Note.
In estimating the horse power by means of the above table, 15 square feet has been allowed for each horse power, and the number of feet in each boiler is given in round numbers. This table is one used in every-day practice by boiler makers.
THE FLUE BOILER.
The Two Flue Boiler.—Fig. 31.
The Six Inch Flue Boiler.—Fig. 32.
THE HORIZONTAL TUBULAR STEAM BOILER.
The great majority of stationary boilers are cylindrical or round shaped, because—
1. The cylindrical form is the strongest.
2. It is the cheapest.
3. It permits the use of thinner metal.
4. It is the safest.
5. It is inspected without difficulty.
6. It is most symmetrical.
7. It is manufactured easier.
8. It resists internal strain better.
9. It resists external strain also.
10. It can be stayed or strengthened better.
11. It encloses the greatest volume with least material.
12. It is the result of many years’ experience in boiler practice.
13. It is the form adopted or preferred by all experienced engineers.
It follows, too, that the horizontal tubular boiler, substantially as shown in [fig. 30], is the standard steam boiler; engineers and steam power owners cling with great tenacity to this approved form, which is an outgrowth of one hundred years’ experience in steam production.
In the plain horizontal tubular boiler shown in cuts, the shell is filled with as many small tubes varying from two inches to four inches in diameter as is consistent with the circulation and steam space. In firing this type of boiler the combustion first takes place under the shell, and the products, such as heat, flame, and gas, pass through the small tubes to the chimney, although in the triple draught pattern of the tubular boiler, the heat products pass, as will hereafter be explained, a second time through the boiler tubes, making three turns before the final loss of the extra heat takes place.
The illustrations on pages 78 and 80 exhibit the gradual advances to the horizontal tubular by the two-flued boiler ([fig. 31]) of the six flues ([fig. 32]) and of the locomotive Portable Boiler ([fig. 33]). The vertical or upright tubular boiler is but another modification of the horizontal tubular.
The Locomotive Portable Boiler.—Fig. 33.
In parts of the vertical boiler there is very little circulation and the corrosion on the inner side is such as to wear the boiler rapidly. In the ash pit, ashes and any dampness that may be about the place also causes rapid corrosion. The upper part of the tubes and tube sheet are frequently injured; for instance, if the tubes pass all the way through to the upper tube sheet, providing there is no cone top, when the fire is first made under the boiler, combustion at times does not take place until the gases pass nearly through the tubes. The water usually being carried below the tube sheet there is a space left above the water line, where there is neither steam nor water, and the heat is so great that the ends of the tubes are burned and crystalized, and the tube sheet is often cracked and broken by this excessive heat before the steam is generated. The first difficulty is experienced in “the legs” of the Portable Locomotive boiler—hence the general verdict of steam users in favor of the round shell, many-tubed boiler.