Elements of a Simple Gas Turbine Plants
Elements of a Simple Gas Turbine Plants: A simple gas turbine plant
is shown in Fig. It consists of compressor combustion and turbine. When the units run the atmospheric air is drawn into the compressor, raised to static pressure several times that of the atmosphere. The compressed air then flows to the combustion chamber, where the fuel is injected. The produces of combustion, comprising a mixture of gases at high temperature and pressure, are passed through the turbine where they expand and develop motive force for turning the turbine rotor. After expansion, the gases leave the turbine at atmospheric pressure.
The temperature of the products of combustion is nearly 1000° to 1500°F, The temperature of the exhaust gases is in the range of 900° to 1100°F. The compressor is mounted on the same shaft as that of the turbine. The major portion of the work developed in the turbine is used to drive the compressor and the remainder is available as net power output.
Turbine drives the compressor and the load. Both impulse and
reaction turbines can be used in gas turbine plants. As compared to steam turbines gas turbines have tew stages because they operate on smaller pressure drops.
Axial flow type turbines are commonly used. The various requirements of
turbines are as follows:
(i) Light Weight
(i) High Efficiency
(iii) Reliability in operation
(iv) Long working life.
Combustion Chamber :
In the combustion chamber, combustion of fuel takes place. The combustion process taking place inside the combution chamber is quite important because it is in this process that energy, which is later converted into work by the turbine, is supplied. Therefore, the combustion products and air so that complete combustion and uniform temperature distribution in the combustion gases may be achieved.
Combustion should take place at high efficiency, because losses incurred in the combustion process have a direct effect on the thermal efficiency of the gas turbine cycle. Further the pressure losses in the combustion chamber should provide sufficient volume and length for complete combustion of the fuel.
Initially the temperature developed in combustion chamber is too high. The
difficulty is avoided by adding a satisfactory amount of air to maintain stable combustion conditions and then the products of combustion are cooled to a temperature suitable for use in gas secondary the combustion chamber. In combustion chamber used for aircraft engines a large quantity of air is used to keep the temperature of combustion chamber to about 650°C. The air fuel ratio may be of the order of 60 :1 in this case
The requirements of a combustion chamber are as follows:
(i) Low-pressure loss
(ii) High combustion efficiency
(iii) Good flame stability
(iv) Low weight
(v) Through mixing of cold air and hot products of combustion to generate
(vi) Low carbon deposit in the turbine, and combustion chamber.
The various compressors used are the reciprocating compressor, centrifugal compressors and axial flow compressor. The reciprocating compressors are not preferred due to the friction in sliding parts, more weight, less speed and inability to handle large volumes of air. For a gas turbine power plant of high output and efficiency generally pressure ratios of 10: 1 or more is used. It is observed that when a single compressor with a pressure ratio not more than 4:1 is required the centrifugal compressor is the most suitable.
It is quite rugged in construction, can operate more efficiently over a wide range of mass rate of flow of air than a comparable axial flow compressor. Centrifugal compressor is mainly used in super chargers and in jet aircraft plants, where lower pressure ratios and small volumes of
air is needed. For higher pressure ratios multi-stage centrifugal compressor does not prove to be as useful as an equipment axial flow compressor. Therefore, when high pressure ratios are needed, axial compressor is advantageous and is always used for industrial gas turbine installations. Further is desirable that more than one compressor should be used when the pressure ratio exceeds ratio 6:1. Although the axial flow compressor is heavier than the centrifugal compressor but it has higher efficiency than the centrifugal compressor. It is important that air entering the compressor should be free from best.
Therefore, air should be passed through a filters are not needed in the closed cycle system. Elements of a Simple Gas Turbine Plants