CHAPTER XVII

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 respecta 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 9A.M.to 10P.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 thevalve 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 therest 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 ownengines required, the current was alwaysfromour pipe into the connected pipes.

Scale 20Diagram from Allen Engine, back end of cylinder, at Fair of American Institute, 1870.

Scale 20

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-waterunder 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 abovediagramrepresents 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 eachcase, of six different loads, exactly the same effective work done that was shown by the brake.

Scale 20Friction Diagram from Allen Engine at Fair of American Institute, 1870.Scale 24Gage Pressure 60Diagram from Allen Engine, Fair of American Institute, 1870, cutting off at ¹⁄₄ stroke.

Scale 20

Friction Diagram from Allen Engine at Fair of American Institute, 1870.

Scale 24Gage Pressure 60

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 subtractedto 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.

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