Wootten Poi/cr.—In the \Vootten locomotive-boiler there is a compar atively shallow fire-box, extended laterally to any limit desired and usu ally located somewhat above or only to a slight degree below the lower line of the waist. Interposed between the fire-box and the flue-sheet is a combustion-chamber, and across its fire-box end extends a fire-bridge, so high as to permit a fire of proper thickness to be carried without allowing fuel to enter the chamber or the tubes. The crown-sheet is stayed directly to the shell by bolts, as are the side-sheets. Figures 8 and 9 (pl. 74) give lengthwise and cross-sections of a \Vootten boiler, which show more clearly than any description the peculiarities of its construction. It has done remarkably good service upon the Philadelphia cX. Reading Railroad and on other lines, particularly with the use of low grades of fuel, which could not well be used in any other type of fire-box.
The Marine Boiler has at one end a fire-box that is surrounded by water, and from which flues extend to a combustion-chamber or "back connection," surrounded by water spaces, and running nearly up to the water-line. From the front part of this chamber, above the flues, there extend horizontal return-tubes to the front of the shell. The gases pass through the flues into the chamber at the back, return through the small flues to the front, and enter the up-take. In marine boilers the flame must necessarily be within the shell. In the drop-flue type (generally internally fired) the products of combustion pass np and over a bridge, along tubes to the back, then down through tubes to the front, then again through still lower ranks of tubes to the front, and then to the up-take.
The smaller the bore of the tubes, the stronger they are for a given thickness of metal, or the thinner they may be to afford a given strength; so that the advantage of small tubes is in the lines either of safety, if the thickness is maintained at a standard, or of cheapness and increased steam ing capacity, if they are made thinner in proportion to their diameter. Some English locomotives and marine boilers have copper fire-boxes, which, though they have great conductive power, are expensive and lack sufficient strength.
Figures 9 and ro (pl. 75) show a double-ended marine boiler built for the United States cruiser " Baltimore." There are at each end four large internally-fired corrugated flues that discharge into combustion-chambers, from which lead small flues through which the gases travel in the reverse direction to that taken in the large flues. The grates are not illustrated in position, and the up-takes and steam chimneys are not shown. Length
wise-screwed stays in the steam-room strengthen the shell in a longitu dinal direction.
The marine boiler shown in Figures 7 and 8 was constructed for the United States revenue steamer " Lot M. Morrill." There are three corrugated flues which are internally fired. The corrugations strengthen the flues transversely, at the same time enabling them to expand and con tract independently of the main shell of the boiler, without causing any injurious strains. The gases of combustion after passing through these tubes pass upward and then reverse their course, going through a number of small tubes that discharge into a common up-take, which is surrounded by steam space. Lengthwise stays of wrought iron supplement the stay ing action of the tubes in counteracting the tendency of the heads to sep arate.
the above-mentioned boilers are composed of cylinders, either large or small, and those with tubes or flues traversing lengthwise have flat heads, which facilitate fastening the ends of the tubes, so as to give strength and tightness. The larger flues arc fas tened in the heads by being flanged and riveted; smaller flues have their ends inserted in holes (drilled of the same size as their external diam eter), and then so expanded by considerable pressure with a suitable tool that they completely fill the holes. Sometimes, besides being expanded in the holes, the ends of the tubes are slightly extended and are then turned over by a special tool, so as to lie against the flat face of the head. This is termed " beading," and in the opinion of some adds to the efficiency with which the tubes act as lengthwise stays to counteract the tendency of the steam to bulge the heads or to force them apart. Sonic classes of boilers—notably, those of locomotive-engines—have tubes smaller than the holes in the tube-plates; to fill the holes metal ferules or rings are placed upon the tube-ends, thus making a more steam-tight joint than could be made by the thin edges of the tube-sheet.
Vertical the boilers above described the shells are horizon tal, but cylindrical boilers are sometimes set vertically, though in such cases the gases of combustion pass vertically only, and generally through a number of small tubes extending from end to end. Such vertical boilers are generally provided with a double-walled extension, styled the " fire box," having its lower end closed by the grate, the space between the two walls of this fire-box being called the "water-leg" and constituting effi cient heating surface. Of this type are a vast number of portable and steam fire-engine boilers.