328 The Hon. Charles A. Parsons [May 4, 



velocity of the blades will approximate to one-half that of the 

 tangential component of the steam issuing from the guide blades. 

 The blades, as we have seen, are curved, with thickened backs, and are 

 smooth ; the steam therefore flows around them, and past them, 

 without much loss by shock or eddy current or frictional loss. The 

 proportions of turbines as regards diameter, height of blade, and 

 blade openings, are calculated so that, under average working condi- 

 tions, the correct expansion of the steam shall be attained, and the fall 

 in pressure and velocity of steam at each turbine of the series shall be 

 such as to secure for it the highest efficiency. 



When a turbine is tested the pressures at many points along the 

 barrel are recorded, and the calculated pressures confirmed and 

 verified by experiment, and these are usually in close accord. As the 

 result of data accumulated from experiments on many turbines, the 

 probable horse-power that will be obtained from a given design of 

 turbine can be predicted with as much accuracy as in the case of the 

 reciprocating engine. The best results that have been obtained from 

 large turbines show that about 70 per cent, of the available energy in 

 the steam is converted into brake horse-power : and where, we may 

 inquire, has the other 30 per cent, gone ? 



The chief losses of efficiency in all steam turbines are due to 

 three principal causes : firstly, to skin-friction of the steam coursing 

 at high temperature through the small openings between the blades ; 

 secondly, to unavoidable leakages ; and, thirdly, to eddy-current losses 

 arising from insufficient blade velocity and errors of workmanship. 



The first of these losses, the friction of the steam, is reduced by 

 superheating, and thus partially removing the fluid frictional loss 

 arising from the drops of condensed water mingled with the steam. 

 In some cases this gain in efficiency is worth the extra cost of the 

 superheater, but, unless intermediate superheaters are used, initial 

 superheat cannot be raised high enough to maintain dryness through- 

 out the major part of expansion without destroying the turbine. 

 Moderate initial superheat, however, is generally used with some gain 

 in economy, which in the compound turbine amounts to 1 per cent. 

 for every 10° F. of superheat. The second loss, which is from 

 leakage, is present in the compound and the sinuous types but not in 

 the De Laval type. The amount of this loss decreases as the size of 

 the engine increases. It is also chiefly consequent on the coefficient 

 of expansion of metals, which is a bugbear to the turbine designer. 

 If a metal with a much smaller coefficient of expansion than steel and 

 iron could be obtained at a reasonable price and of suitable qualities 

 for the construction of turbine cases, drums and shafts, a considerable 

 increase of economy could be obtained, as it Avould allow of smaller 

 working clearances and less leakage. The third loss, from insufficient 

 blade-velocity, is not present to a material extent in the larger com- 

 pound or sinuous course turbines, but is present, as already explained, 

 to a considerable extent in the single-wheel type. 



