Online Encyclopedia

STEAM

Online Encyclopedia
Originally appearing in Volume V25, Page 850 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
STEAM ENGINE second, 8 in the third and 8 in the fourth, or 34 stages in all. The low pressure turbine (fig. 63) comprises 28 more stages stepped as shown in the figure. The reversing turbine which is seen on the left-hand side in fig. 63, at the place where the rotor is reduced in diameter, has 26 stages in 4 steps. These turbines have a total normal horse power of 12,500, and run at 450 revolutions per minute. 128. Longitudinal Forces in Marine Turbines.—In a marine steam turbine the 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 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 brass fixed in the case in close proximity to the faces of projecting rings on the rotor (fig. 64) with a longitudinal clearance of o•015 in. This form of dummy is suitable for the end near the thrust block, where exact longitudinal adjustment is possible, but the astern turbine in fig. 63 requires a different form because some longitudinal play is necessarily brought about there by differences in expansion of the rotor and stator. Accordingly, the astern dummy is of the " radial " form shown in fig. 65 where the fine clearance is round the circumference of the brass rings set in the rotor and stator alternately. The whole dummy includes about sixteen of these rings. 129. 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 series, on the middle shaft. In very large vessels four shafts are used, and the turbines form two independent sets one on each side of the ship. The 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 " Mauretania " where the low pressure turbines have drums 188 in. in diameter, are about 172 ft. in diameter over all and 5o ft. 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 War-Ships.—In turbines for the propulsion of war-ships it is necessary to secure a fairly high economy at speeds greatly short of those for which the turbines are designed when working at full power, for the normal cruising speed of such vessels is usually from half to two-thirds of the speed at full power. To counterbalance the reduced blade velocity, when running under these conditions, the number of rows of blades has in some cases been augmented by adding what are called cruising turbines, which are connected in series with the 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 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 group of turbines is used in series: when the speed is increased a larger amount of power is got by admitting steam 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 torpedo-boat destroyers and small cruisers. In some large cruisers and battleships a four-shaft 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 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 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 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 air, or for delivering it under pressure. The turbine-driven fans and blowers designed by Mr Parsons are themselves compound turbines driven reversed in such a manner as to pro-duce a cumulative difference in the pressure of the air that is to be impelled. An interesting field for the application of steam turbines is to economize the use of steam in non-condensing engines of the older type, by turning their exhaust to the supply of a turbine provided with an efficient condenser. It is a characteristic of the turbine that it is able to make effective use of low pressure steam. No condensing piston and cylinder can compete with it in this respect; for the turbine continues to extract heat energy usefully when the pressure has fallen so low that frictional losses and the inconveniences attaching to excessive volume make it impracticable to continue expansion to any good purpose under a piston. 132. Parsons Vacuum Augmenter.—For the same 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 influence in making a marked reduction of steam economy. In the region of usual condenser pressures a difference of r in. changes the steam consumption by about 5%. With this in mind Mr Parsons has invented a device called a vacuum augmenter, shown in fig. 68. The condensed water passes to the air-pump through a pipe bent to form a water-seal. The air from the condenser is extracted by means of a small steam 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 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 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 net gain results. 133. Rateau and Zolly Turbines.—Professor Rateau has designed a form of steam turbine which combines some of the849 features of the Parsons turbine with those of the De Laval. He divides the whole drop into some twelve or twenty-four stages and at each stage employs an impulse wheel substantially of the De Laval type, the steam passing from one stage to the next through a diaphragm with nozzles. This form can scarcely be called an independent type. It has been applied as an exhaust steam turbine in conjunction with a regenerative thermal 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 scale, acts in a precisely similar manner to that of Rateau: it differs only in mechanical details. 134. Combined Reciprocating and Turbine Engines.—The combination of a reciprocating engine with a turbine is suggested by Parsons for the propulsion of cargo or other low-speed steamers where the speed of the 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 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 consideration confines the practical application of turbines to vessels whose speed is over say 15 knots. But by restricting the turbine to the lower 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 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-hour than aay form of reciprocating engine using steam under similar conditions. Trials made in April 1900, by the 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 electrical horse-power-hour. The best piston engines when 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 light loads. The figures below illustrate this by giving the results of a series of trials of the same machine under various loads. Load in kilowatts . . Steam used per kilo- watt-hour in pounds Still better results have been obtained in more 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 Newcastle-on-Tyne Electric Supply Company, showed a consumption of only 13.19 lb of steam per kilowatt-hour, with steam of zoo lb pressure by gauge and 67° C. superheat (temperature 264.7° C.), the vacuum being 29•o4 in. (barometer 30 in.). It is interesting to compare this performance with the ideal amount of work obtainable per 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 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 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 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 Mersey to Sandy Hook on the 5th of July in the same year. The " Liverpool" made her maiden voyage in the following October. But the " British Queen " did not make her initial attempt till the 1st of July 1839. Trouble overtook all three of these early Atlantic lines, and they soon ceased to exist. Perhaps the most serious factor against them was the success of Mr Samuel Cunard in obtaining the government contract for the conveyance of the mails from Liverpool to Halifax and Boston, with a very large subsidy. The Cunard Line was enabled, and indeed, by the terms of its contract, obliged, to run a regular service with a fleet of four steamships identical in size, power and 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 port could not then maintain two steamship lines, and the steamship " Liverpool," with another somewhat similar vessel which they had built, fell into the hands of the P. & O. Company. The Great Western Steamship Company proceeded to build the " Great Britain," an iron screw steamship, which in every way was before her time, and were swamped by financial difficulties, their " Great Western " being sold to the West India Royal Mail Company, to whom she became a very useful servant. The " Great Britain " (which was stranded in Dundrum Bay in September 1846, owing to her captain, Hosken, being misled by a faulty chart and mistaking the lights) eventually drifted into the Australian trade. The London company put a second ship, the " President," on their station. She was lost with all hands, no authentic information as to her end ever being obtained. Her mysterious fate settled the fortunes of her owners, and the " British Queen " was transferred to the Belgian flag. Steam navigation across the Atlantic was now an accomplished fact. But all the three pioneers had been borne down by the difficulties which attend the carrying out of new departures, even when the general principles are sound. Constant improvement has been the watchword of the ship-owner and the ship-builder, and every decade has seen the ships of its predecessor become obsolete. The mixed paddle and screw leviathan, the " Great Eastern," built in the 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 fuel that the steamship, previously the conveyance of mails and passengers, began. to compete with the sailing vessel in the carriage of cargo for long voyages; the 'seventies brought better accommodation for the passenger, with the midship saloon, improved state-rooms, and covered access to smoke-rooms and ladies' cabins; the early 'eighties saw steel replacing iron as the material for ship-building, and before the close of that decade the introduction of the twin-screw rendered break-downs at 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 vogue were no longer useful for their primary purposes, and degenerated first into derrick props and finally into mere 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 provision of train-toboat facilities at Liverpool and Southampton in the, 'nineties did away with the inconveniences of the tender and the cab. The introduction of the turbine engine at the beginning of the loth 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 bell signalling tends to diminish the risk of stranding and collision, whilst wireless telegraphy not only destroys the isolation of the sea but tends output was determined by electrical means. Direct measurements of the useful mechanical power (the " brake " power) may, however, be obtained by applying a torsion 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 & Johnson's torsion meter two light gun-metal wheels are fixed on the shaft as far apart as is practicable, generally 15 or 20 ft., and their relative angular displacement is found by comparing the inductive effects produced on fixed coils by magnets which are carried on the wheels. In Hopkinson & Thring's torsion meter a short length of shaft—a foot or so—suffices. A small mirror is carried by a collar fixed to the shaft, and a second collar fixed a little way along is geared to the mirror in such 'a way as to deflect the mirror to an extent proportional to the twist: the deflexion is read by means of a lamp and scale fixed alongside. As the shaft revolves the light reflected from the mirror is momentarily seen at each revolution and its position along the scale is easily read. (J. A. E.)
End of Article: STEAM
[back]
STEAK
[next]
STEAM (0. Eng. steam, vapour, smoke, cf. Du. steam;...

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.