1.2 Cylinder and exhaust arrangements
For turbines driving electrical generators, the limit of a single-cylinder turbine is around 100 MW, depending on the design concepts, the initial steam conditions (pressure and temperature), whether or not a reheat cycle is used, the exhaust conditions and the speed of rotation.
Frequently turbines of this size are designed and built on a modular basis, with the inlet sections, blading sections, and exhaust sections selected from a range of standard designs to suit a range of output powers, initial and exhaust conditions and special applications such as extraction steam for process heating or district heating.
For larger machines, multi-cylinder designs are used (Fig 1.4). The number of cylinders depends on a similar list of terminal conditions and design considerations. A typical turbine of 500-900 MW output in a fossil-fired power station (coal, oil, or gas) or a gas-cooled nuclear station would have one HP turbine, one intermediate pressure (IP) turbine and two (LP) turbines, rotating at 3000 or 3600 r/min, depending on the grid frequency (see Section 1.3 of this chapter). The IP and LP turbines would probably be double-flow.
In a power station with a water-cooled reactor (PWR, BWR, CANDU, etc.), the initial steam is at lower pressure and temperature, so the steam mass flow rate and volumetric flow rate are likely to be much higher for a given output. The turbine might then have one double-flow HP turbine and two or three LP turbines.
The turbine with a number of cylinders on a single shaft is described as a tandem-compound machine (Fig 1.5 (a)). The other main type is the cross-compound machine (Fig 1.5 (b)), in which the turbine cylinders are mounted on two separate parallel shafts driving two separate generators. The steam connections and the auxiliary systems are arranged as for a single generating unit. This design of plant has been more widely used where the grid frequency is 60 Hz and the available LP turbine blading results in a large number of LP cylinders being required: the use of a very long shaft can be avoided by the use of a cross-compound machine. Further, the number of LP cylinders can be reduced if the LP turbine shaft rotates at 1800 r/min, while the HP shaft rotates at 3600 r/min.
The arrangement of the LP turbine and its connection to the condenser depends critically on the location of the condenser, and the orientation of the condenser tubes with respect to the turbine axis.
The condenser has traditionally been mounted below the turbine, with the condenser tubes either axial or transverse. During the 1960s and 1970s, many 500 MW and 660 MW turbines were built in the UK with side-mounted condensers and axial tubes. Variants of this design are called the pannier condenser (Fig 1.6 (a)) and the integral condenser (Fig 1.6 (b)). The main objective was to reduce the overall height of the turbine, with a view to reducing the height and size of the turbine hall. Because the tubes were axial, the condenser steam space could be sectionalised, thus allowing a lower condenser pressure in the cold end. This provided a small efficiency benefit.
The main disadvantage of this design is that the condenser becomes an important part of the turbine in terms of structural integrity, loading, foundations, etc. This makes the condenser design dependent on the number and size of LP turbines, thus inhibiting the concept of modular design and complicating the design interface between a turbine maker and a number of possible condenser makers. This design may also complicate access to the turbine for maintenance, for example, to the bearing pedestals.
Later plant in British stations has therefore reverted to the use of underslung condensers with transverse tubes (Fig 1.6 (c)), using a connecting duct between the turbine outlet flange and the condenser inlet flange. Because the condenser tubes are normally much longer than the width of the turbine casing, this duct is trapezoidal in shape.
On some turbines built between 1920 and 1960, where the available last stage blading provided a limitation on turbine output or efficiency, the Baumann exhaust turbine (Fig 1.7) has been used by certain manufacturers, particularly Metropolitan-Vickers in the UK. In this design, the penultimate turbine stage is divided: the steam flow through the outer annular part of the stage is led directly to the condenser, while that flowing through the inner part flows through the final stage on its way to the condenser. Because the two parts of the penultimate stage moving-blading have different duties, there is a discontinuity in the blade profile which makes it a difficult concept to use in the most highly-rated turbines.