“In the early days of electric lighting the speed of dynamos was far above that of the engines which drove them, and therefore belts and other forms of gearing had to be resorted to. To make a high-speed engine, therefore, was of considerable importance, and this led to the possibilities of the steam turbine being considered. It was at once seen that the speed of any single turbine wheel driven by steam would be excessive without gearing, and in order to obtain direct driving it was necessary to adopt the compound form, in which there were a number of turbines in series, and thus, the steam being expanded by small increments, the velocity of rotation was reduced to moderate limits. Even then, for the small sizes of the dynamos at that time in use, the speed was high, and therefore a special dynamo had to be designed. Speaking generally, an increase of speed of a dynamo increases its output, and therefore it was obvious that such a high-speed dynamo would be very economical of material.

WESTINGHOUSE-PARSONS STEAM TURBINE.
A and S, steam inlets. B, exhaust. P, P, P, balance pistons. T, adjustment bearing. R, relief valve. V, primary admission valve. VS, secondary admission valve.

[Enlarged illustration] (223 kB)

“These considerations led, in 1884, to the first compound steam turbine being constructed. It was of about 10 horse-power and ran at 300 revolutions per second, the diameter of the armature being about three inches. This machine, which worked satisfactorily for some years, is now in the South Kensington Museum. Turbines afterward constructed had two groups of 15 successive turbine wheels, or rows of blades, on one drum or shaft within a concentric case on the right and left of the steam inlet, the moving blades or vanes being in circumferential rows projecting outwardly from the shaft and nearly touching the case, and the fixed or guide blades being similarly formed and projecting inwardly from the case and nearly touching the shaft. A series of turbine wheels on one shaft were thus constituted, and each one complete in itself is like a parallel-flow water turbine, the steam, after performing its work in each turbine, passing on to the next, and preserving its longitudinal velocity without shock, gradually falling in pressure as it passes through each row of blades, and gradually expanding. Each successive row of blades was slightly larger in passage way than the preceding to allow for the increasing bulk of the elastic steam, and thus the velocity of flow was regulated so as to operate with the greatest degree of efficiency on each turbine of the series. . . . It constituted an ideal rotary engine, but it had limitations. The comparatively high speed of rotation necessary for so small an engine, made it difficult to avoid a whipping or springing of the shaft, so that considerable clearances were found obligatory, and leakage and loss of efficiency resulted. It was perceived that these defects would decrease as the engine was enlarged, with a corresponding reduction of velocity. In 1888 therefore several turbo-alternators were built for electric lighting stations, all of the parallel-flow type and non-condensing. In 1894 the machines were much improved, the blade was bettered in its form, and throughout greater mechanical strength was attained. . . . To-day (1905) under 140 pounds steam pressure, 100° Fahr. superheat, and a vacuum of 27 inches, the barometer being at 30 inches, the consumptions are in round numbers as follows:—A 100-kilowatt (134 horse-power) plant takes about 25 pounds of steam per kilowatt-hour at full load, a 200-kilowatt (268 horse-power) takes 22 pounds, a 500-kilowatt (670 horse-power) takes 19 pounds, a 1,500-kilowatt (2,010 horse-power) 18 pounds, and a 3,000-kilowatt (4,020 horse-power) 16 pounds (or 12 pounds per horse-power-hour). Without superheat the consumptions are about 10 per cent. more, and every 10° Fahr. of superheat up to about 150° lowers the consumption about 1 per cent.

“A good vacuum is of great importance in a turbine, as the expansion can be carried in the turbine right down to the vacuum of the condenser, a function which is practically impossible in the case of a reciprocating engine, on account of the excessive size of the low-pressure cylinder, ports, passages and valves which would be required. Every inch of vacuum between 23 and 28 inches lowers the consumption about 3 per cent. in a 100-kilowatt, 4 per cent. in a 500-kilowatt, and 5 per cent. in a 1,500-kilowatt turbine, the effect being more at high vacua and less at low.”

Marine Steam Turbines.

In 1894 Mr. Parsons launched his “Turbinia,” the first steamer to be driven by a turbine. Her record was so gratifying that a succession of vessels, similarly equipped, were year by year built for excursion lines, for transit across the British Channel, for the British Royal Navy, and for mercantile marine service. The thirty-fifth of these ships, the “Victorian” of the Allan Line, was the first to cross the Atlantic Ocean, arriving at Halifax, Nova Scotia, April 18, 1905. She was followed by the “Virginian” of the same line which arrived at Quebec, May 8, 1905. Not long afterward the Cunard Company sent from Liverpool to New York the “Carmania” equipped with steam turbines, and in every other respect like the “Caronia” of the same owners, which is driven by reciprocating engines of the best model. Thus far the comparison between these two ships is in favor of the “Carmania.” The new monster Cunarders, the “Lusitania” and the “Mauretania,” each of 70,000 horse-power, are to be propelled by steam turbines. The principal reasons for this preference are thus given by Professor Carl C. Thomas:—

Decreased cost of operation as regards fuel, labor, oil, and repairs.

Vibration due to machinery is avoided.