Scotch Marine Boiler – Diagram, Parts, Working, Advantages, Disadvantages

Scotch Marine Boiler

The marine steam boilers of the scotch or tank-type are used for marine works, particularly, due to their compactness, efficiency in operation, and their ability to use any type of water. It does not require brickwork settings and external flues.

Scotch marine boiler is a famous boiler, the first one having been installed in a ship in about 1862 and up until around 1900 was practically the only type boiler found aboard merchant or Navy ships. At that time water tube boilers began to come into use but for a number of years the Scotch marine still remained the dominant boiler. With the advent of modern high-pressure power plants the water tube boiler became a necessity. However, there are still a considerable number of older American ships with Scotch boilers.

Main Components of Scotch marine boilers :


The fuel in the Scotch boiler is burned in a cylindrical steel furnace located inside the water space of the boiler. The number of furnaces depends on the size of the boiler, there usually being three or four.

Combustion Chamber-

The furnace opens into a combustion chamber which is simply a rectangular steel box standing on end and surrounded with water.

In the combustion chamber the unburned gases, given off from the burning fuel in the furnace, mix with air and burn.


The tubes are made of seamless drawn steel, a popular size being 3 1/4 inch outside diameter which is the way all boiler tubes are measured.

The Scotch marine boiler has a two-pass (or more) arrangement of tubes that run horizontally to allow the heat inside the tubes to travel back and forth. It also has an internally fired furnace with a cylindrical combustion chamber.


A small proportion of the tubes, scattered among the firetubes, are staytubes. They are heavier tubes and are threaded into the tube sheets to give added support to the flat tube sheets and heads.

Working Of Scotch Marine Boiler :

It has a drum of diameter from 2.5 to 3.5 meters placed horizontally. These steam boilers may be single-ended or double-ended. The length of a single-ended steam boiler may be up to 3.5 meters while for double-ended up to 6.5 meters. A single-ended boiler has one to four furnaces which enter from fronb.end of the boiler. A double-ended boiler has furnaces on both of its ends and may have furnaces from two to four in each end.
A single-ended scotch marine steam boiler is fired by four furnaces, as shown in Fig.  The furnaces are generally corrugated for strength. Each furnace has its own combustion chamber. There are fine flat plates in the combustion chamber, which require staying, i.e. the top plate, backplate, two side plates, and the tube plate. There are a number of smoke tubes placed horizontally and connect the combustion chamber to the chimney. The front and back plates of the shell are strengthened by longitudinal stays.

Scotch Marine boiler Diagram : 

Scotch marine boiler diagram
Scotch marine boiler diagram

The combustion chamber walls form the best heating surface. The furnace tubes, smoke tubes, and the combustion chamber, all being surrounded by water, give a very large heating surface area in proportion to the cubical size of the boiler. The water circulates around the smoke tubes. The level of water is maintained a little above the combustion chamber. The flue gases, from the combustion chamber, are forwarded by draught through the smoke tubes, and finally up the chimney. The smokebox is provided with a door for cleaning the tubes and smokebox.

Advantages of the Scotch Marine Firetube Boiler :

Advantages of the scotch marine boiler are as follows,

1. Most of these boilers are built with a lot of heating surfaces, usually 5 square feet of heat transfer surface per boiler HP. This provides higher steady-state efficiency and good resistance to fouling. A Watertube boiler will usually have less heat transfer surface, but often add economizers to overcome this difference. This is why Firetube boilers typically have the best steady-state efficiency.
2. Because a large diameter shell contains the water and steam, these boilers have a large steam chest and disengaging area that provides good steam quality without the addition of mechanical separation.
3. The furnace can be made large enough to support a variety of different fuels and emission requirements.
4. The large shell space allows for a fair amount of sediment accumulation without hurting the vessel, providing more flexibility in water treatment and blowdown maintenance. This is why the Firetube boiler is considered to be more forgiving.
5. The boiler has large energy content (a combination of the steam and water at saturated temperature) that allows it to generate a higher steam flow rate for short periods with minimal steam pressure variations.

Disadvantages of the Scotch Marine Firetube Boiler :

The Scotch marine boiler has the following disadvantages,

1. Most Scotch Marine Firetube Boilers require a slow warmup cycle that can take several hours. Multiple pass boilers are subject to “Thermal Shock”, and if the warmup cycle is not done properly, failures will occur over time. The larger (longer) the boiler, the bigger
the problem, because the difference of growth between the furnace, tubes and shell will be larger.
2. The longer warmup cycle often requires backup boilers to be kept hot for quicker response. This adds cost and complicates the operation of a plant.
3. Because the steam pressure is contained by the large-diameter shell, it has a limited pressure capacity, usually around 300 PSI.
4. The large steam and water content means that more energy is required to bring the unit on-line. If the boiler operates infrequently, that energy (or a portion of that startup energy) will be lost in the off time, which is reflected in the lower dynamic efficiency.

Application of Scotch Marine Boiler

  • This boiler is commonly used in marine works. The reason for this is its compactness, efficiency in operation, and ability to operate in any type of water.
  • This boiler is used on ships.

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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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