From this point of view the present activity in the construction and development of the steam-turbine is of interest to engineers and power users. The steam consumption of a mod ern steam-turbine of moderate size compares very favorably with that of the better class of large reciprocating engines, but what is of greater importance is the evident superior steam economy under variable loads. The steam con sumption per horse-power hour varies little from one-third to full load; at over-loads the econ omy, as shown by numerous tests, may be even better. This feature predestines the steam-tur bine to the special field of electric lighting and power generation, where it must inevitably be come a formidable rival of the larger-sized, slow-speed reciprocating steam-engine.
The turbine is not only able to utilize a fairly large fraction of the possible gain due to reduction of back pressure, but is generally much cheaper to build than piston engines, especially for very low back pressures which require in piston engines abnormally large low pressure cylinders. Thus the practice has been established of using turbines for the low ranges of pressure in preference to piston engines, and of combining reciprocating engines and tur bines to work together — the turbine taking the exhaust steam from the engine at whatever re lease pressure it is discharged, usually from atmospheric to 25 pounds absolute. This prac
tice has been found especially valuable in adding capacity to existing overloaded reciprocating engines. In some cases the power has been doubled by this arrangement.
The high rotative speed of the steam-turbine is a prominent factor in favor of its adoption in connection with electrical generators, since the cost of the generator end of the equipment ought eventually to be very materially reduced; but for many lines of work the high rotative speed of the present types of steam-turbine is prohibitive, nor can it be adapted successfully to belt-driving, except by the use of gearing.
The economies already obtained with both the steam-turbine and the gas-engine have brought each into a prominence which is at least suggestive of the important developments that are taking place in methods of obtaining and using power.