4 THEORETICAL ANALYSIS OF INTERCOOLED, REHEAT, AND COMPOUND CYCLES 4

4 THEORETICAL ANALYSIS OF INTERCOOLED, REHEAT, AND COMPOUND CYCLES

4.1 IDEAL INTERCOOLED CYCLE

Specific output of a simple cycle gas turbine can be increased by decreasing the compression work. This can be accomplished by dividing the compression process into stages and cooling the compressed air between each stage. In an ideal case, the compressed air is cooled to the inlet temperature.

Figure 4.1 shows an intercooled cycle consisting of a two-stage compressor with an intercooler. Intercooling increases the specific output of the system.

Fig 4.1 Intercooled Cycle

A simple thermodynamic analysis of an ideal, intercooled cycle with a perfect intercooler gives the following relation for thermal efficiency, , at the condition of maximum specific output:³⁷

		(Eq. 4.1)
where:
	T_r = cycle temperature ratio (turbine inlet tamp/compressor inlet temp).

	k = ratio of specific heats, and
	P_r = cycle pressure ratio.

Figure 4.2 compares thermal efficiency of an ideal simple and intercooled cycle (Eq. 4.1 as a function of P_r). Thermal efficiency decreases for the intercooled cycle. In an actual cycle, intercooling tends to lower the average exhaust temperature; however, thermal efficiency is improved in a cycle with relatively high compression ratios.

Fig. 4.2 Thermal Efficiency of Ideal Simple and Intercooled Cycle

4.2 IDEAL REHEAT CYCLE

The specific output of an ideal simple cycle can be increased by dividing the expansion process into steps and reheating expanded gases between each step. The expansion process may take place between separate turbines or between different stages of a multistage turbine.

Figure 4.3 shows a reheat cycle consisting of a two-stage turbine with a second combustion chamber between stages.

Fig. 4.3 Reheat Cycle

Thermodynamic analysis of the reheat cycle shown in Figure 4.3 results in the following relation for thermal efficiency at the condition of maximum specific output.³⁷

(Eq. 4.2)

In deriving Eq. 4.2, it was assumed that the reheat combustor was a perfect device. Figure 4.4 shows the thermal efficiency obtained from Eq. 4.2 and the thermal efficiency of a simple cycle as a function of pressure ratio. The reheat cycle shown here has an even larger reduction in thermal efficiency than the intercooled cycle.

Fig. 4.4 Thermal Efficiency of an Ideal Simple and Reheat Cycle

Intercooled and reheat cycles have a lower efficiency than that of a simple-cycle gas turbine at low-to-moderate pressure ratios. The main effect of intercooling or reheating is to reduce the size and cost of the basic equipment by increasing the specific power of the turbine. However, the lower cost of the basic unit is offset by the added cost of an intercooler or additional combustor and a more complicated turbine design.

4.3 EFFECT OF INTERCOOLING OR REHEAT WITH REGENERATION

Figure 4.5 shows the effect of intercooling (single stage) combined with regeneration and reheat (single stage) combined with regeneration.³ It indicates that intercooling is a more efficient process than reheat when combined with regeneration. The broken curve in Fig. 4.5 shows the thermal efficiency of the regenerative cycle.

Fig. 4.5 Comparison of Simple, Combined Intercooled Regeneration, Combined Reheat Regeneration and Regenerative Cycle Thermal Efficiencies Vs Pressure Ratio

Addition of an intercooler to a simple-cycle gas turbine reduces the flexibility of the system, in that the system no longer is independent of cooling water. Further observation of Fig. 4.5 indicates that at a lower pressure ratio, the regenerative cycle offers system efficiencies slightly lower than those for combined intercooling and regeneration. A regenerative-cycle system is less complicated and therefore costs less.

At high-pressure ratios, combined intercooling and regeneration become more desirable. However, at high-pressure ratios, regenerator and intercooler are more susceptible to leakage which ultimately translates into an increase in unit downtime and increased maintenance cost. Such complexities have made the regenerative cycle the most economical system compared to combined intercooled regeneration and reheat regeneration units. No manufacturer currently offers any combination of intercooled or reheat cycle with regeneration as a standard packaged unit.

4.4 COMPOUND CYCLE

A compound cycle consists of an intercooler, reheater, and regenerator. Such a combination results in high thermal efficiencies over a wide range of pressure ratios and produces high specific outputs. Figure 4.6 shows the thermal efficiency of a compound cycle. The compound plant is larger than a

Fig. 4.6 Comparison of Simple and Compound Cycle Thermal Efficiencies Vs Pressure Ratio

conventional simple-cycle plant because of the addition of the intercooler, reheater and regenerator. Compound cycles have been used only in experimental plants because the higher efficiency and specific output does not warrant the additional plant complexity caused by added equipment. The main advantage of a compound plant over a conventional steam plant is its moderate use of cooling water, which is offset by added equipment cost. In the 1978 market, the regenerative cycle offers the most economical gas turbine cycle.