Gas turbines

07.01.2010 18:57

Gas turbines

The gas turbine is in many respects similar to the steam turbine. It also possesses one or more impellers, which turn around an axle. A compressed gas pushs now the impellers. Gas turbines are of importance in power stations, but also in ships and locomotives, and in form of the jet engine also in airplanes, of course.

open cycle

A gas turbine is mostly built with open cycle. After the gas - mostly normal air - has passed through the turbine, it is not used again, but transferred to the environment for cooling. Therefore it is also particularly suitable for airplanes, which don't have water for cooling in contrast to ships.

Grafic: turbine in power station enlarge (41 K, 500 * 250) Gas turbine in power stations The engine above shows the kind of a gas turbine, how it is used e.g. in power stations. The oversize induction and exhaust pipes are to clarify that enormous amounts of air are needed. After the air is sucked in, it is compressed and heated up, which is closer described below. The turbine drives a generator (on the right) for the production of electric current. Red arrows mean "high pressure, high temperature", blue ones "normal pressure, low temperature".

Compression

In the gas turbine first the air gets sucked in and compressed. This compression is done by compressors, likewise impellers, which press air together on small space. In modern turbines several compressors are usually needed, e.g. one low and two high pressure compressors. The compressors stress a part of the won energy and are usually coupled with their own drive turbine therefore.

Burn

Of course the pressure and the temperature of the gas rises after the compression. This compressed air is now additionally heated up, either by an outside burn, or by injecting fuel directly into the turbine, as it was tried e.g. in car gas turbines. The gases are now at huge pressure and are very hot. They go now through the usual multi-level turbine and perform work giving rotation energy to the turbine. When the air leaves the turbine, its temperature is still high.

The jet turbine

The aircraft gas turbine was developed during and after the 2^nd World War and replaced the piston engines of the airplanes gradually. These jet engines have a gas turbine in their core likewise, as they were described above. Additionally the so called turbofan engine possesses an additional impeller, a fan, which is much larger than the other ones and it is located at the very front of the engine. You can see this fan if you look directly into an engine from the front. The large quantity of air which is sucked in by the fan in such a way, is divided in modern engines: One part passes through the turbine, the other part goes around the turbine, it "bypasses" it and is therefore also called "bypass air". It meets at the end again with the other jet. The large amount of air heated up gives the necessary thrust to the turbine now.

 

Grafic: jet turbine, highly simplified. The empty spaces are flowed through by bypass air. enlarge (41 K, 800 * 612), animated (24 K, 400 * 160), animated, big (73 K, 500 * 280), Mini jet turbine (for your website) (9 K, 150 * 84)

Combined engines

In power stations the warm exhaust air of the gas turbine is often used for the production of steam for a steam turbine, that produces energy, too. A combination of gas and steam turbine leads to efficiencies of over 50 percent. The most modern power stations use further a so called heat-force-coupling: The waste heat from the condenser of the steam turbine is used for heating. Like this, about 90 percent of the energy gets used. Unfortunately these systems are also accordingly expensive and are used just in individual power stations of the industrial nations therefore.