SINGLE AND TWO SHAFT GAS TURBINES

There are basically two gas turbine driving methods, known as ‘single-shaft’ and ‘two (or twin) shaft’ drives. In a single-shaft gas turbine, all the rotating elements share a common shaft. The common elements are the air compressor, the compressor turbine and the power turbine. The power turbine drives the generator.

In some gas turbines, the compressor turbine and the power turbine are an integral component. This tends to be the case with heavy-duty machines.

The basic arrangement is shown in Figure 2.3.

In a two-shaft gas turbine the compressor is driven by a high pressure turbine called the compressor turbine, and the generator is driven separately by a low pressure turbine called the power turbine

The basic arrangement is shown in Figure 2.4.

Two-shaft systems are generally those which use aero-derivative engines as ‘gas generators’, i.e. they produce hot, high velocity, high pressure gas which is directed into the power turbine. Some light industrial gas turbines have been designed for either type of drive. This is achieved by fitting a removable coupling shaft between the two turbines. Some points to consider with regard to the two types of driver are:-

a) High speed of rotation tends to improve the compressor and turbine efficiency. Hence, with two separate shafts, the best thermodynamic performance from both turbines and the compressor is obtainable.

b) Using aero-derivative machines means that a simple ‘add on’ power turbine can be fitted in the exhaust streams of the aero engine. This enables many manufacturers to design a simple power turbine and to use a particular aero engine.

c) Two-shaft machines are often criticized as electrical generators because of their slower response to power demands in comparison with the single-shaft machines. This can be a problem when a two-shaft machine may have to operate in synchronism with other single-shaft machines or steam turbine generators. Sometimes the slower response may affect the power system performance during the starting period of large motors. A power system computerized stability study should be carried out to investigate these types of problem.

Some of the recent aero engines could be called ‘three-shaft’ arrangements because within the gas generator there are two compressor turbines and two compressors.