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sides. The rings interleave in the manner shown, the edges of the grooves being bent down so as practically to make contact with the walls of the grooves in the adjacent rings. An extremely effective and compact labyrinth is thus formed.

The efficiency of the Ljungstrom turbine is remarkably high for machines of moderate capacity. Independent tests of a I.5OO-K.W. machine, after 15 months' service, have shown a steam consump- tion of 1 1-95 lb. per K.W.H., with steam at 208 Ib. per sq. in. abs. and 569 F. temperature, and a vacuum of 1-29 in. Hg. The no-load consumption of the same machine was only 1340 lb. per hour, or 7'5 % of the full-load consumption.

The appearance of a complete Brush-Ljungstrom turbo-alternator is shown in fig. II.

FIG. 9

Steam Conditions in Turbines. The steam consumption of a tur- bine depends not only upon the excellence of its mechanical design but upon the amount of heat in every pound of steam delivered to the turbine which is available for conversion into work. The avail- able heat may be increased by increasing the pressure and tempera- ture of the entering steam and by lowering the pressure at which it is exhausted. Progress in these directions is limited by construc- tional difficulties, but nevertheless striking advances have been made. The best practice of the time may be exemplified by the lo,ooo-K.W. machine installed in 1910 at the Carville station of the Newcastle Electric Supply Co., which operated with steam at 190 lb. per sq. in. .gauge pressure and a superheat of 150 F. at the stop valve, and a vacuum of one in. of mercury. Under these con- ditions there was an available heat drop of 407-2 B.Th.U. per lb. of steam. In 1916 a machine of n,ooo-K.VV. was installed in the same station with a stop-valve pressure of 250 lb. gauge, a superheat of 244 F. and a vacuum of one in. of mercury. This change in steam conditions increased the heat drop to 450-2 B.Th.U. per Ib. of steam. In 1921, a machine having an economical rating of 25,000 K.W., installed at Manchester, utilized a stop-valve pressure of 350 lb. gauge, a superheat of 264 F. and a vacuum of 0-9 in. of mer- cury, thus working with an available heat drop of 484-7 B.Th.U. per lb. of steam. It may be taken that modern practice sanctions steam pressures up to 350 lb. per sq. in., temperatures up to 700 F.

and vacua as high as 29-1 in., with the barometer at 30 inches. Nc commercial reciprocating engine could work under such steam con- ditions with anything like the efficiency a turbine would show in similar circumstances.

Speeds of Turbines. The principal use' of steam turbines on land being to drive electric generators, the speed at which these can bi run controls to a large extent the speeds for which turbines can be designed. Continuous current turbo generators are comparatively small in size and few in numbers, and as these are almost exclusively driven through reduction gearing on account of the difficulties o'l commutation at high speeds, their characteristics do not materially affect the design of the turbines. All large land type turbines arc directly coupled to alternators and as the frequency of alternation ia

FIG. 10

standardized in Great Britain at 50 and 25 cycles per second, and in the United States and Canada at 60 cycles per second, the speeds of turbines have to be correspondingly standardized. If F denotesj the frequency, and N the number of pairs of poles of the alternator,

then =^ denotes the only possible speed, in revolutions per

minute, at which the turbine can be run. In Great Britain the standard turbine speeds are therefore 3,000, 1,500, 1,000 and 750 revs, per minute, while for 60 cycles they are 3,600, 1,800, 1, 200 and | 900 revs, per minute. It is naturally desirable to build any turbine for the highest speed at which the desired output can be economically ; obtained. Considerations of stress limit the dimensions for a jjiven speed, and the dimensions limit the volume of steam which can be >