December i6, 1909J 



NATURE 



205 



volumes of steam does not occur in the case of the steam 

 turbine as in the case of the reciprocating engine, and it 

 has been found that with the steam turbine the gain due 

 to vacuum goes steadily on up to the highest attainable 

 vacua. Between 25" and 26", or 26" and 27", there is a 

 gain of about 4 per cent. ; a further gain of S per cent, 

 is made with the vacuum increased to 28", and a still 

 further gain of 6 per cent, to 7 per cent, when it is in- 

 creased to 29". 



This is more easily understood if we consider that the 

 theoretical power to be derived from the steam is almost 

 proportional to the logarithm of the e.xpansions, and thus 

 practically the same power can be obtained working from 

 400 lb. to I lb. absolute, or 28" vacuum, as from 200 lb. 

 to \ lb., or 29" vacuum. In each case there are 400 

 expansions by pressure, and in each case the theoretical 

 consumption of steam by Clausius' cycle would be about 

 93 lb. per kilowatt hour. With 150° F. superheat this 

 would come down to 87 lb., and under the conditions of 

 200 lb. pressure and 29" vacuum with 150° F. superheat, 

 13. 2 lb. per kilowatt hour has actually been obtained 

 with an overall efficiency, including the alternator, of 

 about 66 per cent., or 715 per cent, on the turbine shaft, 

 allowing for the electrical losses. Prof. Ewing, in his 

 book on "The Steam Engine," gives a list of principal 

 results obtained from condensing reciprocating engines, and 

 in no case does the ratio of the consumption of steam by 

 Clausius' cycle, compared with that used per indicated 

 horse-power, exceed 64 per cent. As the ratio of brake 

 horse-power to indicated horse-power is never more than 

 90 per cent., this means an efliciency at the engine shaft 

 of not more than 58 per cent. When it is remembered 

 that the figure obtained in the case of the turbine was 

 715 per cent., and further that the reciprocating engine is 

 unable to take advantage of high vacua, it is easily seen 

 where the advantage of the turbine, especially in large 

 sizes, comes in. 



The other advantages were dealt with of absence of 

 vibration, reduced cost of repairs and maintenance, and 

 space occupied, this last being only in some cases one-third 

 or one-fourth of that necessary for reciprocating engines. 



In very large sizes, also, it has been found practically 

 impossible to make reciprocating engines satisfactory, and 

 It may not, perhaps, be generally known that one of the 

 reasons which led the Cunard committee to adopt turbines 

 for the great express steamers Ltisitania and Mauretania 

 was the fact that the engineering difficulties of the 

 enormous reciprocating engines required made the problem 

 almost impossible of solution without the use of turbines. 



Steam turbines may be divided into two great divisions, 

 single and compound. In the former class there is the 

 De Laval, in which the whole of the expansion is carried 

 out in a single jet, but in order to get efficiency the speed 

 of revolution is so high that gearing has to be resorted to, 

 and this limits this type of turbine to small sizes. 



The second class is that universally adopted for all large 

 turbines, in which the expansion of the steam is carried 

 out in stages. 



The compound turbine naturally divides itself into two 

 subclasses, those in which the expansion of the steam 

 takes place both in the fixed and moving blades, and those 

 in which it takes place in the fixed blades only. Included 

 in the former class is the Parsons, while the latter con- 

 tains the Rateau, Zoelly, Curtis, and various others. In 

 the Rateau and Zoelly, which strongly resemble one 

 another, the velocity of the steam at each stage is taken 

 up by a single row of blades mounted on a wheel, and in 

 the Curtis by a wheel having two or more rows of moving 

 blades with guide blades between. There are also various 

 combinations of these, especially those with a Curtis high- 

 pressure part and a Parsons low-pressure, but as yet they 

 have not come largely into use. 



A description is then given of the various tvpes of 

 turbine, and also the method of calculating the blading and 

 other particulars, along with some practical rules for their 

 design, and it is shown that, with these limitations, turbines 

 can be constructed with similar stresses and dimensions to 

 give outputs varving as the square of their dimensions 

 and inversely as the square of the speed of revolution. 



Now it can be shown that alternators also obey the same 

 I ule of varying inversely as the square of the speed, and 

 NO. 2094, VOL. 82I 



thus it will be seen that alternators coupled to turbines go 

 up in size together, and that, apart from the trouble there 

 is due to being compelled to have an even number of poles, 

 alternators of the maximum size for that speed have 

 similar turbines attached to them, and thus there is no 

 limit to the size of turbo-alternator. In the case, how- 

 ever, of continuous current dynamos, the output of a 

 dynamo (as it is chiefly limited by commutation conditions 

 which depend principally on the ampere turns on the arma- 

 ture per inch diameter) is practically only proportional to 

 the speed, and it is easily seen that a limit is soon reached 

 where the speed of the turbine is too low for economical 

 conditions. 



However, by using tandem dvnamos it will be seen that 

 the output is doubled, and this enables tandem turboi- 

 dynamos up to about 4000 kilowatts to be economically 

 built. 



In the second lecture various applications of and 

 auxiliaries to the steam turbine were described. 



The design of condensers has been especially influenced 

 by the introduction of steam turbines. .•\s has been shown, 

 in the old days of reciprocating engines, the condenser 

 giving 25" vacuum was quite good enough, but nowadays, 

 on account of the great improvement in economy of steam 

 turbines, with higher vacua, it is common to have between 

 28" and 29". 



The maximum vacuum which can be obtained from a 

 condenser is the vacuum due to the temperature of the 

 outlet water, and the closer to this we can get the vacuum 

 actually obtained the better. There are two ways of 

 expressing this difference : one is by pressure and the 

 other is by temperature, and for condenser work the 

 latter is the more convenient. When it is remembered 

 that from about 24" to 27" each inch of vacuum makes 

 4 per cent, difference in the steam consumption of a tur- 

 bine, between 27" and 28" about J per cent., and from 

 28" to 29" 6 per cent, or 7 per cent., or that, approxim- 

 ately, 3° F. difference in the temperature of the exhaust 

 means an increase or decrease of about i per cent, in steam 

 consumption, it is easily understood how important it is 

 to keep the difference of temperature between the outlet 

 water from the condenser and the temperature due to the 

 vacuum as small as possible. This difference in good 

 modern condensers, when condensing, say, 12 lb. per square 

 foot per hour, can be kept as low as <;° F. or 6° F. 



Another way of looking at the efficiency of the condenser 

 is the B.T.U. transmitted per square foot of cooling surface 

 per hour per 1° F. difference of temperature, and this 

 figure can in well-constructed condensers be as high as 

 1000 to 1200 B.T.U. 



It is in connection with the extracting of air thoroughly 

 from the condenser that the greatest improvements have 

 been made of late years, and amongst these dry air pumps 

 and the vacuum augmentor are especially prominent. This 

 latter consists simply of a jet of steam drawing the air 

 and vaoour from the condenser and delivering it through 

 a small auxiliary condenser to the air pump, and thus, 

 although the air pump may only produce a vacuum of, 

 say, 27" or 28", there may be a vacuum of 28" to 29" 

 in the condenser, and in practice this appliance has been 

 found most satisfactory. The effect of using this vacuum 

 augmentor has been in some cases to bring up the con- 

 ductivitv from about 250 or 300 to between 800 and 1000, 

 or to reduce the loss of temperature from some 26° F. to 

 5° F., a gain in temperature of, say, 21° F., or 7 per 

 cent., in the consumption of the turbine. 



When it is remembered that the steam jet of the vacuum 

 augmentor only uses about 0-6 per cent, of the _ steam 

 used by the turbine, it is easily seen that the gain due 

 to the better vacuum is vastly more than the loss due to 

 the steam jet. 



One great field for turbines which has onlv within the 

 last counle of vears come into prominence, although it was 

 patented by Mr. Parsons some years ago, is the use of 

 exhaust turbines, that is, turbines taking steam at atmo- 

 spheric pressure from reciprocating engines or other 

 machinery, and utilising the power contained in it in an 

 pxhnust turbine. When it is remembered that there is as 

 much power in steam working from atmospheric pressure 

 down to a 27" vacuum as between Ko lb. down to atmo- 

 spheric pressure, it is easily seen that the power of an 



