Chapter 6
Thermodynamic Performance


The pressure ratios for the maximum specific output and efficiency can vary greatly, depending on whether the gas turbine cycle features a recuperator, inter coolers or even intermediate heating. The turbine inlet temperature also pushes up the optimal pressure ratio. The pressure level of the cycle, on the other hand, has no effect. However, since a closed process can be designed for different gases, this also gives rise to different pressure ratios. The higher the molecular weight of the working medium, the fewer the isentropic exponent κ, which results in lower temperature ratios in the flow machinery at a constant pressure ratio. Since the temperature ratio influences the process efficiency, gases with several atoms such as CO2 need much higher pressure ratios than helium, which has a single atom. If we consider helium, with a turbine inlet temperature of 850°C and a cooling water temperature of 15°C, the diagrams in Fig. 72 show the thermal efficiency of various cycle designs as a function of the specific output. The pressure ratios are shown as points.

Cycle A has no recuperator, intercooler or intermediate heating. B stands for the recuperator, with 1 for an 85% degree of recuperation, 2 for 90% and 3 for 95%. C stands for a compressor intercooling, with the number beside it indicating the number of intercooler stages. Finally, D stands for intermediate expansion heating, with the numbers indicating the number of intermediate heating stages. Figure 73 contains enthalpy-entropy diagrams of the four processes analyzed and Fig. 74 is a cycle diagram of the fourth process. The sealing and cooling helium flows are also shown (dotted lines) as included with the flow pressure drops in the cycle system in the calculations for the results shown in Fig. 72.

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