BOILERS

Fire-tube boilers. These are boilers with straight tubes that are surrounded by water and through which the products of combustion pass. The tubes are usually installed within the lower portion of a single drum or shell below the water-line.

Water-tube boilers. These are boilers in which the tubes, themselves contain steam or water, the heat being applied to the outside surface. The tubes are usually connected to two or more drums set parallel to, or across, the centerline. The drums are usually set horizontally.

Tube shape and position. The tubular heating surface may be classified: 1) by form — either straight, bent, or sinuous or 2) by inclination — horizontal, inclined, or vertical.

Firing. The boiler may be either a fired or an unfired pressure vessel. In fired boilers the heat applied is a product of fuel combustion. A nonfired boiler has a heat source other than combustion.

Circulation. The majority of boilers operate with natural circuktion. Some utilize positive circulation in which the operative fluid may be forced "once through" or controlled with partial recirculation.

Furnace position. The boiler is an external combustion device in that the combustion takes place outside the region of boiling water. All heat must be transferred through the heating surface to reach the water. The relative location of the furnace to the boiler is indicated by the description of the furnace as being internally or externally fired: 1) the furnace is internally fired if the furnace region is completely surrounded by water-cooled surfaces; 2) the furnace is externally fired if the furnace is auxiliary to the boiler or built under the boiler.

General shape. During the evolution of the boiler as a heat producer many new shapes and designs have appeared. Some of these boilers have become popular and are widely recognized in the trade, including the following:

1. Fire-tube boilers — horizontal return tubular, short firebox, compact, locomotive, vertical tube (steam jenny), Scotch type, and residential units.

2. Water-tube boilers — both horizontal straight tube and bent tube units. The horizontal straight tube boiler may have a box type header made of steel plate, or a sectional header each section of which connects the tubes in a single vertical row. The bent tube boiler may have one to four drums. If the drum is parallel to the tubes, the boiler is long — longitudinal drum; if across the tubes, it is a cross drum. If the furnace is enclosed with water-cooled "surfaces, it is a waterwall (water-cooled) furnace.