Among the mechanics I associated with, the injector was spoken of as a machine having a secret method of action. There was supposed to be some part inside, where a vacuum was formed, which sent the water into the boiler. We were all familiar with the vacuum of condensing engines, and it was a convenient solution of the mystery. I remember committing what we regarded as a heinous sin, in trying to find out the secret of the injector. On a Sunday forenoon in a Scotch town, where it was considered next to a crime to miss going to church, three of us stole quietly into an engine-house, and took an injector apart, when we knew the foreman, like a pious man, was listening to the sermon. All our plans had been laid the previous day, and the necessary tools laid conveniently at hand. The feeling of disappointment we experienced at finding nothing to explain the working of the thing, is still vivid in my memory.
THE PRINCIPLE OF THE INJECTOR’S ACTION.
The principle of the injector’s action is that of induced currents, which is much more popularly understood to-day than it was twenty-five years ago. A current of any kind has a tendency to induce a movement in the same direction of any body it passes over or touches. Thus, we are all familiar with the fact that a current of air called wind, passing over the surface of a body of water, sets waves in motion, and dashes the water high up on the shore above its original level. In the same way, a jet of steam moving very rapidly, when injected into a body of water under favorable conditions, imparts a portion of its motion, and starts momentum sufficient to overcome the original pressure of the steam. That is how the injector is made to force water into a boiler against the same pressure the steam is starting from. The principle is now utilized in the arts for many useful purposes. The ordinary locomotive blast, blowers, steam siphons, steam-jets, jet exhausters, and argand burners, are common instances of its application.
When the action of the injector is closely examined, its mystery as a source of power disappears; for it is found that an amount of heat equal to the mechanical equivalent of work done, is used up during the operation of feeding. Thus, when a given quantity of heat units pass from the throttle to work the injector, the whole of the heat does not return to the boiler along with the feed-water, as was first supposed to be the case; but a portion of heat representing the foot pounds of work done is dissipated, besides other losses incident to leakage, radiation, and convection.
DIFFERENT FORMS OF INJECTOR.
There are a great many different forms of injectors in use, but all of them conform to certain elementary principles in their mode of action. Steam passes from the boiler through the steam-pipe and receiving-tube A ([Fig. 1]) at a high velocity, and, combining with the water at the point B, condenses, but imparts considerable momentum to the water, which rushes along into the delivery pipe, raises the check-valve, and passes into the boiler.
Fig. 1.
The tubes of the injector are arranged so that a concentrated jet of steam shall impinge upon the water to force it through passages that are designed in the best form for maintaining the solidity of the current. The speed imparted to the water represents the work performed by the steam, and upon this velocity depends the successful action of the injector. As the current of water for starting the injector could not be induced against the constant pressure on the check-valve, which equals the pressure at the throttle, an overflow is provided where the water can flow unchecked till the necessary momentum is obtained, when the overflow is closed. All deviations in construction from the elementary injector shown in [Fig. 1] are made for the purpose of extending the action of the injector under varied conditions, for making it work automatically under different pressures of steam, and for improving its capacity to lift water above its natural level.