Thermodynamics

510 | Thermodynamicsw turbinew compressorBack workw netFIGURE 9–34The fraction of the turbine work usedto drive the compressor is called theback work ratio.produce 21.6 MW (29,040 hp). The regeneration also reduces the exhaust temperaturefrom 600°C (1100°F) to 350°C (650°F). Air is compressed to 3 atmbefore it enters the intercooler. Compared to steam-turbine and dieselpropulsionsystems, the gas turbine offers greater power for a given size andweight, high reliability, long life, and more convenient operation. The enginestart-up time has been reduced from 4 h required for a typical steampropulsionsystem to less than 2 min for a gas turbine. Many modern marinepropulsion systems use gas turbines together with diesel engines because of thehigh fuel consumption of simple-cycle gas-turbine engines. In combined dieseland gas-turbine systems, diesel is used to provide for efficient low-power andcruise operation, and gas turbine is used when high speeds are needed.In gas-turbine power plants, the ratio of the compressor work to the turbinework, called the back work ratio, is very high (Fig. 9–34). Usuallymore than one-half of the turbine work output is used to drive the compressor.The situation is even worse when the isentropic efficiencies of the compressorand the turbine are low. This is quite in contrast to steam powerplants, where the back work ratio is only a few percent. This is not surprising,however, since a liquid is compressed in steam power plants instead ofa gas, and the steady-flow work is proportional to the specific volume of theworking fluid.A power plant with a high back work ratio requires a larger turbine toprovide the additional power requirements of the compressor. Therefore, theturbines used in gas-turbine power plants are larger than those used in steampower plants of the same net power output.Development of Gas TurbinesThe gas turbine has experienced phenomenal progress and growth since itsfirst successful development in the 1930s. The early gas turbines built in the1940s and even 1950s had simple-cycle efficiencies of about 17 percentbecause of the low compressor and turbine efficiencies and low turbine inlettemperatures due to metallurgical limitations of those times. Therefore, gasturbines found only limited use despite their versatility and their ability toburn a variety of fuels. The efforts to improve the cycle efficiency concentratedin three areas:1. Increasing the turbine inlet (or firing) temperatures This hasbeen the primary approach taken to improve gas-turbine efficiency. The turbineinlet temperatures have increased steadily from about 540°C (1000°F) inthe 1940s to 1425°C (2600°F) and even higher today. These increases weremade possible by the development of new materials and the innovative coolingtechniques for the critical components such as coating the turbine bladeswith ceramic layers and cooling the blades with the discharge air from thecompressor. Maintaining high turbine inlet temperatures with an air-coolingtechnique requires the combustion temperature to be higher to compensate forthe cooling effect of the cooling air. However, higher combustion temperaturesincrease the amount of nitrogen oxides (NO x ), which are responsible forthe formation of ozone at ground level and smog. Using steam as the coolantallowed an increase in the turbine inlet temperatures by 200°F without anincrease in the combustion temperature. Steam is also a much more effectiveheat transfer medium than air.