D. With Respect to Repetition of Flow through Blades

Single Pass, and Re-entry or Repeated Flow.— Practically all turbines are single pass, except the tangential-flow type just described and the re-entry turbine. The blades are set radially, and the steam after passing through them once, following its discharge from the nozzle, is returned by a reversing chamber and again sent through them.

Re-entry turbines also fall into the velocity-stage classification.

E. With Respect to Rotational Speed

60-cycle, 50-cycle, and 25-cycle Generators, Directly Connected to Turbines. Rotational speed requirements are extremely rigid in turbines which are direct-connected to electric generators, as these must be a-c units, except in the smallest sizes, and must therefore run at speeds corresponding to the standard numbers of cycles per second, or

A two-pole unit for 60-cycle service runs at 3,600 rpm, and this is the maximum possible speed, as no generator can have less than two poles. A four-pole machine runs at 1,800, a six-pole at 1,200, and an eight-pole at 900 rpm. For 25-cycle current, a two-pole machine runs at 1,500 and a four-pole at 750 rpm. For 50-cycle current, standard in Europe and somewhat used in the United States, the speeds would be 3,000, 1,500, and 750 rpm.

Geared units may have practically any speed ratio between the turbine and the driven machine, so that the turbine may be designed for its own most efficient speed. Many small-capacity geared outfits are used to drive small generators, fans, pumps, etc., and a few large ones are used for electric generators, but practically the only field for the large geared unit has been in ship propulsion, where high propeller efficiency requires slow or moderate propeller speed (90 rpm for slow-speed ships up to 450 rpm for those of high speed). Single-reduction gearing, consisting of a pinion on the turbine shaft running with a large gear on the propeller shaft, is used for moderate-speed reduction, say, 8 or 9 to 1. In double-reduction gearing, another pinion and gear are added in series, allowing a speed reduction of 60 or 80 to 1.

F. With Respect to Relative Motion of Rotor or Rotors

Single-motion Turbine. All the turbines except the types of radial flow mentioned above have only one rotor in each casing or cylinder and are single-motion turbines.

Double-motion Turbines. Besides the radial-flow double-motion turbine having two rotors turning in opposite directions, axial-flow double-motion turbines are occasionally used. Two ordinary axial-flow wheels carrying single rows of blading pointing in opposite directions are mounted separately on concentric shafts, one being hollow. Steam issuing from the nozzles passes through the blading on one wheel and on exit immediately enters the blading on the second wheel, which turns in the opposite direction. The arrangement allows large power to be produced by a light and compact machine and has been used for propelling naval torpedoes, the working fluid being highly compressed and heated air. The two propellers turn in opposite directions, one being mounted on the hollow shaft and placed slightly ahead of the other.

(Church E. F.,)