Scotch Marine Boiler: 7 Important Facts You Should Know


What Is a Scotch Marine Boiler?

A Scotch Marine boiler is an example of a fire tube boiler wherein the flue gases flow through tubes that are placed in a tank of water. The Scotch Marine boiler has adapted its working principle from the Lancashire boilers which consist of numerous furnaces to increase the heating capacity. Two main differences set Scotch Marine Boiler apart from Lancashire boilers and they are

  • Scotch Marine boilers are made up of multiple fire tubes to increasing the heating capacity per cross-sectional area
  • Scotch Marine boilers are half the size of Lancashire boilers as the path for the flue gas to flow has been packed in the available location.

Scotch Marine Boiler Design

A Scotch Marine boiler is a fire tube boiler that is used in ships. The boiler is shaped like a horizontal cylinder with furnaces at the lower portion of the boiler. Above the furnace, there are many fire tubes. The heat and smoke from the furnace flow to the boiler. The fire tubes have a capping at the end which is known as a smoke box on the outer surface of the boiler shell.

Scotch Marine Boiler Parts

The Scotch Marine Boiler consists of the following parts which are

  • A Furnace: The space below the boiler is where the furnace is located. A boiler with a single end is usually equipped with four furnaces. The furnaces are corrugated for strength, and they have separate combustion chambers for each furnace.
  • A Combustion Chamber: This is the area below the shell of the boiler where the fuel is burnt to generate steam from the water. This chamber is made up of four layers of plates which are top plate, backplate, tube plate, and two-sided plates.
  • A Smoke Box: This unit has several tubes packed together horizontally and connects the combustion chamber and the chimney. The flue gases from the combustion chamber pass through these tubes.
  • A Chimney: It is used for releasing the flue gases from the combustion chamber into the environment.
  • A Boiler Shell: This part of the boiler consists of cylindrical plates which are welded together to contain sufficient water and steam. The layout also helps in protecting the inner parts of the boiler. The fitting of the boiler is usually attached to the boiler shell.

Scotch Marine Boiler Diagram

Image Credit: Image from page 1026 of “Canadian forest industries 1901-1… | Flickr

Scotch Marine Boiler Working

Scotch Marine boilers can be single-ended or double-ended. A single-ended boiler is usually composed of somewhere between one to four furnaces while a double-ended boiler has furnaced on either end of the boiler with somewhere between two to four boilers. It is equipped with a horizontally placed drum which is about 2.5 to 3.5 meters in diameter.

The water that passes through the tubes is heated by the flue that is burned in the combustion chamber of the furnace. The flue gases which are produced from the burning of fuel passes on the outer surface of the tubes to the chimney. This adds additional heat to the tubes thereby providing faster steam generation and better quality of steam. The flue gases are then released into the environment through the furnace

Advantages of Scotch Marine Boiler

The main advantages of Scotch Marine Boiler are as below:

1. The shell of these boilers is large which makes it easy for blowdown and water treatment processes.

2. The working fluid for this type of boiler is primarily water of any type not necessarily purified or distilled water.

3. This type of boiler has high efficiency making it suitable for heavy load.

4. The cost of construction is minimal as it does not require a brickwork model or any external flues.

Disadvantages of Scotch Marine Boiler

The main disadvantages of Scotch Marine Boiler are as follows

1. This type of boiler cannot easily control the variations in load.

2. The steam quality is not comparable to water tube boilers

3. They require larger floor space depending on the load that needs to be handled

4. The diameter of the boiler is the reason why the pressure from this type of boiler cannot exceed 300psi.

Frequently Asked Interview Questions and Answers

1. How does a scotch marine boiler work?

The working principle of a scotch marine boiler is simple. The fuel that is used to heat the water will be burnt in a combustion chamber. The fuel is fed to the combustion chamber through a fire hole. Through the convection heat transfer process, the heat produced by burning the fuel is transferred to the water in the chamber surrounding the combustion process.

The water is then converted into steam and supplied to the steam turbine. The flue gases that were produced during the combustion process are released into the environment via the smoke tube into the boiler chimney. Here, water carried away the heat in the exhaust gases passing through the smoke tubes

2. What are the types of boilers?

Several types of boilers are used in the industries. A list of the commonly used boilers are jotted down below:

  • Shell and Tube boiler
  • Lancashire boiler
  • Locomotive boiler
  • Wet Back Boilers
  • Dry Back Boilers
  • Cornish Boiler
  • Scotch Marine Boiler
  • Packaged Boiler
  • Reversal Chamber
  • Two-Pass Boiler

3. Which is better fire tube boiler or water tube boiler? | Between fire tube and water tube boilers which has more advantages and why?

Among, the two types of boilers i.e., the fire tube boiler and the water tube boiler, the water tube boiler is more efficient than the latter due to the following reasons

  • The amount of water that is used in the water tube boiler is comparatively lesser than the amount used in a fire tube boiler which in turn results in the quicker generation of steam and less fuel requirement in the case of the water tube boiler.
  • Since they require less amount of water for the boiling process, the design of the boiler is compact and environmentally friendly.
  • They respond quickly to changes in load i.e., need of steam. Units that are configured in terms of modules can be charged up and down depending on the required amount of steam.
  • Their increased efficiency and excellent performance are also attributed to their ability to last longer in comparison to their counterpart.
  • It is safer to operate a water tube boiler in comparison to a fire tube boiler as they are internally fired.
  • They occupy small floor space and are usually used in large power plants because of their efficiency and higher steam production.

4. What improvements did the Yarrow Water Tube Boiler have over the Scotch Marine Boiler?

The Yarrow Water Tube Boiler was built with a different concept of working than the Scotch Marine Boiler. A Scotch Marine boiler produces large volumes of steam which is at low pressure. On the other hand, a Yarrow Water Tube Boiler was built to produce fewer volumes of high-pressure steam. It is difficult to compare the merits and demerits of these two types of boilers.

A Yarrow Water Tube boiler is usually used for marine applications with a newly installed turbine not because of the high pressure it produces but also because of its compact sizing and limited maintenance that is required for it to work. While Scotch Marine boilers are often installed in equipment with piston technology which had to undergo timely repair and involves high maintenance.

5. Can anyone explain to me about marine boiler and types of marine boiler

Marine boilers are used for marine applications, whereby the heat produced by the fuel is used for running the ship. The working principle of a marine boiler is to change the state of fluid from liquid to vapor. The temperature of the fluid in the boiler is transformed from liquid to vapor in an enclosed vessel to avoid loss of energy into the surrounding. The heating is carried out in furnaces to ensure that heat is transferred to the operating fluid.

There are two main categories of marine boilers which are

  • Water Tube Boilers
  • Fire Tube Boilers

6. What is the function of a pressure gauge in a boiler?

The primary function of a pressure gauge in any type of boiler is to indicate the pressure build-up inside the drum of the boiler which is often represented as kN/m2.

7. What are stay tubes in a boiler?

In a fire tube or water tube boiler, stay tubes provide support and stability to the endplates and the normal tubes. The stay tubes have a greater wall thickness and strong construction to withstand the high surrounding temperature. They have a greater diameter and are welded to the plates on both the upper well as the lower region. The steam is collected in a region made up of shell-like structures and an internal cone.

8. Why are fire tube boilers not suitable for high pressures?

Fire-tube boilers require large volumes of water due to which the steam pressure is produced after a long time. Since both steam and water are contained in a single vessel, the pressure of the steam produced by this type of boiler is not very high and not very dry.

9. What are the different types of water tube boilers and fire tube boilers?

The different types of water tube boilers used in industries are as follows:

  • Sterling boiler
  • Simple Vertical Boiler
  • Babcock and Wilcox Boiler

The different types of fire tube boilers used are as given below:

  • Immersion Boiler
  • Scotch Marine Boiler
  • Cochran Fire Tube Boiler
  • Lancashire Boiler
  • Cornish Fire Tube Boiler
  • Locomotive Boiler

10. Why are economizers used in boilers?

Economizers are devices used in boilers that use the heat from exhaust gases to preheat the cold water entering the boiler. They are heat exchangers aimed at increasing the temperature of the fluid beyond its boiling point and thereby reducing energy used.

There are other types of boilers such as Benson Boiler, Cochran Boiler, Babcock and Wilcox boiler, and AFBC boilers that are widely used in the industry