CCCXLV.

NOTE ON THE STEAM TURBINE.

BY J. BURKITT WEBB, HOBOKEN, N. J.

(Member of the Society.)

IF steam is to be used in turbines, it will be well to have clear conceptions of the fundamental principles of their construction and action. Judging from various papers before this Society, and the discussions upon them, and from similar productions met with elsewhere, the subject is but imperfectly understood. Some of these papers, indeed, claim that standard writers on the subject have been poorly informed thereupon, and have fallen into errors detected (only too) readily by their authors; while others admit that the standard authors are or may be right, but claim that their presentation of the subject is faulty.

I shall not now attempt to discuss all the principles laid down by Rankine and others, but wish to call attention to one of special interest in connection with the steam turbine, in the hope of provoking an interesting discussion thereon.

When steam flows out from under a considerable pressure it attains a very high velocity. In the production of this velocity the steam expands from the high to the low pressure, and the mechanical energy thus produced, and existing in the form of kinetic energy in the moving steam, would seem to be obtained with a high degree of economy. The problem, therefore, is to some extent the same as in a water turbine: having given a stream of fluid at a certain velocity, to abstract as much as possible of the energy from it by allowing it to react upon a moving wheel. Now the primary condition of economy is that the fluid shall leave the wheel with only enough velocity left in it to get it out of the way; and, in the case of the steam turbine, this requires an almost if not quite impracti cable velocity for anything like a great difference of pressures.

The subject not only includes a consideration of that form of passages through the wheel which will allow it to move with the least velocity, but of the best method of constructing it to resist

centrifugal force, and how to balance and lubricate it. The advantages of a successful turbine are too apparent to need mention, and I hope to hear of progress in this direction.

DISCUSSION.

Mr. Ambrose Swasey.—I would like to speak of a steam-turbine on which a gentleman is working in Cleveland. I had occasion to cut some gearing for it some little time ago, and I would say that he has run it up to twenty-five thousand revolutions per minute. I do not take any one else's statement for that; I timed it with an indicator. He has run the turbine for several months, developing a great deal of power with it. It is about six inches in diameter, and he uses the steam expansively, on a similar principle to the one mentioned by Prof. Webb; but he has a very high velocity, the highest I ever saw. In fact, I did not suppose that gearing could be run as fast. The size of the large gear is twelve inches in diameter, about twenty pitch, and the pinion is one inch in diameter. The first one was made of rawhide, and that lasted pretty well; but the heat from the steam softened it after a while, and then we got one of vulcanized fibre. That has done very hard work and has worked for a long time. 18,000 is the usual speed

for it.

Prof. Webb.-Did the gentleman stay near it himself?

Mr. Swasey.-It stood on a table geared up to run, and I stood very near to it, and he was running it at 19,000 when I first indicated the speed; and he said he had run it that way for several days. I said, "How fast have you run it?" and he opened the valve and let it go at 25,000. It was a simple matter to indicate its speed, because it was all geared up. All I had to do was to indicate one

of the shafts.

Prof. Denton.-What is the diameter of the largest rotating part?

Mr. Swasey.-About six inches; the steam is taken in the center and there is a stationary disk on each side, so that the wheel is perfectly balanced. He has developed, as he claims, as much economy as a common slide-valve engine at that speed. It was certainly very interesting to me, because I did not suppose that it could be run as fast as that.

Prof. Webb.-Will you state just how the gearing is proportioned?

Mr. Swasey. The gears are 20 pitch, one inch face; the pinion is vulcanized fibre, and the gears are 12 inches in diameter and of bronze. There are two of them, one on each side. He had considerable trouble about the shaft heating in the first place; but then he made it hollow, and arranged it so that it drew the air through the center; moreover, the shaft is connected in such a way that no pressure comes on the bearings. The shape of the series of blades is such that as steam passes from one to the other it gets to a larger diameter, and so on, so that the steam is used expansively. The size of the shaft is about seven sixteenths of an inch.

Prof. Denton.-As I understand, this machine has one theoretical advantage and one theoretical disadvantage. The phenomena of cylinder condensation, which in a reciprocating engine we know is the great expense of steam, is here eliminated, I believe. In the reciprocating engine we have the cooling of the surfaces at exhaust and the condensation during admission. There is no such action there; but there must be some friction in going from each of those sections to the other. I did not hear Prof. Webb speak of that— whether that friction is not lost. But the disadvantage I see in the device is the clearance that those vanes must have in the casing. That rotating vane must clear that casing, in going at that speed, by some sensible amount. There must be considerable clearance allowed to keep it from touching the sides. Now, the slightest clearance on such a circumference will waste lots of steam. I have in mind a rotary engine of the ordinary style carrying a revolving vane on a hub, and the latter is the dividing line between the exhaust side and the live steam side. Now, one thirty-second of an inch variation of distance between the hub and the abutment will let through 250 pounds of steam an hour, or 80 lbs. of steam per hour per H. P. The hub is 5 inches long. In other words, you have a crack there about of an inch by 5 inches. That is the sole cause of the rotary engine using more steam than an ordinary engine. There is no mystery about rotary engines using more steam than others. It is nothing but leakage. I have made this rotary engine go just as well as an ordinary slide-valve, by fitting it closely and then let it come back to this leaky state, and you get your consumption up again.

Prof. Webb.-I understand from Mr. Swasey that we do not know exactly the shape of the vanes of the Cleveland turbine, and I want to make the suggestion that the speed of the turbine might be lowered by altering the shape of the vanes. As to leakage, I

suppose, if it is built properly there need be no more than one thousandth of an inch clearance, and then there would not be much steam escaping past the ends of the vanes. Perhaps fifteen to twenty-five thousand revolutions will not prove necessary, because the compound principle of these turbines affords a means of reducing the speed; by increasing the number of rings of vanes the speed can be reduced.

Prof. Wood.-I would like to ask Mr. Swasey whether the enlargement for expansion was along or parallel to the axis, or whether the steam passed around and then radially outward.

Mr. Swasey.-There are concentric disks. The steam passes from a smaller disk to one of larger diameter, but on a radial plane. Prof. Webb.-Since the adjournment of the Erie meeting, I have learned the following particulars about the Dow steam-turbine.

Steam passes radially outward through a succession of buckets and passages, there being six compoundings. The diameter of the working wheels is 53 inches, spindle or shaft g of an inch diameter, weight of moving parts 7 lbs. 7 oz.; highest measured speed, 35,000 revolutions per minute (so that the outer circumference traveled nearly 9 miles per minute). In pumping water with a boiler pressure of 70 lbs., it was estimated, from the work done, that it developed 20 horse-power with less than 27 pounds of wet steam per horse-power per hour.

For the benefit of those who have given no attention to this subject, I will briefly describe two successful steam-turbines.

One of these was used many years since to run a wood-planer, and, being coupled directly to the shaft, it ran at the same speed, or at about 4,000 revolutions per minute. It was essentially a Barker's mill run by steam, no attempt being made to use it expansively. Now, the speed was much too low for an economical use of the steam; but, as there was an abundance of shavings for fuel, this was a small disadvantage, and the simplicity and convenience of the thing made it successful. (A more detailed description of this wheel and its mode of action was given with the assistance of previously prepared blackboard sketches and simple calculations of the amount of steam used and the horse-power produced.)

Rankine refers to this wheel in his Steam Engine, page 538, in the following words:

"The REACTION STEAM ENGINE, in a rude form, is described in the Pneumatics of Hero of Alexandria. It was improved and

brought into use to a limited extent by Mr. Ruthven. Its principle and mode of action are analogous to those of a reaction water wheel.

"The FAN STEAM ENGINE, invented by Mr. William Gorman, is analogous in its principle and mode of action to an inward flow water-turbine. An engine of this kind was used at the Glasgow City Saw Mills, and was considered equal in efficiency to an ordinary high-pressure engine."

Successful steam-turbines are now being built in England in which the expansion of steam from 150 lbs. boiler pressure to that of the atmosphere is utilized. Two such turbines may be seen in operation on the steamer City of Berlin. They are duplicates, each one of them capable of running the three hundred or more incandescent lights that are used. The horse-power of each turbine is about thirty, and they run at some nine thousand revolutions per minute. The maximum diameter of the revolving part is about nine inches, and the shaft is coupled directly to the armature of the dynamo, which is in line with it and runs at the same speed. The whole machine, including the dynamo, occupies seven or eight feet in length by eighteen inches square.

(A detailed description was given with the assistance of previously prepared blackboard sketches. Those who desire further information can find cuts and description in Industries-London and Manchester, Friday, January 13, 1888, Vol. IV., No. 81.)

APPENDIX.

The following facts with respect to the flow of steam and centrifugal force may be of interest :-

In Rankine's Steam Engine, page XIV, is the following:

"Outflow of steam. When the external absolute pressure is less than 3-5ths of the internal, calculate the outflow as if the external absolute pressure were equal to 3-5ths of the internal.

"For a rough approximation, let p, be the internal and P2 the external absolute pressure; q, the weight of outflow per unit of area per second; then, when p1 = or < & P2, J = P1⁄2 ÷ 70 nearly ; and when p2> } P21

On page 564 of the same work is a table of the number of cubic feet in a pound of steam for all pressures.