Boilers vary considerably in detail and design. Most boilers may be classified and described in terms of a few basic features or characteristics. Some knowledge of the methods of classification provides a useful basis for understanding the design and construction of the various types of naval boilers.

In the following paragraphs, we have considered the classification of naval boilers according to intended service, location of fire and water spaces, type of circulation, arrangement of steam and water spaces, number of furnaces, burner location, furnace pressure, type of superheaters, control of superheat, and operating pressure.

INTENDED SERVICE

A good place to begin in classifying boilers is to consider their intended service. By this method of classification, naval boilers are divided into two classes, PROPULSION BOILERS and AUXILIARY BOILERS. Propulsion boilers are used to provide steam for ships' propulsion and for vital auxiliaries' services. Auxiliary boilers are installed in diesel-driven ships and in many steam-driven combatant ships. They supply the steam and hot water for galley, heating, and other hotel services and for other auxiliary requirements in port.

LOCATION OF FIRE AND 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, FIRE-TUBE BOILERS and WATER-TUBE BOILERS. In the fire-tube boilers, the gases of combustion flow through the tubes and thereby heat the water that surrounds the tubes. 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 naval ships are of the water-tube type. Auxiliary boilers may be either fire-tube or water-tube boilers.

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 CIRCULATION BOILERS.

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 naval boilers are designed for accelerated natural circulation. In such boilers, large tubes (3 inches 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 naval boilers.

Forced circulation boilers 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.