Fig. 16.—The Giffard injector.
A form of injector very commonly used is Giffard's (Fig. 16). Steam is allowed to enter by screwing up the valve V. As it rushes through the nozzle of the cone A it takes up water and projects it into the "mixing cone" B, which can be raised or lowered by the pinion D (worked by the hand-wheel wheel shown) so as to regulate the amount of water admitted to B. At the centre of B is an aperture, O, communicating with the overflow. The water passes to the boiler through the valve on the left. It will be noticed that the cone A and the part of B above the orifice O contract downward. This is to convert the pressure of the steam into velocity. Below O is a cone, the diameter of which increases downwards. Here the velocity of the water is converted back into pressure in obedience to a well-known hydromechanic law.
An injector does not work well if the feed-water be too hot to condense the steam quickly; and it may be taken as a rule that the warmer the water, the smaller is the amount of it injected by a given weight of steam.[2] Some injectors have flap-valves covering the overflow orifice, to prevent air being sucked in and carried to the boiler.
When an injector receives a sudden shock, such as that produced by the passing of a locomotive over points, it is liable to "fly off"—that is, stop momentarily—and then send the steam and water through the overflow. If this happens, both steam and water must be turned off, and the injector be restarted; unless it be of the self-starting variety, which automatically controls the admission of water to the "mixing-cone," and allows the injector to "pick up" of itself.
For economy's sake part of the steam expelled from the cylinders of a locomotive is sometimes used to work an injector, which passes the water on, at a pressure of 70 lbs. to the square inch, to a second injector operated by high-pressure steam coming direct from the boiler, which increases its velocity sufficiently to overcome the boiler pressure. In this case only a fraction of the weight of high-pressure steam is required to inject a given weight of water, as compared with that used in a single-stage injector.
[1] "The Steam-Engine," p. 3.
[2] By "weight of steam" is meant the steam produced by boiling a certain weight of water. A pound of steam, if condensed, would form a pound of water.