drum. Thus a circulation is set up. The efficiency of a water-tube boiler depends in a large measure on proper circulation. As the distance of grate to smoke stack is rather short, most types of water-tube boilers have a system of baffle plates for conducting the gases among the tubes to increase the distance of travel. The economy depends in large measure on efficient baffling.
The boilers are fitted with a casing made of fire brick, asbestos, or other non-conducting material held in place by thin sheet metal.
Water-tube boilers have all the attachments enumerated for the cylindrical fire-tube boiler. In addition nearly all except Babcock & Wilcox boilers have automatic feeding apparatus. Bellville boilers are fitted with reducing valves. Some types, especially Bellville, are fitted with feed heaters or economizers placed above the boiler proper, where the feed water is heated before entering the boiler. Others are fitted with superheaters. Most types have an arrangement of steam or air service for the removal of soot.
Down-tube boilers are those in which the steam-generating tubes discharge into a steam drum below the water line. Priming boilers are those where these tubes discharge at or above the water line. Such a boiler as the Schultz appears to be neither one nor the other.
Large-tube boilers use tubes varying from three to five inches in diameter. Small-tube boilers use tubes one to two inches in diameter.
Express boiler is a term applied to rapid steaming small-tube boilers, capable of large power on small weight and using heavy forced draught. This type is rather less economical and is chiefly used for very fast vessels such as torpedo boats.
Up to the present time the Babcock & Wilcox and Dürr of large-tube type, and the Thornycroft, Yarrow, and Normand of express type, have given the greatest satisfaction in service. The Niclausse and Bellville appear to give satisfaction in some services and dissatisfaction in others.
THORNYCROFT BOILER—DARING TYPE.
The Babcock & Wilcox and Bellville are straight-tube priming boilers. The former is shown in section on the plate accompanying the article on Boiler (q.v.). The Niclausse, Dürr, Yarrow, and D'Allest are straight-tube ‘drowned’ tube boilers. The Thornycroft and the Schultz are express bent-tube priming boilers. The Normand is an express bent-tube ‘drowned’ tube boiler. Of these the Yarrow, D'Allest, and Babcock & Wilcox are simplest and the Bellville most complicated.
YARROW BOILER.
The systems of forced draught in use are the closed ash pit and closed fire room. Of induced systems generally fitted with air heaters we have various patent forms. Superheating is somewhat used, but does not meet with great satisfaction, owing to increased cost and weight and the rapid deterioration of such attachments.
The economic results and other data of water-tube boilers varies so much with different types and conditions that no average results can fairly be taken. As a rule there is a larger ratio of heating surface than in Scotch and a decrease in weight, so that among some of the best types the weight per indicated horse power, including water, is 50 to 90 pounds. See Shipbuilding; Steam; Steam Engine; Ship, Armored; Navies; Transportation.
STEAM PUMP. See Pumps and Pumping Machinery.
STEAMSHIP. See Steam Navigation.
STEAM SHOVEL. A modified form of dredge adapted for excavating material on dry land. The steam shovel was invented in 1840 by an American named Otis, but it did not come into general use until about 1865. The large increase in railway construction in the United States created an active demand for a rapid excavating machine. As will be seen from the accompanying outline drawing, the steam shovel consists essentially of a strong frame mounted on wheels to form a base to which the working parts are attached. The digging mechanism consists of a crane hinged to a mast or support at the front end of the car, and a dipper handle and dipper carried by the crane. The operating machinery consists of a main engine, which hoists the dipper and swings the crane, and of a thrusting mechanism for forcing the dipper into the earth. The operation begins with the dipper at the position shown by the dotted lines in the illustration. The engine man begins to hoist and thus swing the dipper
