Page 394 - Advanced thermodynamics for engineers

384 CHAPTER 17 GAS TURBINES

Propellor Combustion
Fuel
Air chamber
Exhaust
2 3
1 4
C T
Power

FIGURE 17.4
Turboprop aircraft gas turbine.

difference across the engine. These engines are discussed in Section 17.3. In certain variants, e.g. the
turboprop and the fan engines, some shaft work is also taken out of the main shaft to drive the propeller
or fan. This modifies the design of the components and the power balances. A schematic of a turboprop
gas turbine is shown in Fig. 17.4.
Gas turbines are being increasingly used for power generation, when they are used in combination
with steam turbines to make a CCGT power plant. A schematic diagram of such a plant is shown in
Fig. 17.5(a). This has been much simplified, and usually more than one gas turbine is coupled to the
steam turbine. The thermal efficiency of such plant is higher than that of the gas turbine or steam
turbine operating on its own. This is because the temperature range of the combined cycle is bigger
than the range of either of the individual components, as can be seen in Fig. 17.5(b) which shows the
cycles for a CCGT. A further advantage of this type of plant is that the load control can be achieved by
changing the number of gas turbines coupled to the steam turbine. For example, if there are four gas
turbines the power output might be controlled down to 75% load by reducing the output of all the gas
turbines. At between 75% and 50% load perhaps only three turbines will be run, and so on. This means
that the gas turbines are always working close to full load – their most efficient operating regime. The
power output of the steam turbine reduces as the load level goes down.

17.2 SIMPLE GAS TURBINE CYCLE ANALYSIS
The simple gas turbine cycle can be examined quite easily by going around the cycle; consider
Fig. 17.2(a) and (b) for the closed cycle gas turbine, and assuming unit mass of fluid:
Compressor work, w 12

w 12 ¼ h 2 h 1 ¼ c p T 2 T 1 (17.1)
Turbine work, w 34

w 34 ¼ h 4 h 3 ¼ c p T 3 T 4 (17.2)
The heat supplied (at constant pressure), q 23 is

q 23 ¼ c p T 3 T 2 (17.3)