[Illustration:  Fig. 78.]

Some Points in Design.

The efficiency of a boiler is governed chiefly (1) by the amount of heating surface exposed to the flames; (2) by the distribution of the heating surface; (3) by the amount of fuel which can be burnt in the furnace in a given time; (4) by avoiding wastage of heat.

The simplest form of boiler, depicted in Fig. 78, is extremely inefficient because of its small heating surface.  A great deal of the heat escapes round the sides and the ends of the boiler.  Moreover, a good deal of the heat which passes into the water is radiated out again, as the boiler is exposed directly to the air.

Fig. 79 shows a great improvement in design.  The boiler is entirely enclosed, except at one end, so that the hot gases get right round the barrel, and the effective heating surface has been more than doubled by fitting a number of water-tubes, aaa, bbbb, which lie right in the flames, and absorb much heat which would otherwise escape.  The tubes slope upwards from the chimney end, where the heat is less, to the fire-door end, where the heat is fiercer, and a good circulation is thus assured.  The Babcock and Wilcox boiler is the highest development of this system, which has proved very successful, and may be recommended for model boilers of all sizes.  The heating surface may be increased indefinitely by multiplying the number of tubes.  If a solid fuel-coal, coke, charcoal, etc.-fire is used, the walls of the casing should be lined with asbestos or fire-clay to prevent the metal being burnt away.

[Illustration:  Fig. 79—­Side and end elevations of a small water-tube boiler.]

The horizontal boiler has an advantage over the vertical in that, for an equal diameter of barrel, it affords a larger water surface, and is, therefore, less subject to “priming,” which means the passing off of minute globules of water with the steam.  This trouble, very likely to occur if the boiler has to run an engine too large for it, means a great loss of efficiency, but it may be partly cured by making the steam pass through coils exposed to the furnace gases on its way to the engine.  This “superheating” evaporates the globules and dries the steam, besides raising its temperature.  The small water-tube is preferable to the small fire-tube connecting furnace and chimney, as its surface is exposed more directly to the flames; also it increases, instead of decreasing, the total volume of water in the boiler.

A Vertical Boiler.

[Illustration:  Fig. 80.—­Details of vertical boiler.]

The vertical boiler illustrated by Fig. 80 is easily made.  The absence of a water jacket to the furnace is partly compensated by fitting six water-tubes in the bottom.  As shown, the barrel is 8 inches long and 6 inches in outside diameter, and the central flue 1-1/2 inches across outside solid-drawn 1/16-inch tubing, flanged ends, and four 1/4-inch stays—­disposed as indicated in Fig. 80 (a) and (b)—­are used.  The 5/16 or 3/8 inch water-tubes must be annealed and filled with lead or resin before being bent round wooden templates.  After bending, run the resin or lead out by heating.  The outflow end of each pipe should project half an inch or so further through the boiler bottom than the inflow end.