Fig. 79.—Fichet-Heurtey producer with rotating bed-plate.
Air-Blast.—The system by which air and steam are injected necessitates the employment of a steam-boiler of 75 pounds pressure. This method of blowing, which is rather complicated, has the disadvantage of varying
in feed with the pressure of the steam in the boiler, which pressure is not easily maintained at a given number of pounds per square inch. Moreover, when more or less resistance is offered by the fuel in the generator the quantity of air which is injected is likely to be diminished in quantity while the quantity of steam remains the same. The result is a change in speed which follows from the modification of the proportions of the two elements. For these reasons some manufacturers have resorted of late years to the employment of fans and blowers.
Fig. 80.—Koerting blower.
Blowers.— The fans or blowers employed vary considerably in arrangement. Most of them are based on the Koerting system (Fig. 80), and comprise essentially (1) a tube through which the steam is supplied under pressure, and (2) a cylindro-conical blast-pipe. The tube is placed in the axis of the blast-pipe at its outer opening. As it escapes under pressure the steam is caught in the blast-pipe and draws with it a certain quantity of air, which can be regulated. It is important that these injection blowers should operate in such a manner that the pressure and the feed of air and steam can be controlled.
Fans.—Mechanical blowers have the advantage of
dispensing with the employment of steam under pressure and the consequent installation of a boiler (Fig. 78). Driven by the engine itself or from some separate source of power, these apparatus are easily placed in position, require no great amount of attention, and utilize but little energy. They are either of the centrifugal type or of the rotary type, exemplified in the Root blower (Fig. 81). The latter system has the advantage of high efficiency, and of enabling comparatively high pressures—19 to 27 inches of water—to be attained, which, however, are used only for special fuels, such as lignite, peat, and the like. The air supplied by the blower, before reaching the fire-box, is superheated, either before or after it is charged with steam.
