Fire-Tube Boilers Return-Tube Boilers Horizontal Many Small Fire Tubes Internally-Fired

A single-furnace boiler has but one combustion chamber. A two-furnace boiler may have a combustion chamber for each furnace or it may have a common combustion chamber. If there is but one boiler on board, it is better to have two combustion chainbers, so that in case a tube bursts, the boiler will not be disabled. If, however, there are several boilers, it is better to have a common combustion chamber for the two furnaces, because the alternate stoking keeps up a more nearly constant pressure of steam and there is less smoke. Three-furnace boilers usually have three combustion chambers, while four-furnace boilers have two. In case four furnaces are used with three combustion chambers, the two center furnaces lead to a common combustion chamber and each outside furnace has one.

Boiler

s. This form of marine return-tube boiler is practically the same as two single-ended boilers placed back to back, but with the rear plates removed. The weight of the rear plates is saved and there is less loss from radiation. This makes the double-ended boiler lighter and cheaper in proportion to the heating surface. Double-ended boilers are often made 16 feet in diameter and 18 feet long.

There are two distinct classes of double-ended return-tube boiler—those having all the furnaces open into one combustion chamber and those having several combustion chambers. The boiler having but one combustion chamber has the disadvantage that if one fire is being cleaned the whole boiler may be cooled by the inrush of cold air. It is better to have a combustion chamber for each furnace or at least have a combustion chamber for the furnaces of each end. The usual method of dividing up the combustion chambers is by water spaces as shown in Fig. 24, which is the section of a boiler a combustion chamber for each furnace.

Internal Furnace Return-Tube Boiler.

Although the returntube boiler is commonly used in marine work, this type, with some changes in detail, is used in plants ashore. Fig. 25 shows the construction and arrangement of parts. The flue is larger in proportion to the diameter than is the case with the marine form; the combustion chamber is partly external to the shell, that is, the rear tube sheet is also the rear end plate. This arrangement does away with the necessity of staying the flat plates of the combustion chamber.

Another form of internal furnace, return-tube boiler is shown in Fig. 26. This boiler usually has two flues extending from the front to the hack head. The grate is placed in the corrugated portion while conical water tubes support the flue back of a bridge wall. The large furnaces and the space around the conical tubes provide a combustion chamber of ample size.

The arrows show the direction of the hot gases. After leaving the internal flue they enter the return tubes which are below the furnace; before leaving the boiler, they pass underneath the shell. By this arrangement the hottest gases are near the water line and the cooler gases in contact with the cold water, thus there is the greatest difference in temperature at all times. At each change in the direction of the hot gases, there is an opportunity for dirt and ash to fall by gravity so that the tubes may remain clean and efficient.

With the exception of the foundation there is no brickwork. The shell is covered with a non-conducting material. This boiler, like the Galloway, has a large steam and water space, thus insuring dry steam and great reserve power.