to a certain extent, the volume of the steam which passes through the remaining wheels, but in the machine illustrated, the makers have employed a special device to permit a reduction of the length of the last row of blades. The steam which enters the last wheel but one, is divided into two parts, that which acts on the outer annulus of the blade ring passing away directly to the condenser, and only that which acts on the inner annulus being afterwards conducted to the final wheel. The blading on the last wheel there- fore only deals with about half the weight of steam which passes through the preceding wheel, and it can handle this amount at a very reduced pressure.
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A rigid coupling is fitted to connect the turbine shaft with the shaft of the alternator, and the turbine shaft is located axially by means of an adjustable thrust block of the Michell type which j takes care of any unbalanced end pressure along the shaft.
The mean diameter of the blading of this machine is 84 in. and the length of the last row of blades is 24 inches. The mean circum- ferential velocity of the blading is 550 ft. per second, the tip velocity of the longest blades being 708 ft. per second. The turbine is designed to work with a stop- valve pressure of 250 Ib. per sq. in., a tempera- ture of 650 F. and a vacuum of 0-9 in. of mercury, thus having an available heat drop of 455-2 B.Th.U. per Ib. of steam. Under these conditions the guaranteed steam consumpion is 10-2 Ib. 'per K.W.H., this figure being the same for both I5,OOO-K.W. and 18,750-KAV. load.
FIG.5
The Ljungstrom Steam Turbine. In the early days of the reac- on turbine, a number of machines were built by the Hon. C. A. arsons in which the steam passed radially outwards between two scs carrying rings of blades projecting axially from their opposed ces, one_ disc being stationary and the other driving the shaft of i electric generator. Mechanical difficulties were experienced, 'incipally due to the distortion of the discs by uneven heating, id the design was soon completely abandoned in favour of the
- ial flow type. In the year 1910 Messrs. Birger and Frederic
Ljungstrom of Stockholm built an entirely new type of radial flow reaction machine which was conspicuous not only for its mechanical merits but for its great efficiency. The Ljungstrom turbine is now being developed in sizes up to 30,000 K.W. capacity, and is manu- factured in Great Britain by the Brush Electrical Engineering Co. and in the United States by the General Electric Company. The steam is admitted between two discs and in its passage from their center to their circumference it passes through concentric blading rings mounted alternately on the faces of the discs. The discs revolve at equal speeds in opposite directions, so that the relative blade speed is twice as great as in an ordinary machine of the same
Fl C. 6
FIG. 7
revolutions and diameter, with the consequence that for equal
efficiency the number of blade rings is only one quarter as great.
Each disc is fastened to the end of a separate alternator shaft, and
as the turbine comes up to speed, the alternators come automatically
into synchronism and operate in parallel so that they act virtually
as a single machine.
87 fO.
FIG. 8
The mechanical construction of the Ljungstrom turbine is unique. Fig. 5 shows a section through a machine to develop 5,000 K.W. at 3,000 revs, per minute, the illustration including the two ends of the alternator shafts, upon which the turbine discs are mounted. The construction will be better understood by reference to figs. 6 10 which show the most important details to a larger scale. The steam enters the turbine through the branched pipe shown in fig. 5 and thence passes to the centre of each disc through the holes marked 2 in figs. 6 and 7, which illustrate the disc alone. It will be seen that the face of the disc contains a number of circumferential grooves. Each groove carries a blade ring, shown to a larger scale in fig. 8, in which I represents the disc; 2 a seating ring; 3 a caulk- ing strip; 4 an expansion ring; 5 and 6 rolling edges; 7 steam pack- ing strips; 8 caulking strips; 9 strengthening ring; 10 dovetail profile ring; II the blade itself. These blade rings are interleaved as they project alternately from the discs, and steam leakage is checked by the thin fins 7. The blades are made from drawn steel strip and are welded solidly into the strengthening rings 10.
The conical steel expansion ring, 4, is a particularly important feature of the blading system, and similar rings will be seen at I in fig. 7, where they serve to connect the three parts of which the disc is composed. The ring of holes shown at 3 in fig. 7 is to admit the extra steam necessary for overload conditions, the inner rings of blading being then short circuited. The pressure of steam in the blading naturally tends to thrust the discs apart. It is therefore balanced by an arrangement of " dummies," or labyrinth discs, as shown in fig. 5. A detail of the labyrinth, to a larger scale, is given in fig. 9. To prevent the high-pressure steam leaking along the shafts, these are fitted with labyrinth packings, a portion of one of these packings being illustrated in fig. 10. The whole packing con- sists of a number of rings keyed alternately to the shaft and to the housing and having deep grooves turned circumferentially in the
