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Ease of installation and commissioning is another reason for gas turbine use. A gas turbine unit can be tested and packaged at the factory. Use of a unit should be carefully planned so as to cause as few start cycles as possible. Frequent startups and shutdowns at commissioning greatly reduce the life of a unit.
Environmental considerations are critical in the design of any system. The system"s impact on the environment must be within legal limits and thus must be addressed by the designer carefully. Combustors are the mostcritical component， and great care must be taken to design them to provide low smoke and low N0x output. The high temperatures result in increasing the N0x emissions from the gas turbines. This resulted in initially attacking the N0x problem by injecting water or steam in the combustor. The next stage was the development of Dry Low N0x Combustors. The development of new Dry Low N0x Combustors has been a very critical component in reducing the N0x output as the gas turbine firing temperature is increased. The new low N0x combustors increase the number of fuel nozzle and the complexity of the control algorithms.
Lowering the inlet velocities and providing proper inlet silencers can reduce air noise. Considerable work by NASA on compressor casings has greatly reduced noise.
Auxiliary systems and control systems must be designed carefully， since they are often responsible for the downtime in many units. Lubricationsystems， one of the critical auxiliary systems， must be designed with a backup system and should be as close to failure-proof as possible. The advanced gas turbines are all digitally controlled and incorporate on-line condition mon-itoring to some extent. The addition of new on-line monitoring requires newinstrumentation. Control systems provide acceleration-time， and tempera-ture-time controls for startups as well as controls various anti-surge valves.At operating speeds they must regulate fuel supply and monitorvibrations，temperatures， and pressures throughout the entire range.
Flexibility of service and fuels arecriteria， which enhance a turbinesystem， but they are not necessary for every application. The energy shortage requires turbines to be operated at their maximum efficiency. This flexibility may entail a two-shaft design incorporating apower turbine， which isseparate and not connected to the Gasifier unit.孔ultiple fuel applicationsare now in greater demand， especially where various fuels may be in shortage at different times of the year.
Categories of Gas Turbines
The simple cycle gas turbine is classified into five broad groups:
1. Frame Type Heavy-Duty Gas Turbines. The frame units are the largepower generation units ranging from 3孔W to 480孔W in a simplecycle configuration， with efficiencies ranging from 30-46%.
2. Aircraρt-Derivative Gas Turbines Aeroderivative. As the name indi-cates， these arepower generation units， which have origin in the aerospace industry as the prime mover of aircraft. These units have been adapted to the electrical generation industry by removing the by-passfans， and adding a power turbine at their exhaust. These unitsrange in power from 2.5孔W to about 50孔W. The efficiencies of these units can range from 35-45%.
3. lndustrial Type-Gas Turbines. These vary in range from about
2.5孔W-15孔W. This type of turbine is used extensively in many petrochemical plants for compressor drive trains. The efficiencies of these units is in the low 30s.
4. Small Gas Turbines. These gas turbines are in the range from about
0.
5孔W-2.5孔W. They often have centrifugal compressors and radial inflow turbines. Efficiencies in the simple cycle applications vary from 15-25%.
5. Micro-Turbines. These turbines are in the range from 20 kW-350 kW.The growth of these turbines has been dramatic from the late1990s， as there is an upsurge in the distributed generation market.
Frame Type .eavy-Duty Gas Turbines
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