The grate surface of such a boiler develops 13 to HI indicated horse power per square foot. Ratio of heating to grate surface,' 30-35 to 1.
chambers connected by a system of tubes either straight or bent. The feed water usually enters the upper or steam drum and is conducted by clown tubes to a lower or water drum: from here the water, becoming heated, rises and passes up through steam-collecting tubes to the steam drum. Thus a circulation is set up. The effi ciency 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 battle plates for conducting the gases among the tubes to in crease the distance of travel. The economy de pends in large measure on efficient battling.
The boilers arc fitted with a casing made of fire brick, asbestos, or other non-conducting ma terial 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 & Wil cox boilers have automatic feeding apparatus. Bellville boilers are fitted with reducing valves. Some types, especially Belltille, are fitted with feed heaters or economizers placed above the boil er proper, where the feed water is heated before entering the boiler. Others are fitted with super heaters. 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 ap pears 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 tor pedo boats.
Up to the present time the Babcock & Wilcox and Diirr of huge-tube type, and the Thorny croft, Yarrow, and Normand of express type, have given the greatest satisfaction in service.
The Niclausse and Bellville appear to give satis faction in some services and dissatisfaction in others.
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 Nielansse, Diirr, 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 Bab cock & Wilcox are simplest and the Bellville most complicated.
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 sat isfaction, 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; NA VIES; TRANSPORTATION.