In all the experiments, steam was superheated from 40° to 50° Cent. (104° to 122 Fahr.), in view of the expansion, which it would at first have to undergo in order to be brought down from the pressure at the boilers to a pressure barely equivalent to that of the atmosphere. But the results contained in Table I. have been so calculated as to take this superheating into account, and they must be sensibly equivalent to those obtained when working with saturated steam, which is the ordinary course in practice at Bruay collieries. It will be seen that the consumption of steam per electric horsepower-hour at full load amounts to about 18 kilogrammes (40 pounds) at 1,600 revolutions per minute, but only to 17 kilogrammes (37 pounds) at 1,800 revolutions, and that with a bad vacuum of only 63 centimetres (24.8 inches) of mercury. If the vacuum had attained 70 centimetres (27·6 inches), which is quite practicable with a good condenser, the above-stated rates of consumption would have been reduced by about 26 per cent., falling consequently to 13 and 12 kilogrammes (28) and 261⁄2 pounds) respectively. The efficiency at 1,600 revolutions, at full load, of turbine and dynamo taken together, amount to 55 per cent., and at 1,800 revolutions it rose to 58 per cent. The structural conditions involved in the fact that the dynamos had been designed to run at a speed never exceeding 1,800 revolutions per minute, alone prevented the writer from increasing the experimental speed. Nevertheless, the diagram which it is possible to plot out from the foregoing results in functions of the speed, allows us to estimate that at 2,500 revolutions per minute an efficiency of 64 per cent. would be obtainable.* In fact, this has been obtained since then, from a similar group of machinery, but of twice the power, and supplied with high-pressure steam. Fig. 13 (Plate IX.) shows the curve of efficiencies in func. tions of the speed. Fig. 14 (Plate IX.) shows the curves of efficiencies at two different speeds in functions of the load, that is, of the electric power produced at the dynamos. It will be observed that the efficiency becomes considerable at one-third load, and on the other hand remains practically unchanged from half-load upwards. Finally, Fig. 15 (Plate IX.) illustrates the total consumption of steam in functions of the load. It shows clearly that the total consumption at zero load, the electric apparatus being excited, does not exceed 12 per cent. of what it is at full load. With other groups of turbines and dynamos, the writer has known it to fall to 10 per cent. This peculiarity of turbines is worthy of note, for it is known that electrogenerative groups, comprizing piston-engines, consume generally when running at zero load about 20 per cent. of their total consumption at full load. The Hon. C. A. PARSONS (Newcastle-upon-Tyne) wrote that the members were much indebted to Prof. Rateau for bringing this matter prominently before them, as there were, undoubtedly, many cases where such a system would probably lead to a great saving in the consumption of coal, and a reduction in the number of boilers required. per cent. * It is possible then to design when needed, low-pressure plant which shall have an efficiency of 64 This, between a steam-pressure equivalent to that of the atmosphere and a vacuum-pressure of 0.08 kilogramme or 1 pound, corresponding to a good vacuum of 70 centimetres (276 inches) of mercury, would show a consumption of only 11 kilogrammes (24 pounds) of saturated steam per electric horsepower-hour, excitation included. |