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Parameters affecting thermodynamic efficiency of simple Gt-cycle and combined cycle

Mumbai University > Mechanical Engineering > Sem 7 > Power Plant Engineering

Marks : 5M

Year: May 2016

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The most important parameters which affect the efficiency and specific output of the simple gas turbine plant are the compressor pressure ration and turbine inlet temperatures. The effects of pressure ration on the performance of the compressor are shown Fig 25.24 (a) and Fig. 25.24 (b) and the effects of the pressure ration and gas turbine inlet temperature on the performance or the gas turbine are shown in fig.25.25 (a) and Fig. 25.25 (b)

It is obvious from Fig.25.24 (a), the optimum pressure ratio with respect to specific output is not the optimum one for thermal efficiency. Designers choose a pressure ratio which maximizes overall economy of the plant. Presently, the inlet temperature of the turbine as high as 13000C with a pressure ratio of 16 are used in modern gas turbine plants.

Performance of combined Cycle Power Plant

1) Compressor Inlet Temperature: The parameters whichaffect the performance of the combined cycle are compressor inlet temperature, pressure ratio, turbine inlet and exit temperatures and compressor and turbine efficiencies. Increasing T1 the compressor work increases but at the same time, heat input is reduced as T2 increases with an increase in T1. Work output of the turbine is not affected by T1. The rate of increase in compressor work is greater than the rate of reduction in heat supply. Therefore, the net effect of increasing T1 is a decrease in the gas turbine cycle efficiency and that also decreases the overall efficiency of the combined plant.

2) Gas Turbine Inlet Temperature Increasing T3 increases the turbine work output and increases the thermal efficiency at a given pressure ratio. But at the same time, heat supply is more with an increase is T3. The rate of increase in turbine work is higher than the rate of increase in heat supply to the combustion chamber for all pressure ratios. Therefore, increase in T3 increases the cycle efficiency.

3) Pressure Ratio: With increase in pressure ratio, the efficiency of the cycle increases at a given

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