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STEAM See also:ENGINE second, 8 in the third and 8 in the See also:fourth, or 34 stages in all. The See also:low pressure See also:turbine (fig. 63) comprises 28 more stages stepped as shown in the figure. The See also:reversing turbine which is seen on the See also:left-See also:hand See also:side in fig. 63, at the See also:place where the rotor is reduced in See also:diameter, has 26 stages in 4 steps. These turbines have a See also:total normal See also:horse See also:power of 12,500, and run at 450 revolutions per See also:minute. 128. See also:Longitudinal Forces in Marine Turbines.—In a marine steam turbine the See also:size of the dummy is reduced so that instead of balancing the whole steam thrust it leaves a resultant force which nearly balances the propeller thrust. Consequently only a small thrust See also:block has to be provided to take any difference there may be between these forces. This thrust block is shown on the extreme right in each figure, beyond the gland and bearing. The dummy (at D in the figures) is made up of some 22 rings of See also:brass fixed in the See also:case in See also:close proximity to the faces of projecting rings on the rotor (fig. 64) with a longitudinal clearance of o•015 in. This See also:form of dummy is suitable for the end near the thrust block, where exact longitudinal See also:adjustment is possible, but the astern turbine in fig. 63 requires a different form because some longitudinal See also:play is necessarily brought about there by See also:differences in expansion of the rotor and stator. Accordingly, the astern dummy is of the " radial " form shown in fig. 65 where the See also:fine clearance is See also:round the circumference of the brass rings set in the rotor and stator alternately. The whole dummy includes about sixteen of these rings. 129. See also:Shaft Arrangement of Marine Turbines.—Fig. 66 shows the usual three-shaft arrangement, with two low pressure turbines in parallel on the wing shafts, and one high pressure turbine, with which they are jointly in See also:series, on the See also:middle shaft. In very large vessels four shafts are used, and the turbines form two See also:independent sets one on each side of the See also:ship. The See also:outer shaft on each side carries a high pressure turbine, and the inner shaft carries the corresponding low pressure turbine and also a turbine for reversing. This arrangement is followed in the " Lusitania " and " See also:Mauretania " where the low pressure turbines have drums 188 in. in diameter, are about 172 ft. in diameter over all and 5o ft. See also:long, and weigh 300 tons. Each turbine has 8 steps with about 16 stages in each step in the high pressure turbine and 8 in the low. They run at 18o revolutions per minute. 13o. Cruising Turbines in See also:War-See also:Ships.—In turbines for the propulsion of war-ships it is necessary to secure a fairly high See also:economy at speeds greatly See also:short of those for which the turbines are designed when working at full power, for the normal cruising See also:speed of such vessels is usually from See also:half to two-thirds of the speed at full power. To counterbalance the reduced blade velocity, when See also:running under these conditions, the number of rows of See also:blades has in some cases been augmented by adding what are called cruising turbines, which are connected in series with the See also:main turbines when the ship is to run at cruising speed. In the three-shaft arrangement the cruising turbines are fitted on the wing propeller shafts, which carry also the low pressure and astern turbines. They form a high and inter- mediate pressure pair through which the steam may pass in series ( Condense See also:Condenser 9--3 - before going on to the main turbines. This arrangement is shown in fig. 67, where C.H.P. and C.I.P. are the two cruising turbines. In cruising at low speeds the whole See also:group of turbines is used in series: when the speed is increased a larger amount of power is got by admitting steam See also:direct to the second cruiser turbine; and finally at the highest speed both cruiser turbines are cut out. The arrangement shown in fig. 67 has been used in some See also:torpedo-See also:boat destroyers and small cruisers. In some large cruisers and battleships a four-shaft See also:system is employed and a longitudinal bulkhead divides the whole group into two independent sets. On each of the outer shafts there is a high-pressure ahead and also a See also:separate high-pressure astern turbine. On each of the inner shafts there is a combined low-pressure ahead and astern turbine and also a cruising turbine. All four shafts can be reversed.
131. Application of See also:Parsons Turbine.—The Parsons was the earliest steam turbine to be made commercially successful, and it has found a wider range of application than any other. Its See also:chief employment is as an electric generator and as a marine engine, but it has been put to a considerable number of other uses. One of these is to drive fans and blowers for exhausting See also:air, or for delivering it under pressure. The turbine-driven fans and blowers designed by Mr Parsons are themselves See also:compound turbines driven reversed in such a manner as to See also:pro-duce a cumulative difference in the pressure of the air that is to be impelled.
An interesting See also: Parsons Vacuum Augmenter.—For the same See also:reason it is especially important in the turbine to secure a good vacuum: any increase in condenser pressure during a turbine test at once shows its See also:influence in making a marked reduction of steam economy. In the region of usual condenser pressures a difference of r in. changes the steam See also:consumption by about 5%. With this in mind Mr Parsons has invented a See also:device called a vacuum augmenter, shown in fig. 68. The condensed See also:water passes to the air-See also:pump through a See also:pipe See also:bent to form a water-See also:seal. The air from the condenser is extracted by means of a small steam See also:jet pump which delivers it into an "augmenter condenser " in which the steam of this jet is condensed. The vacuum in the augmenter condenser is directly produced by the See also:action of the air-pump. The effect of this device is to maintain in the main condenser a higher vacuum than that in the augmenter condenser, and consequently a higher vacuum than the air-pump by itself is competent to produce. This is done with a small See also:expenditure of steam in the jet, but the effect of the augmented vacuum on the efficiency of the turbine is so beneficial that a considerable See also:net gain results. 133. Rateau and Zolly Turbines.—See also:Professor Rateau has designed a form of steam turbine which combines some of the849 features of the Parsons turbine with those of the De See also:Laval. He divides the whole drop into some twelve or twenty-four stages and at each See also:stage employs an impulse See also:wheel substantially of the De Laval type, the steam passing from one stage to the next through a See also:diaphragm with nozzles. This form can scarcely be called an independent type. It has been applied as an exhaust steam turbine in See also:conjunction with a regenerative thermal See also:accumulator which enables steam to be delivered steadily to the turbine although supplied from an intermittent source. The Zol1y turbine, which has found considerable application on a large See also:scale, acts in a precisely similar manner to that of Rateau: it differs only in See also:mechanical details. 134. Combined Reciprocating and Turbine Engines.—The See also:combination of a reciprocating engine with a turbine is suggested by Parsons for the propulsion of See also:cargo or other low-speed steamers where the speed of the See also:screw shafts cannot be made high enough to admit of a sufficient blade velocity for the efficient treatment in the turbine of high-pressure steam. With a small speed of revolution blade velocity can be got only by increasing the diameter of the spindle, and a point is soon reached when this not only involves an unduly large size and See also:weight of turbine, but also makes the blades become so short (by augmenting the circumference of the annulus) that the leakage loss over the tips becomes excessive. This See also:consideration confines the See also:practical application of turbines to vessels whose speed is over say 15 knots. But by restricting the turbine to the See also:lower See also:part of the pressure range and using a piston and cylinder engine for the upper part a higher economy is possible than could be reached by the use of either form of engine alone, the turbine being specially well adapted to make the most of the final stages of expansion, whereas the See also:ordinary reciprocating engine in such vessels makes little or no use of pressure below about 7 lb per sq. in. 135. Consumption of Steam in the Parsons Turbine.—In large sizes the Parsons turbine requires less steam per horse-power-See also:hour than aay form of reciprocating engine using steam under similar conditions. Trials made in See also:April 1900, by the See also:present writer, of a 2000 h.p. turbine coupled to an electric generator showed a consumption of 181 lb per kilowatt hour, with steam at 155 lb per sq. in. superheated 84° F. Since I kilowatt is 1'34 h.p. this consumption is equal to 13.6 lb per See also:electrical horse-power-hour. The best piston engines when See also:driving dynamos convert about 84% of their indicated power into electric power. Hence the above result is as good, in the relation of electric power to steam consumption, as would be got from a piston engine using only 11.4 lb of steam per indicated horse-power-hour. An important characteristic of the steam turbine is that it retains a high efficiency under comparatively See also:light loads. The figures below illustrate this by giving the results of a series of trials of the same See also:machine under various loads. Load in kilowatts . . Steam used per kilo- See also:watt-hour in pounds Still better results have been obtained in more See also:recent examples, in turbines of greater power. A Parsons turbine, rated as of 3500 but working up to over 5000 kilowatts tested in 1907 at the Carville power station of the See also:Newcastle-on-See also:Tyne Electric Supply See also:Company, showed a consumption of only 13.19 lb of steam per kilowatt-hour, with steam of zoo lb pressure by See also:gauge and 67° C. superheat (temperature 264.7° C.), the vacuum being 29•o4 in. (See also:barometer 30 in.). It is interesting to compare this performance with the ideal amount of See also:work obtainable per See also:pound of steam, or in other words with the ideal " heat drop." At the temperature and pressure of supply the total heat II is 709. After expansion to the pressure corresponding to the stated vacuum (0.96 in.) the total heat of the wet mixture would be 486, the dryness being then o•792, if the expansion took place under ideal adiabatic conditions. Hence the heat drop Ii—I2 is 223 See also:units, and this represents the work ideally obtainable under the actual conditions as to temperature and pressure of supply and exhaust. Since i kilowatt-hour is 1896 thermal units (lb—degree C.), each pound of steam was generating an amount of electrical energy See also:equivalent to 896 or 143.7 thermal units, and the electric 13.19 output consequently corresponds to 64.1% of the ideal work. If we allow for the loss in the electric generator by taking the electrical output as 92 % of the mechanical power, this implies that 70% of the ideal work in the steam was mechanically utilized. 136. Torsion Meters for Power.—No measurement corresponding to the " indicating " of a piston engine is possible with a 1450 18.1 1250 18.5 See also:I000 19'2 750 20.3 500 1250 22.6 34'0 steam turbine. In the tests that have been quoted the useful Packet Company, and despatched her on the first steam voyage from the See also:Mersey to Sandy See also:Hook on the 5th of See also:July in the same See also:year. The " See also:Liverpool" made her See also:maiden voyage in the following See also:October. But the " See also:British See also:Queen " did not make her initial See also:attempt till the 1st of July 1839. Trouble overtook all three of these See also:early See also:Atlantic lines, and they soon ceased to exist. Perhaps the most serious See also:factor against them was the success of Mr See also:Samuel See also:Cunard in obtaining the See also:government See also:contract for the See also:conveyance of the mails from Liverpool to See also:Halifax and See also:Boston, with a very large See also:subsidy. The Cunard See also:Line was enabled, and indeed, by the terms of its contract, obliged, to run a See also:regular service with a See also:fleet of four steamships identical in size, power and See also:accommodation. It thus offered conveyance at well-ascertained times and by vessels of known speed. The other companies, with their small fleets of isolated ships and their irregular departures, could not continue the competition. The Atlantic Steamship Company of Liverpool found that the See also:port could not then maintain two steamship lines, and the steamship " Liverpool," with another somewhat similar See also:vessel which they had built, See also:fell into the hands of the P. & O. Company. The See also:Great Western Steamship Company proceeded to build the " Great See also:Britain," an See also:iron screw steamship, which in every way was before her See also:time, and were swamped by See also:financial difficulties, their " Great Western " being sold to the See also:West See also:India Royal See also:Mail Company, to whom she became a very useful servant. The " Great Britain " (which was stranded in Dundrum See also:Bay in See also:September 1846, owing to her See also:captain, Hosken, being misled by a faulty See also:chart and mistaking the See also:lights) eventually drifted into the Australian See also:trade. The See also:London company put a second ship, the " See also:President," on their station. She was lost with all hands, no See also:authentic See also:information as to her end ever being obtained. Her mysterious See also:fate settled the fortunes of her owners, and the " British Queen " was transferred to the Belgian See also:flag. Steam See also:navigation across the Atlantic was now an accomplished fact. But all the three pioneers had been See also:borne down by the difficulties which attend the carrying out of new departures, even when the See also:general principles are See also:sound. See also:Constant improvement has been the watchword of the ship-owner and the ship-builder, and every See also:decade has seen the ships of its predecessor become obsolete. The mixed See also:paddle and screw See also:leviathan, the " Great Eastern," built in the See also:late 'fifties, was so obviously before her time by some fifty years, and was so under-powered for her size, that she may be left out of our reckoning. Thus, to speak roughly, the 'fifties saw the iron screw replacing the wooden paddle steamer; the later 'sixties brought the compound engine, which effected so great an economy in See also:fuel that the steamship, previously the conveyance of mails and passengers, began. to compete with the sailing vessel in the See also:carriage of cargo for long voyages; the 'seventies brought better accommodation for the passenger, with the midship See also:saloon, improved See also:state-rooms, and covered See also:access to See also:smoke-rooms and ladies' cabins; the early 'eighties saw See also:steel replacing iron as the material for ship-See also:building, and before the close of that decade the introduction of the twin-screw rendered break-See also:downs at See also:sea more remote than they had previously been, at the same time giving increased safety in another direction, from the fact that the duplication of machinery facilitated further subdivision of hulls. Now the masts of the huge liners in See also:vogue were no longer useful for their See also:primary purposes, and degenerated first into See also:derrick props and finally into See also:mere See also:signal poles, while the introduction of boat decks gave more shelter to the promenades of the passengers and removed the navigators from the distractions of the social side. The See also:provision of See also:train-toboat facilities at Liverpool and See also:Southampton in the, 'nineties did away with the inconveniences of the See also:tender and the See also:cab. The introduction of the turbine engine at the beginning of the loth See also:century gave further subdivision of machinery and increase of economy, whereby greater speed became possible and comfort was increased by the reduction of vibration. At the same time the introduction of submarine See also:bell signalling tends to diminish the See also:risk of stranding and collision, whilst wireless telegraphy not only destroys the See also:isolation of the sea but tends
output was determined by electrical means. Direct measurements of the useful mechanical power (the " See also:brake " power) may, however, be obtained by applying a torsion See also:dynamometer to the shaft. Devices are accordingly used in marine turbines for determining the horse-power from observations of the elastic twist in a portion of the propeller shaft as it revolves. In Denny & See also: Additional information and CommentsThere are no comments yet for this article.
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