Short Answer:

The classifications of boilers are based on different factors such as the path of water and hot gases, pressure of steam, type of fuel used, and purpose of application. The most common types are fire-tube boilers and water-tube boilers, depending on the flow of water and gases.

In simple words, boilers can be divided into various categories depending on how they generate steam, the pressure they work at, and their design. This classification helps in selecting the right type of boiler for power generation, industrial heating, or other applications efficiently and safely.

Detailed Explanation :

Classifications of Boilers

A boiler is a closed vessel used to convert water into steam using heat from fuel combustion. Since boilers are used for various applications such as power generation, heating, and industrial processes, they are designed in different forms and sizes. Therefore, boilers are classified based on several characteristics such as the flow of fluids, pressure, fuel used, furnace position, and purpose.

The classification helps engineers and designers choose a suitable boiler type for a particular purpose to achieve high efficiency, safety, and cost-effectiveness. Below are the main classifications of boilers explained in detail.

1. According to the Flow of Water and Hot Gases

This is one of the most common classifications of boilers. It is based on the direction in which water and hot flue gases move within the boiler.

1. Fire-Tube Boiler

• In a fire-tube boiler, hot gases pass through tubes, and water surrounds the tubes.
• Heat from the hot gases is transferred through the tube walls to the surrounding water, which then converts into steam.
• These boilers operate at low to medium pressures (up to 25 bar).
• Examples: Cochran boiler, Lancashire boiler, Locomotive boiler.

Advantages:

• Simple design and construction.
• Low cost and easy maintenance.
• Suitable for small power and heating applications.

Disadvantages:

• Not suitable for high-pressure operations.
• Slow steam generation rate.

1. Water-Tube Boiler

• In a water-tube boiler, water flows inside tubes, and hot flue gases surround the tubes.
• These boilers can generate steam at much higher pressures and temperatures than fire-tube types.
• Examples: Babcock and Wilcox boiler, Stirling boiler, Lamont boiler.

Advantages:

• Produces high-pressure steam.
• High efficiency and quick steam generation.
• Suitable for large power plants.

Disadvantages:

• Expensive and complex design.
• Requires skilled operation and maintenance.

2. According to the Pressure of Operation

Boilers are also classified according to the pressure at which they generate steam.

1. Low-Pressure Boilers

• These boilers produce steam at pressures below 10 bar.
• Used for heating and small-scale industrial processes.
• Example: Cochran boiler, Cornish boiler.

1. Medium-Pressure Boilers

• Generate steam between 10 bar and 25 bar.
• Commonly used in small power plants and process industries.
• Example: Lancashire boiler, Locomotive boiler.

1. High-Pressure Boilers

• Operate at pressures above 25 bar.
• Used in modern thermal power plants for generating large amounts of steam.
• Examples: Babcock and Wilcox boiler, Benson boiler, Lamont boiler.

3. According to the Type of Fuel Used

Boilers are also classified based on the type of fuel used to produce heat.

1. Solid Fuel Boilers

• Use solid fuels such as coal, wood, or biomass for combustion.
• Commonly used in older power plants and industries.
• Example: Lancashire boiler, Cornish boiler.

1. Liquid Fuel Boilers

• Use liquid fuels such as diesel oil or furnace oil.
• Common in industries where liquid fuels are readily available.
• Example: Oil-fired Cochran boiler.

1. Gas-Fired Boilers

• Operate using natural gas or LPG as fuel.
• Highly efficient and produce cleaner combustion.
• Example: Gas-fired vertical boilers.

1. Electrical Boilers

• Use electric heating elements to convert electrical energy into heat.
• Environmentally clean and used for small-scale applications.

4. According to the Method of Firing

The firing method describes how fuel is burned in the boiler.

1. Internally Fired Boiler

• The furnace is located inside the boiler shell.
• Example: Cochran boiler, Lancashire boiler.

1. Externally Fired Boiler

• The furnace is placed outside the boiler shell, and heat is transferred through tubes.
• Example: Babcock and Wilcox boiler.

5. According to the Axis of the Boiler

Boilers can also be classified based on their orientation:

1. Vertical Boilers

• The shell is positioned vertically.
• Compact design and occupies less space.
• Example: Cochran boiler.

1. Horizontal Boilers

• The shell is placed horizontally.
• Suitable for high steam generation rates.
• Example: Lancashire boiler, Babcock and Wilcox boiler.

6. According to the Number of Tubes
7. Single-Tube Boiler

• Has only one flue gas tube.
• Example: Cornish boiler.

1. Multi-Tube Boiler

• Contains multiple tubes for greater heat transfer area and efficiency.
• Example: Lancashire boiler, Locomotive boiler.

7. According to the Circulation of Water
8. Natural Circulation Boiler

• Water circulation is due to natural convection currents created by temperature differences.
• Example: Lancashire boiler, Babcock and Wilcox boiler.

1. Forced Circulation Boiler

• Uses a pump to circulate water through tubes.
• Produces steam at higher pressure and rate.
• Example: Lamont boiler, Benson boiler.

8. According to the Use or Application
9. Stationary Boilers

• Fixed in one place and used for power generation in industries and power plants.
• Example: Lancashire boiler, Babcock and Wilcox boiler.

1. Portable Boilers

• Can be easily moved and used for temporary or mobile applications.
• Example: Locomotive boiler, Marine boiler.

1. Marine Boilers

• Used in ships and vessels to generate steam for propulsion.
• Example: Scotch marine boiler.

1. Locomotive Boilers

• Mounted on trains and used to generate steam for engine movement.
• Example: Locomotive boiler.

9. According to the Type of Steam Generated
10. Saturated Steam Boiler

• Produces steam that is saturated, i.e., contains moisture.
• Example: Simple fire-tube boilers.

1. Superheated Steam Boiler

• Produces dry steam at a temperature higher than its saturation point using a superheater.
• Example: Benson boiler, Lamont boiler.

Conclusion

The classifications of boilers are made based on different design, operational, and application characteristics such as pressure, fuel type, and fluid flow. This classification helps in selecting the most efficient and suitable boiler for a specific industrial or power generation need. Boilers such as fire-tube and water-tube are the most common, while modern plants prefer high-pressure, superheated, and forced-circulation boilers for better performance. Understanding boiler classifications ensures safe operation, high efficiency, and proper maintenance in various engineering applications.