Mr. Allen’s Invention of his Boiler. Exhibition at the Fair of the
American Institute in 1870.
At that time the “Field boiler tubes” were attracting considerable
attention in London. These were designed to prevent the water from
being lifted from the closed bottom of vertical tubes over the fire,
which would cause them to be burned out. The Field tubes were smaller
internal tubes, provided at the upper end with three wings which
centered them in the middle of the external tubes, in which they
reached nearly to the bottom. They were made slightly bell-mouthed
at the top. The circulation was down the internal tube and upwards,
through the annular space. The bell mouth prevented these currents from
interfering with each other. One morning Mr. Allen said to me that
he had an idea that by inclining the tubes at a small angle from the
vertical a better circulation would be got than in the Field tubes.
He thought the steam as fast as formed would all go to the upper side
of the inclined tubes, and would rush up along that surface without
driving the water before it, and so the water would always be at the
bottom of the tube, no matter how hard the boiler was fired. I was
struck with the idea and determined to test it. I got the largest
test-tube I could find, 1¹⁄₄ inches in diameter and 15 inches long, and
set it in an adjustable support, and applied the flame of four Bunsen
burners, bunched together, at the bottom. In a vertical position the
water was instantly thrown clean out of the tube. At about the angle
of 20 degrees Mr. Allen’s idea was completely realized. The bubbles of
steam united in a continuous stream on the upper side and rushed up
with no water before them. With the most rapid generation of steam the
water remained solid at the bottom of the tube. The sight was a very
interesting one. I reasoned that if this satisfactory result was got
under a short column of water, and only the pressure of the atmosphere
and in a small tube, it could certainly be relied upon under a column
of water several times longer, under a pressure of several atmospheres
and in a much larger tube. The greater the pressure the smaller the
bubbles of steam would be. Those formed under one atmosphere were about
as large as kidney beans.
Mr. Smith was anxious to have us exhibit the engine at the Fair of the
American Institute in New York in the fall of 1870. This Institute was
then at the height of its usefulness, and its annual fairs were crowded
with exhibits and attracted wide attention. Mr. Allen and I consulted
about it, and on account of the liability of getting more hot water
than steam from the queer boilers that might be exhibited, we agreed
that, as the engine would have to be tested for economy, it would not
be safe to exhibit unless we could make a boiler according to Mr.
Allen’s plan to supply the steam. With this boiler we could certainly
get dry steam, and felt confident of getting it superheated.
Our recommendation to that effect was adopted, and we prepared to
exhibit two engines, one of them 16 inches diameter of cylinder by
30 inches stroke to make 150 revolutions per minute, and the other 6
inches in diameter by 12 inches stroke to make 300 revolutions per
minute, and a boiler. We also made to drive our own shop, to take the
place of the portable engine and boiler, an engine of the smaller size
above named, except that the cylinder was, by thickening its walls,
made 5 inches in diameter only. This was because this size would be
ample for the power we required, and I would be able to show the effect
of inertia of the heavy reciprocating parts in producing smooth and
silent running, much better than with a 6-inch cylinder, which would
have about 50 per cent. larger area with no greater weight in the
reciprocating parts, except only in the piston. This exhibition, as we
shall see, became of great importance. We made also an Allen boiler for
ourselves, of four sections; really, as it proved, three or four times
as large as we needed, but we could not well make it smaller.
This exhibition at the American Institute was in every respect a great
success, not a drawback of any kind about it. The little engine was
used by Merrill & Sons to drive their exhibit of forging machinery,
hammers and drops. The large engine gave motion to a miscellaneous
exhibit of machinery in motion. The exhibition of machinery in motion
closed each day for an hour from 12 to 1, and again from 6 to 7, but
I ran these engines continuously from 9 A.M. to 10 P.M., to show that
high speed asked no favors. There were five boilers, including our own,
from the start. The other four were smaller than ours. Another boiler,
the largest of all except ours, was started later, as will be told.
Ours had a brick flue and chimney, but only 30 feet high. Those of the
others were iron. There were a number of other engines and pumps and
pulsometers, all steam eaters.
It was found impossible to keep up steam. It fell to half pressure
every day before stopping time came.
One morning, about a week after the opening, on my arrival my friend
Mr. Lee, who was superintendent of the machinery department, came to me
and said, “Do you know what they are all saying about here?” “No,” I
replied. “Well,” said he, “you ought to know. It is that your engines
use all the steam, and your boiler does not make any, and that is
where all the trouble is.” I replied: “I am ready for them. You see
that valve up there. I put it in expressly to meet whatever questions
might arise. By closing it I can shut my system off from the general
steam connections and run my two engines from my own boiler, and will
try to get on without their assistance.” So a ladder was brought and
I went up and shut the valve. Directly my pressure rose to 70 pounds,
the pressure allowed; my automatic damper closed as nearly as it was
permitted to do, and the steam began to blow off. To prevent this, the
fireman had to set his door a little way open, and in this condition we
ran all day. In the rest of the show the steam ran down until at noon
there was barely 15 pounds pressure, but the wrath of the exhibitors of
machinery driven by other engines was blowing off. After the noon hour
the additional boiler was started and helped them a good deal, so that,
starting with 70 pounds at 1 o’clock, at 5 o’clock they still had 25
pounds pressure.
Mr. Lee asked me several times during the day to open the valve and
I refused to do it. Finally, at about 5 o’clock, he said to me, “If
you don’t open that valve, I shall.” “Well,” said I, “there will be a
number of the managers of the Institute here at this hour, I presume,
and if you will send for them and have them come here and see the state
of the case for themselves then I will open the valve.” So this was
done. Half a dozen of these gentlemen came and made an inspection of
the boilers and said to me: “We are quite satisfied. It is evident that
you have been supplying most of the steam and using very little.” So I
opened the valve and there was no further trouble. The assistance of
the large boiler added that day prevented any serious fall of pressure
afterwards.
A few days after the above incident a committee of the managers waited
on me and said: “We have heretofore had a good deal of trouble with
our steam supply, and would like next year to have a boiler that we
can rely upon. What will you ask to leave this boiler here for our use
next season?” I agreed with them for three hundred dollars, and so the
boiler remained for the next exhibition, when there will be something
more to be said about it and views of it will be shown. That winter
Barnum wintered his animals in that building, and paid me three hundred
dollars more for the use of the boiler to warm it. In my ignorance of
business these items of good luck came in very handy. Mr. Allen said he
never heard of a new thing so successful from the start.
The remark respecting my exhibit of engines and boiler at the fair of
the American Institute in 1870, that there was not a drawback of any
kind about it, must, however, be qualified in one respect. I was not
able to run my 16×30 engine at the speed of 150 revolutions per minute,
as I had intended.
A blunder had been made in the size of the driven pulley on the line of
shafting. It was smaller than specified, because the contractor for the
shafting put on a pulley he had, and this was not observed till we were
running, when it was too late to change it. The exhibitors of machinery
in motion all complained that their machines were running too fast, and
after two or three days the directors ordered me to reduce the speed
of my engine to 125 revolutions per minute, at which speed it was run
through the rest of the fair. I was much disappointed, but consoled
myself with thinking that perhaps this speed would please the general
public better than the higher one, the engine even then being three or
four times too large for its work.
The boiler gave me at the engine steam superheated 23 degrees all
the time. This I proved by transposing the thermometers. I had two
thermometers, duplicates, one on the steam-chest and the other on the
first boiler drum, in which the steam was not superheated. The former
indicated 23 degrees higher temperature. When these were exchanged the
same difference continued to be shown.
I was greatly interested in observing in my own and other engines the
relative amounts of initial cylinder condensation, as this was shown in
the steam blown from the indicator stop-cocks. I had one of these on my
steam-chest, and the steam blown from this was not visible until three
or four inches above it. That blown from the stop-cocks on my cylinder
had a very little tinge of white, showing the superheating to have been
lost and a slight initial condensation to take place. As the piston
advanced, the blowing steam became invisible, showing re-evaporation,
through the falling of the boiling-point on the expansion.
On other engines, of which several were exhibited, the observation
showed large amounts of initial condensation. From one of them I
remember the blowing steam looked like a white painted stick.
I observed that the steam only lost three degrees of its superheat in
passing through 25 feet of 6-inch pipe from the boiler to the engine.
For this comparison I placed a thermometer on the second steam drum, in
which the steam was superheated, where it showed about 26 degrees of
superheat. This measured the rate at which the heat was lost through
the felt covering of the pipe, and suggested an excellent method of
comparing the protective value of different coverings under absolutely
the same conditions.
The superheating of the steam for our own engine was not affected by
the connection of our steam-pipe with those of the other engines.
The explanation of this phenomenon seemed to be that as our boiler
generated far more steam than our own engines required, the current
was always _from_ our pipe into the connected pipes.
[Illustration: Diagram from Allen Engine, back end of cylinder, at Fair
of American Institute, 1870.]
I was here first made alive to the enormous waste of steam in the
feed-pumps, a separate one for every boiler, including our own. In
these the steam has to follow full stroke, at a pressure sufficient, on
the larger area of the steam piston, to overcome the pressure in the
boiler. Moreover, the extreme heat interval between the temperatures
of the entering and the exhaust steam and the slow motion, permitting
the walls of cylinder, heads and piston to be cooled very deeply by the
exhaust, produces the condensation of probably from five to ten times
as much steam as is usefully employed, differing according to the rate
of piston motion. I began to rather admire the practice of the English,
who knew nothing about boiler feed-pumps, except those on the engine,
and I certainly wonder that the genius did not arise long before he
did, who first thought of exhausting the feed-pump into the feed-water
under atmospheric pressure only, so returning to the boiler all the
heat received in the pump that is not converted into the work of
overcoming the boiler pressure and the atmospheric resistance or lost
in external radiation.
The above diagram represents the performance of this engine in its
regular work. It shows distinctly the compression curve, the points
of cut-off and release, and the back pressure required to expel the
exhaust. It will be seen that the expansion fell to 5 pounds below
the atmosphere. I have added to it a line representing the waste room
in ports and clearance, and the theoretical expansion curve plotted
according to the law of Mariotte, showing the expansion terminating 2.5
pounds above this curve, from the re-evaporation already noted and the
heat abandoned by the steam as the pressure fell.
After the close of the fair this engine was run on several days,
under a variety of loads applied by a Prony brake, in the presence
of a number of engineers and others who had been invited to witness
the trials. Of the diagrams taken on these trials, I find that I have
preserved only the two here shown, namely, a single friction diagram
from the back end of the cylinder, on a scale of 20 pounds to the inch,
and a diagram showing large power, taken from the front or crank end,
on a scale of 24 pounds to the inch. The former shows the trifling loss
from friction in this engine. I have measured this card, and find the
mean effective pressure, or difference between the areas showing the
excess of the forward over the back pressure, to be 1.1 pounds on the
square inch, which, assuming the opposite card to be equal with it, was
the friction of the engine. The exhaust line shows the power required
to reverse the direction of motion of the exhaust, which at the end of
the stroke was rushing back into the cylinder.
The latter is especially interesting as showing the identity of the
expansion curve with the theoretical, three points on which are marked
by the crosses. The sharp reaction of the indicator while the crank was
passing the dead center will also be observed.
After this trial I made a careful comparison of the diagrams taken
under the different loads with the friction diagrams, and found the
uniform results to be that the friction diagrams subtracted from the
diagrams taken under the load left in each case, of six different
loads, exactly the same effective work done that was shown by the brake.
[Illustration: Friction Diagram from Allen Engine at Fair of American
Institute, 1870.]
[Illustration: Diagram from Allen Engine, Fair of American Institute,
1870, cutting off at ¹⁄₄ stroke.]
From this I concluded that in these engines the use of the friction
brake is unnecessary; the friction is sensibly the same under all
loads, and the friction diagram only needs to be subtracted to learn
from the diagram the amount of effective work done.
The verdict of the judges, President Barnard of Columbia College,
Thomas J. Sloane, the proposer of the gimlet-pointed wood screw, now
in universal use, in place of the flat-ended screws formerly used, and
inventor of the special machinery required for their manufacture, and
Robert Weir, engineer in the Croton Aqueduct department, may be summed
up in the single expression from their report, “The performance of this
engine is without precedent.” For its success I was largely indebted,
first, to the remarkable circulation and steam-generating power of the
boiler, and, second, to the superheating of the steam in the second
drum.