Power Plant Engineering

What is a Scotch Marine Boiler? How Does It Work?


If we see the construction and design of a Scotch marine boiler, then it is quite similar to the Lancashire Boiler, which we completed earlier. Like the Lancashire boiler, it has also a greater heating surface due to multiple separate furnaces. Also, it differs from the Lancashire boiler in two aspects: one is by using a small diameter tube and another one is the length of the boiler. Today we will discuss the working features of the Scotch marine boiler. Let’s start with the definition.

What is a Scotch Marine Boiler

According to the position of the water and hot gases, the Scotch Marine Boiler is a sub-type of Fire tube Boiler. Similarly, if we consider the axis of the boiler shell, then it is considered a horizontal boiler. Apart from that, the combustion chamber is present inside the boiler, so this is also an internally fired boiler. Normally we could transport these boilers from one place to another, so it is also Mobile boiler.

It is a multi-tubular boiler because multiple tubes are present inside the boiler. The flow of feedwater inside the boiler is done by natural circulation. Two furnaces are present inside this boiler so it is also a Dual furnace boiler. This boiler is commonly known as the Scotch boiler and is widely used in the marine sector. So this type of boiler does not require any brickwork settings and external flues.

Scotch Marine Boiler Diagram


Scotch Marine Boiler Parts

The important parts of this boiler are described below.

  • Furnace: Normally the firebox is placed over the grate surface of this boiler. In other words, you can say that it plays a mediator role between the fire tubes and the combustion chamber. This furnace is designed in such a way that it deflects back the un-burnt charge particle and directs the flue gases into the combustion chamber.
  • Grate: It is like a base or floor section inside the combustion chamber, where the fuel (solid fuel e.g. coal) is burnt. This base plate is made up of cast iron. Why? Because of its high heat capacity. You can see that there is some space between them so that air can pass through them. The whole surface area of a grate where the fuel is burnt is known as a grate surface.
  • Combustion Chamber: This is a closed chamber, that is placed inside the boiler and is connected with the fire tubes respectively. Through this chamber, the hot flue gases pass into the fire tubes. This chamber provides greater heat to the boiler because of its greater surface.
  • Smokebox: This is a box-like structure where the smoke of burnt fuel and exhaust steam from the shell gets collected. These are exhausted into the atmosphere through the short chimney.
  • Chimney: One part of the chimney is opened to the atmosphere and another part is connected to the smokebox of the boiler. The hot flue gases come out from fire tubes to the smokebox and exhaust out to the atmosphere through this open chamber which is also called a chimney. Here the size of the chimney is very small.
  • Fire tubes: The fire tubes are horizontal tubes whose one end is joined with the combustion chamber and another with the smokebox. The flue gases flow from the combustion chamber to the firetubes respectively and then exhaust out into the atmosphere via a chimney. These firetubes are used to exchange heat energy from their surface to the water by the convection process. As this is a firetube boiler, the regular flow of hot gases could damage its strength and longevity. So to make these tubes more reliable, they are made up of seamless steel e.g. carbon steel or alloy steel. The materials of these tubes are different for low-pressure, medium-pressure as well and high-pressure boilers.
  • Stay tubes: It is designed in such a way that it supports the endplates of the boiler. Here these tubes are heavier as compared to the fire tubes because the wall thickness of the stay tube is higher than the normal flue tubes.
  • Safety valve: The safety valve is a mechanical device and also the most important part of a scotch marine boiler’s construction. It preserves human life as well as property like ships. Normally it prevents the explosion when there is an excessive internal pressure of steam inside a boiler. This mechanical device is designed in such a way that it automatically opens and releases the excess internal pressure. Then it closes automatically when the internal pressure decreases to the prescribed value.
  • Whistle valve: This valve is used for controlling and operating a whistle when the inside pressure of a boiler is more than the outer atmospheric pressure. Basically, this is an alert sound for an operator.
  • Pressure gauge: This indicates the pressure of steam in the boiler. It is mounted on the top portion of a Scotch Marine Boiler. The main purpose of this mechanical device is that it used for monitor the internal pressure of a boiler. In most cases, Bourdon-type pressure gauges are widely used in a steam boiler.
  • Manhole: This hole is present in the upper part of the boiler. It is provided for inspection, cleaning, and repair of the interior part of the boiler shell. But the question is why is it named so? Because through this hole, a man is entered into a boiler, and he can easily identify the integral defects. The size of the manhole is equal to the size of a human.
  • Soot blower: Here soot means a black powder consisting of amorphous carbon or you may say some kind of dust. A soot blower is a mechanical device that is used for removing the soot that is deposited on the internal tubes of a boiler. It is directly related to the efficiency of a boiler. That’s why we should keep these tubes clean so that the hot gas passes smoothly.

Scotch Marine Boiler Types

Generally, the Scotch Marine Boiler is classified into two types. e.g.

According to the placement of the furnace

  • Single-ended scotch marine boiler: It contains one to four furnaces i.e. corrugated fire tubes and these are placed at the front part of the boiler. In standard cases, the length of a single-ended steam boiler can be 3.5 meters.
  • Double-ended scotch marine boiler: It contains two to four furnaces at both ends of the scotch boiler. In standard cases, the length of a double-ended steam boiler can be 6.5 meters.

According to the design

  • Wetback: In this type, the rear surface of the combustion chamber is water-jacketed and it is also considered as a heating surface.
  • Dryback: In this type, the rear surface of the combustion chamber is an open box or you may see that it is surrounded by a sheet metal jacket. Here the chances of heat loss are more as compared to the wetback type boiler. Thus efficiency also reduces here.

Scotch Marine Boiler Application

Scotch Marine Boilers are mostly used in marine works and ships. Why? because it can work in any type of water. For this, it shows an idle efficiency.

Scotch Marine Boiler Working

When it comes to the working principle, the process is very simple as shown in the above figure. The oil burner injects fuel into the furnace through the fire hole. The furnaces are corrugated which gives strength to the boiler and also ensures the boiler’s long useful life. Each furnace has its own combustion chamber. The fuel is ignited in the furnace and entered into the combustion chamber. A great heating surface area is present inside the Scotch Marine Boiler, which includes the flue tubes, smoke tube, and combustion chamber.

The heat energy is generated by the burning of fuel and finally, the heat is transferred to the surrounding water through the surface of the combustion chamber, flue tubes, and smoke tubes respectively. Now the surrounding water is converted into saturated steam. This steam is stored inside a rectangular shell, placed just above the flue tubes. The outlet/ delivery pipe is connected to a steam turbine, which is driven by the steam. Then the flue or combustion gases move towards the chimney via flue tubes. Here it exhausted out and mixed with the atmosphere.

Advantages of Scotch Marine Boiler

The advantages of this boiler are given below.

  • Generally in this type of boiler brickwork settings and external flues are not required.
  • This boiler can work in any type of water.
  • That’s why this is widely used in the marine sector.
  • When it comes to performance, it has good efficiency.
  • Also, this is compact in size.

Disadvantages of Scotch Marine Boiler

The disadvantages of this boiler are given below.

  • It usually requires more floor area.
  • This boiler has a limited pressure capacity.

This is all about the “Scotch Marine Boiler”. Thanks for giving your valuable time. If you have any doubts related to this boiler please don’t hesitate, to ask in the comments section below. If you have any prior experience related to this boiler and know more about it, please share it with us. We promise that we will solve your doubts as soon as possible. And if you really like this article, don’t forget to share it with your colleagues, and friends. Good Luck!

MechStudy Team

MechStudy Team

About Author

Leave a comment

Your email address will not be published. Required fields are marked *

You may also like

Power Plant Engineering

What is a Steam Boiler? How Does it Work?

A steam boiler represents a closed vessel designed to transform fluid, typically water, into steam through the utilization of heat
Power Plant Engineering

What is a Fire Tube Boiler? How Does It Work?

In a Fire Tube Boiler, hot flue gases typically flow through small tubes that are surrounded by or immersed in