Gas turbine

A gas turbine is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. (Gas turbine may also refer to just the turbine element.)

Theory of operation

Gas turbines are described thermodynamically by the Brayton cycle, in which air is compressed isentropically, combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.

Related Topics:
Thermodynamically - Brayton cycle - Isentropic - Combustion

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As with all cyclic heat engines, higher combustion temperature means greater efficiency. The limiting factor is the ability of the steel, ceramic, or other materials that make up the engine to withstand heat and pressure. Considerable engineering goes into keeping the turbine parts cool. Most turbines also try to recover exhaust heat, which otherwise is wasted energy. Recuperators are heat exchangers that pass exhaust heat to the compressed air, prior to combustion. Combined cycle designs pass waste heat to steam turbine systems. And combined heat and power (co-generation) uses waste heat for hot water production.

Related Topics:
Heat engine - Efficiency - Recuperator - Heat exchanger - Combined cycle - Steam turbine - Combined heat and power

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Mechanically, gas turbines can be considerably less complex than internal combustion piston engines. Simple turbines might have one moving part: the shaft/compressor/turbine/alternator-rotor assembly (see image above), not counting the fuel system.

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More sophisticated turbines may have multiple shafts (spools), hundreds of turbine blades, movable stator blades, and a vast system of complex piping, combustors and heat exchangers.

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The largest gas turbines operate at 3,000 or 3,600 rpm to match the AC power grid. They require a dedicated building. Smaller turbines, with fewer compressor/turbine stages, spin faster. Jet engines operate around 10,000 rpm and micro turbines around 100,000 rpm.

Related Topics:
Rpm - AC - Power grid - Jet engine

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Thrust bearings and journal bearings are a critical part of design. Traditionally, they have been hydrodynamic oil bearings, or oil-cooled ball bearings. This is giving way to hydrodynamic foil bearings, which have become common place in micro turbines and APU’s (auxiliary power units.)

Related Topics:
Thrust bearing - Journal bearings - Hydrodynamic oil bearings - Ball bearings - Foil bearing

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