Here is a list of important steam boilers:- 1. Cochran Boiler 2. Lancashire Boiler 3. Locomotive Boiler 4. Babcock and Wilcox Boiler 5. Oil-Fired Packaged Boiler.
1. Cochran Boiler:
Cochran boiler is one of the best types of multi-tubular vertical fire tube boiler. It has a number of horizontal smoke tubes. It is internally fired, natural circulation fire tube boiler (Fig. 3.3).
Following are the data relating to Cochran boiler:
Shell diameter: 2.8 m
Height: 5.8 m
Working pressure: 7 bars (maximum 20 bars)
Steam generating capacity: 3500 kg/hr
Heating surface: 120 m2
Efficiency: 65% for best fuel
Lump size coal is supplied to the combustion chamber over the grate through the fire door. The amount of air required for combustion is supplied by natural draught produced by chimney. The hot gases or flue gases formed after combustion are passed through horizontal tubes after being collected in the combustion chamber and then they are discharged to atmosphere through chimney.
The entire outer surface of the tubes forms the heating surface. The water which is in the shell surrounds the heated tube and gets evaporated. The boiler shell is fitted with large number of boiler mounting.
2. Lancashire Boiler:
Lancashire boiler is a horizontal-type fire tube boiler. It is internally fired natural circulation boiler (Fig. 3.4). The working pressure for such a boiler is about 12 bar. The rate of steam generation is approximately 8000 kg/hr. The boiler is much suitable for sugar mills, textiles, and chemical plants.
The constructional details of a Lancashire boiler are shown in Fig. 3.4. The boiler consists of a cylindrical shell kept horizontally on bricks wall supports. Two large flue tubes of diameter about half that of boiler shell are placed inside. The setting of brick support is so well constructed that it forms two numbers of side flues and one bottom flue outside the shell.
It is made for the purpose of heating shell from outside as well. All the three flue channels are connected with chimney. The boiler drum is filled with water to three-fourths of its total volume. The remaining space of shell is left for the purpose of collecting steam. The boiler is mounted with essential mountings and accessories.
3. Locomotive Boiler:
Locomotive boiler is a horizontal shell-type portable boiler as shown in Fig. 3.5. It is used for railways. It consists of a shell in which water is kept for steam generation. The furnace is of rectangular shape and coal is burnt on grate. The air required for the combustion enters through the slots of the grate. The gases formed after combustion pass through large number of horizontal tubes and collected in smoke box and then discharge to the atmosphere through stake (short chimney).
Generally, locomotive boiler is used for running reciprocating steam engine. The exhaust steam from the engine enters the smoke box through the large number of vertical tubes fitted with nozzles. The steam while passing through it gets expanded which produces vacuum.
The velocity of hot gases is increased. This drives the gases to go away through the stake. A door is provided in the smoke box for inspection and cleaning. The boiler is fitted with large number of important boiler mountings for its safe operation.
4. Babcock and Wilcox Boiler:
Babcock and Wilcox boiler is an externally fired water tube boiler as shown in Fig. 3.6. It consists of a horizontal drum which holds water and steam. In this boiler, the circulation of water takes place by natural means due to difference in density.
The drum is supported on brick structure and iron girder. The boiler is capable of generating steam in the range of 20-40 bar pressure and the rate of steam generation is about 200 kg/hr. There are different types of Babcock and Wilcox water tube boilers. In some cases, the rate of steam generation may be 4000 kg/hr.
Generally, these types of boilers are used in medium-size thermal power plants. The boiler has large number of inclined tubes, the end of which is connected with two boxes known as uptake and downtake headers. The headers are connected with drums holding water and steam. Water from drum comes down to the downtake header and then goes to the uptake header through those inclined tubes.
The boiler has a moving chain grate which is formed by an endless chain and this is driven by electrical motor. The moving chain grate is placed near the uptake header just below it. The coal of nearly 20 mm size falls on the moving grate by means of hopper.
The coal is burnt on grate. After complete burning, the ash falls below at the other end of grate. The hot gases formed after the combustion of fuel move upward through the tubes to drum and move downward again in a zigzag path.
The movement of flue gas is deviated by baffle plate to remain hot gas for a longer time and finally the gas moves to the chimney through the exit door. The boiler is fitted with a superheater placed inside the furnace to get the steam superheated by the heat available with the hot gases.
The superheater is made by a large number of steel U-tubes connected with a box. The end of the box is connected with drums. The steam produced inside the boiler is first allowed to pass through the superheater tube before the use. The boiler is fitted with a large number of boiler mountings for safe operation.
5. Oil-Fired Packaged Boiler:
Generally, this type of unit is oil-fired and is smaller as compared to Lancashire fired tube boiler as shown in Fig. 3.7. For such a boiler, LDO or furnace oil is used as the fuel for the combustion substituting coal.
The main shell of the boiler is similar to that of horizontal tubular boiler. There is a large diameter tube to accommodate the furnace. The shell is generally mounted on channel/rail which can be easily erected and installed connecting chimney for the disposal of flue gases.
This type of boiler can replace other types of fire tube boilers due to its compactness and ease in maintenance. The efficiency is quite good. The place of operation of such type of boiler is very neat and clean as compared to the coal-fired boiler.