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Encyclopedia Britannica - Main :: VIR-WAT |
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WATER MOTORS . The subject of hydraulic transmission of power is treated generally under POWER TRANSMISSION (Hydraulic), and the present article is confined to water motors. Hydraulic Lifts.The direct-acting lift is perhaps the simplest of all machines using pressure-water, but as the height of the lift increases, certain problems in construction become exceedingly difficult to cope with, notably those due to the great increase in the weight and displacement of the ram. In fact, with a simple ram it is not possible to lift beyond a certain height with a given pressure and load. It becomes, therefore, necessary to balance in some way the varying displacement of the ram if economy
In one form, shown in fig. 1, the lift cylinder is in hydraulic connexion with a pair of short cylinders placed one above the other, the pistons working in them being connected together by a common rod. Below the piston of the upper cylinder is an annular space E (surrounding the common piston rod) with a capacity equal to the maximum displacement of the lift-ram, while the corresponding annular area C of the piston of the lower cylinder is just large enough when subjected to the working water pressure to enable the work of lifting the net load to be done and any friction to be overcome. The area B of the top side of the upper piston is proportioned in such a way that when under the full water pressure the dead weight of the ram and cage is just balanced when the former is at the bottom of its stroke. With this arrangement the lift - ram and the two balance pistons are always in equilibrium, or, in other words, the ever-changing displacement of the lift-ram is automatically in balance. To work the lift, pressure-water is admitted to the annular space C above the lower of the two balance pistons (the space B above the upper one is always in communication with the pressure-water), and the combined pressure on the two pis-tons is sufficient to lift the cage, ram and load. As the ram ascends it apparently increases in weight, but this is balanced by the greater pressure on the two balance pistons as they descend, owing to the in-crease of the head of water acting on them. To allow the lift-ram to descend, the pressure-water in C above the lower balance piston is discharged through the exhaust into the drain, while that above the upper piston is simply pushed back into the pressure main. As an illustration
economy
In another system of hydraulic balance (fig. 2) the ram A has an annular area so proportioned that when it is connected with the water in an elevated tank (usually placed somewhere in the roof of the building), the hydraulic pressure upon it just balances the weight of the ram and cage. Here again, since the Intensity of the pressure on A becomes greater as it descends owing to the increased head, the apparent increase of weight of the lift-ram as it rises is automatically balanced ; water from the high-pressure system is admitted down the hollow ram B and does the work of lifting the live load. Since the introduction of deep-level electric railways in Londonand elsewhere, hydraulic passenger lifts on a large scale have beer brought into use for conveying passengers up and down from the street level to the underground stations. Direct-acting Water Motors.Owing to the difficulty of securing a durable motor with a simple and trustworthy means of automatically regulating the quantity of water used to the power needed at various times from the motor, not much advance has been recently made in the use of water motors with reciprocating rams or pistons. Probably the most successful one has been a rotary engine
In this engine
movement
pulley
Water Wheels.The Pelton water wheel (fig. 5) has proved a most successful motor when very high heads are available, heads of 2000 feet having been used occasionally. Such machines have been extensively em-ployed in America, and have also lately been used in Great Britain, worked by the FIG. 2.-Hydraulic high-pressure water supplied in large towns. Balancing. The wheel carries a series of cups placed at equal distances around the circumference. A jet or jets of water impinge on the cups, the interiors of which are shaped in such a way that the jet is discharged parallel to its original direction. If the linear velocity of the cups in feet a second is V,, and the linear velocity of the jet is V2, then the velocity of the jet relative to the cup is V2VI feet a second, and if the whole energy of the water is to be given up to the cups, the water must leave the cup with zero absolute velocity. But its velocity relative to the cup, as it passes back-wards, is (V2--VI), and since the forward velocity of the cup is V1, the absolute velocity of the water is (V2-V1)+V1or2V1-V2. This will become zero if VI is 1V2, that is, if the linear velocity of the cup-centres is one-half that of the jet of water impinging upon them. The theoretical efficiency of the wheel FIG. 3.Section of Rigg's Water- would then be Too %. The Engine. actual efficiency of these wheels when used with high falls is from 8o to 86 %; when used in connexion with high-pressure water in London an efficiency This engine was fully described in Engineering, vol. xlv, n. 61. of 70% has been obtained, and when a dynamo is driven directly by them about 66 % of the hydraulic energy has been converted into electric energy. Pelton wheels are very sensitive to variation of load, and considerable trouble was experienced at first in securing adequate governing when they were used to generate electric energy; but this difficulty has been overcome, and they have been rendered most efficient machines for use with high falls, where ordinary turbines would be difficult to manage owing to the excessive speed at which they would run. In a small installation in the United States water is brought in a 36-in. pipe a distance of 1800 ft., and supplies six Pelton wheels each 28 in. in diameter, running at 135 revolutions a minute under a head of 130 ft. The total power developed is 600 H.P., and though the load factor varies very greatly in this case, the differential type of governor used secures perfect control of the running of the wheels. Turbines.The tu rbine has now become one of prime movers employed by man, and in the United States of America and on the continent of Europe 2 its use has enormously increased of recent
In the " Hercules " turbine, shown in fig. 6, the flow is what is called mixed, that is, it is partly a radial inward and partly an axial flow machine. On entering, the water flows at first in a radial direction, and then gradually, as it passes through the wheel, it receives a downward component which becomes more and more important. Professor Thurston has published the results of a test 1 This and some of the other drawings have been taken from Blaine's Hydraulic Machinery. 2 The following statistics of turbine construction in Switzerland are taken from Schweizerische Bauzeitung (1901), p. 128, which, in the same volume at p. 53, contains a valuable article on the most ;mportant improvements in turbines and their regulation shown n the Paris Exhibition of 1901 :of one of these, which gave an efficiency of 87 % at full load and 70%a at about three-fifths full load. Period. Number Total H.P. Average of H.P. Turbines. 18441869 767 36,894 48 18691879 1006 66,688 661 18791889 1840 133,579 721 18891899 2231 400 474 1791 Totals . 5844 637,635 Another turbine of the mixed flow type is the " Victor," which consists of three partsthe outer guide case, and, inside this, the I register gate, and the wheel. The gate regulates the speed of the wheel by varying the quantity of water; when fully open it merely forms a continuation of the guide passages, and thus offers no obstruction to the flow of the water, but by giving it a movement
horizontal
York
horizontal
Probably the most important application of turbines to the generation of power on a great scale is that at Niagara Falls. The water is tapped off from the river Niagara about i m. above the falls and brought by a canal to the power- house. The wheel-pit is 18o ft. in depth, and is connected with the river below the falls by a tail-race, consisting of a tunnel 21 ft. high and 18 ft. Io in. wide at its largest section. The original turbines were of the " Fourneyron " type, and a pal: were mounted on each vertical shaft, the two being capable of giving out 5000 H.P. with a fall of 136 ft. Each pair of wheels is built in three storeys, and the outflow of the water is controlled by a cylindrical gate or sluice, which is moved up and down by the action of the governor. As the pair of wheels and the big vertical shaft (which is of hollow steel 38 in. in diameter) with the revolving part of the dynamo mounted on the upper end of the shaft weigh about 152,000 ib, a special device, since adopted in other similar power plants, was designed to balance in part this dead weight. The water passes from the penstock through the guide blades of the upper wheel, and in doing so acts in an upward direction on a cover of the upper wheel, which thus becomes, as it were, a balance-piston. The total upward pressure on this piston is calculated to be equal to 150,000 lb; hence the shaft-bearings are practically relieved from p_*essure when the wheels are running. Another turbine which has come into extensive use is the " Francis, ' an exceedingly efficient turbine on a low fall with large quantities of water. At Schaffhausen two of them with a fall of 121 ft. de veloped 430 H.P., when the older turbines only gave 26o H.P., the efficiency of the Francis turbine being in this case 86 % at full load and 77% at half load. A recent
III~I_li Ilia ll~IH! 1fl off for various purposes. When complete the turbine weighed about 140 tons. There is a regulating arrangement, by which one-half of the guide-passages can be shut off in pairs from the water, and at the same time air is freely admitted into these unused passages by pipes which pass through the hinges of the controlling shutter. Tests of a turbine of this slow-moving type showed an effciency of 82 % at full gate, and one of 75 % when half of the passages in the guide-blades were closed by the shutters, as described above. As an illustration
AuTxoRITIEs.For further information concerning the construction and employment of water motors, the reader is referred to the following papers and textbooks:Prot. Inst. Mech. Eng. (1882), p.119 (1889), p. 350; (1895), p. 353. (These papers contain full accounts of recent forms of liftsEngineering, vol. lxvii. pp. 91, 128, 16o, " Power Station at Niagara ' ; vol. lxxii. pp. 391-767, " Governing of Water Wheels."Prot. Inst. Civil Eng., vol. lxxxvi. p. 6o, " Mersey Railway Lifts "; vol. xciii. p. 596, " Experiments on Jonval and Girard Turbines at Alching "; vol. xcvi. p. 182, " Hydraulic Canal Lifts "; vol. cii. p. 154, " Keswick Water-Power Electric Station "; vol. cxii. p. 410, ` Hydraulic Works at Niagara "; vol. cxviii. p. 537, " A 12-Mile Transmission of Power Generated by Pelton Wheels "; vol. cxxiii. p. 530, " The Pelton Water Wheel "; vol. cxxiv. p. 223, " The Niagara Power Works "; vol. cxxvi. p. 494, " The Rheinfelden Power Transmission Plant "; vol. cxli. E. " Electric Transmission Plants in Transvaal," p. 307, " Turines "; vol. cxlii. p. 451, " Electrical Installations at Lausanne "; vol. cxly. p. 423, " Water Power at Massena "; vol. cxlvii. p. 467, " Some Large Turbine Installations."Wood, Theory of Turbines; Bovey, Hydraulics; Bjorling, Hydraulic Motors; Blaine, Hydraulic Machinery; Bodmer, Hydraulic Motors; Unwin, " Water Motors " (Lectures on Hydro-Mechanics, Inst. Civil Eng., 1885). (T. H. B.) WATER-OPOSSUM, or YAP0cK (Chironectes minimus), the single representative of the genus. This animal is distinguished from other opossums by its webbed hind-feet, non-tuberculated soles, and peculiar coloration. Its ground colour is light grey, with four or five sharply contrasted brown bands passing across its head and back, giving it a very peculiar mottled appearance; the head and body together are about 14. in. long, and the tail measures a little more. It is almost wholly aquatic in its habits,living on small fish, crustaceans and other water animals; its range extends from Guatemala to southern Brazil. End of Article: WATER MOTORS If you wish, you can link directly to this article.
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