Turbines have developed to their present forms along various lines according to steam conditions, condensing water available, types of service, and requirements as to cost and economy; and this process is still going on, with all builders continually changing, experimenting with, and improving their product.¹ There has been a growing amount of standardization of the smaller sizes of machines, but not until the urgent requirements of the Second World War forced it was there a serious attempt to standardize the operating conditions and certain phys­ical characteristics of the units in the range from  11,500 to 60,000-kw rating. However, this standardization is not so rigid but that further development is continually taking place, with the use of higher temperatures and pressures and improvements in heat rates, paralleling the continued improvements in improvement of the electric generator, condenser, and steam-generating equipment associated with the steam turbine in power plant use.

Definitions. A steam turbine may be defined as a form of heat engine in which the energy of the steam is transformed into kinetic energy by means of expansion through nozzles, and the kinetic energy of the result­ing jet is in turn converted into force doing work on rings of blading mounted on a rotating part.

The usual turbine consists of four fundamental parts: the rotor which carries the blades or buckets; the stator consisting of cylinder and casing, which are often combined and within which the rotor turns; the nozzles or flow passages for the steam, which are generally fixed to the inside of the cylinder; and the frame or base for supporting both the stator and the rotor, the latter being carried in bearings. Cylinder, casing, and frame are often com­bined, particularly in small turbines.

Accessories necessary for the successful continuous commercial opera­tion are a controlling or governing system for adjusting the energy supply to the turbine to suit the load to be carried² and for maintain­ing constant speed, a lubricating system, piping for steam supply and exhaust, and (for economical power generation in large quantities) a condensing system.

Classifications of Steam Turbines

Steam turbines may be classified in the following ways:

A. With respect to form of steam passage between the blades:

a) impulse

1) simple, or single-stage

2) velocity-stage, Curtis

3) pressure-stage, Rateau

4) combination pressure- and velocity-stage

b) reaction, Parsons

c) combination impulse and reaction

В. With respect to general arrangement of flow:

a) single-flow

b) double-flow

c) compound, two- or three-cylinder, cross- or tandem-connected

d) divided-flow

C. With respect to direction of steam flow relative to plane of rotation:

a) axial-flow

b) radial-flow

c) tangential-flow

D. With respect to repetition of steam flow through blades:

a) single-pass

b) reentry or repeated flow

E. With respect to rotational speed:

a) for 60-cycle generators

b) for 50-cycle generators (European practice)

c) for 25-cycle generators

d) for geared units and for direct-connected or electric-drive marine units, no especial speed requirements.

F. With respect to relative, motion of rotor or rotors:

a) single-motion

b) double-motion

G. With respect to steam and exhaust conditions;

a) high-pressure condensing

b) high-pressure non-condensing

c) back-pressure

d) superposed or topping

e) mixed-pressure

f) regenerative

g) extraction, single

h) extraction, double

i) reheating or resuperheating

j) low-pressure

(Church E. F., Steam Turbines, N. Y. 1972)