Short Answer:

A water-tube boiler is a type of boiler in which water flows inside the tubes and hot flue gases flow outside the tubes. The heat from the flue gases is transferred through the tube walls to the water, which turns into steam. This type of boiler is suitable for high-pressure and high-temperature applications.

In simple words, a water-tube boiler uses tubes filled with water, and the hot gases from fuel combustion surround these tubes. It can produce steam at higher pressure and temperature, making it ideal for power plants and large industrial applications where large amounts of steam are required.

Detailed Explanation :

Water-Tube Boiler

A water-tube boiler is an advanced type of boiler designed for generating steam at high pressure and high temperature. In this boiler, water circulates inside the tubes, and hot flue gases flow outside these tubes. The heat from the combustion gases is transferred through the metal tube walls to the water, converting it into steam.

Water-tube boilers are mainly used in thermal power plants, marine engines, and industrial processes where large quantities of steam are required. They are preferred over fire-tube boilers when high efficiency and rapid steam generation are needed. Some common examples of water-tube boilers include the Babcock and Wilcox boiler, Stirling boiler, and Lamont boiler.

1. Working Principle of Water-Tube Boiler

The working of a water-tube boiler is based on the transfer of heat energy from hot flue gases to water flowing inside the tubes.

The process can be explained as follows:

1. The fuel (such as coal, oil, or gas) is burned in the furnace, producing hot gases.
2. These hot gases flow over the tubes filled with water.
3. As the hot gases pass across the tubes, heat is transferred from the flue gases to the water through conduction and convection.
4. The water in the tubes absorbs this heat and gradually converts into steam.
5. The generated steam is collected in the steam drum at the top of the boiler.
6. The steam can be saturated or superheated depending on the design and use.

In most modern water-tube boilers, forced circulation of water is used to enhance heat transfer and improve efficiency.

2. Construction of Water-Tube Boiler

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

1. Steam Drum:
   • A large cylindrical vessel located at the top of the boiler.
   • It stores steam and maintains the separation between water and steam.
2. Water Drum (Mud Drum):
   • Located at the bottom of the boiler.
   • It collects sediments and impurities from the circulating water.
3. Water Tubes:
   • Tubes that carry water from the water drum to the steam drum.
   • These tubes are surrounded by hot flue gases, which heat the water inside.
4. Furnace:
   • The combustion chamber where fuel is burned to produce heat.
   • It is lined with refractory material to resist high temperature.
5. Superheater:
   • Used to heat the steam further after it is produced, increasing its temperature and efficiency.
6. Economizer:
   • A heat recovery device that preheats the feedwater using exhaust flue gases before entering the boiler.
7. Air Preheater:
   • Preheats the air used for combustion, improving fuel efficiency.

These parts work together to achieve continuous water circulation, efficient heat transfer, and high steam generation.

3. Working of Water-Tube Boiler

The operation of a water-tube boiler involves the following steps:

1. Fuel Combustion:
   • The fuel is burned in the furnace, producing hot gases at high temperature.
2. Heat Transfer:
   • The hot gases flow over the water-filled tubes, transferring heat to the water inside.
3. Steam Formation:
   • As water absorbs heat, its temperature increases until it boils and converts into steam.
4. Steam Collection:
   • The generated steam moves upward into the steam drum, where water droplets are separated from dry steam.
5. Steam Supply:
   • The dry steam is then directed to turbines, engines, or industrial equipment for use.
6. Circulation:
   • In natural circulation boilers, water moves due to density differences between hot and cold water.
   • In forced circulation boilers, a pump is used to circulate water rapidly through tubes.

This cycle continues as long as fuel is supplied and water is available, ensuring a constant steam supply.

4. Features of Water-Tube Boiler

• Water flows inside the tubes, and hot gases surround the tubes.
• Produces high-pressure steam (above 25 bar).
• Generates steam quickly due to large heating surface area.
• Can be designed for superheated steam production.
• Suitable for large power plants and industrial operations.

5. Advantages of Water-Tube Boiler

1. High Pressure Operation:
   • Can produce steam at very high pressures and temperatures.
2. High Efficiency:
   • More efficient heat transfer because water is inside small tubes, providing a large surface area.
3. Rapid Steam Generation:
   • The small amount of water and large heating area result in quick steam production.
4. Compact and Lightweight:
   • Occupies less floor space and is lighter than fire-tube boilers of the same capacity.
5. Safe Operation:
   • Contains less water, so the risk of explosion is reduced.
6. Easy to Transport and Install:
   • Modular designs allow easy transportation and assembly.
7. Better Control:
   • Pressure, temperature, and flow can be easily controlled for different load conditions.

6. Disadvantages of Water-Tube Boiler

1. High Initial Cost:
   • More expensive to manufacture and install than fire-tube boilers.
2. Complex Design:
   • Consists of many tubes, drums, and components, making it complicated.
3. Requires Skilled Operation:
   • Needs trained personnel for operation and maintenance.
4. Difficult Cleaning:
   • The small water tubes are difficult to clean and maintain.
5. Frequent Inspection Required:
   • Scaling or leakage in tubes can affect performance and safety.

7. Examples of Water-Tube Boilers

1. Babcock and Wilcox Boiler:
   • A stationary, high-pressure, horizontal boiler widely used in industries and power plants.
2. Stirling Boiler:
   • Consists of multiple drums and a large heating surface for improved efficiency.
3. Lamont Boiler:
   • A forced circulation boiler used for high-pressure operations.
4. Benson Boiler:
   • A supercritical boiler that operates without a steam drum.

8. Applications of Water-Tube Boiler

Water-tube boilers are widely used in:

• Thermal power plants for electricity generation.
• Marine boilers for propulsion and auxiliary power.
• Industrial plants for process heating and power.
• Chemical and refinery industries requiring high-pressure steam.
• Supercritical and ultra-supercritical power systems for high efficiency.

Conclusion

A water-tube boiler is a high-pressure steam-generating boiler in which water circulates inside tubes surrounded by hot gases. It provides efficient heat transfer, rapid steam generation, and safe operation at high pressures, making it suitable for power plants and industrial use. Although it is more expensive and complex than fire-tube boilers, its higher efficiency and performance justify its use in modern engineering applications. The Babcock and Wilcox boiler is one of the most widely used examples of this type.