Gas turbine or combustion 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. It may also refer to the turbine component.
Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine??™s blades, spinning the turbine which powers the compressor and, for some turbines, drives the mechanical output. The energy given up to the turbine comes from the reduction in the temperature and pressure of the exhaust gas.
Gas turbines are described thermodinamically by the Brayton cycle, in which air is compressed isentropically (equal), combustion occurs at constant pressure, and expansion over the turbine occurs isentropically back to the starting pressure.
Types of gas turbines are jet engines, aeroderivative gas turbines, amateur gas turbines, auxilliary power units, and industrial gas turbines. Airbreathing jet engines are gas turbines optimized to produce thrust from the ehaust gases, or from ducted fans connected to the gas turbines. Jet engines that produce thrust primarily from the direct impulse of exhaust gases are often called turbojets, whereas those that generate most of their thrust from the action of a ducted fan are often called turbofans or (rarely) fan-jets. Gas turbines are also used in many liquid propellant rockets, the gas turbines are used to power a turbopump to permit the use of lightweight, low pressure tanks, which saves considerable dry mass.
Aeroderivative gas turbines are also being used in electrical power generation due to their ability to startup, shut down, and handle load changes more quickly than industrial machines. They are also used in the marine industry to reduce weight. The GE LM2500 and LM6000 are two common models of this type of machine.
Amateur gas turbines increase the number of gas turbines .These are the commercial turbines acquired through military surplus or scrapyard sales, then operated for display as part of the hobby of engine collecting.
Auxiliary power units (APUs) are small gas turbines designed for auxiliary power of larger machines, such as those inside an aircraft. They supply compressed air for aircraft ventilation (with an appropriate compressor design), start-up power for larger jet engines, and electrical and hydraulic power.
Industrial gas turbines differ from aeroderivative in that the frames, bearings, and blading is of heavier construction. Industrial gas turbines range in size from truck-mounted mobile plants to enormous, complex systems. They can be particularly efficient that is 60% when waste heat from the gas turbine is recovered by a heat recovery steam generator to power a conventional steam turbine in a combined cycle configuration. It can be turned on and off within minutes, supplying power during peak demand.
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion.It has almost completely replaced the reciprocating piston steam engine primarily because of its gretaer thermal efficiency and higher power-to-weight ratio. The turbine generates rotary motion, therefore it is particularly suited to be used to drive an electrical generator. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.
There are two types of steam turbines: Steam supply and exhaust condition, and casing or shaft arrangements. Steam supply and exhaust condition types include condensing, non-condensing, reheat, extraction and induction.
Non-condensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, and desalination (remove salt from water) facilities where large amounts of low pressure process steam are available.
Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser.
Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion.
Extracting type turbines are common in all applications. In the turbines, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled.
Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.
Casing or shaft arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications.