45
40
35
30
25 Pressure ratio aircraft Pressure ratio industrial 20
15
10
5
0
1940 1950 1960 1970 1980 1990 2000 2010
Y
ear
Figure 1-.. Development of en.ine pressure ratio over the years.
1600 1400 1200 1000
800
600
400
200
0
1940 1950 1960 1970 1980 1990 2000 2010
YEAR
Figure 1-.. Tren. in improvement in firin. temperature.
parallel eachother， as both growths are necessary to achieving the optimum thermal efficiency.
The increase in pressure ratio increases the gas turbine thermal efficiency when accompanied with the increase in turbine firing temperature. Figure 1-5 shows the effect on the overall cycle efficiency of the increasing pressure ratio and the firing temperature. The increase in the pressure ratio increasesthe 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.
An Overview of .as Tur.ines 11
Tamb=15°C EFF. COMP =87% EFF. TURB. =92%
70
60
50 Overall Eff.@ 800 C
Overall Eff.@1000 C
40 Overall Eff.@1200 C
Overall Eff.@ 1300 C
30 Overall Eff.@ 1350 C
Overall Eff.@1400 C
20 Ideal Cycle
10
0 0 5 10 15 20 25 30 35 40 PRESSURE RATIO
Figure 1-5. Overall cycle efficiency.
In thepast， the gas turbine was perceived as a relatively inefficient power source when compared to other power sources. Its efficiencies were as low as15% in the early1950s， today its efficiencies are in the 45-50% range， which translates to a heat rate of 7582BtujkW-hr (8000 kJjkW-hr) to 6824 BTUj kW-hr (7199 kJjkW-hr). The limiting factor for most gas turbines has been the turbine inlet temperature. With new schemes of cooling using steam orconditionedair， and breakthroughs in blademetallurgy， higher turbine temperatures have been achieved. The new gas turbines have fired inlet temperatures as high as 2600 0F (1427 0C)， and pressure ratios of 40:1 with efficiencies of 45% and above.
Gas Turbine Design Considerations
The gas turbine is the best suited prime mover when the needs at handsuch as capitalcost， time from planning to completion， maintenancecosts， and fuel costs are considered. The gas turbine has the lowest maintenance and capital cost of any major prime mover. It also has the fastest completion time to full operation of any other plant. 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 60s.
The design of any gas turbine must meet essential criteria based on operational considerations. Chief among these criteria are:
1. High efficiency
2. High reliability and thus high availability
3. Ease of service
4. Ease of installation and commission
5. Conformance with environmental standards
6.
Incorporation of auxiliary and control systems， which have a high degree of reliability
7. Flexibility to meet various service and fuel needs
A look at each of these criteria will enable the user to get a better under-standing of the requirements.
The two factors， which most affect high turbine efficiencies， are pressureratios and temperature. The axial flow compressor， which produces the high-pressure gas in the turbine， has seen dramatic change as the gas turbine pressure ratio has increased from 7:1 to 40:1. The increase in pressure ratio increases the gas turbine thermal efficiency when accompanied with the increase in turbine firing temperature. The increase in the pressure ratioincreases 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 bladesand creates large drops in cycle efficiency and performance. In somecases， itcan lead to compressorsurge， 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.
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