Gas Turbine

Gas Turbine, also combustion turbine, engine that employs gas flow as the working medium by which heat energy is transformed into mechanical energy. Gas is produced in the engine by the combustion of certain fuels. Stationary nozzles discharge jets of this gas against the blades of a turbine wheel. The impulse force of the jets causes the shaft to turn. A simple-cycle gas turbine includes a compressor that pumps compressed air into a combustion chamber. Fuel in gaseous or liquid-spray form is also injected into this chamber, and combustion takes place there. The combustion products pass from the chamber through the nozzles to the turbine wheel. The spinning wheel drives the compressor and the external load, such as an electrical generator.

In a turbine or compressor, a row of fixed blades and a corresponding row of moving blades attached to a rotor is called a stage. Large machines employ multistage axial-flow compressors and turbines. In multishaft arrangements, the initial turbine stage (or stages) powers the compressor on one shaft while the later turbine stage (or stages) powers the external load on a separate shaft.

The efficiency of the gas-turbine cycle is limited by the need for continuous operation at high temperatures in the combustion chamber and early turbine stages. A small, simple-cycle gas turbine may have a relatively low thermodynamic efficiency, comparable to a conventional gasoline engine. Advances in heat-resistant materials, protective coatings, and cooling arrangements have made possible large units with simple-cycle efficiencies of 34 percent or higher.

The efficiency of gas-turbine cycles can be enhanced by the use of auxiliary equipment such as intercoolers, regenerators, and reheaters. These devices are expensive, however, and economic considerations usually preclude their use.

In a combined-cycle power plant, the considerable heat remaining in the gas turbine exhaust is directed to a boiler called a heat-recovery steam generator. The heat so recovered is used to raise steam for an associated steam turbine. The combined output is approximately 50 percent greater than that of the gas turbine alone. Combined cycles with thermal efficiency of 52 percent and higher are being put into service. Gas turbines have been applied to the propulsion of ships and railroad locomotives. A modified form of gas turbine, the turbojet, is used for airplane propulsion. Heavy-duty gas turbines in both simple and combined cycles have become important for large-scale generation of electricity. Unit ratings in excess of 200 megawatts (MW) are available. The combined-cycle output can exceed 300 MW.

The usual fuels used in gas turbines are natural gas and liquids such as kerosene and diesel oil. Coal can be used after conversion to gas in a separate gasifier.

See also Airplane; Jet Propulsion; Turbine.