Natural circulation water-tube boilers are classified as DRUM-TYPE BOILERS or HEADERTYPE BOILERS, depending on the arrangement of the steam and water spaces. Drum-type boilers have one or more water drums (and usually one or more water headers as well). Header-type boilers have no water drum; instead, the tubes enter many headers which serve the same purpose as water drums.

What is a header, and what is the difference between a header and a drum? The term header is commonly used in engineering to describe any tube, chamber, drum, or similar piece to which tubes or pipes are connected in such a way as to permit the flow of fluid from one tube (or group of tubes) to another. Essentially, a header is a type of manifold or collection point. As far as boilers are concerned, the only distinction between a drum and a header is size. Drums maybe entered by a person while headers cannot. Both serve basically the same purpose.

Drum-type boilers are further classified according to the overall shape formed by the steam and water spaces-that is, by the tubes. For example, double-furnace boilers are often called M-type boilers because the arrangement of the tubes is roughly M-shaped. Single-furnace boilers are often called D-type boilers because the tubes form a shape that looks like the letter D.

NUMBER OF FURNACES

All boilers commonly used in the propulsion plants of naval ships may be classified as either SINGLE-FURNACE BOILERS or DOUBLEFURNACE BOILERS. The D-type boiler is a single-furnace boiler; the M-type boiler is a double-furnace (divided-furnace) boiler.

BURNER LOCATION

Naval boilers are also classified on the basis of where their burners are located. Most burners in naval propulsion plants are located at the front of the boiler. These are called FRONT-FIRED BOILERS. Other ships, such as the AO-177 and LKA-113 class ships, have their burners on the top of the boilers. These are called TOP-FIRED BOILERS.

FURNACE PRESSURE

Another convenient boiler classification is based on the air pressure used in the furnace. Most boilers in use in naval propulsion plants operate with a slight air pressure (seldom over 5 psig) in the boiler furnace. This slight pressure is not enough to justify calling these boilers pressurizedfurnace boilers. However, some boilers installed on naval ships are truly pressurized-furnace boilers. They are called PRESSURE-FIRED or SUPERCHARGED BOILERS. These furnaces are maintained under a positive air pressure of about 65 psia (about 50 psig) when operated at full power. The air pressure in these boiler furnaces is maintained by special air compressors called superchargers.

TYPE OF SUPERHEATERS

On almost all boilers used in the propulsion plants of naval ships, 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 RADIANTTYPE SUPERHEATERS.

CONTROL OF SUPERHEAT

A boiler that provides some means of controlling the degree of superheat independently of the rate of steam generation is said to have CONTROLLED SUPERHEAT. A boiler in which such separate control is not possible is said to have UNCONTROLLED SUPERHEAT.

Normally, the term superheat control boiler is used to identify a double-furnace boiler. The term uncontrolled superheat boiler is used to identify a single-furnace boiler.