Lancashire Boiler
It is a stationary, fire tube, internally fired boiler. The size is approximately from 7-9 meters in length and 2-3 meters in diameter.
Construction of Lancashire Boiler:
It consists of
- Cylindrical shell
- Furnace tubes, bottom flue and side flues
- Grate
- Fire bridge
- Dampers
Cylindrical shell
It is placed in horizontal position over a brick work. It is partly filled up with water. The water level inside the shell is well above the furnace tubes.
Furnace tubes, bottom flue and side flues:
Two large internal furnace tubes (flue tubes) extend from one end to the other end of the shell. The flues are built-up of ordinary brick lined with fire bricks. One bottom flue and two side flues are formed by brick setting, as shown in the figure.
Grate
The grate is provided at the front end of the main flue tubes. Coal is fed to the grate through the fire hole.
Fire bridge:
A brickwork fire bridge is provided at the end of the grate to prevent the flow of coal and ash particles into the interior of the furnace (flue) tubes. Otherwise the coal and ash particles carried with gases form deposits on the interior of the tubes and prevent the heat transfer to the water.
Dampers:
Dampers is in the form of sliding doors are placed at the end of the side flues to control the flow of gases from side flues to the chimney flue.
Working of Lancashire boiler
Coal is fed to the grate through the fire hole and is burnt. The hot gases leaving the grate move along the furnace (flue) tubes upto the back end of the shell and then in the downward direction to the bottom flue. The bottom of the shell is thus first heated.
The hot gases, passing through the bottom flue, travel upto the front end of the boiler, where they divide into two streams and pass to the side flues. This makes the two sides of the boiler shell to become heated. Passing along the two side flues, the hot gases travel upto the back end of the boiler to the chimney flue. They are then discharged into the atmosphere through the chimney.
With the help of this arrangement of flow passages of hot gases, the bottom of the shell is first heated and then its sides. The heat is transferred to water through the surface of the two flue tubes (which remain in water) and bottom and sides of the shell.
The arrangement of flues increases the heating surface of the boiler to a large extent.
Dampers control the flow of hot gases and regulate the combustion rate as well as steam generation rate.
The boiler is fitted with necessary mountings. Pressure gauge and water level indicator provided at the front. Safety valve, steam stop valve, low water and high steam safety valve and man-hole are provided on the top of the shell.
High steam low water safety valve:
It is a combination of two valves. One is lever safety valve, which blows-off steam when the working pressure of steam exceeds. The second valve operates by blowing-off the steam when the water level falls below the normal level.
Blow-off clock:
It is situated beneath the front portion of the shell for the removal of mud and sediments. It is also used to empty the water in the boiler during inspection.
Fusible plug:
It is provided on the top of the main flues just above the grate. It prevents the overheating of the boiler tubes by extinguishing the fire when the water level falls below a particular level. A low water level alarm is mounted in the boiler to give a warning when the water level falls below the preset value.
Salient features of Lancashire Boiler
The arrangement of flues in this boiler increases the heating surface of shell to a large extent.
It is suitable where a large reserve of steam and hot water is needed.
Its maintenance is easy.
Superheated can be easily incorporated into the system at the end of the main flue tubes. Thus overall efficiency of the boiler can be increased.
Note : The simple vertical Boiler, Cochran and Lancashire Boilers discussed till this post are Fire tube boilers. In the upcoming posts, I will write about water tube boilers namely Babcock and Wilcox Boiler.
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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
1. 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.
