PART SIXTH.

Steam-Boilers.

Steam-boilers embrace a great variety of designs;* in fact, any vessel in which steam is generated for mechanical purposes

may be termed a steam-boiler, regardless of shape or form. The

* For a full description of all the steam-boilers in use at the present day, their peculiarities of design, construction, care, and management, see Roper's "Use and Abuse of the Steam-Boiler."

most common forms of marine-boilers in use at the present day are the horizontal and vertical, fire- and water-tubulars. The watertubular is fast disappearing, and is now rarely to be found except in the United States Navy, or those of other countries. Its gradual disappearance arises from the fact that it is more expensive to build and to repair, is more dangerous, and requires extra care and management. If a tube splits or becomes leaky in the fire-tubular boiler, the difficulty may be met by plugging, and the vessel can proceed on its way; but if the same accident occur in a water-tubular, it would be necessary to blow out the boiler. The same principle which was embodied in the Montgomery water-tubular marine-boiler was

introduced into the Dimpfel locomotive-boiler, but soon fell into disuse in both cases. The fire-box, fire-tubular marine-boiler, with

combustion-chamber at the back end and superheater in the uptake, is the type of boiler most generally in use on the steamships of the different lines sailing out from the seaports of this country as well as those of other nations.

Aside from the choice among engineers between the two forms, there is a wider difference in their proportion than in anything else connected with the steam-engine. While all generally agree that, in proportioning a marine-boiler, there should be sufficient grate-surface to consume the maximum quantity of coal required for the engine for which that boiler was intended to furnish steam, and that there should be sufficient heating-surface to absorb the heat evolved by the fuel; yet, when it comes to laying down proportions, one engineer allows twice as many square feet of heating-surface to one square foot of grate-surface as another. Watt's proportions for land- and marine-boilers varied from 9.5 to 10 feet of heating-surface to 1 square foot of grate-surface. Maudsley and Miller allowed 10 square feet of heating-surface to 1 square foot of grate-surface in the boilers of the celebrated ocean steamer Great Western, and from 10 to 12 square feet of heating-surface to 1 square foot of grate-surface in other marine-boilers that they constructed about the same time; so that neither they nor Watt seemed to have any fixed rule, nor did there appear to be any among naval constructors either in this country or England.

This may be seen from the fact that the U. S. gun-boat Massachusetts had 34 feet of heating-surface to 1 square foot of gratesurface, while the Vixen, with the same-sized engine, had only 16 to 1. The merchant-steamer Constitution had 66 square feet of heatingsurface to one square foot of grate-surface, while the Franklin, a steamship of nearly the same capacity, with engines of the same power, had only 28 to 1. The boilers of the celebrated steamships of the Collins Line, which have made such fast time between New York and Liverpool, had 33 square feet of heating-surface to 1 square foot of grate-surface, while in the boilers of the steamships of the Cunard Line the heating-surface varies from 18 to 37 square feet to 1 square foot of grate-surface. The Mary Powell,

one of the fastest river-boats in American waters, has 17 square feet of heating-surface to 1 square foot of grate-surface. In proportioning the heating-surface to the cubic contents of the cylinder, the same variation seems to exist which shows there is no recognized proportion for either. The steamship Massachusetts, U. S. N., has 77 square feet of heating-surface to 1 cubic foot of cylinder, while the Powhatan has less than 15 square feet, and the San Jacinto has a trifle over 12. The merchant-steamer Union had one hundred and eighteen square feet of heating-surface to 1 cubic foot of cylinder, while the Isaac Newton had only 10 to 1. The steam-tug Rescue had 63 square feet of heating-surface to 1 cubic foot of cylinder, while the Anglo-Saxon had only 10 to 1.

The average proportion of heating-surface to grate-surface of 345 steamships, tugs, and ferry-boats examined was about 30 square feet of heating-surface to 1 square foot of grate-surface, while an examination of a great number of steamships, tug, and ferry-boats in this country, England, and France, showed that the average proportion of heating-surface to 1 cubic foot of cylinder was about 28. In stationary boilers the heating-surface varies from 12 to 30 to 1 square foot of grate-surface, while in some patented sectional boilers there are 60 to 70 square feet of heating-surface to one square foot of grate-surface, the average for locomotive-boilers being about 60 square feet of heating-surface to 1 square foot of grate-surface.

To proportion a marine-boiler understandingly, it is necessary to know the size of the engine and of the boat or ship, the load to be propelled, and the speed at which it is to move. The engineer can determine the pressure and volume of steam required, and decide on the degree of expansion, the quantity of grate- and heating-surface, and in relation to these two latter conditions, as shown in the foregoing paragraphs, the field has a very wide latitude. But he must be sure that the boiler possesses sufficient strength to resist in safety the maximum pressure to which it will ever be exposed; that it contains sufficient grate-surface for the combustion of the necessary quantity of fuel under any circum

stances; that it has sufficient heating-surface to evaporate the necessary quantity of water; that it is capable of containing a sufficient supply of water and steam to prevent undue fluctuation, and that it affords convenient facilities for the repair or renewal of any of its parts. After the foregoing conditions are determined on, another object of great importance to be considered is making the boiler as light and compact as possible. The term heatingsurface, when applied to steam-boilers, means all that part of the fire-box, crown-sheet, tube-sheets, and flues with which the fire and flame come in contact in their escape from the furnace to the domchimney.

is

9 Martin's upright tubular-boiler sometimes used for marine purposes. Its only advantage is economy of space; its first cost is more than that of the ordinary horizontal marine tubularboiler, and it is not more efficient. The capacity of the steam-room is about one-third the capacity of the boiler. Hi The quantity of steam that can be generated in any boiler in a given time is dependent

upon a great variety of circumstances,

such as the kind of boiler, its condition as to dirt, scale, etc., the