26o 



NATURE 



[April 20, 191 1 



ran at 18,000 revolutions, and had fcli^htly c!. "' ' 

 ings. 'Ihe turbine t(-fth or bladis were lik< 



teeth, set al an angle and sliarpcned at tl>e fruiu , -p, 



the guide blades wrre similar. Gradually the form of the 

 blades was improved —curved blades with thickened backs 

 were introduced. The blades were cut off to length from 

 brass material rolled and drawn to the required section, 

 and inserted into a groove with soft brass packing distance 

 pieces between and caulked up tightly, and dummy laby- 

 rynth packings of various types introduced. The design 

 was improved so as to reduce steam leakages and provide 

 for greater expansion ratios. 



The construction of a suitable dynamo to run with the 

 turbine involved nearly so much trouble as the turbine 

 itself ; the chief features were the adoption of very low 

 magnetic densities in the armature core and small 

 diameters and means to resist the great centrifugal forces. 

 The dynamo was also mounted in elastic bearings. Now 

 that the turbine has found its most suitable field in large 

 powers, and the speed of revolution is consequently re- 

 duced, elasticity in the bearings is less essential, and in 

 large land plants and in marine work rigid bearings are 



Fig. I.— Turbines being completed. (From Engineering.) 



now universal. I have said that steam behaves like an 

 incompressible fluid in each turbine of the series, but as 

 it is highly elastic, its volume increases with the succession 

 of small drops of pressure, and the turbines have to be 

 made larger and larger. This enlargement is secured by 

 increasing the height of blade, by increasing the diameter 

 of the succeeding drums, and by altering the angles and 

 openings between the blades. All three methods are 

 generally adopted to accommodate the expanded volume of 

 one hundredfold in the condensing turbine. 



Now as to the best speed of the blades. It will be 

 easily seen that in order to obtain so much power as 

 possible from a given quantity of steam, each individual 

 row of blades must work under appropriate conditions. 

 This, as has been found by experiment, requires that the 

 velocity of the blades relatively to the guide blades shall 

 be about one half the velocity of the steam, or, more 

 accurately, equal to one half the velocity of issue from 

 rest due to the drop of pressure in guides or moving blades. 

 The curve for efficiency in relation to the velocity ratio 

 has a fairly fiat top, so that the range of velocity ratio 

 for high efficiency is wide, and the speed of the turbine 

 may be varied considerably about that for maximum 

 efficiency without materially affecting the result. 



In compound turbines the efficiency of the initial rows 



NO. 2164, VOL. 86] 



- " ' ' :• <»s ?^r (. • : 



, and, fu 



. ^. , ,, ,1 cent, cit ^y 



in the steam is <>n lu the chaft. '1! jn 



curve may be < .ipproximately by />. /',, 



where pcPt '* ^'^^' drop in pressure across aiu lu; uim; ; 

 pv is obviously not tjuite constant, but if a mean value is 

 assumed the error is small. The '- ' r-- 



fore lies between the adiabatic and ,r 



steam, but nearer the former, and 



assumptions are found by experiment to be of mu 

 importance than the errors in workmanship and in. 

 tions of materials that are unavoidable in pracLital 

 mechanics. The differential thermal expansions of the 

 metal of which the turbine is mad- ' ' ' 



for large working clearances and lo 

 every available means is taken to n)::.„„. 

 In turbine design, the expression of ; 

 between the steam and blades may be r- : 

 integral of the square of the velocity of eacli row ; 

 the turbine, which is a coefficient called K. If 

 instance, as usual in land turbines, equals i5o,o<' 



we kno 



with a . 



pressure o( 200 

 lb. and a good 

 vacuum the 

 velocity ratio is 

 0-55, and the 

 turbine is work- 

 ing close up to 

 its speed for 

 maximum effici- 

 ency. In large 

 marine work, 

 where 

 and spa 

 of impo;.„... 

 K varies fron 

 S o , 000 t ■• 



120,000, or ev- 

 t o 140,00' 



With K - 



80,000, a loss of 

 efficiency o l 



about 9 p* : 

 cent, below t"- 

 highest 

 able is a 

 With K 



120,000, the d' - 

 ficit is only abou: 

 I J per cent. 



There a r • 



many forms of 



turbines now on 



the market, but 



we need only consider four chief types, which are : — 



First, the compound reaction turbine, with which we 

 have been dealing, representing more than 90 per cent, of* 

 all marine turbines in use in the world, and about half 

 the land turbines driving dynamos. 



Secondly, the de Laval, which is only used for small 

 powers. 



Thirdly, the " multiple impulse compounded," or Curtis, 

 which has been chiefly used on land, but which has been 

 fitted in a few ships. 



Lastly, the compound reaction type, with one or more 

 " multiple impulse elements " added to replace the reaction 

 blading at the high-pressure end. 



We may dismiss the numerous other types as simply 

 modificat'ons of the original t}-pe, without any scientifit 

 interest. 



Let me explain the latter types. The multiple impulse 

 principle is the only substantial innovation since the 

 compound reaction and the de Laval turbines came into 

 use. It was proposed by Pilbrow in 1842, and first 

 brought into successful operation by Curtis in 1896. 

 .\ little consideration should be given to it as involving 

 some characteristic points of difference from what has 

 been said about reaction blading. It will be seen that 

 Curtis used the de Laval divergent nozzle, and that he 



