Boilers: classification, performance parameters, Draught and its calculations,

Boilers, classification of boilers, working of different boilers, boiler mountings and accessories, performance parameters of a boiler, heat balance sheet, draught and its calculations

1. Boilers Prepared by Ankur Sachdeva Assistant Professor, ME

2. What is a Boiler? A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. A boiler, also called a steam generator is an engineering device that generates steam at constant pressure. It is a closed vessel, generally made of steel in which vaporization of water takes place. Heat required for vaporization may be provided by the combustion of fuel in furnaces, electricity, nuclear reactors, hot exhaust gases, solar radiations, etc.

3. Classification of Boilers

4. Classification of Boilers 1 . Based upon the orientation/axis of the shell: When the axis of the boiler shell is horizontal the boiler is called horizontal boiler. Example: Lancashire boiler, Locomotive boiler, Babcock and Wilcox boiler etc. If the axis is vertical, the boiler is called a vertical boiler Example: Cochran boiler. If the axis of the boiler is inclined, it is known as an inclined boiler. Lancashire Boiler Cochran Boiler

5. Classification of Boilers 2. Based upon the utility of the boiler: Stationery boiler, such boilers are stationery and are extensively used in power plants, industrial processes, heating, etc. Example: ( Babcock and Wilcox Boiler, Benson Boiler) (ii) Portable boiler, such boilers are portable and are of small size. These can be of the following types, Locomotive boilers , which are exclusively used in locomotives. Marine boilers , which are used for marine applications

6. Classification of Boilers 3. Based on the type of firing employed: According to the nature of the heat addition process boilers can be classified as, Externally fired boilers, in which heat addition is done externally i.e. furnace is outside the boiler unit. Example: Babcock and Wilcox boiler, Locomotive boiler, etc. (ii) Internally fired boilers, in which heat addition is done internally i.e. furnace is within the boiler unit. Example: Lancashire boiler, Cochran boiler, etc .

7. Classification of Boilers 4. Based upon the tube content: Based on the fluid inside the tubes, boilers can be, Fire tube boilers : These boilers have hot gases inside the tube and water is outside surrounding them. Examples: Cornish boiler, Cochran boiler, Lancashire boiler, Locomotive boiler, etc . (ii) Water tube boilers: These boilers have water flowing inside the tubes and hot gases surrounding them. Examples: Babcock-Wilcox boiler, Stirling boiler, La-Mont boiler, Benson boiler, etc. Fire tube boilers Water tube boilers

8. Classification of Boilers 5. Based on the circulation of water and steam: According to the flow of water and steam within the boiler circuit the boilers may be of the following types, Natural circulation boilers: In these boilers, the circulation of water/steam is caused by the density difference which is due to the temperature variation. Example: Babcock and Wilcox boiler ( ii) Forced circulation boilers: In these boilers, the circulation of water/steam is caused by a pump i.e. externally assisted circulation. Example: La-Mont boiler, Benson boiler, etc.

9. Classification of Boilers 6. Based on the Pressure inside the boiler Low-Pressure Boiler- If the working pressure of steam inside the boiler is below 80 bar. Example: Lancashire Boiler High-Pressure Boiler- If the working pressure of steam inside the boiler is between 80 bar and 160 bar. Example: Babcock & Wilcox Boiler Modern Boiler- If the working pressure of steam inside the boiler is above 160 bar Example: Benson Boiler Once Through /Super-critical Boiler: T he entire feedwater is transformed into steam in a single pass through the boiler and the working pressure of steam inside the boiler is above 221 Bar. Example: Ramsin Boiler

10. Difference between Fire Tube and Water-Tube Boilers

11. Difference between Fire Tube and Water-Tube Boilers

12. Requirements of a Good Boiler It should be capable of generating steam at the desired rate at the desired pressure and temperature with minimum fuel consumption and cost. The boiler should have strength to withstand excessive thermal stresses. Boiler should occupy less floor area and space. It should be equipped with all necessary mountings. Boiler should have the capability to get started quickly from cold. It should have sufficient steam and water storage capacity to meet fluctuations in demand and to prevent fluctuation in steam pressure or water level. The boiler should have a constant and thorough circulation of water.

13. Selection Criteria of a Boiler for an Application Steam pressure requirement Steam temperature requirement Steam generation rate Initial cost and constraints Running and maintenance costs Availability of fuel and water Inspection and maintenance requirements.

14. Boiler Mountings & Accessories

15. What is a Boiler Mounting? Boiler mountings are mechanical devices  that  are considered necessary to operate the boiler smoothly & and safely. They are usually mounted on the surfaces of the  boilers. These are the parts of the system that are mounted on the boiler’s own body  for  protection of the boiler and for complete control of the steam generation cycle. Various boiler mountings are: Water level indicator Pressure Gauge Safety valve Fusible plug Blow-off-cock Feed-check-valve Steam stop valve

16. Water Level Indicator   It indicates the water level inside the boiler vessel. It shows the level in the boiler drum. Construction Normally two water level indicators are fitted into the boiler. These are fitted at the  front end side of every boiler . The water level indicator consists of three cock as steam cock, water cock, drain cock, and glass tube. The steam  cock connects or disconnects the glass tube with steam space. The water cock  connects or disconnects the glass tube with water in the boiler. The drain cock is used to drain out the water in from the glass tube at intervals to ensure that the steam and water cock are clear in operation. The glass tube is protected by means of a cover which is specially made.

17. Water Level Indicator Working When the steam cock and water cock opened, steam rushes from the upper passage, and water rushed from the lower from passage to the glass tube.  This will indicate the level of water in the boiler.  Two balls are placed at the junction of the metal tube. Under normal operating conditions, the balls are kept closed.  I n case the glass tube is broken, steam will rush from the upper passage and water from the lower passage due to pressure difference between boiler pressure at atmospheric pressure. The balls are carried along the passage to and of the glass tube and then closed passages.  This position of the ball is shown in Fig by a dotted circle. Thus flow of water and steam out of the boiler is prevented. 

18. Pressure Gauge A pressure gauge is used to indicate the pressure of steam in the boiler. It is generally mounted on the front top of the boiler. Pressure gauge is of two types as ( i ) Bourdon Tube Pressure Gauge & (ii) Diaphragm type pressure gauge. Both these gauges have a dial in which a needle moves over a circular scale under the influence of pressure. At atmospheric pressure it gives zero reading. Some gauges indicate only the positive pressure but some are compound and indicate negative pressure or vacuum also. Looking at the gauge, the boiler operator can check the safe working pressure of the boiler and can take necessary steps to keep the pressure within safe limits.

19. Bourdon-Tube Pressure Gauge Construction & working The bourdon tube is an elliptical spring material tube made with special quality bronze. One end of the tube is connected to the gauge connector and the other end is closed and free to move. A needle is attached to the free end of the tube through a small gear mechanism. With the movement of the tube under pressure, the needle rotates on the circular scale. The movement of the tube & and hence needle is proportionate to the rise in pressure and so calibrated with scale. The pressure gauge connector is attached to the boiler shell through a U-tube siphon and three-way cocks. In the U-tube, condensate remains filled and so live steam does not come in direct content of the bourdon tube but it pushes or exerts pressure on the condensate which further stretches the bourdon tube. Steam is not allowed direct contact with the gauge due to the high-temperature effect on the pressure recording.

20. Safety Valves Safety valve is used to guard the  boiler  against the excessive high  pressure   of steam inside the drum. If the  pressure  of steam in the boiler drum exceeds the working pressure then the safety valve allows blow-off of the excess quantity of steam to the atmosphere. Thus, the pressure of steam in the drum falls. The escape of steam makes an audio noise to warn the boiler attendant. There are four types of safety valve: Dead weight safety valve. Spring-loaded safety valve Lever-loaded safety valve High steam and low water safety valve

21. Spring-Loaded Safety Valve Construction It consists of a cast iron body having two branch pipes.  Two separate valves are placed over the valve seat. A lever is placed over the valve by means of two pivots. The lever is held tight at its proper position by means of a spring.  One end of spring is connected with the lever while other end with the body of the valve.  The valve is kept on it seats with help of spring force.

22. Spring-Loaded Safety Valve Working In the normal condition, the downward force due to spring is higher than upward force applied by steam.  The valve is closed due to spring force.  When steam pressure exceeds the normal limit, the upward force due to steam pressure becomes higher than the downward force due to spring.  Thus the valves are lifted from their seats opening the passages for steam to release out of the boiler.

23. Fusible Plug The function of the fusible plug is to protect the boiler from damage due to overheating of boiler tubes by low water levels. Construction As shown in the figure, it is simply a hollow gun metal plug screwed into the firebox crown. This hollow gun metal plug is separated from the main metal plug by an annulus fusible material. This material is protected from the fireside by means of a flange.

24. Fusible Plug Working When the water in the boiler is at its normal level, the fusible plug remains submerged in water and its temperature does not exceed its melting temperature, because its heat is transferred to water easily. If under some unwanted condition, the water level comes down to an unsafe limit; the fusible plug is exposed to steam in place of water. On the other side, it is exposed to fire. So its temperature exceeds its melting point due to very low heat transfer to steam and it melts down. Immediately steam and water under high-pressure rush to the firebox and extinguish the fire.

25. Blow-Off Cock It is a controllable valve opening at the bottom of water space in the boiler and is used to blow off some water from the bottom which carries mud or other sediments settled during the operation of the boiler. It is also used to completely empty the water when the boiler is shut off for cleaning purposes or for inspection and repair. Construction and working It has a casing having a passage with one side flange to connect with the boiler shell. The passage is blocked by a cone-shaped plug having a cross rectangular hole. Sealing is made with top and bottom asbestos packing filled in grooves on the plug. The shank of the plug passes through a gland and stuffing box in the cover. On the top portion of the shank a box spanner can be fitted to rotate the shank and plug by 90  to either open or close the blow-off-cock

26. Feed Check Valve The feed check valve is fitted in the feed water line of the boiler after the feed pump. Its function is to allow the water to flow in the boiler when the discharge pressure of the feed pump is more than the inside steam pressure of the boiler and prevent the backflow in case the feed pump pressure is less than the boiler pressure. The feed check valve is fitted slightly below the normal water level in the boiler. Construction & Working In the casing of the valve there is a check valve that can move up or down on its seat under the pressure of water. When the supply pressure of feed water acting at the bottom of the check valve is higher, the valve lifts up and allows the water to fill in the boiler. When supply pressure drops by stopping of feed pump, the boiler pressure acts on the top of the valve and it sits on its gun metal seat and stops the backflow of the boiler water out of the boiler shell.

27. Steam Stop Valve It is fitted over the boiler in between the steam space and steam supply line. Its function is to regulate the steam supply from the boiler to the steam line. Construction and working Its casing has a L-shaped steam flow passage. It consists of a valve and valve seat to stop or allow the steam flow. The valve is attached to a spindle and handle. Spindle passes through packing in the stuffing box to prevent leakage. The spindle has external threads in the top portion and moves in the internal threads of a fixed nut. By rotating clockwise and anticlockwise the spindle and valve move down and up thus closing or opening the valves.

28. What are Boiler Accessories? Boiler accessories are devices that are installed along the boiler and the surrounding area to increase the efficiency of the boiler. These are not an indispensable part of the boiler, and thus without installing  these  devices, the boiler operation can be completed at low efficiency. Various boiler accessories are: Feed pump Economizer Air Pre-heater Superheater

29. Relative Position of Boiler Accessories

30. Feed Pump A feed pump is placed near the boiler and is used to feed water to the boiler working at a high pressure. The job of the feed pump is not just to put the water in the boiler but as the boiler is working at high pressure, the discharge pressure of the feed pump must be sufficiently higher than this to push the water inside the boiler. Construction & Working The feed pump used in the boiler is of two types: ( i ) Reciprocating type and (ii) Rotary type. Both these types are positive displacement types to discharge against high pressure. The discharge pressure of a single-stage centrifugal pump is not high enough to overcome the high pressure of the boiler so a multistage centrifugal pump is used as a boiler feed pump.

31. Feed Pump Construction & Working In stationary low-pressure boilers used in processing industries, a multistage centrifugal pump run by an electrical motor is more suitable. In a multistage centrifugal pump, a number of centrifugal casings are so attached to each other that the  impeller  of each is mounted on the same shaft run by an electrical motor, and discharge of 1 st  stage goes to 2 nd  stage and of 2 nd  to 3 rd  stage and so on. As shown in Fig, in each stage, the pressure of water goes on increasing and the discharge pressure of the final stage is so high as to overcome the internal pressure of the boiler. These pumps have independent working and have smooth operation.

32. Economizer An economizer is a specially constructed heat exchanger for harnessing the heat energy of outgoing flue gases and utilizing it in preheating of boiler feed water. It saves the heat energy and so the fuel and decreases the operating cost of the boiler by increasing its thermal efficiency. Construction & Working Economizers are of two types : ( i ) External type and (ii) Internal type. The external type economizer is constructed and installed apart from the boiler and the flue gases from the boiler are directed to flow through it before escaping through the chimney. It is employed for boilers of medium pressure range. Here a number of vertical tubes made of cast iron are connected to common headers at the bottom and top.

33. Economizer Construction & Working Feed water flow into the bottom header and then through the vertical tubes flow out from the top header. Hot flue gases escaping from the boiler are directed to flow across the outside surface of tubes thus indirectly heating the feed water flowing inside. To avoid deposit of soot over the tube surface, tubular scrapers are fitted over the tubes. These are operated by chain and pulley system and while moving up and down slowly scrap the soot over the wall of tubes and so increase the heat transfer rate. An internal tube economizer is fitted inside the boiler and is an integral part of it.

34. Advantages of Economizer 1.    Stresses produced in the boiler material due to temperature difference of boiler material and feed water are reduced because of increase in feed water temperature. 2.    Evaporative capacity of boiler increases as less heat will be required to generate steam if feed water temperature is already high due to preheating. 3.    Overall efficiency of boiler increases because of more steam produced per kg of fuel burnt.

35. Air-Preheater The function of an air pre-heater is to further utilize the heat of flue gases after coming out of the economizer to preheat the air used in a furnace or oil burner. Construction It is a plate type or tubular type or storage heat exchanger, in which flue gases pass through the tubes on one side of the plate and air passes on the other side. In storage type, a rotor fitted with mesh or matrix alternatively comes in the passage of flue gases and air thus exchanging heat.

36. Superheater The superheater is a heat exchanger whose function is to superheat the wet or saturated steam up to the desired temperature by transfer of heat energy. The superheaters are either located in the passage of hot flue gases or directly over the furnace depending upon its design and application. In modern steam power plants, the superheating and reheating of steam is done in the range of 440°C to 650°C. The 40% of the total heat generated in a modern boiler is utilized by the superheaters. The advantages of superheating and reheating of steam are : (a) It improves the overall efficiency of the plant. (b) It reduces the moisture content in the last stages of the turbine. 6. It helps in preventing the erosion of blades and corrosion of various components of steam turbines. 7. The superheaters are made of special high-strength steels in case of superheating is done above 450°C but less costly carbon steel can be used for temperatures below 450°C

37. Types of Superheaters Radiant superheaters are the type of superheaters that are installed in a radiation zone just after the furnace. In high-pressure boilers when the temperature difference in saturated and superheated steam is around 100˚C, superheaters must be installed in a radiant zone. Convective superheaters are installed after the convective bank of tubes. When the temperature difference between saturated and superheated steam is not more than 50˚C, superheaters must be installed in a convective zone.

38. Babcock and Wilcox Boiler It is a water tube, inclined tube, stationary, externally fired natural circulation boiler.

39. Babcock and Wilcox Boiler Diameter of boiler drum: 1.2 to 1.8 m Length of boiler shell: 6 m to 9 m It is a water tube, inclined tube, stationary, externally fired natural circulation boiler. This boiler can generate steam up to a pressure of 40 bar. It can generate steam at the rate of 20,000 kg/h to 40,000 kg/h. It was discovered by George Herman Babcock and Stephen Wilcox in the year 1967 and was named after its discoverer as the Babcock and Wilcox boiler. It is suitable for meeting the demand of increased pressure and large evaporation capacity or large-sized boiler units It has three main parts: ( i ) Steam and water drum (ii) Water tubes and (iii) Furnace

40. Working of Babcock and Wilcox Boiler First the water starts to come in the water tubes from the drum through the down-take header. The water present in the inclined water tubes gets heated up by the hot flue gases. The coal burning on the grate produces hot flue gases and it is forced to move in a zigzag way with the help of baffle plates. As the hot flue gases come in contact with water tubes, it exchanges the heat with water and converts it into steam. The steam generated is moved upward and through up take header it gets collected at upper side in the boiler drum. An anti-priming pipe is provided in the drum. This anti-priming pipe filters the water content from the steam and allows only dry steam to enter into superheater. The superheater receives the water free steam from the anti-priming pipe. It increases the temperature of steam to desired level and transfers it to the steam stop valve. The superheated steam from the steam stop valve is either collected in a steam drum or made to strike on the steam turbine for electricity generation.

41. Lancashire Boiler Characteristics Lancashire boiler is        - a stationary type,        - fire-tube,        -  horizontal straight tubes,        -  internally fired,        - natural circulation boiler.

42. Lancashire Boiler

43. Lancashire Boiler It is a horizontal, low-pressure, stationary, internally-fired, natural circulation fire tube boiler. It has a circular shell connected to end plates supported by gusset plates. Two fire tubes run throughout the length of the boiler. Hot gases start from the grate area, enter into fire tubes, and come out at the back of the boiler from where these gases flow towards the front of the boiler through the bottom flue. Upon reaching the front these hot gases flow through the side flues and enter the main outlet. Outlet passage may also be used commonly by more than one boiler. Working pressure in these boilers is in the range of 0.7 MPa to 2 MPa and the efficiency of the boiler is about 65%–70%. Size of these boilers depends upon the size of the shell which may be 2 m to 3 m in diameter and 6m to 10m in length.

44. Cochran Boiler It is vertical, stationary, internally fired, fire tube boiler.

45. Cochran Boiler This is a fire tube boiler of vertical type and came up as a modification over the simple vertical boiler in order to maximize the heating surface. Total heating surface area is 10–25 times the grate area. It has a cylindrical shell with a hemispherical crown. Hemispherical geometry offers maximum volume space for a given mass of material and is also very good for strength and maximization of radiant heat absorption. Fire box is also of hemispherical form. Flue gases flow from the fire box to a refractory material lined combustion chamber through a flue pipe. After coming out of fire tubes hot gases enter into a smoke box having a chimney upon it. As the fire box is separately located so any type of fuel such as wood, paddy husk, oil fuel etc. can be easily burnt. These boilers are capable of generating steam up to pressure of 20 bar and steam generating capacity from 20 kg/ hr to 3000 kg/hr. Boilers have dimensions ranging from 1 m diameter and 2 m height to 3 m diameter and 6 m height. Efficiency of such boilers ranges between 70 and 75%.

46. LA MONT Boiler It is a forced circulation, high-pressure water tube boiler. The circulation of the water is maintained by a  centrifugal pump . This pump is driven by a steam turbine using the steam from the boiler. In this La Mont Boiler, the separator drum is kept outside the boiler. From the hot flue gases produced from the combustion of the fuel, the heat is supplied to the water in the evaporator tubes, superheater tubes, economizer tubes, and the air in the air heater tubes. Then the waste gases move to the atmosphere through the chimney. The separator drum separates the steam from the water. Steam passes from the evaporator and feed water is fed from the economizer to the drum. The steam is separated from the water. The water is again pumped to the evaporator by the centrifugal pump via the distributing header.

47. LA MONT Boiler The separated steam is sent to the superheater which receives the heat from the flue gases flowing from the combustion chamber. This superheated steam is then delivered out through a stop valve. An economizer is provided in the water circuit to preheat the feed water using the hot gases leaving the boiler. The feed water is sent inside the boiler through the economizer. The centrifugal pump is used to circulate the water to the economizer. The centrifugal pump delivers the feed water to the headers at a pressure of 2.5 atm above the drum pressure. In the evaporator, water is distributed through the nozzles. The steam is going to the superheater before it goes to the prime mover. A choke is usually fitted at the entrance to each unit, in order to give a secure uniform flow of feed water through the pipelines.

48. Disadvantages of LA MONT BOILER The major disadvantage is the formation and attachment of bubbles on the inner surfaces of the heating tubes. This reduces the heat flow and steam generation.

49. BENSON BOILER In 1927, the Benson boiler was developed by Benson in West Germany. It was the first super-critical drumless boiler. It is a high-pressure, vertical, fire tube boiler. This boiler has no drum and is designed to operate at a critical pressure of 225 bar. WORKING The fuel is burnt on the grate and the hot flue gases flow over the radiant evaporator, convection superheater, convection evaporator, economizer, and air preheater and then pass through the chimney. The feed water is pumped through the economizer tubes and receives heat from the flue gases. Then this heated water flows into the radiant superheater where it receives further heat from the flue gases and gets evaporated. The remaining water is evaporated in the convection superheater. The steam now becomes  saturated steam   in the convection superheater. Then the steam is delivered out through the stop valve.

50. BENSON BOILER The water is passed to the radiant evaporator through the economizer. In the economizer, the major amount of water is converted into steam. The remaining water is evaporated in the final evaporator absorbing the heat from the hot gases by convection. The main disadvantage is salt deposition in this system in the transformation zone when all remaining water is converted into steam. To avoid this, for every 4000 hrs., after working periods, the boiler is cleaned with high-pressure water. The maximum pressure obtained from the Benson boiler is 500 atm .

51. Advantages of BENSON BOILER In this system, there is no drum. So the total weight of the Benson boiler is reduced by 20% when compared to other boilers. The erection of this boiler is easier and quicker. Transformation is easy. It occupies very little space. It can be started very quickly since it has welded joints. It is an economical one. Sudden fall of demand creates circulation problems due to bubble formation in the natural circulation boiler which never occurs in the Benson boiler. Around only 4% of blow-down losses are occurred in the Benson boiler. There is no explosion hazards.

52. LOEFFLER BOILER The major difficulty experienced in the Benson boiler is the deposition of salt and sediment on the inner surfaces of the water tubes. The deposition reduced the heat transfer and ultimately the generating capacity. This further increased the danger of overheating the tubes due to salt deposition as it has high thermal resistance. The difficulty was solved in the Loffler boiler by preventing the flow of water into the boiler tubes. Most of the steam is generated outside from the feed water using part of the superheated steam coming out from the boiler. The pressure feed pump draws the water through the economizer and delivers it into the evaporator drum. About 65% of the steam coming out of the superheater is passed through the evaporator drum in order to evaporate the feed water coming from the economizer.

53. LOEFFLER BOILER The steam circulating pump draws the saturated steam from the evaporator drum and is passed through the radiant superheater and then the convective superheater. About 35% of the steam coming out from the superheater is supplied to the H.P. steam turbine. The steam coming out from the H.P. turbine is passed through the reheater before being supplied to the L.P. turbine. The amount of steam generated in the evaporator drum is equal to the steam tapped (65%) from the superheater. The nozzles that distribute the superheated steam through the water into the evaporator drum are of special design to avoid priming and noise. This boiler can carry higher salt concentration than any other type and is more compact than indirectly heated boilers having natural circulation. These qualities fit it for land or sea transport power generation. Loffler boilers with a generating capacity of 94.5 tonnes/hr and operating at 140 bar have already been commissioned.

54. VELOX BOILER

55. VELOX BOILER When the gas velocity exceeds the sound velocity, the heat is transferred from the gas at a much higher rate than rates achieved with sub-sonic flow. The advantages of this theory are taken to obtain the large heat transfer from a smaller surface area in this boiler. Air is compressed to 2.5 bars with the help of a compressor run by a gas turbine before being supplied to the combustion chamber to get the supersonic velocity of the gases passing through the combustion chamber and gas tubes and high heat release rates. The burned gases in the combustion chamber are passed through the annulus of the tubes. The heat is transferred from gases to water while passing through the annulus to generate the steam. The mixture of water and steam thus formed then passes into a separator which is so designed that the mixture enters with a spiral flow. The centrifugal force thus produced causes the heavier water particles to be thrown outward on the walls. This effect separates the steam from the water.

56. VELOX BOILER The separated steam is further passed to the superheater and then supplied to the prime-mover. The water removed from the steam in the separator is again passed into the water tubes with the help of a pump. The gases coming out from the annulus at the top are further passed over the superheater where its heat is used for superheating the steam. The gases coming out of the superheater are used to run a gas turbine as they carry sufficient kinetic energy. The power output of the gas turbine is used to run the air compressor. The exhaust gases coming out from the gas turbine are passed through the economizer to utilize the remaining heat of the gases. The extra power required to run the compressor is supplied with the help of an electric motor. Feed water of 10 to 20 times the weight of steam generated is circulated through the tubes with the help of a water circulating pump. This prevents the overheating of metal walls.

57. Performance of a Boiler The performance of the boiler reduces with time due to Poor combustion, Heat transfer fouling, Poor operation and maintenance Deterioration of fuel quality and water quality

58. Performance Parameters of Boilers Parameters used to measure the performance of a particular boiler: Evaporative Capacity Evaporation Ratio Parameters used to measure the performance of different boilers: Boiler Efficiency Equivalent Evaporation

59. Evaporative Capacity and Evaporation Ratio Evaporative capacity of a boiler is defined as its capacity to generate steam in a unit time. It is measured in kg/h. Evaporation ratio is the ratio of the amount of steam generated to the amount of fuel consumed in unit time. Typical Examples: Coal-fired boiler: 6 i.e., 1 kg of coal can generate 6 kg of steam Oil-fired boiler: 13 i.e., 1 kg of oil can generate 13 kg of steam

60. Boiler Efficiency It is ratio of heat actually used for steam generation and total heat available due to combustion of fuel in boiler. Boiler efficiency = m f is the mass of fuel burnt per hour, C.V. is calorific value of fuel used (kcal/kg) m is mass of steam generated per hour Enthalpies h and h w are that of final steam and feed water, kcal/kg  

61. Equivalent Evaporation There exists a large variety of boilers in terms of their arrangement, efficiency, steam generation rate, steam condition, type of fuel used, firing method and draught, etc. For comparing one boiler with another any of the above parameters cannot be considered as they are interdependent. For comparing the capacity of boilers working at different pressures, temperatures, different final steam conditions, etc. “equivalent evaporation is used. Equivalent evaporation refers to the quantity of dry saturated steam generated per unit time from feed water at 100˚C to steam at 100˚C at the saturation pressure corresponding to 100˚C. Sometimes it is also called equivalent evaporation from and at 100˚C.

62. Equivalent Evaporation Heat supplied for generating steam at 100˚C from water at respective saturation pressure is 538.9 or 539 kcal/kg = 2257 kJ/kg Equivalent evaporation in(kg/kg of fuel) =   Enthalpy of final steam shall be;

63. Boiler Trial Boiler trial refers to running the boiler under test conditions for its performance estimation. It gives the steam generation capacity of boiler, thermal efficiency of plant and heat balance sheet of the boiler. Under trial the boiler is run for quite long durations so as to attain steady state. Generally the boilers are run for 4 to 6 hours duration for the boilers of oil fired type and coal fired types. Duration of boiler run for attaining steady state changes from boiler to boiler. Observations are taken after the boiler attains steady state for a duration ranging from 10–15 minutes. Measurements are made for fuel supply, combustion analysis, steam generation rate and its quality/state, flue gas and their analysis, temperature and pressure at salient locations and all other measurements as required for heat balance sheet preparation.

64. HEAT BALANCE SHEET OF A BOILER

65. HEAT BALANCE SHEET OF A BOILER

66. HEAT BALANCE SHEET Total heat supplied by fuel: Q = m f .CV m f is the mass of fuel burnt per hour, CV is calorific value of the fuel used (kcal/kg). Heat used for steam generation- Q steam = m steam (h – h w ) m steam is mass of steam generated per kg of fuel burnt, h is enthalpy of final steam produced h w is enthalpy of feed water.

67. Heat Lost Due to Incomplete Combustion Heat loss due to incomplete combustion = Heat released when carbon burns into CO 2 – Heat released when carbon burns into CO. During complete combustion of carbon into CO 2 , 3.38 x 10 4 KJ of heat is released. With incomplete combustion of carbon into CO , 1.012 x 10 4 KJ of heat is released by burning one kg, of carbon. Hence heat loss due to incomplete combustion of one kg of carbon shall be = 2.368 x 10 4 KJ per kg of carbon. where f CO and f CO2 are percentage by volume of CO and CO2 present in flue gases, f C is fraction of carbon present in per kg of fuel. Heat loss due to incomplete combustion = x f c x 2.368 x 10 4 , kJ/kg of fuel  

68. Heat Lost to Dry Flue Gases A large portion of heat getting lost goes along with flue gases. Flue gases leaving boiler comprises of dry flue gases and steam. Heat loss with dry flue gases can be given by, Q dry flue = m dfg C pg (T g – T a ) where m dfg is mass of dry flue gas per kg of fuel, C pg is specific heat of dry flue gas and T g and T a are temperature of flue gas and air entering combustion chamber.

69. Heat Lost to Steam in Hot Gases Steam is produced due to burning of hydrogen present in fuel into water vapour. Heat lost with steam in flue gases shall be Q steam in flue = m s (h s1 – h f1 ) where m s is mass of steam produced per kg of fuel, h s1 is enthalpy values of steam at gas temperature T g and partial pressure of vapour in flue gas. h f1 is enthalpy of water at mean boiler temperature.

70. Heat Lost in Unburnt Fuel Some portion of heat may get lost in unburnt fuel, which could be given by the product of mass of unburnt fuel per kg of fuel and its calorific value Q unburnt = m ubf . CV where m ubf is mass of unburnt fuel per kg of fuel CV is calorific value of fuel

71. Heat Lost Due to Moisture in Fuel Moisture present in fuel shall also cause the loss of heat. This moisture shall get evaporated and superheated as fuel is burnt. For evaporation and superheating of moisture latent and sensible heat requirement shall be met from heat available in boiler due to burning of fuel. Heat lost due to moisture is given by: Q moisture = m moist (h s2 – h f2 ) where m moist is mass of moisture per kg of fuel burnt, h s2 enthalpy of final steam produced. and h f2 is enthalpy of water at boiler furnace temperature.

72. Heat loss due to convection, radiation and other unaccountable losses In a boiler heat also gets lost due to convection, and radiation from the boiler’s surface is exposed to the atmosphere. The heat loss may also be there due to unconsumed hydrogen and hydrocarbon etc. Q unaccounted = (m f . CV) – ( Q steam + Q incomplete + Q dry flue + Q steam in flue + Q unburnt + Q moisture )

73. HEAT BALANCE SHEET (per minute basis)

74. Boiler Draught Draught refers to the pressure difference created for the flow of gases inside the boiler. The boiler unit has a requirement of the expulsion of combustion products and supply of fresh air inside the furnace for continuous combustion. The obnoxious gases formed during combustion should be discharged at such a height as will render the gases unobjectionable. A chimney or stack is generally used for carrying these combustion products from inside of the boiler to outside, i.e. draught is created by the use of a chimney. Draught may be created naturally or artificially by using some external device.

75. Importance of Boiler Draught

76. Classification of Boiler Draught

77. Natural Draught It is produced employing a chimney. The natural draught is produced by a chimney due to the fact that the hot gases inside the chimney are lighter than the outside cold air i.e. density difference between hot gases inside a chimney and cold atmospheric air. In a boiler unit the combustion products (hot) rise from the fuel bed through the chimney and are replaced by fresh air (cold) entering the grate. The intensity of draught produced by a chimney depends upon flue-gas temperature and height of the chimney. The draught produced by a taller chimney is large as the difference in weight between the column of air inside and that of air outside increases with height.

78. Natural Draught Calculations = Density of air, Kg/m 3 = Density of combustion gases in kg/m 3 . The density of air at temperature ,Ta = 353/T a = 1.293 x 273 /T a The density of flue gases is given by 𝜌 g = Draught is given by- P = 353 gH Draught is given by in (terms of water column in mm) h= 353 H  

79. Condition for Maximum Discharge Through Chimney Head created by hot combustion gases is given by- )  

80. Chimney Efficiency Chimney efficiency is defined as the ratio of “energy with a unit mass of gas in natural draught” and “the extra heat carried by the same mass of gas due to high temperature in a natural draught as compared to that in artificial draught”. Where, T g,a is temperature of flue gases in artificial draught Tg temperature of flue gases in natural draught. C p, g is specific heat of hot flue gases in J/kg K

81. Advantages of Natural Draught No external power is required. Less capacity investment. The maintenance cost is low as there is no mechanical part. Chimney keeps the flue gases at a high place in the atmosphere which prevents the contamination of the atmosphere. It has a long life.

82. Disadvantages of Natural Draught The maximum pressure available for producing natural draught by the chimney is hardly 10 to 20 mm of water under the normal atmospheric and flue gas temperatures. The available draught reduces with increases in outside air temperature and for generating enough draught, the exhaust gases have to be discharged at relatively high temperatures resulting in the loss of overall plant efficiency. Thus maximum utilization of Heat is not possible.

83. Artificial Draught When the draught is produced by some external agency i.e. mechanical fan/blower or by steam jet itself, it is called artificial draught. Artificial Draught is generally required in modern boiler installations, which require a total static draught of 30 to 350 mm, which is impossible to produce by installing a chimney because the maximum draught that can be practically produced in a chimney is 12 mm of water. In modern commercial boilers, more value of draught is required to increase the heat transfer coefficient and hence the thermal efficiency. Artificial draught is must to use to overcome the flow resistance offered by large flue passages through the boiler and also a number of other accessories pre-heater, economizer, super heater etc. The Artificial Draught may be either mechanical draught (which is produced by fan/blower) or steam jet draught (which is produced by using a high velocity jet of steam).

84. Mechanical Draught

85. Forced Draught System It is the arrangement in which high-pressure air is delivered to the furnace so as to force flue gases out through stack. It is positive pressure draught in which fan is placed at base of boiler before grate. Air under pressure may be fed to stokers or grate for which a fan/blower is put at the bottom of furnace. Due to pressurised air the pressure inside furnace becomes more than atmospheric pressure so it should be properly sealed, otherwise gas may leak.

86. Induced Draught System In this system, the Blower or Induced Draught fan is located near the base of the chimney. The air is sucked in the system, by reducing the pressure through the system below the atmosphere. The flue gases, generated after combustion are drawn through the system, and after recovering heat in the economizer, and air-preheater, they are exhausted through the chimney to the atmosphere. Here it is to be noted that the draught produced is independent of the temperature of hot gases, so the gases may be discharged as cold as possible after recovering as much heat as possible.

87. Comparison Between Forced Draught and Induced Draught

88. Advantages of Forced Draught over Induced Draught The size and power required by the I.D. fan is more because this fan handles more gases. Since the I.D. fan handles hot gases, water-cooled or air-cooled bearings are to be used. F.D. fan consumes less power and normal bearing can be used

89. Balanced Draught System This system uses a combination of forced draught and induced draught instead of forced or induced draught alone. Forced draught fan ensures a complete supply of air for proper combustion after overcoming all resistances while induced draught fan takes care of post combustion resistances, thus ensuring complete removal of flue gases.

90. Steam Draught System It is a very simple and easy method of producing artificial draught without the need of an electric motor. It may be forced or induced depending on where the steam jet is installed. Steam under pressure is available in the boiler. When a small portion of steam is passed through a jet or nozzle, pressure energy converts to kinetic energy and steam comes out with a high velocity. This high-velocity steam carries, along with it, a large mass of air or flue gases and makes it to flow through the boiler. Thus steam jets can be used to produce draught and it is a simple and cheap method. The steam jet is directed towards a fixed direction and carries all its energy in kinetic form. It creates some vacuum in its surroundings and so attracts the air of flue gases either by carrying along with it. Thus it has the capacity to make the flow of the flue gases either by carrying or inducing towards the chimney. It depends on the position of the steam jet.

91. Forced Steam Jet Draught Steam from the boiler after having been throttled to a gauge pressure of 1.5 to 2 bar is supplied to the jet or nozzles installed in the ash pit. The steam emerging out of nozzles with a great velocity drags air along the fuel bed, furnace, flue passage, and then to the chimney. Here steam jet forces the air and flue gases to flow through the boiler hence it is forced steam jet draught.

92. Induced Steam Jet Draught The jet of steam is diverted into a smoke box or chimney. The kinetic head of the steam is high but the static head is low i.e. it creates a partial vacuum that draws the air through the grate, ash pit, and flues and then to the motor box and chimney. This type of arrangement is employed in locomotive boilers. Here steam jet is sucking the flue gases through the boiler so it is Induced Steam Jet Draught.

93. Advantages of Artificial Draught over Natural Draught Easy control on combustion of fuel and evaporation of water. Significant increase in capacity or evaporation power of the boiler. Increase in fuel burning capacity of the grate. Fuel consumption decreases sufficiently. Improvement in the efficiency of the plant. Reduced chimney height. Prevention of smoke. Low-grade fuel can be used.

94. Draught Losses Loss due to the frictional resistance offered by flue gas passage to the flow of flue gases. Loss due to bends in gas flow circuit, which also offer flow resistance. Loss due to friction head in grate, economizer, super heater etc. Loss due to flow resistance offered by chimney. Loss due to imparting some velocity to flue gases, which is required to increase heat transfer in boiler and also to throw away the flue gases from chimney.