The interior of every boiler is divided into water-space and steam-room, which terms explain themselves. The portions of the boilers expwed to the hot gases of combustion constitute the heating surface, which is "direct" if it lies directly over the fire-grate or "indirect" if it receives the radiated heat indirectly.
.Slac• or the stack or chimney (which has the double purpose of increasing the draught and of carrying the products of com bustion to a height at which they will not be offensive) there should be a "damper," generally a flat plate, which is hung by one edge or balanced upon pivots and may be made to close the flue or open it to any degree desired. In some cases there is employed an automatic regulator, in which steam-pressure corresponding to that in the boiler operates a pis ton or a diaphragm and through suitable mechanism closes the damper in proportion to the rise of the steam-pressure. Where possible, boiler-chim neys should be tall, so as to give them good draught under all circum stances. On damp days or with unfavorable winds many boilers set with insufficient height and cross-area of flue fail to generate steam to full or desired capacity. Rapidity of combustion may be decreased not only by closing the damper in the chimney or in the passage leading thereto, but also by closing the ash-door below the grate or by opening the fire door above the grate. Considerable increase of combustion may be effected by a blower above or under the grate or by a steam-ejector in the stack.
The Feed is either by pump or by injector ( /5/. 79, fgs. 1-3). The pump, which may be driven by a special engine or by the main engine, needs no special description. The injector is a device by which the momentum of a jet of steam from a boiler is made to drive a stream of water into the boiler which supplies that steam. That a jet of steam should supply motive force to force into the boiler which supplies it a jet of water that is increased by the condensation of the working-jet itself is apparently a paradox, but these devices are no longer myster ies. The injector has the advantage (shared by an independent feed pump) of being able to supply the boiler whether or not the main engine is running. Whatever type or types of feeding device are used, there should be two appliances, each able to supply at least twice as much water as the boiler can evaporate under the most favorable circumstances.
feed-water should be heated, where this can be done, by waste steam or by the gases of combustion after they have left the boiler. A good heater raises the temperature of the feed-water to 'So° and even to 212° from the normal 62° Fahr. Figure 7 (fii. 77) shows an arrangement of feed-heater which generally goes by the name of "econo mizer." In it the hot gases of combustion, passing from the battery of boilers, go through an arrangement of vertical tubes, through which the feed-water is circulated before going to the boilers. By this means the water is heated nearly to 212° Fahr., thereby greatly assisting the work of evaporation. Such devices are especially good where there is insuf ficient heating surface in the boiler proper, or where, from the improper position or dirty condition of such surfaces, the water in the generator proper does not sufficiently absorb from the products of combustion the heat which they contain.
are sometimes supplied with horizontal mud drums, connected with the main shell (or with the lower shell if there are two or more), and placed out of the reach of the hot gases of combustion, so that they will have in them but little circulation and be but little exposed to injury from flame. From these drums the mud, scale, and other impurities which may settle therein can be easily removed. But where the feed-water contains large quantities of mineral substances it is absolutely necessary to have a purifier (generally a vessel containing filter ing material such as coke, shavings, etc.), in which exhaust steam from the boiler heats the feed to a high enough temperature to make it drop the liine-salts, the most common mineral substances found in water.
The Alcchazzical shown in Figure 6 has in the boiler, with its mouth facing front, a funnel, mostly above the water-line, but with its apex below low-water level. From the point there rises and leaves the boiler a tube, which enters at one side of the top of a hollow cast-iron sphere, having a vertical partition extending downward halfway across it. From the other side of this sphere a tube goes through the boiler, entering below the steam-line. The circulation sweeps the mechan ically suspended impurities backward and carries them into the sphere, where they are dropped in the settling-chamber, from which they may be blown off, the water returning to the boiler.