Page:EB1922 - Volume 32.djvu/817

Rh efficiently utilized, so that in practice a fairly definite limit of power corresponding to each speed is obtained.

Turbo alternators have been satisfactorily built, having a maxi- mum continuous rating of over 6,000 K.W. at 3,600 revs., the limit of economical rating for this speed being at the present time about 5,000 K.W. At 3,000 revs, per minute the maximum continuous rating is about !3,75oK.W., the economical output being 1 2 ,500 K.W., the machine built in 1921 for the Liverpool corporation being of this size. There are several turbines with a maximum continuous rating of 30,000 K.W. running at 1,800 revs, per minute, and at 1,500 revs, per minute, a continuous rating of 35,000 K.W. appears to be about the present limit, both for impulse and reaction machines. Machines of this size and speed were installed in Chicago in 1918, and in Paris in 1921. In machines of 30,000 K.W., and over it is not uncom- monly the practice to use two or more generators, the whole unit really consisting of mechanically independent high- and low-pressure turbines. Certain units built by the Westinghouse Co. in the United States have a maximum rated output of even 60,000 K.W., but these in fact consist of three independent turbo generators, through which the steam passes in series. This multiplication of cylinders and shafts is of course the usual custom in connexion with marine turbines.

The practice of dividing a turbine into two parts, namely a high- and a low-pressure cylinder arranged in tandem, was first intro- duced many years ago and the design has been standardized for the larger machines of the reaction type. It has the advantage that the separate casings are shorter and less liable to distortion than an equivalent single casing, while by .making the low : pressure drum of larger diameter and of the double flow type, the requisite area for the enormous volume of the low-pressure steam is conveni- ently provided for. The importance of this will be realized from the fact that in a modern turbine the ratio of expansion of the steam may be over 800- 1. Fig. I shows a section through a two-cylinder tandem turbine as constructed by the Parsons Co., and fig. 12 illus- trates the appearance of a two-cylinder side by side arrangement as used with gearing for marine purposes.

Governing of Steam Turbines. The speed regulation of turbines is effected by a centrifugal governor driven by worm gearing from the main shaft, which acts in the case of all reaction machines by con- trolling the pressure at which steam is admitted to the casing. In machines constructed either wholly or partially on the impulse principle, the governor may open up successively extra nozzles or groups of nozzles as the load increases. Loads in excess of the maximum economical load are sometimes provided for by admitting steam to the turbine at some intermediate point, thus raising the pressure there above the normal full load pressure and enabling the turbine to do more work, although at a somewhat reduced efficiency. The by-pass valves for this purpose may be hand operated, but as a rule they are under the control of the governor and are thus auto- matically opened when the extra steam is required to maintain the speed. In view of the close governing required on turbo generators and of the size and weight of the valves which have to be operated, it is the universal practice to employ a relay arrangement on all but the smallest machines, the governor merely controlling the position of a small balanced piston valve which admits oil under pressure to one side or the other of a piston which does the actual ivork of operating the valves. The pressure oil is supplied from the ubrication system of the turbine.

Bearings and Lubrication. The old sleeve bearing, originally levised by Sir Charles Parsons and employed on his earlier machines, las been entirely superseded and turbine bearings are now con- itructed on ordinary lines, differing only from slow-speed bearings n their proportions and in the provision necessary for their proper ubrication. The bearings are made in two halves, split horizontally, he interior working surfaces being of white metal cast and anchored nto the " steps " which are of cast iron or bronze. These are usually itted with shimplates to provide a fine vertical and lateral adjust- nent, and are frequently supported in spherical seatings to permit >f a certain amount of self-alignment. Safety strips, often of bronze, vhich normally lie slightly below the surface of the white bearing netal, are usually provided. These are intended to carry the weight >f the shaft safely in the event of the white metal being melted out, .nd thus prevent injury to the blading until the machine can be topped. In all turbine bearings the important thing is to insure a opious supply of lubricating oil, not so much for lubrication as to arry off the heat generated by friction and to maintain the bear- ngs at a reasonable working temperature. Water-cooled bearings ave been used by some makers, but the most approved practice is

rely on the flow of oil through the bearing to keep its tempera- ure down. Oil is usually delivered to the bearings at a pressure of bout 15 Ib. per sq. in., a gauge being provided on each bearing to idicale whether the pressure is being maintained. On modern urbines an automatic device operated by the oil pressure is fitted,
 * 'hich shuts the machine down in case of any failure of the oil supply.

Bearings up to 8-in. diameter are usually bored larger than the baft to the extent of about 0-004 m - f r every in. of shaft diame-
 * r. In larger bearings the clearance is proportionately less. This

.Dmewhat large clearance enables the heat to be carried away by le continuous wash of fresh cool oil. The shaft, when running, is

1 ept out of metallic contact with the bearing by a thin film of oil

791

continually dragged underneath it by its rotation. It is this film which supports the shaft, and the pressure of the latter on the bear- ing must therefore not be greater than the film can stand. Theory and experiment both indicate that the greater the surface velocity of the shaft, the more effectively is the film established, and the greater therefore the permissible load on the bearing. But the fact that bearings have to start from rest, when the film is imperfect, imposes a practical limit to the load which can be imposed.

A formula connecting permissible pressure with velocity, given by Mr. F. H. Clough, is P = 17 V V, in which P denotes the pressure in pounds per sq. in. of projected area, and V = velocity of surface of shaft in ft. per second. This is said to be applicable to bearings of normal design in which the length is from twice to three times the diameter. Many designers, however, use the rule that PXV must not exceed 5,600, a simple rule which gives good results in prac- tice, and probably has a considerable margin of safety when the speeds are high and when there is no vibration. One large manu- facturing firm is said to take the permissible pressure per sq. in. of projected area as ranging from 167 to 235 when the velocity ranges from 20 to 73-5 ft. per second. Modern practice is to give P a value not exceeding 150 Ib. in bearings where the velocity is not greater than 30 to 35 ft. per second, and the temperature compara- tively low, say, 100 to 1 10 F. Such conditions would apply to low speed marine turbine bearings. The bearings of land turbines usually work at temperatures from 120 F. to 160 F., but the latter temperature should not be exceeded, as not only is the oil injured, but its viscosity is so low that the supporting film is thinned and the margin of safety becomes low.

For the heat generated in a turbine bearing Stoney gives the

formula B.Th.U. per hour =

190 /. d. v '32

in which / and d are

respectively the length and diameter of the bearing expressed in in., v is the velocity of the surface of the shaft in ft. per second, and t is the temperature on the Fahrenheit scale. The same authority quotes the following formula as often used in slow-speed marine practice: B.Th.U. per hour = IXdXv 1 ' 3 *. Treating the heat which escapes by radiation and conduction as negligible, these formulae give the heat which has to be carried away by the oil and extracted by the oil cooler. This heat of course is the equivalent of the work lost by friction in the bearing. The increase of temperature of the oil passing through the bearing should not exceed 10 to 20 F., and if the specific heat of oil be taken at 0-31 the minimum quantity of oil required for each bearing may be readily calculated. In practice it is advisable to increase this calculated fig. by from 30 to 50%, to allow a margin for steam heat travelling along the shaft and other contingencies.

Mechanical Gearing of Turbines. The De Laval steam turbine, consisting of a single impulse wheel running at a speed of 30,000 to 10,000 revolutions per minute according to the size, has always contained reduction gearing as an integral part of the machine because such speeds are far too high for driving ordinary machinery. Turbines of this type have, however, only been built for powers up to a few hundred horse-power, and although the use of reduction gear may be dated from the introduction of the Laval turbine in 1886, it never became a recognized practice for large powers until it was developed by Sir Charles A. Parsons as the solution of the prob- lem of marine propulsion. De Laval had shown that it was possible to transmit power satisfactorily through mechanical gearing running with a circumferential velocity of over 100 ft. per second. The gears he used were of the double helical type with a spiral angle of 45 degrees. The reduction ratio was usually about 10:1, and the pitch of the teeth varied from 0-15 in. to 0-26 in., according to the power of the turbine. The De Laval gear embodied all the features which have been found! necessary to the successful performance of modern gears transmitting several thousand horse-power through a single pinion. The double helical form of tooth of comparatively fine pitch has been retained, as this design eliminated end thrust and insured silent running by reason'of the number of teeth simultaneously in contact. Ample lubrication of the teeth by means of oil jets was also employed by De Laval, who succeeded in producing durable and satisfactory gears which had an efficiency of about 97 per cent. These gears are used up to about 600 H.P. which is the commercial limit of the type of turbine for which they are designed.

Steam turbines of any type, designed with due regard to efficiency and cost of manufacture, require to run at a far higher speed of revolution than is practicable for screw propellers, especially when the latter are employed to drive ships of moderate speed. The coupling of a turbine, therefore, directly to a propeller shaft involves a compromise in design, in which the speed is greater than desirable for the propeller yet so low as to require the turbine to be of greater size and weight and of lower efficiency than it would otherwise be. In the case of high-speed vessels direct coupling afforded a commer- cially acceptable solution of the problem of turbine propulsion, and for vessels of eighteen knots speed and over, such as warships, passenger liners and cross-channel boats, the direct coupled turbine soon became the recognized driving power. But ordinary cargo vessels andtramp steamers, with an average speed of 10 or 12 knots, were outside the practical field of the steam turbine until speed reduction gearing was available to couple a high-speed turbine with