2. Boiler is a closed vessel in which water under pressure is transferred into steam by the application of heat. In the furnace, the chemical energy in the fuel is converted into heat. It is the function of the boiler to transfer this heat to the water in the most efficient manner. A boiler should be designed to absorb the maximum amount of heat released in the process of combustion.
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4. 1.Fire Tube boiler In boilers of this type most of the work is done by the heat transfer from hot combustion products flowing inside the tubes to the water surrounding the tubes. They may also be called as shell boilers. Fire tube boilers feature simple and rugged construction and are relatively cheaper. The maximum design working pressure for fire tube boilers is limited to 17-kg/square centimeter (approx.)
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7. Packaged Boiler Package boilers are generally of shell type with fire tube design More number of passes-so more heat transfer Large number of small diameter tubes leading to good convective heat transfer. Higher thermal efficiency
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10. Pulverized Fuel Boiler Tangential firing Coal is pulverised to a fine powder, so that less than 2% is +300 microns, and 70-75% is below 75 microns. Coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Combustion temperatures from 1300-1700°C Particle residence time in the boiler is 2-5 seconds Most popular system for firing pulverized coal is the tangential firing using four burners corner to corner to create a fire ball at the center of the furnace
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12. Fluidized bed Combustion (FBC) boiler Further, increase in velocity gives rise to bubble formation, vigorous turbulence and rapid mixing and the bed is said to be fluidized. Coal is fed continuously in to a hot air agitated refractory sand bed, the coal will burn rapidly and the bed attains a uniform temperature Distributed air is passed upward through a bed of solid particles The particles are undisturbed at low velocity.As air velocity is increased, a stage is reached when the particles are suspended in the air
14. 3. Modern power station boilers. Modern power station boilers are all water tube boilers consisting essentially of a combustion chamber and an economiser where heat is transferred to boiler water, a drum where separation of steam and water takes place, superheater where superheat is given to saturated steam, gas ducts and air ducts supporting structure, pipe lines with fittings.
15. Convection Pass Modern boilers are generally two pass units, the second pass being the convection pass where heat recovery units like primary superheater , economiser and air heater are placed. The convection pass is made of steel duct or in the form of tube wall.
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17. FURNACE: Furnace is the primary part of boiler where the chemical energy available in the fuel is converted to thermal energy by combustion. Furnace is designed for efficient and complete combustion. Major factors for efficient combustion are time of fuel inside the furnace, temperature inside the furnace mixing between fuel and air. The Other important feature of the furnace construction is to reduce the air infiltration. This is to ensure sufficient air through the burners for efficient combustion, prevention of heat losses.
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20. The tubes which makeup the walls contain a mixture of steam bubbles and water. This mixture being low dense than water in the down comers, rises rapidly and reaches back to the drum and its place is taken by the water flowing through down comers. This produces what we call is natural circulation. The steam and water mixture which is returned to the drum is separated so that water only (with no steam bubbles) is returned to the down comers, and steam only(with no water droplets) passed to the super heaters.
21. 2) Steam circuit Dry steam from the boiler drum goes to the various superheater sections. Steam from the boiler drum passes through the superheater connecting tubes to the primary superheater , which is positioned in the convection pass. The steam then flows from the primary superheater outlet header to the secondary superheater located in the combustion chamber. Steam then goes to the final superheater which is located in the combustion chamber in the outlet section, it then leaves the final super heater outlet header and passes to the main stem pipe which has a boiler stop valve.
22. 3)Air/gas circuit To burn the fuel in the combustion chamber air is required. After combustion, the hot gases are to be evacuated from furnace through the heat absorbing surfaces. This air and gas flow is created by the boiler draught system, which may be either natural or mechanised. The air drawn from the atmosphere is first routed through an air heater where air is heated by the outgoing flue gases. The hot air is then admitted to the furnace through wind box. In coal fired boilers part of this hot air is used for drying the coal in the pulvariser and transporting the pulvarised coal to furnace. The gases pass through the radiant heat release zone and then through various superheaters and reheaters (in reheat boilers). Normally there will be a primary superheater and secondary superheater.
23. After passing through the air heater the flue gas goes to the chimney. In between the air heater and chimney it is customary to provide precipitator to remove the flyash from the flue gas (especially in coal fired boilers) and induced draught fans to suck out the flue gases from the furnace (in balanced draught/induced draught boilers). water is the working medium which transfers the heat energy available in the fuel to the turbine in the form of steam. 22° C reduction in flue gas temperature increases boiler efficiency by 1%
24. Water is chosen as the medium because of the following reasons. a) its easy availability b) its low viscous property c) it has high specific heat d) Its non-reactivity with surfaces with which it comes in to contact.
25. The following can be termed as boiler pressure parts. 1. Boiler drum 2. Water walls 3. Superheaters 4. Reheaters and 5. Economisers
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27. BOILER DRUM The drum acts as reservoir for water & saturated steam and also provides separation and purification of steam. The feed water to the drum reaches the drum from the boiler feed pump via the economizer. A stronger material for use in boiler drums is Ducal W30.
28. Methods of Steam Separation: 1. By Gravity separation This is employed for boilers having low generation rates. 2. By use of Baffles These are in the form of obstacles in the direct path of steam towards outlet.
29. WATER WALL SYSTEM: In the boiler the walls of the combustion chamber are formed by tubular wall sections which not only form the enclosure for the furnace but also provides the evaporating surface for the feed water. The water from the boiler drum is admitted in to the water wall tubes through the downcomers and bottom ring headers. As the water circulates through the waterwall tubes, which receive heat from the furnace radiation, water partially evaporates into steam. Water-steam mixture then return back to the boiler drum.
30. SUPER HEATERS: Super heaters (SH) are meant for raising the steam temperature above the saturation temperature. The superheated and reheated steam temperature around 540°C and pressure 165 bar. i) SH (Reheater also) can classified into convection and radiation type as per heat transfer process. The super heaters and reheaters which are placed above the furnace and can view the flame are called radiant type. ii) Super heater may be classified also according to the shape of the tube banks and the position of the headers, such as pendant SH, platen SH, horizontal SH, Ceiling SH, wall SH etc. iii) They may be classified according to their stages of superheating they perform, like primary SH, Secondary SH, Final SH etc.
31. Reheaters: Reheaters (RH) are provided to raise the temperature of the steam from which part of energy has already been extracted by HP turbine.
32. De-superheaters: Though super heaters are designed in such a way that heat absorbed by radiant and convection super heaters always try to maintain the steam temperature constant in practice the necessary control is achieved by using de-super heater. All modern boiler contact type de-super heaters by which feed water are sprayed directly into the steam for required cooling. Amount of feed water to be sprayed is controlled by automatic control system which is designed to maintain a set final steam temperature. Provision of manual control is also there for emergency.
33. ECONOMISERS: The economiser absorbs heat from the flue gas and adds it mainly as sensible heat to the feed water. The material used in the manufacture of furnace wall tubes for coal fired boiler is ordinary carbon steel but in the 500 MW oil fired units the major proportion of the furnace is constructed from the 1% Cr. ½% Mo Alloy. In 660 MW units also this material is used for whole of the furnace.
44. Why Boiler Blow Down ? When water evaporates Dissolved solids gets concentrated and Solids precipitates on tubes. Reduces the heat transfer rate
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47. The quantity of blow down required to control boiler water solids concentration is calculated by using the following formula: ( Continuous Blow down) TDS in feed water 300 ppm Steam 3 T/hr TDS(T) =0 TDS (C) =3000 ppm Allowable ) Blow down flow rate=300x10%/3000 =1% :=1% of 3,000 = 30 kg/hr Blow down(B) Total Dissolved solids
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49. There are two methods of assessing boiler efficiency. 1) The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel. 2) The Indirect Method: Where the efficiency is the difference between the losses and the energy input. Performance Evaluation of Boilers Boiler Efficiency Evaluation Method 1. Direct Method 2. Indirect Method
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51. Boiler efficiency ( ): = Q x (H – h) x 100 (q x GCV) Where Q = Quantity of steam generated per hour (kg/hr) H = Enthalpy of saturated steam (kcal/kg) h = Enthalpy of feed water (kcal/kg) q = Quantity of fuel used per hour (kg/hr) GCV = Gross calorific value of the fuel (kcal/kg) Boiler efficiency ( )= 8 TPH x1000Kg/Tx (665 – 85) x 100 1.8 TPH x 1000Kg/T x 3200 = 80.0%
52. What are the losses that occur in a boiler? Feed water Blow down
