Classification of boilers

1. Horizontal, Vertical or Inclined Boiler.

-If the axis of the boiler is horizontal, the boiler is called horizontal, 

-If the axis is vertical, it is called vertical boiler and 

-If the axis is inclined it is called as inclined boiler.

2. Fire Tube and Water Tube

-In the fire boilers, the hot gases are inside the tubes and the water surrounds the tubes.
Examples: Cochran, Lancashire and Locomotive boilers.
-In the water tube boilers, the water is inside the tubes and hot gases surround them.
Examples: Babcock and Wilcox,
 Yarrow boiler etc.

3. Externally fired and internally fired

-The boiler is known as externally fired if the fire is outside the shell.
Examples: Babcock and Wilcox boiler boiler
-In case of internally fired boilers, the furnace is located inside the shell.
Examples: Cochran, Lancashire boiler etc.

4. Forced circulation and Natural Circulation

-In forced circulation type of boilers, the circulation of water is done by a
forced pump.
Examples: Lamont, Benson Boiler etc.
-In natural circulation type of boilers, circulation of water in the boiler takes
place due to natural convention currents produced by the application of
heat.
Examples: Lancashire, Babcock and Wilcox boiler etc.

5. Higher Pressure and Low Pressure Boilers
-The boiler which produce steam at pressures of 80 bar and above are
called high pressure boilers.
Examples: Babcock and Wilcox, Benson Boiler etc.
-The boilers which produce steam at pressure below 80 bar are called low
pressure boilers.
Examples: Cochran, Cornish, Lancashire and Locomotive boiler etc.

6. Stationary and Portable

-Stationary boilers are used for power plant steam, for central station utility
power plants, for plant process steam etc.
-Mobile boilers or portable boilers include locomotive type, and other small
units for temporary use at sites.

7. Single Tube and Multi Tube Boiler

The fire tube boilers are classified as 

-single tube and 

-multi-tube boilers,
depending upon whether the fire tube is one or more than one.
Examples: Cornish, simple vertical boiler are the single tube boiler and rest
of  the boilers are multi-tube boiler.


8. According to passes
-single pass
-Multi pass

Parts of boiler

Shell: The shell of the boiler is the main container usually of cylindrical shape which contains water and steam.

Furnace: A furnace is another important part of the boiler. This may be a grate to burn coal or a burner to atomize and burn liquid fuel. Suitable area and volume should be provided of efficient combustion.

Water flow path: The water flow path is the path followed of absorption of heat from hot gases and conversion into steam. The water should be free from dissolved material in order to reduce the scaling of the heating surface.

Gas flow path: The hot gas path either in fire tube or in water tube should be arranged in such a way that maximum heat of hot gases should be transferred to the water for steam generation. The boiler efficiency mainly depends upon the gas flow path.

Steam path: In most of the boiler, the steam is taken out preferable at the top of the shell to avoid water partials being carried with the steam. To reduce the water particles carried by the steam, it mountings, water level indicator.

Fittings: The valves and gauges which are necessary for the safety of the boiler are blow-off-cock and fusible plug are some of the mountings.

Accessories: Some equipment like economizer, air pre heater and super heater are attached to the boiler to improve its overall efficiency. The economizer and air pre heater are used to extract maximum heat from the flue gases and superheater is used to increase the temperature of steam above saturation temperature. 

Definition of boiler:

It is a closed vessel in which the heat produced by the combustion of fuel is transformed to water for its conversion into steam at the desired temperature and pressure.

Babcock and Wilcox boiler

Babcock and Wilcox is a water-tube boiler is an example of horizontal inclined tube boiler it also a High Pressure Boiler.

Characteristic:

1. Horizontal Boiler
2. Muli tube Boiler
3. Water tube Boiler
4. Straight tube Boiler
5. Externally Boiler
6. Multi tube Boiler
7. Natural draft Boiler
8. Natural circulation Boiler
9. Coal fired Boiler
10. Stationary Boiler
11. Low & Medium pressure Boiler

Construction: Babcock and Wilcox boiler with longitudinal drum. It consists of a drum connected to a series of front end and rear end header by short riser tubes. To these headers are connected a series of inclined water tubes of solid drawn mild steel. The angle of inclination of the water tubes to the horizontal is about 15° or more.

Working: The fire door the fuel is supplied to grate where it is burnt. The hot gases are forced to move upwards between the tubes by baffle plates provided. The water from the drum flows through the inclined tubes via down take header and goes back into the shell in the form of water and steam via uptake header. The steam gets collected in the steam space of the drum. The steam then enters through the antipriming pipe and flows in the superheater tubes where it is further heated and is finally taken out through the main stop valve and supplied to the Steam turbine or Steam engine when needed.

The pressure of steam in case of cross drum boiler may be as high as 100 bar and steaming capacity upto 27000 kg/h.At the lowest point of the boiler is provided a mud collector to remove the mud particles through a blow-dawn-cock.
Advantages:

1. It uses both solid as well as liquid fuel for burning.
2. The drought losses as compared to other boiler in minimum.
3. As compared to other boiler the evaporation 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 generation large quantity of steam.
   

Cochran boiler

Characteristics: 

1. Vertical Boiler
2. Multi tube Boiler
3. straight tube Boiler 
4. low pressure Boiler
5. Coal fired Boiler  
6. Single tube Boiler
7. Natural draft Boiler
8. Natural circulation Boiler
9. Stationary Boiler
10. internally fired Boiler    
11. Water tube Boiler

Working: The fuel is burnt on the grate and ash is collected and disposed of from ash pit. The gases of combustion produced by burning of fuel enter the combustion chamber through the flue tube and strike against fire brick lining which directs them to pass through number of horizontal tubes, being surrounded by water. After which the gases escape to the atmosphere through smoke box and chimney.

Construction: Cochran boiler consists of a cylindrical shell with a dome shaped top where the space is provided for steam. The furnace is one piece construction and is seamless. Its crown has a hemispherical shape and thus provides maximum volume of space.
Advantages:

• The minimum floor area is required.
• Cost of construction is low.
• it can be moved and stet up take it to different location.
• Boiler has self contained furnace . No brick work setting is neccessary.
• Any type of flue can be used.

Disadvantages:

• Steam raising capacity is less due to vertical design.
• Difficult in cleaning and inspection due to vertical design.
• The capacity and pressure are limited.
• The boiler requires high head room.