A Simple Gas Turbine Plant

A gas turbine plant may be defined as one “in which the principal prime-mover is of the turbine type and the working medium is a permanent gas”.

Simple gas turbine plant

Refer to Fig. simple gas turbine plant consists of the following part

1. Turbine.

2. A compressor mounted on the same shaft or coupled to the turbine.

3. The combustor.

4. Auxiliaries such as starting device, auxiliary lubrication pump, fuel system, oil system and the duct system etc.

A modified plant may have in addition to above an intercooler, regenerator, a reheater etc. The working fluid is compressed in a compressor which is generally rotary, multistage type. Heat energy is added to the compressed fluid in the combustion chamber. This high energy fluid, at high temperature and pressure, then expands in the turbine unit thereby generating power. Part of the power generated is consumed in driving the generating compressor and accessories and the rest is utilised in electrical energy. The gas turbines work on open cycle, semiclosed cycle or closed cycle. In order to improve efficiency, compression and expansion of working fluid is carried out in multistages.

Energy Cycle for a Simple-Cycle Gas Turbine

shows an energy-flow diagram for a simple-cycle gas turbine, the description of which is given below :

Energy flow diagram for gas-turbine unit

— The air brings in minute amount of energy (measured above 0°C).

— Compressor adds considerable amount of energy.

— Fuel carries major input to cycle.

— Sum of fuel and compressed-air energy leaves combustor to enter turbine.

— In turbine smallest part of entering energy goes to useful output, largest part leaves in exhaust.

Shaft energy to drive compressor is about twice as much as the useful shaft output.

Actually the shaft energy keeps circulating in the cycle as long as the turbine runs. The important comparison is the size of the output with the fuel input. For the simple-cycle gas turbine the output may run about 20% of the fuel input for certain pressure and temperature conditions at turbine inlet. This means 80% of the fuel energy is wasted. While the 20% thermal efficiency is not too bad, it can be improved by including additional heat recovery apparatus.