Short Answer:

A fire-tube boiler is a type of boiler in which hot gases pass through tubes, and the water surrounds these tubes. The heat from the hot gases is transferred through the tube walls to the surrounding water, converting it into steam. It is commonly used for low and medium-pressure applications.

In simple words, a fire-tube boiler is one where the hot flue gases from fuel combustion flow inside tubes, while the water to be heated fills the outer shell. These boilers are simple in construction, easy to operate, and widely used in small power plants, heating systems, and industries.

Detailed Explanation :

Fire-Tube Boiler

A fire-tube boiler is one of the oldest and simplest types of steam-generating boilers. In this boiler, the hot gases produced by burning fuel pass through a series of tubes that are surrounded by water. The heat from these gases is transferred through the walls of the tubes to the water, thereby converting it into steam. This type of boiler is mainly used where low to medium steam pressure is required.

The fire-tube boiler is compact, reliable, and easy to maintain. It is ideal for small-scale industries, workshops, and heating systems due to its low initial cost and simple design. Some common examples of fire-tube boilers include the Cochran boiler, Lancashire boiler, and Locomotive boiler.

1. Working Principle of Fire-Tube Boiler

The working principle of a fire-tube boiler is based on heat transfer from hot flue gases to water. When fuel (coal, oil, or gas) is burned in the furnace, it produces hot gases. These gases are made to flow through tubes that are surrounded by water in the boiler shell.

1. The furnace burns the fuel and produces hot flue gases.
2. These hot gases pass through the fire tubes, which are surrounded by water.
3. As the hot gases move through the tubes, heat is transferred through the tube walls to the surrounding water by conduction and convection.
4. The water gradually absorbs heat and turns into steam.
5. The generated steam is collected in the upper part of the boiler shell, known as the steam space, and is then sent for use in engines, turbines, or industrial processes.

The process continues as long as fuel is supplied and combustion takes place in the furnace.

2. Construction of Fire-Tube Boiler

A typical fire-tube boiler consists of the following main components:

1. Boiler Shell:
   • The outer cylindrical body that contains water and steam.
   • Made of steel and designed to withstand pressure.
2. Furnace or Combustion Chamber:
   • The space where the fuel is burned to produce hot gases.
   • It is usually lined with fire bricks to protect the shell from direct heat.
3. Fire Tubes:
   • Tubes through which the hot flue gases flow.
   • These tubes are surrounded by water and transfer heat to it.
4. Grate:
   • A platform made of metal bars located at the bottom of the furnace where solid fuel (like coal) is burned.
5. Smoke Box and Chimney:
   • The smoke box collects the exhaust gases after they pass through the fire tubes.
   • The gases then escape into the atmosphere through the chimney.
6. Mountings and Accessories:
   • Mountings such as pressure gauge, safety valve, water level indicator, and stop valve ensure safe operation.
   • Accessories like economizer and air preheater improve boiler efficiency.

These components work together to ensure effective combustion and efficient steam generation.

3. Working of Fire-Tube Boiler

The working process of a fire-tube boiler can be summarized in simple steps:

1. Fuel is supplied to the furnace, where it burns and generates hot gases.
2. These gases pass through the fire tubes, transferring heat to the surrounding water.
3. The heat causes the water temperature to rise until it begins to boil and form steam.
4. The steam collects in the upper space of the boiler shell.
5. The steam is then supplied through a steam outlet to the required area (such as a turbine, engine, or heating system).
6. The flue gases after giving up most of their heat are discharged through the chimney.

The entire process is continuous, provided fuel and water are adequately supplied.

4. Features of Fire-Tube Boiler

• The heat transfer takes place from hot gases to water through metal tubes.
• It works under low to moderate pressure (up to 25 bar).
• Steam generation rate is relatively low compared to water-tube boilers.
• The design is compact and simple.
• Requires less maintenance and is easy to operate.
• Suitable for small-scale power and heating applications.

5. Examples of Fire-Tube Boilers

Some commonly used fire-tube boilers are:

1. Cochran Boiler:
   • A vertical, multi-tubular boiler.
   • Compact and suitable for small-scale industries.
2. Lancashire Boiler:
   • A horizontal, internally fired boiler with two large fire tubes.
   • Widely used in textile and sugar industries.
3. Cornish Boiler:
   • Similar to Lancashire but has a single large flue tube.
   • Simple and efficient for small power needs.
4. Locomotive Boiler:
   • Used in railway engines.
   • Multi-tubular and portable design.

6. Advantages of Fire-Tube Boiler

1. Simple Construction: Easy to design, construct, and operate.
2. Low Initial Cost: Economical for small-scale applications.
3. Compact Size: Requires less floor space.
4. Low Maintenance: Fewer moving parts and simple cleaning.
5. Reliable Operation: Can be operated easily without highly skilled labor.

7. Disadvantages of Fire-Tube Boiler

1. Low Pressure Limitation: Not suitable for high-pressure operations.
2. Slow Steam Generation: Requires more time to generate steam.
3. Limited Capacity: Produces less steam compared to water-tube boilers.
4. Risk of Explosion: Since large amounts of water are under pressure, a failure can be dangerous.
5. Not Suitable for Large Power Plants: Used only for small and medium applications.

8. Applications of Fire-Tube Boiler

Fire-tube boilers are widely used in:

• Textile and food industries for process heating.
• Power generation in small plants.
• Marine and locomotive engines.
• Heating systems for buildings and workshops.
• Industrial operations requiring moderate steam pressure.

Conclusion

A fire-tube boiler is a type of boiler in which hot gases from fuel combustion pass through tubes that are surrounded by water. It is simple, compact, and suitable for low and medium-pressure steam generation. Although it has limitations in steam capacity and pressure, its reliability, ease of maintenance, and low cost make it ideal for small industries and heating applications. Examples like the Cochran and Lancashire boilers demonstrate its practicality and effectiveness in various engineering uses.