Fig. 23.
It will be seen from these drawings that there are three internal cylindrical furnaces at each end of these boilers, making in all six furnaces per boiler. The firing takes place at both ends. The flame and hot gases from each furnace, after passing over the bridge wall enter a flat-sided rectangular combustion chamber and then travel through tubes to the front uptake (i.e. the smoke bonnet or breaching), and so on to the chimney.
The sides of the combustion chambers are stayed to each other and to the shell plate of the boiler; the tops are strengthened in the same manner as the crowns of locomotive boilers, and the flat plates of the boiler shell are stayed together by means of long bolts, which can be lengthened up by means of nuts at their ends. Access is gained to the uptakes for purposes of cleaning, repairs of tubes, etc., by means of their doors on their fronts just above the furnace doors. The steam is collected in the large cylindrical receivers shown above each boiler. The material of construction is mild steel.
The following are the principal dimensions and other particulars of one of these boilers:
- Length from front to back, 20 feet.
- Diameter of shell, 15 feet 6 inches.
- Length of furnace, 6 feet 10 inches.
- Diameter of furnace, 3 feet 10 inches.
- Length of tubes, 6 feet 9 inches.
- Diameter of tubes, 31⁄2 inches.
- No. of tubes, 516.
- Thickness of shell plates, 15⁄16.
- Thickness of tube plates, 3⁄4.
- Grate area, 1261⁄2 square feet.
- Heating surface, 4015 square feet.
- Steam pressure, 80 lbs. per sq. inch.
[Fig. 24] is a sketch of a modern marine boiler, which is only fired from one end, and is in consequence much shorter in proportion to its diameter than the type illustrated in figs. [22] and [23].
Marine boilers over nine feet in diameter have generally two furnaces, those over 13 to 14 feet, three, while the very largest boilers used on first-class mail steamers, and which often exceed fifteen feet in diameter, have four furnaces.