THE INJECTOR.
This consists, in its most simple form, of a steam nozzle, the end of which extends somewhat into the second nozzle, called the combining or mixing nozzle; this connects with, or rather terminates in, a third nozzle or tube, termed “the forcer.” At the end of the combining tube, and before entering the forcer, is an opening connecting the interior of the nozzle at this point with the surrounding space. This space is connected with the outside air through a check valve, opening outward in the automatic injectors, and by a valve termed the overflow valve. The injector nozzles are tubes, with ends trumpet mouthed to receive and deliver the fluids with the least possible loss by friction and eddies.
As a thermodynamical machine, the injector is nearly perfect, since all the heat received by it is returned to the boiler, except a very small part which is lost by radiation; consequently its thermal efficiency should be in every case nearly 100 per cent.
Note.—The operation of the injector is based on the fact, first demonstrated by Giffard, that the motion imparted by a jet of steam to a surrounding column of water is sufficient to force it into the boiler from which the steam was taken, and, indeed, into a boiler working at a higher pressure. The steam escaping from under pressure has, in fact, a much higher velocity than water would have under the same pressure and condition. The rate of speed at which steam travels—taking it at an average boiler pressure of sixty pounds—when discharged into the atmosphere, is about 1,700 feet per second. When discharged with the full velocity developed by the boiler pressure through a pipe, say an inch in diameter, the steam encounters the water in the combining chamber. It is immediately condensed and its bulk will be reduced say 1,000 times, but its velocity remains practically undiminished. Uniting with the body of water in the combining tube, it imparts to it a large share of its speed, and the body of water thus set in motion, operating against a comparatively small area of boiler pressure, is able to overcome it and flow into the boiler. The weight of the water to which steam imparts its velocity gives it a momentum that is greater in the small area in which its force is exerted than the boiler pressure, although its force has actually been derived from the boiler pressure itself.
Fig. 513.—See page [252].
Fig. 514.—See page [251].