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How many times have we all watched the man on the engine operate
the injector to fill the boiler? There would be some steam coming
from the overflow followed by some water then that unmistakable
sound of an injector working. Steam from the boiler was being used
to put water into the same boiler. Isn’t that perpetual motion?
Not quite, as we shall see.
The principles involved and their discovery go back many years
to perhaps as far as Bernulli and Venturi. If we are not careful,
however, we’ll be having Adam and Eve as the inventors of the
atom bomb. So let us begin with outlining just what it is that the
injector does. That is, let’s dissect the steps.
First, it must draw water from the supply tank and bring it to
the injector. We can call that priming although many will recognize
it as lift. Then the pressure is increased to something above that
of the boiler onto which the water is to be injected. For the first
part, the prime or lift, we can thank the Marquis Mannuary
d’Ectot, who in 1818, was granted a French patent for a steam
jet apparatus for pumping water from one height to another. Or,
said another way, from one pressure to another.
His device contained all of the components that we now associate
with an injector. That is, a combining tube and a delivery tube.
But he was not thinking of a boiler feed device. It was Henri J.
Giffard that put it all together and was the one to receive a
patent from the French government in May, 1858, for a boiler feed
device. At first his development was looked upon with skepticism
for it was thought that he was trying to invent a perpetual motion
machine. Truth did triumph and he received an award from the
Academy of Science in Paris; their mechanical grand prize for
1859.
But just what had prompted M. Giffard to develop such a device?
But of course it was for his airship! True. Giffard is the inventor
of the dirigible. The steer able balloon or dirigere from the
Latin. On September 24, 1852, he flew his 143 foot long by 39 foot
diameter cigar shaped balloon powered by a 3 horsepower steam
engine weighing 350 pounds some seventeen miles. As we all know, it
really was necessary for internal combustion engines having much
greater horsepower to weight ratios to be developed before flight
was to become completely practical. But, as Giffard was a man of
great technical capability it is fitting that he should be the one
credited with devising a boiler feed device of such physical
simplicity but of such thermodynamic complexity.
The need must have been acute for the device took the industrial
world by storm. Patent rights were taken up by Messrs. Sharp,
Stewart & Company in England in 1859. By 1860, William Sellers
& Company of Philadelphia were offering them for sale from
their factory. Although the basic working parts of the injector
have remained the same as in the original device, there have been
many improvements added over the years. Injectors for specific
services such as railway locomotives were built to quite different
designs than those for, say, a traction engine. The ones used even
today on portable boilers and our favorite traction engine are
probably the simplest, trouble free, device of anything used around
steam.
Although they appear simple in construction, don’t let that
be deceiving. The design of the nozzles; the taper, their diameter
and the relative diameters between combining and delivery tubes is
critical. The diameter of the steam tube outlet has to be larger
than the delivery tube, or the injector will not work. There are at
least two popular explanations of the mechanism of operation. But
these are simplistic explanations. There is only one true thremo
dynamic and hydraulic explanation which is all tied up in
Bernulli’s Theorem and which at this stage of my career I am
not certain that I could go to the blackboard and go through
again.
We can, however, look at the at E letting atmospheric pressure
lift the water from the supply thus filling from the supply pipe A
through valve B enters the steam nozzle where it expands thus
reaching a high velocity at C. When velocity increases in a closed
system then pressure must decrease and vice versa. So there is a
vaccum formed at E letting atmospheric pressure life the water from
the supply thus filling the space with water. The steam from the
jet is condensed and joins with the supply. The ratio here is about
one pound of steam to 13 pounds of water. The impact of the steam
increases the velocity of the water. As the mixture passes along
the enlarging passage of H its velocity decreases and therefore its
pressure increases. By selecting the various dimensions correctly
it is possible to achieve a pressure at the end of the delivery
tube that is greater than boiler pressure and therefore a flow will
take place into the boiler.
Perhaps this is the time to add a few numbers to the discussion
to help with the understanding of the operation. We can take the
case of a Case (J. I., that is, with pun intended) where 120 pounds
pressure is common. At this pressure the velocity of the steam in
the jet at C would be around 1,400 feet per second at the time
valve B is first opened. But, as steam flow is established and a
vacuum is formed of perhaps 20′ then the velocity could reach
3,500 feet per second. By the time the steam and water have
combined and have passed through the expanding tube H the combined
velocity will be around 170 fps. Let us suppose that we ran boiler
water through a nozzle and did so at the 120 psi pressure. We would
find the water velocity was only about 133 fps. We have, therefore,
a margin between the velocity of the feed water and the velocity
equivalent of the static water in the boiler. QED, the injector
works. And it is not a perpetual motion machine. The differential
velocity is, shall we say, wasted. Or, put another way, there is no
loss of heat units, they all go back into the boiler. But we have
changed the conditions and we have not used the total potential.
Should you wish to look it up in your old Funk & Wagnall’s,
it’s spelled entrophy.
If instead of an injector we had installed a simplex
reciprocating steam pump for our boiler feed then the seeming
paradox would not exist. We could have the boiler feed itself by
simply letting the steam cylinder have a larger diameter than the
water cylinder. The water side pressure would be higher than the
steam side by the ratio of the areas of the two cylinders. Now,
then, think of the diameter of the steam jet in the injector and
that of the inlet to section H of the diagram. When the injector
begins to work there is always some water in the overflow just
before the water flows to the boiler. When steam is first condensed
by the lifted water there is a water piston formed in the
condensing tube being pushed by steam issuing from a nozzle of
greater area. Think of these as the simplex pump. It is just
another way of our explaining the operation.
The next time that you watch an Iron Man start his injector,
just remember, it is a technological spin-off from the aviation
field. We have come a long way in the past hundred or so years on
the basic underlying theories developed by the men of early
science.
