single or double ended return-tube boiler fitted with two, three, or four corrugated furnaces. Boilers vary from 9 to 20 feet in diameter, and 9 to 18 feet in length for single-ended, and 17 to 21 feet for double-ended. A cut of a single-ended boiler is shown; aa are the shell-plates, made in two courses of three sections each, with butt joints; bb, corrugated furnaces, either of Fox, Purves, or Morrison patent, into which the grate and bridge wall are fitted; c, stays supporting boiler front and combustion chamber; DD, tube sheets with tubes, e, expanded into them; some of tliese are stay tubes screwed into tube sheets which they help to support. F is the combustion chamber, where the gases of combustion are finally mixed and burned. The products of combustion pass through the tubes to the uptake and then to the smoke pipe. The floating surface is composed of the crown sheets (top of furnaces), top and sides of combustion chamber, and tube surface, the tube surface being by far the larger portion. H, H, H are steam space stays supporting the boiler ends. The back and sides of the combustion chamber are supported by short stay bolts, I, and the top by girder stays, J. The steam space is fitted with a dry pipe which collects the steam and discharges through the stop valve to the steam pipe.
SINGLE-ENDED CYLINDRICAL BOILER.
The furnace front is fitted with furnace and ash-pit doors, the ash pit being the part of the furnace below the grate. Manholes, K, are fitted to obtain access to boiler for cleaning, etc. The coverings for manholes are called manhole plates. Practically all parts of a boiler, except the grate, furnace doors, and bridge wall, are built of mild steel.
The attachments of a boiler like the one shown are: main and auxiliary stop valves in the steam pipe; check valves through which feed water enters; surface and bottom blow valves, by means of which the boiler is blown down or pumped out; two water columns to show the height of the water; pressure gauge; spring safety valve; and circulating apparatus (generally a hydrokineter).
The grate surface of such a boiler develops 13 to 16 indicated horse power per square foot. Ratio of heating to grate surface, 30-35 to 1. Weight per I. H. P. including water, 90 to 120 pounds. Maximum coal burnt per square foot grate surface per hour about 40 pounds ordinarily 15 to 20 pounds. The efficiency of the boiler is about 70 to 75 per cent. in best condition.
BELLVILLE BOILER—ECONOMIZER TYPE.
In water-tube boilers the water is contained within the tubes, and as these are relatively small great pressures can be carried and the boiler may be considerably lighter. Most types of water-tube boilers have a number of steam and water 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 down tubes to a lower or water drum; from here the water, becoming heated, rises and passes up through steam-collecting tubes to the steam