Popular Science Monthly/Volume 56/April 1900/Steam Turbines and High-Speed Vessels
|STEAM TURBINES AND HIGH-SPEED VESSELS.|
ALL heat engines at present in use take in heat from a source at a high temperature and discharge most of it at a lower temperature, the disappearance of heat in the process being the equivalent of the work done by the engine. In all cases at the present time the source of heat is from fuel of some kind, and after working the engine the residue is, discharged in the case of the steam engine either to the condenser or in the exhaust steam when non-condensing. In the gas engine it is discharged in the waste gases and into the water jacket around the cylinder.
The earliest records of heat engines are found in the Pneumatics of Hero of Alexandria, about 200 b. c. He describes a reaction steam turbine, a spherical vessel mounted on axes supplied with steam through one of the trunnions from a boiler beneath; the steam escaping through two nozzles diametrically opposite to each other and tangential to the sphere, causing the sphere to rotate by the reaction or momentum of the issuing steam, and analogous to a Barker's water wheel.
Thus, the first engine deriving its motive power from fuel was a crude form of steam turbine, and though it could have been applied to useful work, and could easily have been made sufficiently economical to replace manual and horse power in many instances, yet it lay dormant till 1629 a. d., when Bianca suggested the same principle in a different form. Bianca's steam turbine consisted simply of a steam jet fed from a boiler impinging against vanes or paddles attached to the rim of a wheel which was blown round by the momentum of the steam issuing from the jet.
The piston engine is, however, of comparatively modern origin, and dates from about the year 1700 a. d. Engines of this class are so well known that it suffices to say that they have been practically the sole motive-power engines from fuel in use from 1700 up to 1845, and have constituted one of the most important factors in the development of modern engineering enterprise.
Air engines were introduced about the year 1845, and although the larger engines of the Stirling type were very economical in fuel, yet, on account of the inherent difficulty of heating large volumes of air within metal chambers or pipes—a difficulty arising from the low conductibility of air and consequently the overheating and
Fig.—1. Longitudinal Diagram of Parson's Steam Turbine with Side of Containing case Removed. The construction of the blades and guide vanes is more clearly shown in Fig. 2. The steam enters at J and exhausts after leaving the low-pressure cylinder C.
burning of the metal—they have only come into commercial use for very small powers.
During the last thirty-five years gas engines have been perfected, and more recently oil engines, and in point of efficiency both convert a somewhat larger percentage of the heat energy of the fuel into mechanical energy than the best steam engines. All successful oil and gas engines are at present internal-combustion engines, the fuel being burned in a gaseous form inside the working cylinder.
Very numerous attempts have, however, been made to construct internal-combustion engines to burn solid fuel instead of gas. Some have been so far successful as to work with good economy in fuel, but the bar to their commercial success has been the cutting of the cylinder and valves by fine particles of fuel. This difficulty is not present when the fuel is introduced in the gaseous or liquid form, and hence the success of gas and oil engines; but could this difficulty be overcome, the solid fuel would be the cheaper to use.
Internal-combustion engines, gas engines, oil engines, cannon, etc., owe their superior economy in fuel to the very high temperature at which the heat is transferred from the fuel to the working substance of the engine, and consequently the great range of temperature in the working substance of the engine. In steam engines the temperature is limited by the practical difficulties of deterioration of metal and materials involved in the construction.
About fifteen years ago I was led by circumstances to investigate the subject of improving the steam turbine. In recent times several attempts had been made to apply steam turbine wheels of the Hero and Bianca types to the driving of circular saws and fans. The velocity of rotation with either of these types must necessarily be very high in order to obtain a reasonable efficiency from the steam, a velocity much in excess of that suitable for the direct driving of almost all classes of machinery; gearing was considered objectionable, and it therefore appeared desirable to adopt some form of turbine in which the steam should be gradually expanded in small steps or drops in pressure so as to keep the velocity of flow sufficiently low to allow of a comparatively moderate speed of rotation of the turbine engine.
The method adopted was to gather a number of turbines of the parallel flow type on to one shaft and contained in one case, the turbines each consisting of a ring of guide and a ring of moving blades, the successive rings of blades or turbines being graduated in size, those nearer the exhaust end being larger than those near the steam inlet, so as to allow a gradual expansion of the steam during its passage through the turbines.
The form of the turbine was that of a rotating drum, with outwardly projecting rings of blades which nearly touched the containing cylindrical case, and on the case inwardly projecting rings of guide blades which nearly touched the drum. In the first examples of the engine there were two groups of turbines right-and left-handed on each side of the steam inlet, the exhaust taking place at each end of the turbine case, so as to completely balance end pressure from the steam. More recently one series of turbines only has been used, those on the other side of the steam inlet being replaced by packing rings or rotating balance pistons which
Fig. 2 shows the Arrangement of Moving Blades and Guide Vanes in a Parsons's Turbine. The top outer cover has been removed. The cylinder containing the revolving barrel has, as will be seen, a greater internal diameter than the diameter of the drum. It is the annular space thus formed through which the steam flows and which contains the revolving blades and the fixed guide blades. Between each two rings of moving blades there is a ring of guide blades, the latter being keyed into the containing case. The vanes are set at an angle, so that the steam acts on them as wind on the sails of a windmill.
balance the end pressure and divert the whole of the steam through the turbines on the other side.
The steam entering the annular space between the shaft and the case passes firstly through a ring of guide blades attached to the case, and is given a rotational direction of flow; it then passes to the succeeding ring of blades attached to the shaft, by which its direction of rotation is reversed, thereby impressing the difference of its rotational momentum in torque to the shaft. The steam then passes to the second ring of guide blades, and the process is repeated, and so on, gradually expanding by small increments at each ring of blades; the succeeding rings of blades get longer and wider, and at intervals the, diameter of the turbine drums, cylinders, and rings are also increased. In condensing turbine engines of the larger size an expansion ratio in the turbines of one hundred-fold and upward is attained before the steam passes to the exhaust pipe and condenser.
The loss of power present in engines of the piston class, due to cylinder condensation arising from the variation of steam pressure in the cylinder, is not present in the steam turbine, as the steam pressure remains constant at each turbine ring and each part of the cylinder and barrel, and the numerous tests of steam consumption that have been made have shown that compound steam turbine engines of moderate sizes when working with a condenser are comparable in steam consumption per effective horse power with the best compound or triple condensing steam engines of the piston type. They have been constructed in sizes up to about one thousand horse power for driving alternators and dynamos, and several sets of about two thousand horse power are nearing completion.
The application of the compound steam turbine to the propulsion of vessels is a subject of considerable general interest, in view of the possible and probable general adoption of this class of engine in fast vessels.
In the turbine is found an engine of extremely light weight, with a perfectly uniform turning moment, and very economical in steam in proportion to the power developed, and, further, it can be perfectly balanced so that no perceptible vibration is imparted to the ship. The problem of proportioning the engine to the screw propellers and to the ship to be driven has been the subject of costly experiments extending over several years, with the result that a satisfactory solution has been found, giving very economical results in regard to pounds of steam consumed in the engines per effective horse power developed in propelling the vessel, results which are equal or superior to those so far obtained with triple-expansion engines of ordinary type in torpedo boats or torpedo-boat destroyers. The arrangement adopted may be best described by saying that instead of placing, as usual, one engine to drive one screw shaft, the turbine engine is divided into two, three, or sometimes more separate turbines, each driving a separate screw shaft, the steam passing successively through these turbines; thus when there are three turbines driving three shafts, the steam from the boiler passes through the high-pressure turbine, thence through the intermediate, and lastly through the low, and thence to the condenser.
As to the propellers, these approach closely to the usual form. It has, however, been found best to place two propellers of approximately the same pitch on each shaft at some considerable distance apart, so that the after one shall not be seriously affected by the
wash of the one in front. The advantage of this arrangement is that a sufficient blade area is obtained to carry the thrust necessary to drive the vessel with a lesser diameter of propeller, and so permitting of a higher speed of revolution of the engines.
The problem was complicated by the question of cavitation, which, though previously anticipated, was first practically found to exist by Mr. Thornycroft and Mr. Barnaby in 1894, and by them it was experimentally determined that cavitation, or the hollowing out of the water into vacuous spaces and vortices by the blades of the propeller, commences to take place when the mean thrust pressure on the projected area of the blades exceeds eleven pounds and a quarter per square inch. This limit has since been corroborated during the trials of the Turbinia.
This phenomenon has also been further investigated in the case of model propellers working in an oval tank of water, and to permit of cavitation at more moderate speeds than would otherwise have been necessary, the following arrangement was adopted: The tank was closed, plate-glass windows being provided on each side, through which the propeller could be observed, and the atmospheric pressure was removed from the surface of the water by an air pump; under this condition the only forces tending to prevent cavitation were the small head of water above the propeller, and capillary attraction.
In the case of a propeller of two inches in diameter, cavitation commenced at about twelve hundred revolutions, and became very pronounced at fifteen hundred. Had the atmospheric pressure not been removed, speeds of twelve thousand and fifteen thousand respectively would have been necessary.
Photographs were taken with a camera made for the purpose, with a focal plane shutter giving an exposure of about one thousandth of a second, the illumination being by sunlight concentrated on the propeller from a twenty-four-inch concave mirror.
Photographs were also taken by intermittent illumination of the propeller from an arc lamp, the arrangement consisting of an ordinary lantern condenser, which projected the beam on to a small concave mirror, mounted on a prolongation of the propeller shaft, the reflected beam being caught by a small stationary concave mirror at a definite position in each revolution and reflected on to the propeller. By this means the propeller was illuminated in a definite position at each revolution, and to the eye it appeared as stationary. The cavities about the blades could also be clearly seen and traced, the photographs being taken with an ordinary camera and about ten seconds' exposure.
A series of experiments was also made with model propellers in water at and just below the boiling point, dynamometric measurements being taken of power and thrust with various widths of propeller blade, the conclusion arrived at being that wide and thin blades are essential for fast speeds at sea, as well as a coarse pitch ratio of propeller.
The first vessel fitted with steam turbine machinery was the Turbinia. She was commenced in 1894, and, after many alterations and preliminary trials, was satisfactorily completed in the spring of 1897. Her principal features are: Length, one hundred feet; beam, nine feet; five-foot draught of water under the propellers; forty-four tons and a half displacement on trial; she is fitted with a water-tube boiler of eleven hundred feet total heating surface, and forty-two square feet of grate area, with closed stoke-holds supplied with air from a centrifugal fan mounted on a prolongation of the low-pressure turbine shaft. The engines consist of three compound steam turbines, high pressure, intermediate, and low pressure, each driving one screw shaft; on each of the shafts are three propellers, making nine in all; the condenser is of the usual type, and has four thousand square feet of surface.
When officially tested by Professor Ewing, F. R. S., assisted by Professor Dunkerley, she attained a mean speed on a measured mile of thirty-two knots and three quarters, and the consumption of steam for all purposes was computed to be fourteen pounds and a half per indicated horse power of the main engines. Subsequently, after some small alterations to the steam pipe, she was further pressed, and is estimated to have reached the speed of thirty-four knots and a half. She was, and still is, therefore, the fastest vessel afloat; she has been out in very rough weather, is an excellent sea boat, and at all speeds there is an almost complete absence of vibration.
In the Turbinia the exceptional speed results principally from two causes: 1. The engines, screws, and shafting are exceptionally light. 2. The economy of steam in the main engines is greater than usual.
At full speed the steam pressure in the boiler is two hundred and ten pounds; at the engines, one hundred and seventy-five; and the vacuum in the condenser twenty-seven inches, representing an expansion ratio in the turbines of about one hundred and ten after allowance has been made for wire-drawing in the exhaust pipe.
The first vessels of larger size than the Turbinia to be fitted with steam turbine machinery are the torpedo-boat destroyer Viper for the British Government, and a similar vessel for Messrs. Sir W. G. Armstrong, Whitworth & Company.
These vessels are of approximately the same dimensions as the
thirty-knot destroyers now in her Majesty's service, but have slightly more displacement. The boilers are about twelve per cent larger, and it is estimated that upward of ten thousand horse power will be realized under the usual conditions, as against sixty-five hundred with reciprocating engines.
The engines of these vessels are in duplicate. Two screw shafts are placed on each side of the vessel, driven respectively by a high-and a low-pressure turbine; to each of the low-pressure turbine shafts a small reversing turbine is permanently coupled for going astern, the estimated speed astern being fifteen knots and a half, and ahead thirty-five knots; two propellers are placed on each shaft.
The latter of these two vessels has commenced her preliminary trials, and has already reached a speed of thirty-two knots. The manipulation of the engines is a comparatively simple matter, as to reverse it is only necessary to close one valve and open another, and, owing to there being no dead centers, small graduations of speed can be easily made.
In regard to the general application of turbine machinery to large ships, the conditions appear to be more favorable in the case of the faster class of vessels such as cross-Channel boats, faster passenger vessels, cruisers, and liners; in such vessels the reduction in weight of machinery, as well as economy in the consumption of coal per horse power, are important factors in the case, and in some vessels the absence of vibration, both as regards the comfort of passengers, and in the case of ships of war permitting greater accuracy in sighting of the guns, is a question of first importance.
As regards cross-Channel boats, the turbine system presents advantages in speed, absence of vibration, and, owing to the smaller diameter of the propellers, reduced draught.
As an instance, a boat of two hundred and seventy feet length, thirty-three feet beam, one thousand tons displacement, and eight feet six inches draught of water could be constructed with spacious accommodation for six hundred passengers, and with machinery developing eighteen thousand horse power; she will have a sea speed of about thirty knots, as compared with the speed of nineteen to twenty-two knots of the present vessels of similar size and accommodation.
It is, perhaps, interesting to examine the possibilities of speed that might be attained in a special unarmored cruiser, a magnified torpedo-boat destroyer of light build, with scanty accommodation for her large crew, but equipped with an armament of light guns and torpedoes. Let us assume that her dimensions are about double those of the thirty-knot destroyers, with plates of double the thickness and specially strengthened to correspond with the increased size—length, four hundred and twenty feet; beam, forty-two feet; maximum draught, fourteen feet; displacement, twenty-eight hundred tons; indicated horse power, eighty thousand; there would be two tiers of water-tube boilers; these, with the engine space, coal bunkers, etc., would occupy the whole of the lower portion of the vessel; the crew's quarters and guns would be on the upper decks. There would be eight propellers of nine feet in diameter revolving at about four hundred revolutions per minute, and her speed would be about forty-four knots.
She could carry coal at this speed for about eight hours, but she would be able to steam at from ten to fourteen knots with a small section of the boilers more economically than other vessels of ordinary type and power, and, when required, all the boilers could be used, and full power exerted in about half an hour.
In the case of an Atlantic liner or a cruiser of large size, turbine engines would appear to present some considerable advantages. In the first place they would effect a reduction in weight of machinery and some increase in economy of fuel per horse power developed, both thus tending either to a saving in coal on the one hand, or, if preferred, some increase in speed.
The advantages are, however, less pronounced in this class of vessel on account of the smaller relative power of the machinery and the large quantity of coal necessary for long voyages, but the complete absence of vibration at all speeds, not to mention many minor considerations of saving in cost and reduced engine-room staff, are questions of considerable importance.
- Abstract of the Presidential Address to the Institution of Junior Engineers, November 3, 1899.
- On her second trial trip the Viper attained a mean speed of 34.8 knots, her fastest trial being over 35 knots, or about 41 statute miles per hour, with an indicated horse power of 11,000. This vessel is of about 350 tons displacement.