It is a water tube boiler used in steam power plants. In this, water is circulated inside the tubes and hot gases flow over the tubes.
Construction of Babcock and Wilcox Boiler
The Babcock and Wilcox Boiler consists of
- Steam and water drum (boiler shell)
- Water tubes
- Uptake-header and down corner
- Grate
- Furnace
- Baffles
- Super heater
- Mud box
- Inspection door
- Damper
Steam and water drum (boiler shell):
One half of the drum which is horizontal is filled up with water and steam remains on the other half. It is about 8 meters in length and 2 meter in diameter.
Water tubes:
Water tubes are placed between the drum and furnace in an inclined position (at an angle of 10 to 15 degree) to promote water circulation. These tubes are connected to the uptake-header and the down-comer as shown.
Uptake-header and down-corner (or downtake-header)
The drum is connected at one end to the uptake-header by short tubes and at the other end to the down-corner by long tubes.
Grate: Coal is fed to the grate through the fire door.
Furnace : Furnace is kept below the uptake-header.
Baffles: The fire-brick baffles, two in number, are provided to deflect the hot flue gases.
Superheater: The boiler is fitted with a superheater tube which is placed just under the drum and above the water tubes
Mud box: Mud box is provided at the bottom end of the down comer. The mud or sediments in the water are collected in the mud box and it is blown-off time to time by means of a blow –off cock.
Inspection doors: Inspection doors are provided for cleaning and inspection of the boiler.
Working Babcock and Wilcox Boiler:
Coal is fed to the grate through the fire door and is burnt.
Flow of flue gases:
The hot flue gases rise upward and pass across the left-side portion of the water tubes. The baffles deflect the flue gases and hence the flue gases travel in the zig-zag manner (i.e., the hot gases are deflected by the baffles to move in the upward direction, then downward and again in the upward direction) over the water tubes and along the superheater. The flue gases finally escape to atmosphere through chimney.
Water circulation:
That portion of water tubes which is just above the furnace is heated comparatively at a higher temperature than the rest of it. Water, its density being decreased, rises into the drum through the uptake-header. Here the steam and water are separated in the drum. Steam being lighter is collected in the upper part of the drum. The water from the drum comes down through the down –comer into the water tubes.
A continuous circulation of water from the drum to the water tubes and water tubes to the drum is thus maintained. The circulation of water is maintained by convective currents and is known as
“natural circulation”.
A damper is fitted as shown to regulate the flue gas outlet and hence the draught.
The boiler is fitted with necessary mountings. Pressure gauge and water level indicator are mounted on the boiler at its left end. Steam safety valve and stop valve are mounted on the top of the drum. Blow-off cock is provided for the periodical removed of mud and sediments collected in the mud box.
Salient features of Babcock and Wilcox Boiler:
- Its overall efficiency is higher than a fire tube boiler.
- The defective tubes can be replaced easily.
- All the components are accessible for inspection even during the operation.
- The draught loss is minimum compared with other boiler.
- Steam generation capacity and operating pressure are high compared with other boilers.
- The boiler rests over a steel structure independent of brick work so that the boiler may expand or contract freely.
- The water tubes are kept inclined at an angle of 10 to 15 degree to promote water circulation.
Advantages and disadvantages of water tube boilers over fire tube boilers:
Advantages water tube boilers
- Steam can be generated at very high pressures.
- Heating surface is more in comparison with the space occupied, in the case of water tube boilers.
- Steam can be raised more quickly than is possible with a fire tube boiler of large water capacity. Hence, it can be more easily used for variation of load.
- The hot gases flow almost at right angles to the direction of water flow. Hence maximum amount of heat is transferred to water.
- A good and rapid circulation of water can be made.
- Bursting of one or two tubes does not affect the boiler very much with regard to its working. Hence water tube boilers are sometimes called as safety boilers.
- The different parts of a water tube boiler can be separated. Hence it is easier to transport.
- It is suitable for use in steam power plants (because of the various advantages listed above).
Disadvantages of water tube boilers
- It is less suitable for impure and sedimentary water, as a small deposit of scale may cause the overheating and bursting of tubes. Hence, water treatment is very essential for water tube boilers.
- Maintenance cost is high.
- Failure in feed water supply even for a short period is liable to make the boiler overheated. Hence the water level must be watched very carefully during operation of a water tube boiler.
The Dead Weight safety valve consists of a valve V which is made of gun metal to prevent rusting. It rests on the gun metal seat S and is fixed to the top of a vertical steam pipe P. The pipe has a flange F at the bottom for fixing at the top of the boiler shell.
