1 Turbine types
Thorough reviews of the development of the steam turbine have been presented by Traupel  and Harris .
1.1 Direction of flow
Almost without exception, modern large steam turbines are of the axial-flow type (Fig 1.1). Even from the early days in the late nineteenth century, this has been the preferred solution for steam turbines of all ratings.
The principal exception to this rule has been the radial-flow turbine (Fig 1.2), originally introduced by Ljungstrom, and developed to its most recent form by Stal-Laval (now ASEA-Stal). In this design, steam is admitted to the centre of the machine, expanding outwards through two contra-rotating rotors to the exhaust at the periphery. The concept was successfully executed and has been applied at ratings up to about 60 MW. ASEA-Stal still offer the radial-flow turbine for small industrial sets up to about 30 MW output. However, the design is not adaptable to large outputs because of the mass of the blades which would be mounted at the outer periphery. The largest application of the radial-flow turbine is for the high pressure (HP) cylinder of a 460 MW unit associated with a boiling water reactor at Oskarshamn in Sweden.
Axial flow is therefore the characteristic feature of modern steam turbines. These are often categorised according to the type of blading used (see Section 5 of this chapter), but in terms of direction of flow, the steam approaches a group of stages at one end, flows axially through the radially-mounted blading and exhausts at the other end of the group of stages. The simplest configuration of blading is in single-flow (Fig 1.3 (a)).
Groups of stages within a turbine cylinder may be arranged for flow in opposing axial directions. The normal situation for this is the double-flow turbine (Fig 1.3 (b)), in which the steam is admitted at the centre of the cylinder and is divided to flow in opposite axial directions towards the ends of the rotor. This arrangement is used to avoid the excessively long blades which would be incurred by a single-flow arrangement. For turbines of large output, it is normal to have several double-flow low pressure (LP) cylinders operating in parallel. The second benefit is that the double-flow cylinder effectively reduces to zero the axial thrust caused by the steam forces on the moving blading.
One or both of these objectives are also the reasons for other arrangements of stage groups within a turbine cylinder. Thrust reduction is the main target for the reversed-flow cylinder (Fig 1.3 (c)), in which the steam flows in one direction through one group of stages and is then ducted (internally or externally) to flow through a second group of stages in the opposite axial direction. In some cases, the second group of stages may have several sections in parallel to accommodate the increasing volumetric flow rate.
In axial turbines, the steam is admitted to the cylinder and exhausted from it, by one or more circular pipes in a radial or near-radial orientation (see Section 6 of this chapter). At the LP turbine exhaust, the connection flange(s) to the condenser will however normally be rectangular, at an appropriate location and orientation to match the condenser design. This always involves radial flow in one or more directions. The steam is therefore required to turn through a right angle to enter the axial-flow blading and to exhaust from it, at the same time redistributing itself around the circumference. The inlet and exhaust areas therefore require sufficient space to allow an orderly flow without undue pressure loss or flow separation. In some cases, particularly low pressure turbine exhausts, flow guides may be incorporated.