3. Steam Boilers - Classification .... Page 1/1

Some knowledge of the methods of classification provides a useful basis for understanding the design and construction of the various types of boilers. In the following paragraphs, the boilers are classified according to:


-> Intended Service,
-> Location of Fire and Water Spaces,
-> Type of Circulation,
-> Operating Pressure
-> Burner Location,
-> Type of Superheaters,


3.1 Intended Service

A good place to begin in classifying boilers is to consider their intended service. By this method of classification, boilers are divided into two classes,


-> PROPULSION BOILERS, which are used on steam turbine propelled vessels and
-> AUXILIARY BOILERS, which are on diesel propelled vessels and are mainly used for generating steam for process heating (e.g. heating of fuel oil bunkers & hotel servces).


3.2 Location of Fire & Water Spaces

One of the basic classifications of boilers is according to the relative location of the fire and water spaces. By this method of classification, boilers are divided into two classes: Firetube & Watertube Boilers as shown below schematically.


Firetube Boiler Schematic
[Will not be covered in this course]

In the fire-tube boilers, the gases of combustion flow through the tubes and thereby heat the water that surrounds the tubes.		 Fire Tube Boiler - Typical Schematic
Watertube Boiler Schematic
[Will be extensively covered in this course]

In water-tube boilers, the water flows through the tubes and is heated by the gases of combustion that fill the furnace and heat the outside metal surfaces of the tubes. All propulsion boilers used in ships today are of the water-tube type. Auxiliary boilers may be either fire-tube or water-tube boilers. 		Water Tube Boiler - Typical Schematic


3.3 Type of Circulation

Water-tube boilers are further classified according to the method of water circulation. Water-tube boilers may be classified as,


-> NATURAL CIRCULATION BOILERS or
-> FORCED CIR- CULATION BOILERS.

Natural Circulation - Typical Schematic In natural circulation boilers, the circulation of water depends on the difference between the density of an ascending mixture of hot water and steam and a descending body of relatively cool and steam-free water.

The difference in density occurs because the water expands as it is heated, and thus, becomes less dense. another way to describe natural circulation is to say that it is caused by convection currents which result from the uneven heating of the water contained in the boiler. natural circulation may be either free or accelerated.

In a boiler with free natural circulation, the generating tubes are installed almost horizontally, with only a slight incline toward the vertical. when the generating tubes are installed at a much greater angle of inclination, the rate of water circulation is definitely increased.

Therefore, boilers in which the tubes slope quite steeply from steam drum to water drum are said to have natural circulation of the accelerated type. most marine boilers are designed for accelerated natural circulation. in such boilers, large tubes (75 mm or more in diameter) are installed between the steam drum and the water drum. these large tubes, or downcomers, are located outside the furnace and away from the heat of combustion. they serve as pathways for the downward flow of relatively cool water.

When enough downcomers are installed, all small tubes can be generating tubes, carrying steam and water upward, and all downward flow can be carried by downcomers.

The size and number of downcomers installed varies from one type of boiler to another, but downcomers are installed in all marine boilers.


Forced circulation boilers (in marine practice, mostly used as exhaust-gas boilers - Covered in COC Motor Course) are, as their name implies, quite different in design from the boilers that use natural circulation. Forced circulation boilers depend upon pumps, rather than upon natural differences in density, for the circulation of water within the boiler. Because forced circulation boilers are not limited by the requirements that hot water and steam must be allowed to flow upward while the cooler water flows downward, a great variety of arrangements may be found in forced circulation boilers.

3.4 Operating Pressure

The term high-pressure boiler is at present used rather loosely to identify any boiler that operates at a substantially higher pressure than 20 bar boilers (20 bars boilers are typically found on oil tankers with diesel propulsion). In general, any boiler that operates at 50 bar or above are referred to as a high-pressure boiler. Many propulsion boilers on LNG ships operate at about 60 bar with steam temperature of around 500 degree Celsius at the superheater outlet.

3.5 Burner Location

Propulsion boilers are also classified on the basis of where their burners are located. Most burners in older propulsion plants were located at the front of the boiler. These are called FRONT-FIRED BOILERS. Presently most of the marine propulsion burners are located on the top of the boilers. These are called TOP-FIRED BOILERS.

3.6 Type of Superheaters

On almost all boilers used in steam turbine propelled vessels, the superheater tubes are protected from radiant heat by water screen tubes. The water screen tubes absorb the intense radiant heat of the furnace, and the superheater tubes are heated by convection currents rather than by direct radiation. These superheaters are called CONVECTION-TYPE SUPERHEATERS. In a few older ships, the superheater tubes are not screened by water screen tubes but are exposed directly to the radiant heat of the furnace. Superheaters of this design are called RADIANT-TYPE SUPERHEATERS.