the above has been evolved the " mniti tubular" type (fig. to), in which there are from fifty to three hundred tubes of comparatively small diameter, extending from one of the flat heads to the other and serving for the passage of the hot products of combustion. In these the gases may go (without passing underneath) directly from front to back, then into the stack or chimney; but it adds to steam-generating capacity and lessens fuel-consumption per given quantity of steam to cause them first to go under the shell from front to back and then to traverse the flues from back to front, the most usual course then being into the stack. Horizontal multitubular boilers generally have the tubes in rather more than the lower half of their space, and completely covered by water.
Departing considerably from the plain horizontal multitubular boiler is one in some respects like a vertical portable, with separate fire-box, laid on its side, the grate being so placed as to be in the bottom in the new position. Its double fire-box contains water in the " legs " or space between its walls. The tubes running from the tube-sheet in the side are entirely covered with water, as is the crown-sheet or top of the fire box. As a general thing, however, the extension is of greater diameter than the cylindrical shell, being continued down below the bottom of the latter and generally having straight vertical sides and an externally convex bottom. This boiler has much in common with the locomotive type, which is considered in detail on pages 285 to 2S7.
3 shows in central lengthwise vertical sec tion a return-tube boiler. The gases, passing over the bridge-wall and the flame-bed, from front to back, return through the flues to the front, where they divide and are sent along the sides to the stack. There is a large central space left between the right- and the left-hand ranks of tubes, to facilitate cleaning and inspection.
Boilers have two cylindrical shells, one above the other, connected by two or more vertical "nozzles," which permit the passage of water- and steam-currents. The water is carried high enough to fill about one-half the upper such or "drum." Modifications of this type have one lower cylinder or water-drum and two upper cylinders or steam drinns, or two lower and one upper. A common source of danger in such boilers is insufficient size of the connecting-tubes for water-circulation. Figure 5 (pi. shows a cylindrical main shell with hemispherical ends and with two cylindrical lower drums, which receive the heat from the hottest gases.
The french or Elephant Boiler (Jig. t), largely in use in Europe, is not seen in America. It usually has three lower cylindrical drums con nected with one drum of much larger diameter. The products of com bustion first act upon the lower three cylinders, then return to the front end along one side of the upper drum, then pass to the stack along the other side. The water-level is about two-thirds of the way up in the upper drum.
The Pairbairn Boiler (fics. 6, 9) is of the elephant type, and is internally fired. There are three cylindrical shells, two parallel and close together, each traversed concentrically by an internal fine, the third shell being above and between these two, to which it is joined by suitable connecting tubes. The gases, after leaving the internal flues of the lower shells, return to the front along the sides and bottom of the two lower cylinders, then pass to the chimney between these cylinders and the upper cylinder.
One " compound tubular " type (fig. 2) has a large lower shell con nected with a smaller upper one, and the lower cylinder has lengthwise tubes throughout its entire volume. The water-level is about half-way up in the upper cylinder.
locomotive-boiler has a horizontal cylindrical shell with a double-walled extension fire-box, sometimes of such diameter as to require a tapering waist-piece to connect it with the main shell. The fire has a direct radiating action upon the sides and the top of the fire-box, from which 'it passes through hundreds of long tubes of small diameter to the smoke-box, at the farther end, and thence to the stack. (The term "farther end" is used because, as locomotives are ordinarily run, that end of the boiler precedes which would be the back if it were of the station ary type.) The locomotive-boiler has the advantages of being self-con tained, of being able to dispense with brick-work, of having an immense heating surface, of having the fire entirely surrounded by water, and of having more space between the crown-sheet and the grate than any other, this latter giving a large combustion-chamber. Figure 5 (/5/. 74) shows a crude form of locomotive-boiler, the dome having a cast-iron top. The flat heads are held by wrought-iron stay-rods, and the flat crown-sheet above the fire is supported by sling-stays.