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Introduction to Marine Boilers: Types and Principles


There are two kinds of marine boilers: water tube boilers and fire tube boilers. Water-tube boilers are used for applications requiring high pressure, high temperature, and high capacity.

The primary boiler design is determined by the application, i.e. A small fire-tube boiler would be sufficient for motor ships. A boiler is a piece of machinery that has been used in shipping since the days when there were no auxiliary engines.

Boilers were used at the time to perform all of the major functions that propulsion and power generation systems now do. Various types of machines performed important tasks on ships as technology progressed; however, an advanced boiler remains an important part of the engine power plant and cargo operations.

The materials used to manufacture the boilers should be such that they can withstand immense steam pressure and extreme temperatures while causing minimal damage to the environment.

Carbon steel is commonly used to construct pressure vessels using the open stove, electric, or pneumatic processes.

Important considerations for boiler manufacturing include testing the strength of the material used in accordance with regulatory requirements.

Principle of Marine Boiler

The marine boiler operates on the basic Marine Engineering principle of using heat energy to change the state of water from liquid to vapour. The water is boiled in enclosed pressure vessels so that the produced steam does not escape into the atmosphere.

The heating source is contained within insulated furnaces to ensure that heat energy is transferred primarily to the water and not to the surrounding area. Depending on the size and type of ship, different boilers are used.

Types of Marine Boilers:

Smoke Tube Boiler

The hot smoke produced by the boiler furnace passes through several tubes surrounded by water in the boiler drum, as the name implies. The heat generated by the smoke in the tubes creates steam. 

In systems with low steam volume and quality, smoking tube boilers are used. There are two types of smoke tube boilers on ships: horizontal or Scotch boilers and vertical smoking tube boilers.

Vertical Smoke Tube Boiler

Vertical smoke tube boilers are more commonly found on ships. The furnace is located at the bottom of the boiler and is connected to the shell via an ogee ring located at the bottom.

The furnace contains the heat generated inside the furnace and protects the ogee ring from overheating. Above the furnace are the combustion chambers with tube stacks, where hot smoke is passed to heat the water drum.

A gusset plate that transfers tension to the boiler’s shell supports the top of the combustion chambers. Fire tube boilers have competitive steam rates of up to 12,000 kg/hr and pressures of up to 18 kg/cm2. A fire-tube boiler is available for use with oil or gas.

Horizontal Smoke Tube Boiler

This is an old type of boiler with a cylindrical furnace inside the shell that is surrounded by water. The furnace in such a boiler is water-cooled, larger in diameter, and thicker. If the boiler is not operated properly, this causes thermal stress. Endplates, as opposed to burner assemblies, are used in horizontal smoke tube boilers.

These flat-end plates, which are supported by a large amount of steam, remain in the upper part of the boiler. The combustion chamber is supported by remaining on top of the girder and stopping the combustion chamber. The combustion chamber’s stagnation also supports the mid-section backplate and tube plate.

Water Tube Boiler

Water tube boilers are steam boilers that use water to heat the tubes as well as hot gases. These boilers, unlike fire tube boilers, can produce high pressure as well as high steam.

Water-tube boilers are second-generation boilers that differ from smoking tube boilers in their construction and operation. Since the boilers’ taps are filled with water rather than smoke, the thermal stress produced is lower than that produced by faucet hose boilers. Where there is a high demand for steam quality and quantity, this type of boiler is used.

All high-pressure boilers are primarily water tube boilers with three major sections:

  • Water Drum: This is located at the bottom of the boiler and transports hot water. The same water is supplied to the steam drum, which produces steam.
  • Steam Drum: Located in the top half of the boiler, it is equipped with external and internal mounting, as well as safety valves and steam stop valves (main and auxiliary) to supply steam to the ship’s system.
  • Tubes: Different types of water tubes with different functions are used in modern water tube boilers. The most common types are:
  • Screen tubes: These tubes, which are installed prior to the superheater, are adjacent to the furnace and absorb the flame as well as heat the generated gas. Because this tube is directly exposed to the furnace’s heat, it has a large diameter to prevent overheating.
  • Superheater tubes: These are used to transport superheated steam from the boiler behind a screen tube.
  • Generating tubes: The generating tubes connect the water and steam drums. Hot water is transported from the water drum to the steam drum, where steam is produced, as the name implies.
  • Downcomers: Downcomers tubes transport hot water that does not enter the water drum to the generating tube, where it is heated before returning the steam to the drum.

Principle of Water Tube Boiler

Water tube boilers work on the convection and radiation principles. The majority of modern water tube boiler designs operate at extremely high pressures with capacities ranging from 4,500 to 120,000 kg/h. Many water tube boilers that use oil or gas as fuel are of “pack” design.

The primary characteristics of a water tube boiler are as follows:

  • Draft provisions that are forced, driven, and balanced aid in improving combustion efficiency.
  • Water treatment plants have a low tolerance for water quality.
  • Thermal efficiency is high.
  • Rating for high pressure
  • Better Steam Quality.
  • Ideal for high-capacity systems.

Composite Boiler

Composite boilers, which use the exhaust heat of the main engines or generator engines to heat water or generate steam, are another common type of boiler found on ships.

The composite boiler is technically a combination of an exhaust gas boiler and an oil-fired boiler. A composite boiler is a two-sectioned water hose or smoking tube boiler. The burner heats one section of the boilers, while

Exhaust Gas Boiler

Heat recovery units and water tube boilers are examples of exhaust gas boilers and economizers. Engine exhaust gases are used to heat water in tubes at a specific temperature and pressure. Circulation pumps are used to constantly circulate hot water through the economizer’s water tubes.

The auxiliary steam boiler serves as a steam separator for the entire system, keeping it warm and ready to go. When the main engine is running at full power, the economizer’s excess steam is thrown back into the hot well via a steam dump condenser.

The exhaust boiler features a vertical register of double steel tubes with welded steel gills. It provides a strong, rigid structure while also preventing vibration damage.

The full exhaust boiler suffers from thermal expansion because hot gases flow from the boiler trunk to heat the water. This is controlled by fitting different tubes and plates. The tube register, tube support, and tube bends are all oriented in the same direction as the exhaust gas flow.

One auxiliary beam is fixed on one side of the inlet header and one on one side of the outlet header. For thermal expansion, the other end is kept free of the supporting beam.

Footplates are installed on boiler foundations to support the structure and absorb thermal expansion. The style has a raised eyeball to the steel structure on the top side for added robustness and support.


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