All injectors are similar in their operation. They are designed to bring a jet of live steam from the boiler in contact with a jet of water so as to cause it to flow continuously in the direction taken by the steam, the velocity of which it in part assumes, back into the boiler and against its own pressure.
There are three distinct types of live steam injectors, the “simple fixed nozzle,” the “adjustable nozzle,” and the “double.” The first has one steam and one water nozzle which are fixed in position but are so proportioned as to yield good results. There is a steam pressure for every instrument of this type at which it will give a maximum delivery, greater than the maximum delivery for any other steam pressure either higher or lower.
The second type has but one set of nozzles, but they can be so adjusted relative to each other as to produce the best results throughout a long range of action; that is to say, it so adjusts itself that its maximum delivery continually increases with the increase of steam pressure. The third type, double injectors, are those in which the delivery from one injector is made the supply of a second, and they will handle water at a somewhat higher temperature than single ones with fixed nozzles. The double injector makes use of two sets of nozzles, the “lifter” and “forcer.” The lifter draws the water from the reservoir and delivers it to the forcer, which sends it into the boiler. All double injectors have fixed nozzles.
The action of the injector is as follows: Steam being turned on, it rushes with great velocity through the steam nozzle into and through the combining tube. This action causes air to flow from the suction pipe, which is connected to the combining tube, with the result that more or less vacuum is formed, thus inducing a flow of water.
Note.—The motive force of the injector is found in the heat received from the steam. The steam is condensed and surrenders its latent heat and some of its sensible heat. The energy so given up by each pound of steam amounts to about 900 thermal units, each of which is equivalent to a mechanical force of 778 foot pounds. This would be sufficient to raise a great many pounds of water against a very great pressure could it be so applied, but a large portion of it is used simply to heat the water raised by the injector. The above explanation will apply to every injector in the market, but ingenious modifications of the principles of construction have been devised in order to meet a variety of requirements.
Fig. 515.—See page [252].
