gland packing. The valve is operated by rotating the hand wheel. Rotating hand wheel in turn rotate the spindle which move the valve up allowing path for steam to flow. BOILER MOUNTINGS
5 ) Pressure Gauge Pressure gauge are fitted to the steam drum and super heater to indicate steam pressure inside. These gauge are fitted on the front top of the boiler shell and represent pressure in bar. A bourdon tube of closed cross section is attached to the steam space on one end through siphon tube. The tube itself contain water to avoid steam to enter into the pressure gauge. The pointer is connected to the threaded gear attached to the spindle. When pressure is applied to the bourdon tube it becomes circular turning the spindle. This cause the pointer to move along with the gear; representing the boiler pressure. 6 ) Water Level Indicator A pair of water level indicator is installed directly to the boiler shell with an additional remote reading gauge installed at convenient position. They are installed directly on the front end of all boiler types; showing water level in boiler drum. It consist of a glass tube with three independent cock ( Steam cock, water cock and drain cock ). Steam and water cock separates the glass tube with boiler steam and water respectively. Drain cock on other hand used to drain water from glass tube. A metal ball is provided on the water side of the gauge glass to avoid subsequent accident and water loss; by water flashing off steam in event of glass rapture / failure. Under normal condition both steam and water cock is open allowing water and steam pressure to balance. In event of incorrect reading we need to blow through; by closing the water cock and opening drain cock. A strong blow will indicate the steam cock is clear; now repeat the process with steam cock closed and water cock opened. Strong blow of steam with hissing sound indicate the water cock is clear. Now close the drain cock and let water fill in; slowly open the steam cock equalizing the pressure. BOILER MOUNTINGS
BOILER MOUNTINGS
BOILER ACCESSORIES The boiler accessories are the devices, which form an integral part of a boiler but are not mounted on it. They include superheater, economiser, feed pump etc. It may be noted the accessories help in controlling and running the boiler efficiently
1) Economiser The combustion gases coming out of the boiler contain a large quantity of heat. Therefore the maximum amount of heat from the gases should be recovered before it escapes to the chimney. In the economiser, heating the feed water does the recovery of heat in the flue gases. The economiser is placed in the path of the gases. They improve the overall efficiency of the boiler by reducing fuel consumption. 2) Air Pre-heater The air preheater is an accessory that recovers the heat in the exhaust gas by heating the air supplied to the furnace of the boiler. Supplying preheated air into the furnace produces a high furnace temperature and accelerates the combustion of the fuel. Thus the thermal efficiency of the plant will be increased. BOILER ACCESSORIES
3) Superheater The superheater is used in boilers to increase the temperature of the steam above the saturation temperature The dry saturated steam generated in the boiler is passed through a set of tubes placed in the path of the flue gases, in which it will be heated further by the hot gas to increase its temperature about the saturation temperature 4)Feed Pump A feed pump is a boiler accessory required to force the feed water at high pressure into the boiler. Commonly used pumps are,  Reciprocating pumps  Rotary pumps The reciprocating pumps are driven directly by coupling them to the steam engine. The rotary pumps are driven by the steam turbines or by electric motors. BOILER ACCESSORIES
DRAUGHT Boiler draught is defined as the difference between absolute gas pressure at any point in a flow passage and the ambient (same elevation) atmospheric pressure. Draught is achieved a small pressure difference which causes the flow of air or gas to take place. It is measured in millimeter (mm) or water. The draught is one of the most essential systems of the thermal power plant which support the required quantity of air for combustion and removes the burnt products from the system. To move the air through the fuel bed and to produce a flow of hot gases through the boiler economiser, preheater and chimney require a difference of pressure. This difference of pressure to maintaining the constant flow of air and discharging the gases through the chimney to the atmosphere is known as draught. Draught can be achieved by the use of chimney, fan, steam or air jet or a combination of these. When the draught is produced with the help of chimney only, it is known as Natural Draught and when the draught is produced by any other means except chimney it is known as Artificial Draught
Purpose of Boiler Draught:  To provide an adequate supply of air for fuel combustion.  For throw out the exhaust gases of combustion from the combustion chamber.  To discharge these gases to the atmosphere through the chimney. DRAUGHT
Measurement of Draught The amount of draught produce depends upon:  The nature and depth of fuel at the furnace.  Design of combustion chamber or firebox.  The rate of combustion required.  Resistance is allowed in the system due to baffles, tubes, superheaters, economizers, air pre-heaters etc. DRAUGHT
DRAUGHT Classification of Boiler Draught
DRAUGHT Types of Boiler Draught In general, the draughts may be classified into the following two types,  Natural Draught  Artificial Draught Natural Draught Natural draught system employs a tall chimney as shown in the figure. The chimney is a vertical tubular masonry structure or reinforced concrete. It is formed for enclosing a column of flue gases to produce the draught. It removes the gases high enough to prevent air pollution. The draught is produced by this tall chimney due to the temperature difference of hot gases in the chimney and cold external air outside the chimney. Artificial or Mechanical Draught It has been seen that the draught produced by the chimney is affected by the atmospheric conditions. It has no flexibility, poor efficiency and tall chimney are required. In most of the modern power plants, the draught applied must be freedom of atmospheric condition, and It should have more flexibility (control) to bear the fluctuation loads on the plant. Today’s steam power plants requiring 20 thousand tons of steam per hour would be impossible to run without the aid of draft fans. A chimney of a reasonable height would be incapable of improving enough draft to eliminate the huge volume of air and gases ( 400 x 103 m 3 to 800 x 10 3 m 3 per minutes). The further advantages of fans are to reduce the height of the chimney needed.
DRAUGHT Determination of the Height of Chimney for a Given Value of Natural Draught For determination of the height of chimney required to produce certain amount of natural draught, we have to determine the density of flue gases inside the chimney, ρg and that of cold air outside the chimney ρa. A shown in fig 29.1, if H is the height of chimney and Pa is the atmospheric pressure at the top level of the chimney then The absolute total pressure at grate level inside the chimney is given as
DRAUGHT Determination of the Height of Chimney for a Given Value of Natural Draught And total pressure at grate level outside the chimney Then draught produced, given by its definition will be equal to Where, ρa and ρg are in kg/m3 g = acceleration due to gravity = 9.81 m/sec2 H = Height of chimney in m Calculation of ρa and ρg We know the combustion chemistry of carbon and hydrogen N/m2 (Pa)
DRAUGHT Determination of the Height of Chimney for a Given Value of Natural Draught From equation (i) & (ii) it is clear that volume of O2 supplied for combustion of a unit quantity of carbon is equal to volume of CO2 produced at same pressure and temperature and volume of O2 supplied for combustion of hydrogen is half of volume of steam produced at same temperature and pressure and volume of N2 supplied remains same at same temperature and pressure, as it does not take part in combustion. Now, the hydrogen content in the fuel is very less because of which only, the volume of combustion product increases. So, it can be neglected and it is safely assumed that volume of products of combustion will be equal to volume of air supplied when both reduced to same temperature and pressure conditions Let m = mass of air required per kg of fuel burnt Let m = mass of air required per kg of fuel burnt Let T = mean absolute temperature of chimney gases in K T1= Absolute temperature of outside air in K
DRAUGHT Determination of the Height of Chimney for a Given Value of Natural Draught Where, P is in N/m2, T in oK and Ra In J/kg K And volume of (m+1) kg of flue gases will also be same at temperature T1 i.e But hot flue gases are at temperature T. So their volume at temperature T will be Here pressure P is in N/m2, Ra in J/kg K and temperature in K Putting the value of density of fresh air at temperature T1 and of hot flue gases at temperature T2 in equation (Considering atmospheric Pressure Pa, Constant) Density of inside flue gases
DRAUGHT Determination of the Height of Chimney for a Given Value of Natural Draught Putting the values of Atmospheric Pressure P = 101300 Pa (N/m2) Ra = Characteristic gas constant for air = 287 J/ kg K and g = Acceleration due to gravity = 9.81 m/sec2 Draught Produced By this formula the theoretical height of chimney required may be determined for producing a certain amount of natural draught, if we know the absolute temperature of flue gases inside the chimney and of air outside the chimney and mass flow rate of air per kg of fuel burnt. Actual Draught produced by chimney is generally less than 12 mm of water. From equation 29.2, it may be seen that the draught produced by the chimney becomes more effective with increase in height of chimney and temperature of flue gases
DRAUGHT Determination of Diameter of Chimney Let us find out the draught produced in terms of height of flue gases i.e. h meters. So, it will be expressed as pd = ρg. g. h And From Equation Putting the values of ρg from sub eqn (ii) Now assuming that draught losses are Nil and all the draught contribute to imparting velocity to flue gases, velocity of flue gases can be given by: If we take into account the frictional pressure losses in chimney and let it be hf, then
DRAUGHT Determination of Diameter of Chimney Where k is a constant the value of which is given as k= 0.825 for brick chimney, 1.1 for steel chimney Also, the mass of flue gases flowing through any section of chimney will be given by In this way, the height and diameter of chimney to produce a given natural draught can be estimated theoretically
DRAUGHT Efficiency of Chimney From equation 29.3, it is clear that for a given height of chimney draught depends on temperature of hot gases leaving the chimney. Natural Draught is directly proportional to outlet temperature of flue gases. It increases with the increase in outlet temperature. But high temperature of hot gases leaving chimney means the loss of heat energy and hence thermal efficiency of boiler. Thus natural draught is created at the cost of thermal efficiency. However in artificial draught system, the flue gases can be made to leave the chimney at a reduced temperature, thus increasing the thermal efficiency or we can say that more heat of flue gases can be harnessed and given to steam. Thus the efficiency of chimney can be calculated on this basis: Let T = Temperature of flue gases in natural draught T2 = Temperature of flue gases in artificial draught Cp = mean sp heat of flue gases in J/kg K Then extra heat carried away by flue gases due to higher temperature required for producing natural draught Cp (T , T2) for one kg of flue gases Draught Produced in terms of height of column of exhaust gases
DRAUGHT Efficiency of Chimney It can give maximum energy to one kg of flue gases = m. g. h Thus efficiency of chimney can be calculated as It is to be noted that even for a very tall chimney, the efficiency, will be less than 1%. So it is a very inefficient mean to create draught. Due to this reason for and for demand of energy efficiency, natural drought is not used in most of the commercial boilers.
PERFORMANCE OF STEAMGENERATORS 1) Equivalent evaporation It is defined as the weight of saturated water at 100C evaporated to dry and saturated steam at 100C by utilizing heat at the same rate as would have been used under the actual working conditions. If H = Total heat of steam at gHw1 = Total heat of feed water in kJ/kg L = Latent heat of steam at atmospheric pressure = 2257 kJ/kg Wa = Weight of steam produced per hour at given working pressure per kg of fuel We = Equivalent evaporation in kg per kg of fuel Then as per the above definition, We L = Wa (H , Hw1) Given working pressure in kJ/kg = Wa .F, where F= = Wa .F, where F
PERFORMANCE OF STEAMGENERATORS 2) Factor of evaporation It is a quantity which when multiplied by the actual amount of steam generated at a given pressure from water at given temperature, gives the equivalent evaporation. Equivalent Evaporation = F x Actual Evaporation Thus factor of evaporation may also be defined as ratio of Equivalent Evaporation to actual Evaporation. 3) Boiler efficiency It is the ratio of heat actually utilized in generation of steam in a given period to the heat supplied by fuel in the same period or it is the ratio of heat utilized in generation of a given quantity of steam to the heat supplied by fuel burnt to produce this steam. Where C = calorific value of fuel in kJ/kg and Wf = weight of fuel per hour. Boiler efficiency is always less than 1 because of some loss of heat through hot gases escaping to atmosphere and also directly to atmosphere by conduction convection and radiation.

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STEAM GENERATORS.pptx

  • 1.
    NAME : MALIKRAFIYAREYAZ CLASS: BE 5TH SEM BRANCH: MECHANICAL ROLLNO: GCET-303-19 (191105040) SUBJECT: APPLIED THERMODYNAMICS
  • 2.
  • 3.
    CONTENTS  INTRODUCTION  CLASSFICATIONOF BOILERS  COMPARISION BETWEEN ‘FIRE-TUBE’ AND ‘WATER-TUBE’ BOILERS  SELECTION OF A BOILER  ESSENTIALS OF A GOOD STEAM BOILER  BOILER TERMS  FIRE TUBE BOILERS  WATER TUBE BOILERS  BOILER MOUNTINGS  BOILER ACCESSORIES  DRAUGHT  PERFORMANCE OF STEAM GENERATORS
  • 4.
    INTRODUCTION A steam generator,popularly known as a boiler, is a closed vessel made of high-quality steel in which steam is generated from water by the application of heat. The water receives heat from the hot gases formed by burning fuel through the heating surface of the boiler. Steam is mainly required for power generation, process heating, and space heating purposes.
  • 5.
  • 6.
    DIFFERENCE BETWEEN FIRETUBE AND WATERTUBE BOILERS FIRE TUBE BOILER WATER TUBE BOILER Hot flue gases flow inside the tube and the water outsides the tubes. Water flows inside the turbine and hot flue gases outside the tube. These boilers are generally internally fired. These boilers are generally extrernally fired. The boiler pressure limited to 20 bar. The boiler pressure is limited to up to 100 bar. The fire-tube boiler has a lower rate of steam production. A higher rate of steam production. Not suitable for larger power plants. Suitable for larger power plants. Involves lesser risk of explosion due to low pressure. The risk of the explosion is higher due to high boiler pressure.
  • 7.
    DIFFERENCE BETWEEN FIRETUBE AND WATERTUBE BOILERS Difficult in transportation. Simple in transportation. They require less skill to operate, as compare to the water tube boiler. They required a skilled operator. They are difficult to repair and cleaning as they are internally fired. They are easy to repair and clean as they are externally fired. They required a large shell diameter. Because the fire tube situated inside the shell. They required a small shell diameter. The efficiency of the fire tube boiler is less as compared to the water tube boiler. The efficiency of the water tube boiler is more. The maintenance of this boiler is costly. It requires regular inspection. They are easy to maintain as they are externally fired. Ex: Cornish Boiler, Lancashire Boiler. Ex: Babcock and Wilcox Boiler.
  • 8.
    DIFFERENCE BETWEEN FIRETUBE AND WATERTUBE BOILERS
  • 9.
    SELECTION OF ABOILER  High Pressure Vessel Mass  High Water Volume  High Efficiency  Quick Startup Times  Low Footprint  Low Heat Flux and High Heating Surface Area  Ease of Clean Out  Effective Turndown  Ease of Maintenance  Large Steam Disengagement Area
  • 10.
    ESSENTIALS OF AGOOD BOILER The essentials of a good boiler :-  The boiler should be reliable and produce the required amount of steam per hour.  The maintenance of the boiler should be easy and cheap.  It should be efficient. It should have minimum fuel consumption and less initial cost.  It should start quickly.  It should be light in weight, occupy less space and it should be able to meet the variable demand of steam.  The joints should be leak proof. The joints should be accessible for inspection and these should be away from flame.  The mud and other deposits should not collect on heated surface since this would affect the heat transfer rates and consequently the steam generation rates.  The refractory material used should be minimum.  The design of boiler allow maximum fluid velocity in order to increase the heat transfer rates between water and flue gases. It should have minimum friction losses.  It should comply with safety regulations according to Indian Boiler Act 1923.
  • 11.
    BOILER TERMS  Shell:-Shell is the main container that contains water for steam usually made up of Steel plates bent into a cylindrical shape. The plates may be welded or reverted together.  Furnace:- Furnace is a firebox and is the space above the grate and below boiler shell suitable area of furnace improves the efficiency of combustion.  Grate:- It is the platform upon which the fuel burnt. Generally, the grate is made up of cast iron bars evenly.  Water flow path:- It is the path followed by the water in the boiler.  Gas flow path:- It is the path followed by the flue gases of combustion. They may be water tubes or fire tube boilers. The gas flow path should be so arranged that it transferred maximum heat to the water to produce steam. This much affects the efficiency of the boiler.  Steam path:- Generally steam path is at the top of the shell to avoid water particles being carried with the steam. To do so, steam separators are used in case of large capacity and high-pressure boilers.
  • 12.
    BOILER TERMS  Fitting( Mountings):- The valves and gauge which are necessary for the safety of the boiler referred to as mountings. Without mountings, a boiler cannot function safely. Example:- Water level indicator, Pressure gauge, Fusible plug, Safety valve, Blow off cock, etc.  Accessories:- The equipment which improves the efficiency of the boiler is known as accessories. Example:-  Economizer and air preheater are used to extract maximum heat from the flue gases.  Super heater used to raise the temperature of steam above the saturation temperature.  Blowing off:- The process of removing the impurities floating on the water surface level is termed as blowing off. Some special device is used to blowing off impurities.  Settings:- Settings are the walls of the Furnace and combustion chamber. These are made up of fire bricks. They form the passage through which the gases pass.  Water space and steam:- The volume of the shell, which occupies the water is known as water space. The level at which the water stands in the boiler shell is known as the water level. The remaining volume of Shell that not occupied by water is called steam space
  • 13.
    FIRE TUBE BOILERS Theseare one of the most fundamental and old designed boilers. These boilers are very famous in the 18th century, and especially applicable for train engines. In this kind of boiler, combustion of heat, as well as gases, flows through the pipe enclosed by water. These boilers may be high-pressure or low- pressure boilers. The measurement of these boilers can be done always with their external diameter. Generally, these boilers are intended for pressures up toward a highest of 250 psi & around 750 horsepower These are one of the most fundamental and old designed boilers. These boilers are very famous in the 18th century, and especially applicable for train engines. In this kind of boiler, combustion of heat, as well as gases, flows through the pipe enclosed by water. These boilers may be high-pressure or low- pressure boilers. The measurement of these boilers can be done always with their external diameter. Generally, these boilers are intended for pressures up toward a highest of 250 psi & around 750 horsepower
  • 14.
    FIRE TUBE BOILERS Constructionand Working Principle of Fire Tube Boiler The construction of a water tube boiler can be equipped with cylindrical shell, vertical, firebox in the base, space for water in the center segment, and space for steam in the higher segment. A fire-grate is located at the fire-box base as well as coal is fired-up on the firebox. For burnt coal, an ash pit is placed at the base of the grate for gathering the ash from the burnt coal, and sometimes it can be detached. One or many cross-tubes are flanged to the space of water that is placed within the box to raise the outside area of heating for improving the water flow. A small chimney is associated with the pinnacle of the firebox for releasing the waste outlets at some larger height. The cleaning of the boiler can be done by the hand holes as well as manholes of the tubes & shell of the boiler.
  • 15.
    Construction and WorkingPrinciple of Fire Tube Boiler  These boilers include a water level indicator; a pressure gauge, steam stop tap security tap, & a manhole like mountings for providing security as well as simplicity of working. The Fuel burns on the grate of the firebox in the boiler and the resultant hot-flue gases are permitted to flow in the region of cross tubes.  The water nearby the cylindrical type firebox also gets heat through radiation as well as convection, therefore steam will be generated. The flow of water in the boiler will be based on the difference of density within water that is formed with the difference of temperature. FIRE TUBE BOILERS
  • 16.
    FIRE TUBE BOILERS Typesof Fire Tube Boiler  The different types of this boiler include the following. 1) Cornish Fire Tube Boiler The first Cornish boiler was approved by an engineer namely “Trevithick a Cornish”. This kind of boiler includes a plane cylindrical shell, as well as a tiny flue pipe holding the heating system, flows through it. 2) Lancashire Fire Tube Boiler The construction of the Lancashire boiler is related to Cornish boiler but as a substitute of one flue pipe, two flue pipes are utilized.
  • 17.
    FIRE TUBE BOILERS 3)Locomotive Fire Tube Boiler The locomotive boiler is an inactive boiler which is used in train engines. This type of boiler is capable while producing steam as well as it is solid. The design of the locomotive boiler is horizontal multi tubular. The main advantage of using this type of boiler is low cost for construction, installation and steam capacity is high. 4) Vertical Fire Tube Boiler The vertical boiler is a simple boiler, and it comprises of a cylindrical shell enclosed a larger section of water and rest of section will be occupied with steam. It contains cross tubes and a furnace at the bottom of the boiler. The combustion gases after heating the water are allowed to escape into the atmosphere.
  • 18.
    FIRE TUBE BOILERS 5)Cochran Fire Tube Boiler The Cochran boiler is a vertical type multi-tubular boiler, and it includes several horizontal fire tubes. The heating system is one part of the construction and it is perfect. 6) Scotch Marine Fire Tube Boiler The Scotch marine boiler is very popular boilers used for high vapor capacities at high forces. This type of boiler includes a huge number of tiny diameter tubes for providing the benefit of the high-heating region of the surface. The boilers are fired up internally and leave from the boiler using a chimney to the environment.
  • 19.
    WATER TUBE BOILERS Awater tube boiler can be defined as a Steam boiler in which the flow of water in the tubes, as well as hot gases, enclose the tubes. Not like fire tube boilers, this boiler attains high-pressures, as well as high-steam capabilities, can be achieved. This is because of condensed tangential pressure on tubes which is known as hoop stress.
  • 20.
    Construction and WorkingPrinciple of Fire Tube Boiler: The working principle of water tube boiler is thermal siphoning (circulation of natural water). Basically, this type of boiler includes two drums namely steam, lower or mud drum. Two drums are associated via two tubes such as down comer and riser. At first, the water is supplied into the steam type drum with the help of a water pump. Whenever the fuel is burned, then hot gases will be generated that are permitted to supply in the shell part of the boiler. The hot gases which are produced by the fuel will replace heat by the water; the water gets changed into steam. Because, the water temperature increases, the concentration will increase automatically. Clearly, the concentration of steam will be lesser than the water. Thus in the steam drum, the water, as well as steam, gets divided obviously due to variation in concentration. Here the traveling of steam will be upward because of low concentration as well as water will travel downward because of high concentration. WATER TUBE BOILERS
  • 21.
    WATER TUBE BOILERS Constructionand Working Principle of Fire Tube Boiler: The flow of hot water at the base of the steam drum will be supplied into mud drum via downcomer tube as well as to heat the water in the mud type drum. Whenever cooler supply water is initiated into steam type drum due to the high concentration of cool water, and it moves down in the downcomer type tube near mud drum. When transferring hot water from the mud type drum to steam type drum using riser tubes then it consequences in normal water circulation in the water tube type boiler. As additional steam is generated in the boiler then the force in the drum enhances & the water supply into the drum reduces which tend for decreasing the flow of steam. In the same way, when the production rate of steam reduces the force in the water- tube boiler reduces, & the water supply into the boiler enhances which tends to enhance the production rate. Like this, the water tube boiler controls the production of steam.
  • 22.
    WATER TUBE BOILERS Typesof Water Tube Boilers The types of water tube boilers include the following. 1) Simple Vertical Boiler This is one type of water tube boiler. In this type of boiler, the axis of direction is perpendicular with respect to the position. The major components of this boiler include ash pit, grate, feed check tap, fire hole, firebox, cross box, hand hole, fusible plug, water gauge, cylindrical shell, steam space, manhole, pressure gauge, steam stop tap, safety tap, chimney. In this type of boiler, using the fire hoke fuel is added into the grate which destroys by fire to generate the warm gases. Ash pit is for collecting the ash which is converted from the fuel. Hot gases increase high and supply their heat toward the water within the cross box, then moves out using the chimney.
  • 23.
    2) Stirling Boiler TheStirling boiler is one type of water tube boiler, used for generating steam (50,000 kg steam/hour and 60 kgf /cm2 pressure) in the large area of the stationary plant. This type of boiler consists of 3 steam drums as well as 2 mud drums. The steam drums are located on the top section of the boiler whereas mud drums are located on the base of the arrangement. The steam drums and mud drums are connected through bent tube banks. When the tubes are turned then the mechanical pressures due to pipes extension throughout heating cannot influence the system. The two drums as well as tubes are designed of steel which will support the total system. The arrangement of the Stirling boiler is enclosed with brickwork. Here, the arrangement of bricks will avoid the heat dissipation in the surroundings WATER TUBE BOILERS
  • 24.
    3) Babcock andWilcox Boilers This is a horizontal straight water tube boiler; it has a steam drum which is made of steel. The two ends of the drum are associated with a series of two end headers with short riser pipes. These are disposed at 15o0 angle to the horizontal axis of the steam drum. The disposed of the arrangement of tubes assists in the supply of water, and the water level in the drum is indicated with a water level indicator. The fire door in the water boiler will be located at the bottom and the fuel will be supplied through this door and burns in a grate. The burning fuel will generate the hot gases that are forced in the grate to supply upward with baffle plates. At the bottom of the water boiler, mud collector is located for removing the mud particles using blow down a cock. Therefore, a nonstop water circulation from the steam drum toward the water tubes is maintained by convective currents because of the difference in concentration and it is called as normal circulation WATER TUBE BOILERS
  • 25.
    BOILER MOUNTINGS  Regularaccidents and boiler break down can happen due to improper maintenance and cleaning. Without boiler mounting one can damage boiler and its surrounding with one mistake followed by a series of events. Without high lift safety valve the shell could explode resulting into great loss of life and machinery; similarly a shell could collapse under vacuum if air vent is not present during cooling down. During cargo operation there is a increased demand for steam and so the mountings help operate the boiler with more and ease and safety. Boiler mountings include:
  • 26.
    1 ) SafetyValves Safety valves are fitted on every boiler to avoid over pressurizing. Normally three safety valves are fitted on the boiler with one on the super heater and rest two on steam drum. In no condition these valves be less than two in number and must lift at a pressure 3% above boiler working pressure irrespective of boiler types. A steam valve is made up of cast iron body with two independent valves fitted on the valve seat. These valves are connected to a lever by means of a pivot held tight to its position by spring. The spring force keep the valve sit shut on the valve seat under normal condition. When the upward pressure exceeds the downward spring force; the valve is lifted and excess steam is released to the atmosphere. 2 ) Vent Valve Vent valve is installed on the boiler shell to vent air from steam drum during starting of boiler. These vent valves also comes handy during boiler shut down as it let fresh air to enter the boiler drum avoiding its collapse under pressure. A vent valve can also be used to release / dump moist steam at start. BOILER MOUNTINGS
  • 27.
    3 ) FusiblePlug A fusible plug is the threaded gunmetal cylinder with conical plug and tappet hole drilled into it. This This hole is then filled with an alloy of low melting point such as tin. The plug can be of either fire actuated or steam actuated type fitted over the combustion chamber. Under normal condition one side of the plug is exposed to extreme temperature while the other submerged under water; keeping it cool. This low melting point alloy can not melt away till submerged and so remain intact even under extreme condition. In event of water level reach below a safe limit and plug tip is exposed to steam; the tin alloy will melt exposing combustion chamber with steam. As steam is not effective coolant and convection medium the tin alloy can’t transfer heat to the steam leading it to melt away. This sudden injection of steam into the furnace will stop the combustion protecting boiler from any damage. 4) Steam Stop valve A steam stop valve is connected to the boiler to stop and regulate steam flow from boiler to the distribution lines. Main steam stop valve on boiler is kept shut to avoid back-flow of steam to the boiler. The flange of the steam stop valve is bolted on top of the steam drum. Valve main body is made of cast iron while the valve seat is made from gun metal. The spindle on one end is connected to the valve while the other end to the handle wheel passing through ( yoke / Gland nut ) and gland packing. The valve is operated by rotating the hand wheel. Rotating hand wheel in turn rotate the spindle which move the valve up allowing path for steam to flow. BOILER MOUNTINGS
  • 28.
    5 ) PressureGauge Pressure gauge are fitted to the steam drum and super heater to indicate steam pressure inside. These gauge are fitted on the front top of the boiler shell and represent pressure in bar. A bourdon tube of closed cross section is attached to the steam space on one end through siphon tube. The tube itself contain water to avoid steam to enter into the pressure gauge. The pointer is connected to the threaded gear attached to the spindle. When pressure is applied to the bourdon tube it becomes circular turning the spindle. This cause the pointer to move along with the gear; representing the boiler pressure. 6 ) Water Level Indicator A pair of water level indicator is installed directly to the boiler shell with an additional remote reading gauge installed at convenient position. They are installed directly on the front end of all boiler types; showing water level in boiler drum. It consist of a glass tube with three independent cock ( Steam cock, water cock and drain cock ). Steam and water cock separates the glass tube with boiler steam and water respectively. Drain cock on other hand used to drain water from glass tube. A metal ball is provided on the water side of the gauge glass to avoid subsequent accident and water loss; by water flashing off steam in event of glass rapture / failure. Under normal condition both steam and water cock is open allowing water and steam pressure to balance. In event of incorrect reading we need to blow through; by closing the water cock and opening drain cock. A strong blow will indicate the steam cock is clear; now repeat the process with steam cock closed and water cock opened. Strong blow of steam with hissing sound indicate the water cock is clear. Now close the drain cock and let water fill in; slowly open the steam cock equalizing the pressure. BOILER MOUNTINGS
  • 29.
  • 30.
    BOILER ACCESSORIES The boileraccessories are the devices, which form an integral part of a boiler but are not mounted on it. They include superheater, economiser, feed pump etc. It may be noted the accessories help in controlling and running the boiler efficiently
  • 31.
    1) Economiser The combustiongases coming out of the boiler contain a large quantity of heat. Therefore the maximum amount of heat from the gases should be recovered before it escapes to the chimney. In the economiser, heating the feed water does the recovery of heat in the flue gases. The economiser is placed in the path of the gases. They improve the overall efficiency of the boiler by reducing fuel consumption. 2) Air Pre-heater The air preheater is an accessory that recovers the heat in the exhaust gas by heating the air supplied to the furnace of the boiler. Supplying preheated air into the furnace produces a high furnace temperature and accelerates the combustion of the fuel. Thus the thermal efficiency of the plant will be increased. BOILER ACCESSORIES
  • 32.
    3) Superheater The superheateris used in boilers to increase the temperature of the steam above the saturation temperature The dry saturated steam generated in the boiler is passed through a set of tubes placed in the path of the flue gases, in which it will be heated further by the hot gas to increase its temperature about the saturation temperature 4)Feed Pump A feed pump is a boiler accessory required to force the feed water at high pressure into the boiler. Commonly used pumps are,  Reciprocating pumps  Rotary pumps The reciprocating pumps are driven directly by coupling them to the steam engine. The rotary pumps are driven by the steam turbines or by electric motors. BOILER ACCESSORIES
  • 33.
    DRAUGHT Boiler draught isdefined as the difference between absolute gas pressure at any point in a flow passage and the ambient (same elevation) atmospheric pressure. Draught is achieved a small pressure difference which causes the flow of air or gas to take place. It is measured in millimeter (mm) or water. The draught is one of the most essential systems of the thermal power plant which support the required quantity of air for combustion and removes the burnt products from the system. To move the air through the fuel bed and to produce a flow of hot gases through the boiler economiser, preheater and chimney require a difference of pressure. This difference of pressure to maintaining the constant flow of air and discharging the gases through the chimney to the atmosphere is known as draught. Draught can be achieved by the use of chimney, fan, steam or air jet or a combination of these. When the draught is produced with the help of chimney only, it is known as Natural Draught and when the draught is produced by any other means except chimney it is known as Artificial Draught
  • 34.
    Purpose of BoilerDraught:  To provide an adequate supply of air for fuel combustion.  For throw out the exhaust gases of combustion from the combustion chamber.  To discharge these gases to the atmosphere through the chimney. DRAUGHT
  • 35.
    Measurement of Draught Theamount of draught produce depends upon:  The nature and depth of fuel at the furnace.  Design of combustion chamber or firebox.  The rate of combustion required.  Resistance is allowed in the system due to baffles, tubes, superheaters, economizers, air pre-heaters etc. DRAUGHT
  • 36.
  • 37.
    DRAUGHT Types of BoilerDraught In general, the draughts may be classified into the following two types,  Natural Draught  Artificial Draught Natural Draught Natural draught system employs a tall chimney as shown in the figure. The chimney is a vertical tubular masonry structure or reinforced concrete. It is formed for enclosing a column of flue gases to produce the draught. It removes the gases high enough to prevent air pollution. The draught is produced by this tall chimney due to the temperature difference of hot gases in the chimney and cold external air outside the chimney. Artificial or Mechanical Draught It has been seen that the draught produced by the chimney is affected by the atmospheric conditions. It has no flexibility, poor efficiency and tall chimney are required. In most of the modern power plants, the draught applied must be freedom of atmospheric condition, and It should have more flexibility (control) to bear the fluctuation loads on the plant. Today’s steam power plants requiring 20 thousand tons of steam per hour would be impossible to run without the aid of draft fans. A chimney of a reasonable height would be incapable of improving enough draft to eliminate the huge volume of air and gases ( 400 x 103 m 3 to 800 x 10 3 m 3 per minutes). The further advantages of fans are to reduce the height of the chimney needed.
  • 38.
    DRAUGHT Determination of theHeight of Chimney for a Given Value of Natural Draught For determination of the height of chimney required to produce certain amount of natural draught, we have to determine the density of flue gases inside the chimney, ρg and that of cold air outside the chimney ρa. A shown in fig 29.1, if H is the height of chimney and Pa is the atmospheric pressure at the top level of the chimney then The absolute total pressure at grate level inside the chimney is given as
  • 39.
    DRAUGHT Determination of theHeight of Chimney for a Given Value of Natural Draught And total pressure at grate level outside the chimney Then draught produced, given by its definition will be equal to Where, ρa and ρg are in kg/m3 g = acceleration due to gravity = 9.81 m/sec2 H = Height of chimney in m Calculation of ρa and ρg We know the combustion chemistry of carbon and hydrogen N/m2 (Pa)
  • 40.
    DRAUGHT Determination of theHeight of Chimney for a Given Value of Natural Draught From equation (i) & (ii) it is clear that volume of O2 supplied for combustion of a unit quantity of carbon is equal to volume of CO2 produced at same pressure and temperature and volume of O2 supplied for combustion of hydrogen is half of volume of steam produced at same temperature and pressure and volume of N2 supplied remains same at same temperature and pressure, as it does not take part in combustion. Now, the hydrogen content in the fuel is very less because of which only, the volume of combustion product increases. So, it can be neglected and it is safely assumed that volume of products of combustion will be equal to volume of air supplied when both reduced to same temperature and pressure conditions Let m = mass of air required per kg of fuel burnt Let m = mass of air required per kg of fuel burnt Let T = mean absolute temperature of chimney gases in K T1= Absolute temperature of outside air in K
  • 41.
    DRAUGHT Determination of theHeight of Chimney for a Given Value of Natural Draught Where, P is in N/m2, T in oK and Ra In J/kg K And volume of (m+1) kg of flue gases will also be same at temperature T1 i.e But hot flue gases are at temperature T. So their volume at temperature T will be Here pressure P is in N/m2, Ra in J/kg K and temperature in K Putting the value of density of fresh air at temperature T1 and of hot flue gases at temperature T2 in equation (Considering atmospheric Pressure Pa, Constant) Density of inside flue gases
  • 42.
    DRAUGHT Determination of theHeight of Chimney for a Given Value of Natural Draught Putting the values of Atmospheric Pressure P = 101300 Pa (N/m2) Ra = Characteristic gas constant for air = 287 J/ kg K and g = Acceleration due to gravity = 9.81 m/sec2 Draught Produced By this formula the theoretical height of chimney required may be determined for producing a certain amount of natural draught, if we know the absolute temperature of flue gases inside the chimney and of air outside the chimney and mass flow rate of air per kg of fuel burnt. Actual Draught produced by chimney is generally less than 12 mm of water. From equation 29.2, it may be seen that the draught produced by the chimney becomes more effective with increase in height of chimney and temperature of flue gases
  • 43.
    DRAUGHT Determination of Diameterof Chimney Let us find out the draught produced in terms of height of flue gases i.e. h meters. So, it will be expressed as pd = ρg. g. h And From Equation Putting the values of ρg from sub eqn (ii) Now assuming that draught losses are Nil and all the draught contribute to imparting velocity to flue gases, velocity of flue gases can be given by: If we take into account the frictional pressure losses in chimney and let it be hf, then
  • 44.
    DRAUGHT Determination of Diameterof Chimney Where k is a constant the value of which is given as k= 0.825 for brick chimney, 1.1 for steel chimney Also, the mass of flue gases flowing through any section of chimney will be given by In this way, the height and diameter of chimney to produce a given natural draught can be estimated theoretically
  • 45.
    DRAUGHT Efficiency of Chimney Fromequation 29.3, it is clear that for a given height of chimney draught depends on temperature of hot gases leaving the chimney. Natural Draught is directly proportional to outlet temperature of flue gases. It increases with the increase in outlet temperature. But high temperature of hot gases leaving chimney means the loss of heat energy and hence thermal efficiency of boiler. Thus natural draught is created at the cost of thermal efficiency. However in artificial draught system, the flue gases can be made to leave the chimney at a reduced temperature, thus increasing the thermal efficiency or we can say that more heat of flue gases can be harnessed and given to steam. Thus the efficiency of chimney can be calculated on this basis: Let T = Temperature of flue gases in natural draught T2 = Temperature of flue gases in artificial draught Cp = mean sp heat of flue gases in J/kg K Then extra heat carried away by flue gases due to higher temperature required for producing natural draught Cp (T , T2) for one kg of flue gases Draught Produced in terms of height of column of exhaust gases
  • 46.
    DRAUGHT Efficiency of Chimney Itcan give maximum energy to one kg of flue gases = m. g. h Thus efficiency of chimney can be calculated as It is to be noted that even for a very tall chimney, the efficiency, will be less than 1%. So it is a very inefficient mean to create draught. Due to this reason for and for demand of energy efficiency, natural drought is not used in most of the commercial boilers.
  • 47.
    PERFORMANCE OF STEAMGENERATORS 1)Equivalent evaporation It is defined as the weight of saturated water at 100C evaporated to dry and saturated steam at 100C by utilizing heat at the same rate as would have been used under the actual working conditions. If H = Total heat of steam at gHw1 = Total heat of feed water in kJ/kg L = Latent heat of steam at atmospheric pressure = 2257 kJ/kg Wa = Weight of steam produced per hour at given working pressure per kg of fuel We = Equivalent evaporation in kg per kg of fuel Then as per the above definition, We L = Wa (H , Hw1) Given working pressure in kJ/kg = Wa .F, where F= = Wa .F, where F
  • 48.
    PERFORMANCE OF STEAMGENERATORS 2)Factor of evaporation It is a quantity which when multiplied by the actual amount of steam generated at a given pressure from water at given temperature, gives the equivalent evaporation. Equivalent Evaporation = F x Actual Evaporation Thus factor of evaporation may also be defined as ratio of Equivalent Evaporation to actual Evaporation. 3) Boiler efficiency It is the ratio of heat actually utilized in generation of steam in a given period to the heat supplied by fuel in the same period or it is the ratio of heat utilized in generation of a given quantity of steam to the heat supplied by fuel burnt to produce this steam. Where C = calorific value of fuel in kJ/kg and Wf = weight of fuel per hour. Boiler efficiency is always less than 1 because of some loss of heat through hot gases escaping to atmosphere and also directly to atmosphere by conduction convection and radiation.