Babcock Wilcox Boiler | Construction and Working , Advantages
Babcock Wilcox Boiler
This boiler is exclusively used when pressure is above 10 bar and capacity requirement is in excess of 7000 kg of steam/hr. It is a water tube boiler. It is used to operate at high pressures.
Characteristics of boiler :
1. It is horizontal.
2. It is stationary.
3. It is water-tube.
4. Number of tubes are many.
5. It is externally fired.
6. It has natural circulation for water,
7. It has a forced circulation for air and hot gas.
8. It uses both solid as well as liquid fuel.
9. It is high pressure boiler.
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Figure illustrates this boiler. The main parts are described as under:
1. Cylinder Shell or Drum :
It consists of a larger cylindrical shell. The diameter of this shell and length is as per the specification. It is made of steel plates 10 to 12.5 mm, bent into circular shape and then welded or riveted together with the help of lap joints. This shell is
longitudinal. This shell or drum is mounted at the top. The head of the drum are forged and are dished to a radius equal to the drum, connections are made with the upper header and down take header.
2. Tubes :
This boiler has a large number of parallel tubes of equal length. The internal diameter of the tube is about 250 mm width 25 mm thick. These tubes are placed at an inclination of 5° to 15° or even 30° with the horizontal axis. This inclination is provided to bring about natural circulation of water. These tubes are connected with uptake and down take header. There are large number of rows of tubes Each one of such rows of inclined tubes is connected to one set of two headers.
3. Headers :
The headers at right end of the water tubes is called the down header and others shown at the left end of the water tubes are called the uptake headers. The headers are provided with holes and the end of the tubes projecting out of these headers are, covered with caps. This will helps in cleaning the tubes. The down take head«s are provided with a blow-off cock to remove mud and other impurities.
The baffle plates are provided along the length of the tubes. They are usually placed in order to make the circulation of hot gases in the sine-wave form.
5. Header connection :
The uptake header are connected to the boiler shell by short tubes. The down take headers are connected by long tubes.
The boiler shell, tubes, headers etc. are suspended from the channel sections and is independent of the brick work.
Below the uptake header furnace or trolley type Grate of the boiler is arranged.
8. Fire door ;
The coal is fed to the grate to this door. The coal feed to the chain grate stoker (Not shown in the Figure) The chain speed is so adjusted that by the time the coal reaches the other end of the grate, its combustion has been completed
9. Ash pit:
This is provided below the grate. This is provided to collect the ashes produced by combustion of coal.
The super-heater is placed in the boiler. This will superheat the steam. It is ‘U’ shape tubes. They are arranged horizontally between the drum and the water tubes (See Figure). When steam is raised from cold boiler the super-heater should be flooded (filled with water) to avoid its burning.
11. Damper and Damper door :-
This is operated with the help of pulley and chain passing over the pulley (Not shown in figure). They are controlling the passage of hot gases from the fire grate to the smoke-box.
12. Smoke-box :
This is fitted with a long chimney for passing hot gases its the atmosphere (Not shown in figure). .
13. Mud box :
It is attached to the bottom of the down take header (the rear header). Any foreign matter held in suspension i« the water gets collected in this box due to gravity. It can be-removed from here time to time. A hand hole is provided to clean the mud box. A Wow down cock is provided for cleaning. (Not shown in figure).
14. Soot door :
To clean the outside of the water tubes and to remove the soot, a door is provided. This will help to access to the interior of the boiler.
15. Brick work :
The entire hanging arrangement of the boiler shell, inclined tubes, super-heater etc. are enclosed in the brickwork structure. These structures are provided with some I doors for inspection. A man-hole is provided at the back of the boiler shell.
The water is fed into the boiler shell through me feed check valve. Due to gravity water passes through the vertical tubes, headers and fills up the inclined tube first. The water then collects in the boiler shell. The water level in the shell is adjusted with the help of the water level indicator. The coal is put on trolley grate (Not shown in figure). The coal is fired. The trolley is then pushed into the fire space. In order to provide air for combustion of coal and circulation of : hot gases, damper is lowered to the furnace level. The baffle plates do not allow
hot gases to pass to the right side. These gases heat the super-heater and then move down. These gases travel in the sine wave form and then move to the smoke-box fitted with a long chimney.
Water in the upper portion of the tubes is heated and rises up. The cold water from the down- take header rises to take its place and water from the boiler drum (shell) flows down.
Thus the natural circulation of water takes place just by placing the water tubes at an angle of 5° to l5° with the horizontal.
Till the temperature of water in the tube and in the drum becomes uniform, the circulation of water continues. Finally the water in the upper portion of the tubes is converted into steam. It is then collected in the boiler shell.
The sine-wave movement of hot gases enables contact with the maximum surface area of water tubes. The maximum amount of heat is transferred to water from the hot gases. The steam collected above the surface of the water in the drum is wet steam. The super-heater is used to convert this wet steam into superheated steam. This superheated steam is then taken out through the steam stop valve fitted on the top of the boiler for different uses.
Advantages of the boiler:
1. It uses both solid as well as liquid fuel for burning.
2. The draught losses as compared to other boilers is minimum.
3. As compared to otter boilers the evaporative capacity is high.
4. The circulation of water is natural.
5. The defective tubes can be replaced easily.
6. It is used in power stations for generating large quantity of steam.
Use: Use in large power stations due to greater operational safety and lesser space area as compared to fire tube boilers, required to generate the same steam at the same pressure.
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