For safety sake, every boiler ought to have two feeds in order to avoid
accidents when one of them gets out of order, and one of these should
be an 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 suction 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 area. This
area is connected with the outside air by a check valve, opening
outward in the automatic injectors, and by a valve termed the overflow
valve.
The operation of the injector is based on the fact, first demonstrated
by Gifford, 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—taking it at an average
boiler pressure of sixty pounds—travels 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 pass 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.
The following cut 101 represents the outline of one of the best of a
large number of injectors upon the market, from which the operation of
injectors may be illustrated.
[Illustration:
S. Steam jet. V. Suction jet.
R. Ring or auxiliary check.
M. Steam valve and stem, handle.
X. Overflow cap.
C-D. Combining and delivery tube.
P. Overflow valve. O. Steam plug.
N. Packing nut. K. Steam valve