Introduction

The history of the gas turbine goes back to 1791, when John Barber took out a patent for ‘A Method for Rising Inflammable Air for the Purposes of Producing Motion and Facilitating Metallurgical Operations’. Many endeavours have been made since then particularly in the early 1900s to build an operational gas turbine. In 1903, a Norwegian, Aegidius Elling, built the first successful gas turbine using a rotary/dynamic compressor and turbines, and is credited with building the first gas turbine that produced excess power of about 8kW (11 hp). By 1904 Elling had improved his design, achieving exhaust gas temperatures of 773K (500 degrees Celsius), up from 673K (400 degrees Celsius), producing about 33kW (44hp). The engine operated at about 20 000 rpm. Much of his later work was carried out (from 1924 to 1927) at Kongsberg, in Norway.

Elling's gas turbine was very similar to Frank Whittle's jet engine, which was patented in 1930 in England. Whittle's design also consisted of a centrifugal compressor and an axial turbine and the engine was subsequently tested in April 1937. Meanwhile, in 1936, Hans von Ohain and Max Hahn, in Germany, developed and patented their own design. Unlike Frank Whittle's design, von Ohain's engine employed a centrifugal compressor and turbine placed very close together, back to back. The work by both Whittle and Ohain effectively started the gas turbine industry.1

Today, gas turbines are used widely in various industries to produce mechanical power and are employed to drive various loads such as generators, pumps, process compressors, or a propeller. The gas turbine began as a relatively simple engine and evolved into a complex but reliable and high efficiency prime mover. The performance and satisfactory operation of gas turbines are of paramount importance to the profitability of industries, varying from civil and military aviation to power generation, and also oil and gas exploration and production.

In the quest to perfect the gas turbine, compressor pressure ratios have increased from about 4:1 to over 40:1 together with high operating temperatures (about 1800K), resulting in thermal efficiencies exceeding 40%. These features make the gas turbine a formidable competitor to other types of prime movers. In increasing the performance of the gas turbine, various engine configurations have evolved and such engine component arrangements and their applications will be discussed. However, the principles of the gas turbine and the main components that are required for these engines will be discussed first.

1.1 The gas turbine

For a turbine to produce power, it must have a higher inlet pressure than that at the exit. A compressor is normally used to provide this increase in pressure into the turbine. If the compressor discharge flow through the turbine is expanded, the turbine power output will be less than the power absorbed by the compressor because of losses in the compressor and turbine. Under these conditions, the whole engine will cease to rotate.

If energy is added into the compressor discharge air, corresponding to the losses in the compressor and turbine, then the system will run but will not produce any net power output. To produce net power from the gas turbine, additional energy needs to be supplied into the compressor discharge air. The energy supplied to the compressor discharge air is normally achieved by burning fuel in the compressor discharge air and this is accomplished in a combustion chamber or combustor, which is located or positioned between the compressor and turbine as shown in Fig. 1.1.

Clearly, the power output from a gas turbine depends on the efficiency of the compressor, turbine and the combustor. The higher the efficiency of these components, the better will be the performance of the gas turbine, resulting in increased power output and thermal efficiency.

The gas turbine has developed over 50 years into a high efficiency prime mover, and compressor and turbine efficiencies (polytropic) above 90% can be achieved today.

From the above discussion, a gas turbine must therefore have at least the following components:

(1) compressor

(2) combustor

(3) turbine.

A gas turbine comprising these components is often referred to as a simple cycle gas turbine. Gas turbines can include other components, such as intercoolers to reduce the compression power absorbed, re-heaters to increase the turbine power output and heat exchangers to reduce the heat input. These types of gas turbines are referred to as complex cycles. Although such complex cycles were developed in the early days of the gas turbine, today, simple cycle gas turbines dominate, and this is due to the high levels of performance achieved by engine components such the compressor, turbine and combustor. However, there is a renewed interest in complex cycle designs as a means of improving the performance of the gas turbine further.

1.2 Gas turbine layouts

Various arrangements of the gas turbine components have evolved over the years. Some are better suited for certain applications such as power generation (constant speed operation of the load, i.e. the generator) and other layouts are more suited to mechanical drive applications where the gas turbine is used to drive a process compressor or a pump (where the speed of the driven equipment can vary with load). In this section, we shall discuss these various arrangements, highlighting their advantages and disadvantages.

1.2.1 Single-shaft gas turbine

A single-shaft gas turbine consists of a compressor, combustor and a turbine as shown in Fig. 1.1. The compressor draws in air and increases its pressure. This compressed air is then introduced into the combustor, where heat is added by burning fuel. The hot, high-pressure gases are then expanded in a turbine to extract useful power. Part of the turbine power output is absorbed by the compressor, thus providing power for the compression process via the shaft connecting the compressor and turbine. The remaining power output from the turbine is used to drive a load such as a generator.

Single-shaft gas turbines are most suited for fixed speed operation such as base-load power generation. Single-shaft gas turbines have the advantage of preventing over-speed conditions due to the high power required by the compressor and can act as an effective brake should the loss of electrical load occur.

1.2.2 Two-shaft gas turbine with a power turbine

The expansion process in the turbine shown in Fig. 1.1 above may be split into two separate turbines. The first is used to drive the compressor and the second is used to drive the load. The mechanically independent (free) turbine driving the load is called the power turbine. The remaining turbine or high-pressure turbine, compressor and the combustor are called the gas generator. Figure 1.2 shows a schematic layout of a two-shaft gas turbine with a power turbine and is probably the most common engine configuration that is employed for gas turbines in general.

The function of the gas generator is to produce high pressure and high temperature gases for the power turbine. Two-shaft gas turbines operating with a power turbine are often used to drive loads where there is a significant variation in the speed with power demand (mechanical drive applications such as gas compression). Examples are pipeline compressors and pumps. The process conditions may be such that the load runs at low speed but absorbs or demands a large amount of power. In such a situation, the power turbine can run at the speed of the load and the gas generator can run at its maximum speed. If a single shaft gas turbine were employed to provide the power requirements for such applications, the whole engine would be constrained to run at the speed of the load thus resulting in poor engine performance due to the low operating speed condition.

Two-shaft gas turbines are also employed in industrial power generation with the power turbine designed to operate at a fixed speed determined by the generator. Unlike a single-shaft engine, the gas generator speed will vary with electrical load. The main advantage is smaller starting power requirements, as the gas generator only needs to be turned during starting, and better off-design performance. The disadvantage is that the shedding of the electrical load can result in over-speeding of the power turbine.

1.2.3 Three-shaft gas turbine with a power turbine

The gas generator (GG), as discussed in Section 1.2.2, can be divided further to produce a two-shaft or a twin spool gas generator. When this is done, the high-pressure GG turbine drives the high-pressure GG compressor, and the low pressure GG turbine drives the low pressure GG compressor. However, there is no mechanical linkage between the high pressure and low pressure shafts in the gas generator. Figure 1.3 shows a schematic layout of a three-shaft gas turbine with a power turbine. The power turbine is still mechanically independent from the gas generator as described in Section 1.2.2.

Such three-shaft arrangements, as with a two-shaft gas turbine with its own power turbine, are widely used in mechanical drive applications. Much higher-pressure ratios and thermal efficiencies may...