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Steam Plant Boiler Introduction

- 1. The boiler as a steam generator.
- 2. Steam Generators/Boilers • “A combination of apparatus for producing, furnishing or recovering heat together with apparatus for transferring the heat so made available to water which would be heated and vaporized to steam form” (ASME). • Basis for classification of boilers o Contents inside the tube o Firing system o Position of drum o Pressure o Nature of water circulation
- 3. Water Tube and Fire Tube Boilers
- 4. Requirements of a Good Boiler • Low cost of installation, operation and maintenance • Easy maintenance • High efﬁciency • Safety • High transportability • High steam production rate • Good quality of steam • Quick steam generation capacity • Meeting ﬂuctuating demand of steam
- 6. Babcock and Wilcox Boiler
- 10. Boiler Accessories • Economiser • Air preheater • Super heater • Steam trap • Steam separator • Injector
- 11. Economiser
- 12. Air Preheater
- 13. Superheater
- 14. Steam Separator
- 15. Boiler Performance Heat loss in the boiler Heat balance sheet
- 17. • 1. Introduction, • 2. Equivalent Evaporation • 3. Boiler Efficiency. • 4. Boiler Trial • 5. Heat Losses in a boiler • 6. Heat balance sheet
- 18. • Introduction • The performance of a steam boiler is measured in terms of its ‘evaporative capacity • In actual practice, the feed water temperature and working pressure varies considerably. • The feed temperature usually adopted is 100° C and the working pressure as normal atmospheric pressure, i.e. 1.013 bar. It is assumed that the boiler is supplied with water at the boiling temperature (100°C) corresponding to the atmospheric pressure.
- 19. Equivalent evaporation • It is the amount of water evaporated from feed water at 100° C and formed into dry and saturated steam at 100° C at normal atmospheric pressure. It is, usually, written as “from and at 100 °C” • As the water is already at the boiling temperature. it requires only latent heat at 1.013 bar to convert it into steam at the temperature (100° C). The value of this latent heat is taken as 2257 kJ/kg. (hfg at 100°C)
- 21. Equivalent evaporation E me = Kg/h or kg/kg of fuel burnt
- 22. Is called the factor of evaporation Fe and is always greater than unity for all boilers. 𝐹𝑒 = ℎ − ℎ𝑓1 2257
- 23. Boiler Efficiency • It may be defined as the ratio of heat actually used in producing the steam to the heat liberated in the furnace. It is also known as thermal efficiency of the boiler.
- 24. Worked example • A boiler was used to produce wet steam at 11 bar with a dryness fraction of 0.8 from water at 12 C. Fuel is supplied to the furnace of the boiler at a rate of 325 kg per hour. The calorific value of the fuel is 35700 kJ/kg. The mass of water supplied to the boiler in 5 hrs 23 minutes = 15630 kg. • The mass of water was measured before and after the steam generation process and it was discovered that in order to restore the original level in the boiler 1200 kg of water was required. • Evaluate: • Actual evaporation per kilogram of coal • Equivalent evaporation from and at 100 C • Thermal efficiency of the boiler.
- 25. Performance of Boilers • Evaporation Rate: It is steam generation rate of boilers which may be expressed in terms of kg of steam per unit heating surface area or kg of steam per cubic metre of furnace volume or kg of steam per kg fuel burnt. • Equivalent Evaporation: It is equivalent of energy of evaporation of 1 kg of water at 100°C to dry and saturated steam at 100°C, standard atmospheric pressure of 1.013 bar. Hence, the equivalent evaporation of 1 kg of water at 100°C needs 2,257 kJ. • Factor of Evaporation: It is ratio of heat absorbed by 1 kg of feed water under working conditions to latent heat of steam at atmospheric pressure.
- 26. • Boiler Efficiency: It is ratio of heat absorbed by water in boiler to heat supplied to boiler per unit time.
- 27. Thank You.
- 29. Worked example • A boiler was used to produce wet steam at 11 bar with a dryness fraction of 0.8 from water at 12 C. Fuel is supplied to the furnace of the boiler at a rate of 325 kg per hour. The calorific value of the fuel is 35700 kJ/kg. The mass of water supplied to the boiler in 5 hrs 23 minutes = 15630 kg. • The mass of water was measured before and after the steam generation process and it was discovered that in order to restore the original level in the boiler 1200 kg of water was required. • Evaluate: • Actual evaporation per kilogram of coal • Equivalent evaporation from and at 100 C • Thermal efficiency of the boiler.
- 30. Boiler trial The main objectives of a boiler trial are: 1. To determine the generating capacity of the boiler. 2. To determine the thermal efficiency of the boiler when working at a definite pressure. 3. To prepare heat balance sheet for the boiler.
- 31. Heat loss in a boiler • We know that the efficiency of a boiler is the ratio of heat utilised in producing steam to the heat liberated in the furnace. • The heat utilised is always less than the heat liberated in the furnace. • The difference of heat liberated in the furnace and heat utilised in producing steam is known as heat lost in the boiler.
- 32. hf1 hf2 h3 = hf + x hfg h4 = hsup 1 2 3 4 Evaporator Economiser Superheater Steam Drum
- 33. Heat lost to dry flue gases per kg of fuel
- 34. Heat lost in moisture present in the fuel It is assumed that the moisture is converted into superheated steam at atmospheric pressure 1.013 bar). •Heat lost in moisture present in the fuel
- 35. heat lost to steam formed by combustion of hydrogen per kg of fuel
- 36. 4 The heat lost due to unburned carbon per kg of fuel
- 37. Heat lost due to incomplete combustion of carbon to carbon monoxide. • This loss generally occurs in the boiler due to insufficient air supply.
- 38. Heat lost due to radiation. • There is no direct method for finding the heat lost due to radiation. This loss is calculated subtracting the heat utilised in raising steam and heat losses from the heat supplied. • A Heat balance sheet shows the complete account of heat supplied by 1 kg of dry fuel and heat consumed. The heat supplied is mainly utilised for raising the steam and the remaining heat is lost.
- 39. Heat balance sheet Heat supplied kJ Heat consumed kJ % Heat supplied by 1 kg of fuel Xf 1 Heat lost to dry flue gases per kg of fuel 2 Heat lost in moisture present in the fuel. 3 heat lost to steam formed by combustion of hydrogen per kg of fuel 4 The heat lost due to unburned carbon per kg of fuel 5. Heat lost due to incomplete combustion of carbon to carbon monoxide. 6. Heat lost due to radiation etc. X6 = Xf – (x1 + x2 + x3 + x4 + x5) X1 X2 X3 X4 X5 X6
- 40. Example • In a boiler the following observations were made • Pressure of steam 10 bar • Steam condensed 540 kg/hr • Fuel used 65 kg/hr • Moisture in fuel 2% by mass • Mass of dry flue gas 9 kg/kg of fuel • Lower calorific value of fuel 32000 kJ/kg • Temperature of flue gases 325°C • Temperature of boiler house 28°C • Feed water temperature 50°C • Mean specific heat (flue gases) 1 kJ/kgK • Dryness fraction of steam 0.95 Draw up a heat balance sheet for the boiler
- 41. • Heat supplied by 1 kg of fuel • =(1 - 0.02) x 32000 = 31360KJ • Heat utilised in raising steam per kg of fuel • From steam tables, for a feed water temp of 50°C • hf = 209.3 kJ/kg kg 31 . 8 65 540 m m me f s
- 42. • Correspondingly for steam at 10 bar • hf = 762.6 kJ/kg, hfg = 2013.6kJ/kg • Heat utilised in raising steam per kg of fuel • Heat carried away by dry flue gas kJ t t c m b g pg g 2673 28 325 1 9 kJ h xh h m h h m f fg f e f f e 20495 3 . 209 6 . 2013 95 . 0 6 . 762 31 . 8 1 1 2
- 43. • Heat carried away by moisture in fuel per kg of fuel • From tables @ 28°C, hb = 117.3 kJ/kg • Heat carried away by moisture in fuel kJ h t c m b g p m 6 . 60 3 . 117 100 325 1 . 2 2676 02 . 0 100 2676 Cp for steam = 2.1 kJ/kgK
- 44. Heat balance sheet Heat supplied kJ Heat consumed kJ % Heat supplied by 1 kg of fuel 31360 1 Heat utilised in raising steam 2 Heat carried away by flue gases. 3 Heat lost in moisture present in the fuel. 4. Heat lost due to radiation etc. X6 = Xf – (x1 + x2 + x3) 20495 2673 60.6 = 8131.4 Totals 31360 31360
- 45. The following particulars were recorded during a steam boiler trial Pressure of steam = 11 bar; mass of feed water = 4600 kg/hr; Temperature of feed water = 75C; dryness fraction of steam = 0.96; coal used 490 kg/hr; calorific value of coal 35700 kJ/kg; Moisture in coal 4% by mass; Mass of hydrogen in fuel 18% by mass mass of flue gases 18.57 kg/kg coal; temperature of flue gases = 300C; Boiler house temperature = 16C; specific heat of flue gases = 0.97 kJ/kg K Draw the heat balance sheet of the boiler per kg of
- 46. In a boiler trial, the following observations were obtained. Pressure of steam = 8.5 bar; mass of feed water = 1520 kg/hr; Temperature of feed water = 30C; dryness fraction of steam = 0.95; coal used 200 kg/hr; Ash and unburnt coal collected = 16 kg/hr Calorific value of ash and unburnt coal = 3780 kJ/kg calorific value of coal 27300 kJ/kg; Moisture in coal 4% by mass; mass of flue gases 17.3 kg/kg coal; temperature of flue gases = 330C; Boiler house temperature = 17C; specific heat of flue gases = 1 kJ/kg K Draw the heat balance sheet of the boiler per kg of coal.
- 48. Heat balance sheet Heat supplied kJ Heat consumed kJ % Heat supplied by 1 kg of (dry) fuel (1-%m)CV 1 Heat utilised in raising steam per kg of steam 𝑚𝑒 ℎ𝑓 + 𝑥ℎ𝑓𝑔 − ℎ𝑓1 2 Heat carried away by flue gases. 𝑚𝑔𝐶𝑝𝑔 𝑡𝑔 − 𝑡𝑏 3 Heat lost in moisture present in the fuel. 𝑚𝑚 ℎ𝑔 + 𝐶𝑝𝑠 𝑡𝑔 − 100 − ℎ𝑏 4 Heat lost in water formed from hydrogen present in the fuel. 𝑚𝐻𝑦𝑑𝑟𝑜𝑔𝑒𝑛 ℎ𝑔 + 𝐶𝑝𝑠 𝑡𝑔 − 100 − ℎ𝑏 4. Heat lost due to radiation etc. X6 = Xf – (x1 + x2 +x3 +x4) Totals