An Introduction to gas turbine

In the name of ALLAH who is the most beneficient and merciful

The aim is to be able to identify and explain the function of gas turbine major engine component and discuss the mode of operation of simple and complex cycle gas turbine engines compared with diesel and steam plant.

Definition of a Gas Turine

A continuous cycle self contained heat engine using a gas as the working fluid.

Breakdown of definition

1.      Energy output. After providing the power to drive the compressor(self contained) the energy output may be in the form of :

·         Jet thrust

·         Shaft power

·         Compressed air from compressor

·         Heat

2.      Self contained. Once running it is not dependent on any other machine.

3.      Gas may be.

·         Air – Open cycle

·         Hydrogen, helium etc closed cycle . In this case heat is added by means of a heat exchanger.

4.      Energy source. This may be provided by

·         Oil fuel

·         Natural gas

·         Sewage gas

·         Waste gas

·         Waste heat (eg blast furnance)etc .

5.      Compressor. This is usually either axial or centrifugal, but free piston and reciprocating compressor have been tried, although the process then is no longer continuous.

6.      Continuous. Unlike a diesel which operates with gas in a similar way the GT uses a constant pressure process which is continuous.

Comparison of cycles

In the services we are concerned with three main forms of providing power (see below). These all have particular advantages and disadvantages, some of which can be readily appreciated by a quick comparision of the ideal cycle on a T-S chart.

The following factors show themselves:

1)      Steam Plant

• Operates across saturation envelope.
• Closed cycle – recovery process needed to condense and re-use steam, hence much additional machinery.
• Small work to compress fluid. Good work from expanding vapour- large exess.

2)      Diesel

·         Fairly good available work output after compressing gas.

·         Open cycle.

·         Non-continuous cycle means valves and moving parts. Stress problems.

3)      Gas turbines

• Very poor specific work output available after compressing gas. High powers only available if:
• Component efficiencies high
• High mass flow
• High mass flow possible because process is continuous.

Conclusions

From the above comparisons the factors which stand out about gas turbines are:

• Unlike The others, an overall power output is possible purely because the constant pressure lines diverge with increase in entropy.
• Its specific output is very small and dependent upon the efficient operation of the various components. Mass flows are high hence small increases in component efficiencies will give relatively large increases in output.