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THE STEAM TURBINE

AS APPLIED TO MARINE PURPOSES

CHAPTER I.

HISTORICAL.

IN the year 1888 I had to select a type of engine to drive a dynamo for lighting a ship. Mr Parsons came to see me, and I was persuaded to order two of his turbines, though the engine had been previously tried with not too satisfactory results. In the course of conversation I asked him why he did not put the engine into a ship to propel it, and his answer was a silent smile. The turbines were fitted and successfully tried, and one incident of the trial impressed me very much. The turbine was running with no load on. All the lights in the ship were off, and to test the capability of the machine I suddenly switched on the whole load, expecting to hear the turbine slow down to a speed that would instantly compel me to switch off the load. To my surprise the machine seemed to take no notice of the load. It sang away with the same pleasant hum, and seemed laughing in its sleeve at me. This kind of contingency had been provided for in the ingenious sensitive governor which Mr Parsons had put on it. The two incidents narrated seemed to me to have a striking relation to each other. Mr Parsons' machine was equally ready to run with a very small load or with the whole load. The machine was very like the man. Then he was only running with the small load of lighting a

ship, but later he took up the whole load of an Atlantic liner's tens of thousands of horse-power, and there is no appearance of difference in the man. He has the same silent smile and the same pleasant hum with the big load he now carries as he had with the little load of lighting a ship. He carries his present great load of work and honours with the same serene air and scientific spirit that he carried his little one of earlier years. He has built up a great industry in what many would say is an uncommercial manner. He might have made turbines so cheaply that he could have driven out all reciprocating engines for electric-light work, but he was content to make as many as served his scientific purpose in its steady development of the steam turbine. A shrewd business American friend of mine came to this country about ten years ago to see this steam-turbine driving dynamos. He said: "Why, that machine can be made much cheaper than an ordinary engine. Why does not Parsons take all the orders?" I said: "He takes all he wants to keep his works going, at the prices the other fellows get for their reciprocating engines." He said: "That is so like you Britishers; if you can get your price, you jog along without any ambition. Why, if I owned that invention I should sell as much below the other fellows as ever I could afford, and I should soon shut up their works, and then I should scoop the whole pool and make my own price." This, however, was not the way of Charles Parsons. He has not looked upon commercial success as the only end. He has with each success found a further possibility of improvement, and with each improvement a new success, and so the process has been repeated. In 1897 he sprang upon an astonished world (at the assemblage of war-fleets which met to celebrate the Diamond Jubilee of Queen Victoria) a little vessel 100 feet long which defied all the patrol boats whose duty it was to keep intruders out of the long lanes between the ships, and he raced down those lines at 35 knots, a speed then beyond almost

the hope of the most sanguine naval architect. Those who saw that little streak of steel flash across the water, and who had a spark of imagination, saw in the near future a revolution in steam propulsion which would make all the skill and experience that the marine engineers had acquired in the reciprocating engine a thing nearly as useless as that of the skill of the armourer who clothed the tin-clad knights of old for the jousts. The term turbine used to

be much more associated with water-power than steam, and it is interesting to understand the water progenitor of the steam turbine of to-day.

Barker's Mill (fig. 1) is a machine in which the rotation is caused by the reaction of a stream of water, or two streams of water, issuing in the line of the peripheral motion of the arms. The water flows in the line of the axis of motion till it reaches the arms, when it diverges at right angles along

[graphic]

Fig. 1. Barker's Mill.

the arms, and, reaching the nozzles, it again diverges at right angles to both the axis and the arms. The number

of turns in the course of the stream, and the long length through which the water has to flow and be subjected to losses by friction, make this elementary form of turbine not very efficient. The motion is given to the arms by the reaction of the water at the nozzles, but before reaching the nozzles the frictional and other losses have reduced the available force.

If the water could act directly on the ends of the arms without going through the axis, a more efficient turbine would

be obtained.

a turbine.

Fig. 2 shows a general sectional elevation of such This is called an axial-flow1 turbine.

The water, admitted above the horizontal floor, passes down through the annular wheel containing the guide blades GG, and thence into the revolving wheel W W. The revolving wheel is fixed to a hollow shaft suspended from the pivot p. The sluices are worked by the hand wheel h, which raises

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them. a a are the sluice rods. Fig. 3 shows the sectional form of the guide-blade chamber and wheel and the curves of the wheel vanes and guide blades, when drawn on a plane development of the cylindrical section of the wheel; a a a are the sluices for cutting off the water; bbb are apertures by which the entrance or exit of air is facilitated as the buckets empty and fill. The turbine is of 48 horse-power on 5.12 feet fall, and the supply of water varied from 35 to 112 cubic feet per second. The efficiency in normal working is given as 73

1 Encyclopædia Britannica, 9th edition.

per cent.

The mean diameter of the wheel is 6 feet, and the speed 274 revolutions per minute. The water acts directly upon the ends of the arms, and a large part of the energy in the water is taken from it by the vanes in the wheel and converted into useful work in the shaft.

In the axial-flow water turbine it will be seen that the water flows through a fixed series of guides and into a moving series of vanes attached to a vertical shaft or spindle. The guides direct the streams so that they impinge on the moving vanes at a favourable angle. The vanes so guide the water that in passing through them its reaction shall take out as much velocity from the water as possible.

The steam turbine of the Parsons type is on the same principle, but instead of being a one-stage turbine as the foregoing, it is a series of such turbines, and is called a many-stage turbine. In the water turbine the object in shaping the blades is to cause the water to enter with no shock and leave with no velocity. The energy in the water, which is proportional to its mass and to the square of its velocity, is taken from it by reducing its velocity, and a considerable reduction can be made in one set of vanes, so that a considerable amount of energy can be taken from the water by one set.

But in the steam turbine the mass of the steam is so small that, in order to possess any considerable amount of kinetic energy (which can be abstracted from it by passing through vanes), it must have a very high velocity. This necessitates a very high speed of blades, for, in order to get maximum efficiency, the velocity of blade must be about one-half that of the steam. If steam issue from a condition of no velocity and a pressure of, say, 150 lbs. to a region of atmospheric pressure, its velocity will be about 4000 feet per second. Hence to take out all the velocity from the steam the vanes must have a peripheral velocity of 2000 feet per second. This is the velocity of the projectile of a 12-inch gun after it has gone three

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