The steam enters from above, the flow being regulated by the handle K.
The steam passes through the tube S and expands in the tube V, where it
meets the water coming from the suction pipe. The condensation takes
place in the tubes V and C, and a jet of water is delivered through
the forcer tube D to the boiler. Connection passages are made to the
chamber surrounding the tubes C, D, and to the end of tube V. If the
pressure in this surrounding chamber becomes greater than that of the
atmosphere, the check valve P is lifted and the contents are discharged
through the overflow.
So long as the pressure in this chamber is atmospheric, the check valve
P remains closed, and all the contents must be discharged through the
tube D.
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 a good result. 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 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 are
fixed nozzle.
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 followed
by the steam, the velocity of which it in part assumes, back into the
boiler and against its own pressure.
As a thermodynamical machine, the injector is nearly perfect, since
all the heat received by it is returned to the boiler, except such a
very small part as may be lost by radiation; consequently its thermal
efficiency should be in every case nearly 100 per cent. On the other
hand, because of the fact that its heat energy is principally used in
warming up the cold water as it enters the injector, its mechanical
efficiency, or work done in lifting water, compared with the heat
expended, is very low.
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 induces a flow of air from the suction
pipe, which is connected to the combining tube, with the result that a
more or less perfect vacuum is formed, thus inducing a flow of water.
After the water commences to flow to the injector it receives motion
from the jet of steam; it absorbs heat from the steam and finally
condenses it, and thereafter moves on into the forcer tube simply as
a stream of water, at a low velocity compared with that of the steam.
At the beginning of the forcer tube it is subjected only to atmospheric
pressure, but from this point the pressure increases and the water
moves forward at diminished velocity.
“POINTS” RELATING TO THE INJECTOR.
In nine cases out of ten, where the injector fails to do good service,
it will be either because of its improper treatment or location, or
because too much is expected of it. The experience of thoroughly
competent engineers establishes the fact that in almost every instance
in which a reliable boiler feed is required, an injector can be found
to do the work, provided proper care is exercised in its selection.
The exhaust steam injector is a type different from any of the
above-named, in that it uses the exhaust steam from a non-condensing
engine. Exhaust steam has fourteen and seven-tenths (14.7) pounds of
work, and the steam entering the injector is condensed and the water
forced into the boiler upon the same general principle as in all
injectors.
The exhaust steam injector would be still more extensively used were it
not for a practical objection which has arisen—it carries over into the
boiler the waste oil of the steam cylinder.
Some injectors are called by special names by their makers, such as
ejectors and inspirators, but the term injectors is the general name
covering the principle upon which all the devices act.
The injector can be, and sometimes is, used as a pump to raise water
from one level to another. It has been used as an air compressor, and
also for receiving the exhaust from a steam engine, taking the place in
that case of both condenser and air pump.
The injector nozzles are tubes, with ends rounded to receive and
deliver the fluids with the least possible loss by friction and eddies.
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 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.
That the condensation of the steam is necessary to complete the process
will be evident, for if the steam were not condensed in the combining
chamber, it would remain a light body and, though moving at high speed,
would have a low degree of energy.
Certain injectors will not work well when the steam pressure is too
high. In order to work at all the injector must condense the steam
which flows into the combining tube. Therefore, when the steam pressure
is too high, and as a consequence the heat is very great, it is
difficult to secure complete condensation; so that for high pressure
of steam good results can only be obtained with cold water. It would
be well when the feed water is too warm to permit the injector to work
well, to reduce the pressure, and consequently the temperature of the
steam supplied to the injector, as low pressure steam condenses much
easier, and consequently can be employed with better result. Throttling
the steam supplied by means of stop valves will often answer well in
this case. The steam should not be cold or it will not contain heat
units enough to allow it to condense into a cross section small enough
to be driven into the boiler. This is the reason why exhaust injectors
fail to work when the exhaust steam is very cold. It also explains why
such injectors work well when a little live steam is admitted into the
exhaust sufficient to heat it above a temperature of 212°.
Leaks affect injectors the same as pumps, and in addition, the
accumulation of lime and other mineral deposits in the jets stops the
free flowing of the water. The heat of the steam is the usual cause of
the deposits, and where this is excessive it would be well to discard
the injector and feed with the pump.
The efficient working of the injector depends materially upon the size
of the jet which should be left as the manufacturer makes it; hence in
repairs and cleaning a scraper or file should not be used.
For cleaning injectors, where the jets have become scaled, use a
solution of one part muriatic acid to from nine to twelve parts of
water. Allow the tubes to remain in the acid until the scale is
dissolved or is so soft as to wash out readily.
The lifting attachment, as applied to any injector, is simply a steam
jet pump. It is combined with the injector proper and is operated by
a portion of the steam admitted to the instrument. Nearly all the
successful injectors on the market are made with these attachments, and
will raise water about 25 feet, if required, from a well or tank below
the boiler level.
Where an injector is required to work at different pressures it must
be so constructed that the space between the receiving tube and the
combining tube can be varied in size. As a rule this is accomplished by
making both combining and receiving tubes conical in form and arranging
the combining tube so that it can be moved to or from the receiving
tube, and the water space thereby enlarged or contracted at will. The
adjustment of the space between the two tubes by hand is a matter of
some difficulty, however; at least it takes more time and patience
than the average engineer has to devote to it, and the majority of the
injectors in use are therefore made automatic in their regulation.
The injector is not an economical device, but it is simple and
convenient, it occupies but a small amount of space, is not expensive
and is free from severe strains on its durability; moreover, where a
number of boilers are used in one establishment, it is very convenient
to have the feeding arrangements separate, so that each boiler is a
complete generating system in itself and independent of its neighbors.