"The aerospace engines have been leaders in most of the technology in the gas turbine. The design criteria for these engines were high reliability, high performance, with many starts and flexible operation throughout the flight envelope. The engine life of about 3,500 hours between major overhauls was considered good. The performance of the aerospace engine has always been rated primarily on its thrust/weight ratio. Increase in engine thrust/weight ratio is achieved by developing the blades of high aspect ratio in the compressor as well as optimizing the pressure ratio and firing temperature of the turbine for maximum work output per unit flow. The industrial gas turbine has always emphasized long life and this conservative approach has resulted in the industrial gas turbine in many aspects giving up high performance for rugged operation. The industrial gas turbine has been conservative in the pressure ratio and the firing temperatures. This has all changed in the last 10 years; spurred on by the introduction of the “aeroderivative gas turbine,” the industrial gas turbine has dramatically improved its performance in all operational aspects. This has resulted in a dramatic reduction of the performance gap between these two types of gas turbines. The gas turbine to date in the combined-cycle mode is fast replacing the steam turbine as the base load provider of electrical power throughout the world. This is even true in Europe and the United States of America where the large steam turbines were the only type of base load power in the fossil energy sector. The gas turbine from the 1960s to the late 1980s was used only as peaking power in those countries; it was used as base load mainly in the “developing countries” where the need of power was increasing rapidly and that the waiting period of three to six years for a steam plant was unacceptable. Figures 1-6 and 1-7 show the growth of the pressure ratio and firing temperature, respectively. The growth of these is parallel to each other, as both growths are necessary to achieve the optimum thermal efficiency. The increase in pressure ratio increases the gas turbine thermal efficiency when accompanied by the increase in turbine’s firing temperature. Figure 1-8 shows the effect on the overall cycle efficiency of the increasing pressure ratio and firing temperature. The increase in the pressure ratio increases the overall efficiency at a given temperature; however, increasing the pressure ratio beyond a certain value at any given firing temperature can actually result in lowering the overall cycle efficiency. In the past, the gas turbine was perceived as a relatively inefficient power source when compared with other power sources. Its efficiencies were as low as 15% in the early 1950s; however, today its efficiencies are in the range of 45–50%, which translates to a heat rate of 7582 BTU/kW h (8000 kJ/kW h) to 6824 BTU/kW h (7199 kJ/kW h). The limiting factor for most gas turbines has been the turbine inlet temperature.With new schemes of cooling, using steam or conditioned air, and breakthroughs in blade metallurgy, higher turbine temperatures have been achieved. The new gas turbines have fired inlet temperatures as high as 2600 ◦F (1427 ◦C) and pressure ratios of 40:1 with the efficiencies of 45% and above.

Gas Turbine Design Considerations

The gas turbine is the best suited prime mover when the needs at hand such as capital
cost, time from planning to completion, maintenance costs, and fuel costs are considered.
The gas turbine has the lowest maintenance and capital costs among major prime
movers. It also has the fastest completion time to full operation among other plants. Its
disadvantage was its high heat rate but this has been addressed and the new turbines
are among the most efficient types of prime movers. The combination of plant cycles
further increases the efficiencies to the low 60% region.
The design of any gas turbine must meet essential criteria based on operational
considerations. Chief among these criteria are as follows:

High efficiency

High reliability and thus high availability

Ease of service

Ease of installation and commission

Conformance with environmental standards

Incorporation of auxiliary and control systems that have a high degree of reliability

Flexibility to meet various service and fuel needs
A look at each of these criteria will enable the user to get a better understanding of
the requirements.
The two factors that mostly affect the high turbine efficiencies are pressure ratios and the firing temperature. The axial-flow compressor, which produces the highpressure gas in the turbine, has seen a dramatic change as the gas turbine pressure ratio has increased from 7:1 to 45:1 in some of the aero gas turbines used today. The increase in the pressure ratio increases the gas turbine thermal efficiency when accompanied with the increase in turbine’s firing temperature. The increase in the pressure ratio increases the overall efficiency at a given temperature; however, increasing the pressure ratio beyond a certain value at any given firing temperature can actually result in lowering the overall cycle efficiency. It should also be noted that the very high-pressure ratios tend to reduce the operating range of the turbine compressor. This causes the turbine compressor to be much more intolerant to dirt build-up in the inlet air filter and on the compressor blades, and it creates large drops in cycle efficiency and performance. In some cases, it can lead to compressor surge, which in turn can lead to a flameout, or even serious damage and failure of the compressor blades, and the radial and thrust bearings of the gas turbine.
The effect of firing temperature is very predominant; for every 100 ◦F (55.5 ◦C) increase in the temperature, the work output increases approximately 10% and gives about a 0.5–1% increase in the efficiency. Higher pressure ratios and turbine inlet temperatures improve the efficiencies on the simple-cycle gas turbine. Figure 1-9 shows a simple-cycle gas turbine performance map as a function of pressure ratio and turbine inlet temperature.

Another way to achieve higher efficiencies is to use the exhaust gases to heat the air leaving the compressor, thus reducing the amount of fuel required to reach the temperature that required to operate the turbine. This is achieved by the use of regenerators or recuperators, which heat the compressor exit air by the exhaust gases from the turbine exit. Regenerators or recuperators are usually used in small- to intermediate-sized gas turbines. Figure 1-10 shows the effects of pressure ratio and temperatures on efficiencies and work for a regenerative or recuperative cycle. Comparing Figure 1-10 with Figure 1-9 shows that increasing the pressure ratio lowers the efficiency in a regenerative or recuperative cycle. The effect of pressure ratio for this cycle is opposite to that experienced in the simple cycle. Regenerators or recuperators can increase efficiency as much as 15–20% of today’s hot section operating temperatures. The use of a regenerator or a recuperator is that they are mostly used in gas turbines with outputs less than 10 MW. In these gas turbines, the airflow is small not requiring a large regenerator or recuperator and the firing temperature is low thus the pressure ratio is usually less than 10:1. The optimum pressure ratio, as seen in Figure 1-10, is about 20:1 for a regenerative system and 40:1 for the simple cycle at today’s higher turbine inlet temperatures that are starting to approach 3000 ◦F (1649 ◦C).
High availability and reliability are the most important parameters in the design of a gas turbine. The availability of a power plant is the percentage of time the plant is available to generate power in any given period. The reliability of the plant is the percentage of time between planed overhauls. The following are the basic definitions of availability and reliability; expanded definitions of these terms are given in Chapter 21. The availability of a power plant is defined as " türkçe çevirisi