Constant pressure gas turbines are now used for generating electricity, driving locomotives and aeroplane propellers, producing the air stream for wind tunnels, and jet propulsion of air craft. In some types of aircraft, the exhaust gases from the main engines are used for driving a gas turbine which, in turn, drives the supercharger. Gas turbines driven by exhaust gases are also used for supplying a pressure charge to diesel engines.
Figure 16.3 Gas turbine utilising exhaust heat
Overall efficiencies of constant pressure gas turbine plants vary between 18%–25%. This is based on a turbine efficiency of 75%–78%. The efficiency of this type of turbine can be improved by using heat from exhaust gases to preheat the compressed air before it enters the combustion chamber. The hot mixture of air and burnt gases is then expanded through the turbine from which they are exhausted into the heat exchanger. The available heat is now given to the high pressure air from the compressor. Such a system is shown in Fig. 16.3.
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