Intercooling and reheating

The way to enhance the power output of a gas turbine can be achieved by intercooling and reheat. 

Intercooling and Reheat Effects. The net work of a gas turbine cycle is given by... 

Fuller analyses of a/s cycles embracing intercooling and reheating were given in a comprehensive paper by Frost et al. [3], but the analysis is complex and is not reproduced here. 

To further understand the thermodynamic philosophy of the improvements on the EGT cycle we recall the cycle calculations of Chapter 3 for ordinary dry gas turbine cycles—including the simple cycle, the recuperated cycle and the intercooled and reheated cycles. 

The Ericsson, Wicks, and ice cycles are modified Brayton cycles with many stages of intercooling and reheat. It has the same efficiency of the Carnot cycle operating between the same temperature limits. The Feher cycle is a cycle operating above the critical point of the working fluid. 

The Intercooled Regenerative Reheat Cycle The Carnot cycle is the optimum cycle between two temperatures, and all cycles try to approach this optimum. Maximum thermal efficiency is achieved by approaching the isothermal compression and expansion of the Carnot cycle or by intercoohng in compression and reheating in the expansion process. The intercooled regenerative reheat cycle approaches this optimum cycle in a practical fashion. This cycle achieves the maximum efficiency and work output of any of the cycles described to this point. With the insertion of an intercooler in the compressor, the pressure ratio for maximum efficiency moves to a much higher ratio, as indicated in Fig. 29-36. 

Fig. 1.10. Temperature-entropy diagram. showing reheat, intercooling and recuperation.

Several of the gas turbine cycle options discussed m this section (intercooling, recuperation, and reheat) are illustrated in Figure 4. These cycle options can be applied singly or in various combinations with other cycles to improve thermal efficiency. Other possible cycle concepts that are discussed include thermochemical recuperation, partial oxidation, use of a humid air turbine, and use of fuel cells. 

An ideal Brayton cycle is modified to incorporate multistage compression with intercooling, and multistage expansion with reheating. As a result of these modifications, does the efficiency increase ... 

Find the temperature of all states, power required by the compressors, power produced by turbine 1, which drives compressor 1, power produced by turbine 2, which drives compressor 2, power produced by power turbine 3, rate of heat supplied by the combustion chamber, rate of heat supplied by the reheater, rate of heat removed from the intercooler, and cycle efficiency. 

The thermal cycle efficiency of a Brayton cycle can be increased by adding more intercoolers, compressors, reheaters, turbines, and regeneration. However, there is an economic limit to the number of stages of intercoolers, compressors, reheaters, and turbines. 

If an infinite number of intercoolers, compressors, reheaters, and turbines are added to a basic ideal Brayton cycle, the intercooling and multicompression processes approach an isothermal process. Similarly, the reheat and multiexpansion processes approach another isothermal process. This limiting Brayton cycle becomes an Ericsson cycle. 

The second step is to develop several conceptual plants (e.g., cycles A, B, and C) to meet the identified need. One of the several plants is described in Example 5.14. In this example, a three-stage regenerative steam Rankine cycle and a four-stage intercool and four-stage reheat air Brayton cycle are combined to meet the need. 

FIG. 29-36 Performance map showing the effect of pressure ratio and turbine inlet temperature on an intercooled regenerative reheat split-shaft cycle. 

The main cause of the high efficiency is the use of a gas turbine with intercoolers, recuperation, reheater, and a 100 C higher turbine inlet teijperature than currently applied. 

Power plants under about 350 MWe cannot use the latest high-efficiency combined cycle technologies. Those below about 250 MWe cannot use particularly high-efficiency steam turbines because of friction losses and leaks in small dimension gas paths. Those below about 100 MWe cannot economically use reheat steam cycles, giving a further efficiency drop. Moving further down in size gives a steady reduction in efficiency of the gas turbine, whichever manufacturer is selected. The scale effect of gas turbine efficiencies is due to flow paths and pressure drops and can only be partly compensated for with additional components such as intercoolers or reheaters. 

This cycle produces an increase of 30% in work output, but the overall efficiency is slightly decreased as seen in Figure 2-15. An intercooling regenerative cycle can increase the power output and the thermal efficiency. This combination provides an increase in efficiency of about 12% and an increase in power output of about 30%, as indicated in Figure 2-16. Maximum efficiency, however, occurs at lower pressure ratios, as compared with the simple or reheat cycles. 

Fig. 1.10 shows the processes of heat exchange (or recuperation), reheat and intercooling as additions to a JB cycle. Heat exchange alone, from the turbine exhaust to the compressed air before external heating, increases and lowers so that the overall... 

In the ultimate version of the reheated and intercooled reversible cycle [CICICIC- HTHTHT- XJr, both the compression and expansion are divided into a large number of small processes, and a heat exchanger is also used (Fig. 3.6). Then the efficiency approaches that of a Carnot cycle since all the heat is supplied at the maximum temperature Tr = T ax and all the heat is rejected at the minimum temperature = r,nin. 

While the lowest and highest optimum pressure ratios are for these two plants, the addition of reheating and intercooling increases the optimum pressure ratios above that of... 

Finally, carpet plots of efficiency against specific work are shown in Fig. 3.16, for all these plants. The increase in efficiency due to the introduction of heat exchange, coupled with reheating and intercooling, is clear. Further the substantial increases in specific work associated with reheating and intercooling are also evident. 

Fig. 3.16 showed carpet plots of efficiency and specific work for several dry cycles, including the recuperative [CBTX] cycle, the intercooled [CICBTX] cycle, the reheated [CBTBTX] cycle and the intercooled reheated [CICBTBTX] cycle. These are replotted in Fig. 6.17. The ratio of maximum to minimum temperature is 5 1 (i.e. T nx 1500 K) the polytropic efficiencies are 0.90 (compressor), 0.88 (turbine) the recuperator effectiveness is 0.75. The fuel assumed was methane and real gas effects were included, but no allowance was made for turbine cooling.

The CHAT cycle may be seen as a low loss evaporative development of the dry intercooled, reheated regenerative cycle [CICBTBTX]. It offers some thermodynamic advantage—increase in turbine work (and heat supplied ) with little or no change in the compressor work, leading to an increased thermal efficiency and specific work output. 

The pressurized hybrid cycle provides the basis for the high electric efficiency power system. Applying conventional gas turbine technology, power system efficiencies in the 55 to 60 percent range can be achieved. When the pressurized hybrid system is based on a more complex turbine cycle— such as one that is intercooled, reheated, and recuperated—electric efficiencies of 70 percent or higher are projected. 

Figure 4.13 Reheat and intercool Brayton cycle T-s diagram.

Figure 4.14 Ideal reheat and intercool Brayton cycle.

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